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A review of the changes in soil quality and profitability accomplished by sowing rotation crops after cotton in Australian Vertosols from 1970 to 2006.

Introduction

Sustainability in any farming system is dependent upon several interacting factors which include climate, soil quality, plant nutrition, management, weed and disease incidence, and economic factors (Greenland and Szabolcs 1994). Soil quality is defined as 'the degree of fitness of a soil for a specific use its ability or capacity to function for a specific purpose' (Doran 2002; Gregorich 2002). In agricultural systems, soil quality is thought of in terms of productive land that can maintain or increase farm profitability, as well as conserving soil resources so that future farming generations can make a living (Gregorich 2002).

Indicators of soil quality include soil structural indices (aggregate stability, porosity) and related measures such as available water holding capacity, strength, and drainage and leaching potential; labile, microbial, and total soil organic carbon (SOC); exchangeable cations and cation exchange capacity; pH; soil nitrates and phosphates; salinity; sodicity; and accumulated toxins such as herbicides and pesticides (Karlen et al. 1992; Doran et al. 1994; Walker and Reuter 1996; Doran 2002; Gregorich 2002). In addition, the presence or absence of soil fauna such as ants and earthworms can be used as indicators of 'good' or 'poor' soil quality (Walker and Reuter 1996). Management systems whereby soil quality can be modified and managed include tillage and stubble management systems, and sowing crop rotations (Karlen et al. 1992). Among the above soil quality indicators, SOC has been proposed as a primary indicator of soil quality (Lal 1997; Reeves 1997). The frequency and amounts of carbon and N inputs needed to replenish soil carbon and N reserves have been suggested as good indicators of long-term sustainability of many cropping systems, and have been incorporated into predictive models of sustainability (Lal 1997; Reeves 1997; Freebairn et al. 1998; Probert et al. 2005). Predictive models derived for dryland clay soils suggest that the first indicators of a system run-down under commercial cropping are increased requirements of fertiliser N (and other nutrients such as P and S) and water to maintain yields. In the longer term, yield and profitability losses also occur (Freebairn et al. 1998; Probert et al. 2005).

A major research program was initiated during the late 1970s due to extensive yield losses caused by widespread deterioration in soil physical and chemical quality associated with tillage, trafficking, and picking under wet conditions. Its primary objective was to develop soil management systems which could improve cotton yields while concurrently ameliorating and maintaining soil structure and fertility (MeKenzie et al. 2003; Constable 2004). An outcome of this research was the identification of cotton-winter crop sequences sown in a 1 : 1 rotation as being able to sustain lint yields while at the same time maintaining soil physical quality and minimising fertility decline. A second phase of research on cotton rotations was initiated during the early 1990s with the main objective of identifying sustainable cotton--rotation crop sequences, viz. crop sequences which maintained and improved soil quality, minimised disease incidence, facilitated soil organic carbon sequestration, and maximised economic returns and cotton water use efficiency in the major commercial cotton growing regions of Australia (McKenzie 1993). Secondary objectives were to identify and provide solutions to any management constraints of the cotton--rotation crop sequences, and optimise growth and yield of the rotation crop through fertilisers, irrigation and tillage systems. The objective of this review is to summarise the key findings of both of these phases of Australian research on cotton--rotation crop sequences with respect to soil quality and profitability, and identify future areas of research. This paper reviews the Australian research conducted between the early 1970s and 2006. Furthermore, as a major proportion of Australian cotton is grown under irrigation, the primary focus of this review is irrigated cotton-based farming systems of Australia.

Australian cotton production systems: Geography, land preparation, and cropping systems

Major Australian commercial cotton (Gossypium hirsutum L.) production areas are located between Emerald (23.53S, 148.18E; average annual rainfall 640mm), west of Rockhampton in central Queensland, and Hay (34.31S, 144.31E; average annual rainfall 364mm) in the Murrumbidgee valley in southern New South Wales. Climatically, they range from the summer-dominant rainfall regions of the subtropics in Queensland to the winter-dominant rainfall regions in southern New South Wales. Approximately 70% of Australian commercial cotton is grown in New South Wales and about 30% in Queensland.

Under normal rainfall, ~80% of Australian cotton is irrigated, with the major areas of irrigated cotton being located in the Gwydir, Namoi, Macquarie, and Lachlan Valleys of New South Wales; and in the St George, Darling Downs, Theodore--Biloela and Emerald districts of Queensland (McKenzie et al. 2003). The most widely used method of irrigation is furrow irrigation (>94%), with smaller areas being grown with drip (4%) and sprinkler (<2%) irrigation systems (Raine and Foley 2002). The major soil types on which cotton is grown are grey, brown and black Vertosols (~75%) and Chromosols, Sodosols, and Dermosols (15%) (MeKenzie et al. 2003). Australian Vertosols have high clay contents (40-80g/100g) and strong shrink-swell capacities, but are prone to deterioration in soil physical quality if incorrectly managed (McGarry 1995; McKenzie et al. 2003). Soil maps and surveys of Australia also suggest that the surface, subsurface or both in a majority of cotton-growing regions in eastern Australia can be described as 'sodic' (Northcote and Skene 1972; Naidu et al. 1995).

Land preparation methods used in irrigated cotton production range from intensive tillage (deep ripping, disc-ploughing, chisel-ploughing, and bed reconstruction every year) to minimum tillage or permanent beds (Schoenfisch 1999; McKenzie et al. 2003). Spacings of 'permanent beds' are commonly either 1 m (also referred to as retained hills) or 2 m ('broad beds') (Cooper 1999). By definition, a permanent bed implies that the bed stays in place for several seasons in comparison with being ploughed down and reconstructed every year as with more intensive tillage systems (McGarry 1995; Cooper 1999; McKenzie et al. 2003). Significant improvement in soil physical, chemical, and biological properties can arise through the long-term use of permanent beds (Hulugalle and Daniells 2005). The term permanent bed does not, however, imply that all bed disturbance is totally excluded. Some pre-sowing light cultivation may be necessary to renovate or reshape beds, and heliothis (Helicoverpa spp.) pupae must be destroyed by bed disturbance (Hulugalle et al. 2006c). Mechanical disturbance of beds facilitates entry of parasitic wasps and other predators of heliothis pupae into the moths' pupating chambers, thereby greatly reducing the numbers of adult moths that emerge in early spring. Advantages and disadvantages of permanent beds with respect to soil quality and profitability in Australian cotton-based farming systems have been reviewed by Hulugalle and Daniells (2005).

Permanent beds were first introduced in the mid 1980s after yields had declined in the Australian cotton industry through soil structure-related causes (McGarry 1995; McKenzie et al. 2003). Surveys conducted during the early 1990s indicated that ~80-90% of Australian cotton growers used some form of 'permanent beds' (McGarry 1995; Cooper 1999), although with the industry-wide recommendation of compulsory post-harvest tillage for heliothis moth pupae control (Rourke 2002), numbers may have fallen since then.

In New South Wales and southern Queensland, cotton is sown during October and picked during April or May of the following year. In central Queensland cotton can be sown and picked earlier. Cropping systems under which irrigated cotton is grown can be broadly classified into 3 groups: (1) cotton monoculture (cotton--winter fallow--cotton), where cotton is sown in the same field every year indefinitely; (2) long-fallow cotton (cotton--winter, summer and winter fallow--cotton), where cotton alternates with a bare fallow; and (3) cotton--rotation crop sequences where cotton alternates with either summer or winter rotation crops (Cooper 1999). In New South Wales and southern Queensland, the crop sequence when a winter rotation crops is sown is cotton--winter rotation crop--summer and winter fallow--cotton, and for a summer rotation crop, cotton--winter fallow--summer rotation crop--winter fallow--cotton. In central Queensland cotton can be sown immediately after the winter rotation crop with no fallow period, due to the warmer temperatures and longer growing period in the subtropical and tropical zones. Fixed crop rotations are more common in irrigated cotton production systems, whereas opportunity cropping is the norm in dryland systems. The first reported use of rotation crops in New South Wales cotton production systems was during the period 1965-72, when wheat (Triticum aestivum L.) was sown after cotton for the first time by several cotton growers in the lower Namoi valley of New South Wales (Cutting 1978) shortly after commercial cotton was first grown in New South Wales during the summer of 1961-62 (Cotton Australia 2006). Following this (1972-77), other crops such as sorghum (Sorghum bicolor Moench.), soybean (Glycine max L.), and sunflowers (Helianthus annuus L.) were also sown as rotation crops. Similar information is not available for the Queensland cotton industry. However, given the longer history of cotton growing in Queensland--commercial cotton production in Queensland commenced during the 1860s (Buchanan 1999; Cotton Australia 2006)--a similar pattern is likely to have occurred with summer rotation crops such as com and sorghum dominating in these summer-dominant rainfall zones. Buchanan (1999) notes that corn (Zea mays L.) was one of the first crops grown by immigrant cotton growers in the Logan shire of Queensland.

A majority of cotton growers favour sowing a cereal or leguminous crop in rotation with cotton. A survey of 155 cotton farms (100% of the irrigated cotton area in the central-west and 54% in the north-west of New South Wales) conducted during December 1992 indicated that 53% and 81% of cotton growers in the central-west and north-west, respectively, of New South Wales frequently sowed rotation crops after cotton (Cooper 1999). A follow-up survey in 1998 indicated that the proportions of growers practicing rotation cropping were similar (~79%) in both regions (Hickman et al. 1998). The most recent survey, which was conducted during the 2005-06 cotton season, indicated that in both New South Wales and Queensland, rotations were used by 82% of cotton growers (Doyle and Coleman 2007). This survey also indicated that 97% of cotton growers in the Namoi valley and 88% in the southern cotton regions (Macquarie and Murrumbidgee valleys) of New South Wales sowed a rotation crop at least once in 3 years, whereas in the northern cotton regions (Darling Downs, Dawson valley, Emerald) only 66% did so. Wheat was the favoured rotation crop in irrigated systems, with 71% of cotton growers who sowed rotation crops in the central-west and 74% in the north-west of New South Wales sowing wheat in either a 1 : 1 or 2:1 cotton-wheat rotation (Hickman et al. 1998; Cooper 1999). Cereal crops also dominate the rotations used by dryland cotton growers, with 51% preferring winter cereals such as wheat and barley, 8% summer cereals such as sorghum and corn, and 16% summer and winter cereals (Walker et al. 2005). The 1992 survey indicated that legumes such as field pea (Pisum sativum L.) and soybean were preferred as rotation crops by 13% of cotton growers in the central-west, wheras 6% in the north-west (increasing to about 10% by 1998) preferred legumes such as soybean, field pea, faba bean (Vicia faba L.), chickpea (Cicer arietinum L.), and dolichos (Lablab purpureus L.) (Hickman et al. 1998; Cooper 1999). Walker et al. (2005) in their 2001 survey of dryland cotton growers noted, however, that only 2% used legumes alone as rotation crops, whereas 16% used a combination of legumes and cereals. Other crops sown in rotation with irrigated cotton include corn and sorghum, and more recently, Namoi woolly pod vetch (Vicia villosa Roth.) (Hickman et al. 1998; Cooper 1999; Rochester and Peoples 2005). Anecdotal evidence suggests that both corn and woolly pod vetch have gained in popularity during the past 5 years.

Soil quality and crop rotations

Research conducted during the 1970s in the lower Namoi valley of New South Wales indicated that soil structural amelioration was better after safflower (Carthamus tinctorius L.) than after wheat (Hodgson and Chan 1984), although later research in the Macquarie valley suggested that major differences were not detectible between the 2 crops (Hulme et al. 1991). Safflower, however, does, have several disadvantages: firstly, it is extremely thorny and can cause some discomfort during harvesting; secondly, harvest usually occurs during January when frequent summer storms can result in wet harvests, which may reduce harvestable yield and grain quality; thirdly, available safflower varieties have low yield potential; and fourthly, it is an alternative host of spider mites (Tetranychus spp.), a pest of cotton (Beale 1982; Anon. 2007; T. Farrell, pers. comm.). Consequently, it has proved to be unpopular with cotton growers (Cooper 1999).

Hearn (1986) reported that without applied N, irrigated cotton yield was greatest after wheat or long-fallow and least after cotton or sorghum, with soybeans intermediate. Response to N was greatest after cotton or sorghum, least after wheat or long-fallow, with soybeans again intermediate. These effects were associated with structural degradation of the subsoil, with wheat rotation crops improving structure the most. Constable et al. (1992) also reported that in comparison with cotton monoculture, irrigated cotton sown after a wheat rotation crop produced more lint of a better quality. This was, again, thought to be due to amelioration of soil structure in the 0.10-0.30m depth interval and better recycling of soil N by the wheat.

The above results may also partly be related to better rainfall conservation (CSD (Cotton Seed Distributors) Extension and Development Team and Hulugalle N 2006), as both cotton--wheat (summer cotton winter wheat--summer fallow--winter fallow--summer cotton) and long-fallow cotton (summer cotton--winter fallow--summer fallow--winter fallow--summer cotton) in northern New South Wales include summer and winter fallows before cotton is sown in the following year. Cotton monoculture (summer cotton winter fallow--summer cotton) or cotton--sorghum (summer cotton--winter fallow--sorghum--winter fallow--summer cotton) includes only winter fallows. The longer fallow periods in clayey Vertosols enable more rainfall to be stored under long-fallow cotton and cotton--wheat than under cotton monoculture or cotton--sorghum (CSD (Cotton Seed Distributors) Extension and Development Team and Hulugalle N 2006).

More recent research has also demonstrated that wheat rotation crops can improve overall soil quality under irrigated and dryland conditions and in a range of climates where cotton is grown. In several long-term on-farm and on-station field experiments, Hulugalle et al. (1999b, 2001, 2002a, 2002b, 2005a, 2007) reported that better soil structure, characterised by a higher air-filled porosity and specific volume, resulted by growing wheat after cotton than by growing legumes such as dolichos, chickpea, faba bean, and field pea (Fig. 1). This is because the intensity of wet/dry cycles is greater with wheat than with legumes, which in turn is related to wheat's higher subsoil root density in most Vertosols and, consequently, its ability to dry out the subsoil to a greater extent (Hulugalle et al. 2001). Intense and frequent wet/dry cycles in swelling soils such as Vertosols are reported to increase soil porosity and improve structural stability (McGarry 1995; Pillai and McGarry 1999).

[FIGURE 1 OMITTED]

Greater profitability, recycling of leached N, and a lower incidence of black root rot (a seedling disease of cotton caused by the fungus Thielaviopsis basicola) are also more likely with wheat than with legumes (Hulugalle 2005; Hulugalle et al. 1999b, 2001, 2002a; Nehl and Allen 2002). Conteh et al. (1998a) also reported that soil organic carbon fractions, particularly the lighter fractions, were higher when wheat rather than legumes were sown in rotation with cotton. The light fraction of the soil organic matter is directly related to soil structural stability, nutrient recycling, and biological activity (Doran et al. 1994; Doran 2002; Six et al. 2002).

Leguminous rotation crops can improve soil aggregate stability and N by fixing atmospheric nitrogen, by reducing N volatilisation and leaching losses, and by decreasing exchangeable sodium content through a process of chemical exchange rather than leaching (Hulugalle and Cooper 1994; Hulugalle et al. 1996, 1999b, 2001; Rochester et al. 1998, 2001; Rochester and Peoples 2005). However, these changes, particularly in sodic soils, are restricted to surface soils. Among legumes, faba bean and Namoi woolly pod vetch increase soil N more than field pea, chickpea, or dolichos (Rochester et al. 1998, 2001; Rochester and Peoples 2005). Two studies, a field study by Rochester et al. (2001) and a laboratory study by Pillai and McGarry (1999), have also suggested that aggregate stability, subsoil strength, and compaction were reduced more by leguminous than by cereal rotation crops. Both these studies, however, were conducted in good quality soil which did not have any major subsoil chemical constraints such as high sodicity or salinity, whereas most of the other research (Hulugalle et al. 1999b, 2001, 2002a) was conducted in on-farm locations where subsoil sodicity was present.

Commonly grown row-crop legume varieties are also more sensitive to salinity and sodicity than cereal crops such as wheat or sorghum (Maas and Hoffman 1976). It is not surprising, therefore, that under most on-farm conditions, where subsoil sodicity commonly occurs, cereal rotation crops perform better than leguminous crops in maintaining soil physical and chemical quality. Legumes in the rotation may, however, reduce cotton yield because: (a) herbicides commonly used with legumes are incompatible with cotton; (b) they can be alternative hosts of seedling disease-causing organisms of cotton such as such as Thielaviopsis basicola, which is the causal agent of black root rot, Fusarium, Rhizoctonia, and Pythium. Namoi woolly pod vetch is an exception in that it is not an alternative host of black root rot of cotton, although it can be a host to other disease-causing organisms of cotton such as Fusarium, Rhizoctonia, and Pythium (Nehl and Allen 2002); (c) their seed material can be allelopathic to cotton (Hulugalle et al. 1998, 2001, 2002a; Nehl and Allen 2002; Fujii 2003). Fujii (2003) suggests that with respect to Namoi woolly pod vetch, allelopathy is mainly due to the chemicals cyanamide and L-cyanoalanine.

As previously noted, sowing leguminous rotation crops can result in significant benefits to the N nutrition of cotton through a combination of N fixation (depending on stubble management, crop harvest index, type, and health, this can range between 50 and 300 kg N/ha) and minimising N losses by volatilisation and leaching (Rochester et al. 1998, 2001; Rochester and Peoples 2005). Seasonal N uptake by Australian cotton crops can range between 67 and 403kgN/ha, of which 52% is exported in lint and seed (Rochester 2007). Consequently, reductions of 30-100% are possible with respect to N fertiliser rates for cotton. Wheat rotation crops, in contrast, can recover N leached with deep drainage during the cotton season (Hulugalle 2005). Applying N fertiliser to the wheat, in addition to increasing wheat grain yield and protein content, can further improve recovery by increasing the depth and density of the wheat root system (Hulugalle and Entwistle 2001). Over 6 years in an on-farm experiment near Wee Waa, northern New South Wales, average N recovery from the subsoil (>0.6m depth) by N-fertilised wheat was 110kgN/ha per cotton-wheat cycle and by unfertilised wheat 76kgN/ha (Hulugalle 2005). Assuming that the cost of anhydrous ammonia is AU$900/t, then the value of N recovered by the fertilised wheat was $99/ha.cycle and by unfertilised wheat $68/ha.cycle. The recovered N, when released into the soil by decomposing wheat stubble, can be used by the following cotton crop.

Review of the literature indicates that virtually all research on soil quality with respect to cotton rotations in irrigated Vertosols has been conducted on 1:1 rotations, i.e. where cotton alternates with a rotation crop. More complex cropping systems where 2 or more rotation crops are sown after cotton have rarely been studied. An exception to this is the cotton--wheat--Namoi woolly pod vetch rotation reported by Rochester and Peoples (2005) and Williams et al. (2005) who observed that nitrogen fertiliser requirements were negligible with a cotton--wheat--vetch--sequence in relation to either cotton--wheat or cotton--vetch. This was due to the better growth of the vetch after wheat, which in turn resulted in more nitrogen fixation (averaging 227kgN/ha) by the former. In contrast to irrigated systems, dryland crop sequences which include 3 or 4 crops and where cotton is a component crop in an opportunity cropping system have been studied extensively by many authors using a combination of simulation models and long-term experiments. The results indicate that water storage during summer and winter fallows and their interaction with stubble retention, in-crop rainfall amount and distribution, and soil physical and chemical quality are the main drivers of profitability (Probert et al. 2005; deVoil et al. 2006; Thomas et al. 2007).

Residual effects of rotation crops on soil quality

The previously discussed changes in soil properties were reported to occur during periods when the cotton--rotation crop sequences were in place. Studies on the residual effects of rotation crops, that is, several years after the cotton--rotation crop sequence was discontinued, are sparse. Hulugalle et al. (1997) reported that improvements in subsoil porosity, water extraction, aggregate stability, and soil organic carbon, and reductions in sodicity, brought about by sowing a cotton--wheat sequence on permanent beds over a 10-year period were detectible even after 5 years of cotton monoculture in an irrigated Vertosol. This same crop sequence also had higher cotton lint yield and numbers of ants. Ants can be used as an indicator species of soil biological quality in cotton-based farming systems as they are strongly influenced by management practices, which have both negative and positive effects on soil quality such as irrigation, cultivation, and broad-spectrum insecticide use (Reid et al. 2003). Similarly, in 2 on-farm experiments conducted in a medium and a heavy clay (both Vertosols), the residual effects of wheat rotation crops were characterised by higher soil organic carbon and better structure up to 5 years after the rotation sequences had been discontinued, whereas higher nitrate-N concentrations were present for 2-3 years after leguminous rotation crops had been sown (Hulugalle et al. 2006a). Soil structure was also better under ex-long-fallow cotton, presumably due to a lower cropping intensity. These authors concluded that residual effects of crop rotations were more likely to occur where the residues of the rotation crops are relatively recalcitrant or where cropping intensity is lower. Weaver et al. (2004) observed that deep drainage in a sodic Vertosol reflected rotation history 3 years after individual rotation treatments had been discontinued due to long-term changes in soil structure and exchangeable sodium percentage. Deep drainage out of the 0.9-m and 1.2-m depths were in the order of ex-cotton-wheat > ex-cotton monoculture >> ex-cotton--dolichos. Leaching of excess soluble salts and exchangeable sodium were also in the same order (Hulugalle et al. 2004; Weaver et al. 2004).

Can soil organic carbon content be increased by sowing rotation crops?

In many cotton farms, soil organic carbon (SOC), a key indicator of soil quality and fertility, has decreased from 11-12 kg/[m.sup.2] in the surface 0.6 m when conversion from grass pasture to irrigated cotton occurred, to values of 5-7 kg/[m.sup.2] (calculated from McGarry et al. 1989; Hulugalle 2000). In many locations this decrease has continued despite frequent sowing of rotation crops (Fig. 2a, Warren; adapted from Hulugalle 2000). Rates of SOC decline are generally in the range 0.22-0.35kg/[m.sup.2].year (Hulugalle 2000). The major causes for this appear to be:

(1) Insufficient amounts of crop residues being returned to the soil. Approximately 2-3 kg/[m.sup.2] of dry matter needs to be returned to the soil to maintain or increase SOC, whereas most cotton-based cropping systems typically return 0.8-1.2kg/[m.sup.2] (Hulugalle and Weaver 2005). An example of this is shown in Fig. 2b for Merah North in north-western NSW, where a decrease in SOC between 1994 and 2001 was followed by an increase. The cropping sequence from 2000 to 2002 was irrigated cotton, followed by irrigated and fertilised wheat, which was, in turn, followed by irrigated sorghum (Hulugalle et al. 2006a). The above-ground dry matter returned to the soil by the 2 cereal crops was of the order of 2.5 kg/[m.sup.2] and by cotton, 0.3kg/[m.sup.2]. Cumulative below-ground additions were of the order of 1.1kg/[m.sup.2] by 2 cereals, and 0.1 kg/[m.sup.2] by cotton (N. R. Hulugalle, unpublished data). The increase in SOC after the sorghum crop may be partly related to the input of more recalcitrant SOC from a [C.sub.4] crop (sorghum) relative to that from a [C.sub.3] crop (wheat) (Skjemstad et al. 1994). Other examples of cropping sequences which return high amounts of dry matter to the soil are cotton--corn, and cotton--vetch and cotton--wheat--vetch sequences on permanent beds, all of which return above-ground crop residues in amounts ranging from 1.5 to 2.5kg/[m.sup.2] (Rochester and Peoples 2005; N. R. Hulugalle, unpublished data). Sowing a wheat rotation crop alone, however, can slow the rate of SOC decrease, particularly if the wheat stubble is not incorporated but is retained as standing stubble (Fig. 3; Conteh et al. 1998a). Much of the SOC in this system is stored in the surface 0.30 m (Hulugalle et al. 2003, 2005a; Conteh et al. 1998a).

(2) Management practices such as intensive tillage operations, burning of crop stubble, excessive water, and N application rates. Modification of existing management practices is one approach by which SOC decline can be minimised. Frequent and deep tillage operations can rapidly increase the rate of SOC decomposition, even if cotton is sown in rotation with a cereal crop which produces significant amounts of crop residues (Constable et al. 1992; Hulugalle and Entwistle 1997; Hulugalle 2000; Hulugalle et al. 1997, 2003, 2005a; Hulugalle and Daniells 2005). Burning of crop stubble, although acknowledged as soil degrading, is practised by many cotton growers (Cooper 1999; CRC for Sustainable Cotton Production, unpublished survey, 1995). A majority of cotton growers in New South Wales tend to slash and incorporate crop stubble with a range of machinery (Schoenfisch 1999), while the proportion of growers regularly burning crop stubble is higher in Queensland, especially in the southern Queensland regions of Dirranbandi and St. George where >95% of cotton growers burn their crop stubble (CRC for Sustainable Cotton Production, unpublished survey, 1995). Australian sources report that stubble burning in Vertosols, as in other soil types, results in degradation of their physical and chemical properties (Dalal 1989; Rochester and Constable 1996; Rochester et al. 1997; Conteh et al. 1998b). Hulugalle and Cooper (1994) noted that both soil fertility and physical properties in cotton fields with a history of stubble burning was poorer than those in fields where stubble had been incorporated. Similarly, in comparison with cotton stubble incorporation, Conteh et al. (1998b) observed significant declines in soil organic matter and its light fractions, but not the total N content, after 3 years of burning cotton stubble. Rochester and Constable (1996) and Rochester et al. (1997) at the same site noted that cotton lint yield, profitability, and N fertiliser recovery were inhibited by removal of cotton stubble in comparison with incorporating it. A steady decline in lint yield and profitability also occurred with time. All these studies focused on burning cotton stubble but not rotation crop stubble in cotton-based farming systems. Controlled experiments which examine the effects of burning rotation crop stubble with other methods of stubble management such as incorporation and retention as surface mulch in cotton-based fanning systems on Vertosols have not, however, been reported in the literature. Soil quality decline due to burning of rotation crop stubble is, therefore, difficult to ascertain, and research needs to be conducted in this area. Nonetheless, given the role played by rotation crop stubble in soil physical and chemical fertility, nutrient recycling, soil fauna and flora conservation, and water conservation, the negative consequences of burning its stubble are likely to be significant (Dalal 1989; Radford et al. 1995; Bell et al. 2006). As an alternative to intensive tillage, permanent beds have been used by many cotton growers. Although they are thought to conserve SOC stocks, only a few studies, the majority of them on-station experiments, have been able to document this (Hulugalle 2000; Hulugalle et al. 1997, 2005a, 2006a). Most on-farm experiments managed by cotton growers were not able to demonstrate that permanent beds were able to reverse declines in SOC stocks (Hulugalle 2000; Hulugalle et al. 1999b, 2001, 2002a). This may be related to the growers' management practices with respect to stubble, N, and irrigation. Both N and irrigation water were applied at higher rates on-farm than on the experiment station (CSD (Cotton Seed Distributors) Extension and Development Team and Hulugalle N 2006). Managing rotation and cotton crop stubble in permanent bed systems usually involves either slashing and incorporating stubble into the beds with shallow cultivation equipment such as Lilliston cultivators and disc-hillers or burning of crop stubble (Schoenfisch 1999) in combination with centre-busting (disturbance of the bed centre with a tine to a depth of a about 0.3 m). In irrigated systems, retention of crop stubble as surface mulch ('standing stubble') is constrained due to: blocking of 'gas knives' by crop stubble during application of anhydrous ammonia as fertiliser, increased waterlogging during irrigation, and inability to incorporate residual herbicides (Henggeler et al. 2000). Management practices to overcome these problems have been reviewed by Hulugalle and Daniells (2005), although large knowledge gaps remain. Sowing cotton into standing wheat stubble can, however, result in soil organic carbon and exchangeable K increasing, and ESP decreasing (Hulugalle et al. 2003, 2005a, Hulugalle et al. 2006b). A decrease in ESP occurs through an increase in deep drainage and leaching under 'standing stubble' systems and is especially advantageous in sodic soils (Hulugalle et al. 2005b, 2006b). Disadvantages associated with sowing cotton into standing rotation crop stubble include the abovementioned management problems, and accelerated deep drainage and nutrient leaching (Hulugalle et al. 2005b).

(3) Seasonal climatic conditions and climatic extremes such as floods and droughts. The wet soil under irrigated fanning systems and the warm to hot conditions which prevail in New South Wales and Queensland during summer facilitate SOC mineralisation (Felton et al. 2000). This leads to rapid decomposition of soil organic matter during the cotton-growing season. Hulugalle et al. (2003, 2004) also observed that following intensive flooding, sharp and very rapid decreases in SOC occurred, even under permanent beds, whereas a steady decline in SOC was more characteristic of extended drought conditions (Fig. 2d, Hulugalle et al. 2007).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Steady decline in SOC is, not, however, universal in Australian Vertosols sown with cotton-based farming systems and exceptions do occur. In some locations, SOC has remained stable at a relatively high level (Fig. 2c, Wee Waa, adapted from Hulugalle 2000). Similar differences in dryland Vertosols under grain crops and pastures in Queensland have been reported by Skjemstad and Dalal (1987) and Skjemstad et al. (1994), who observed that rapid losses of SOC occurred in grey Vertosols, whereas SOC in black Vertosols was more recalcitrant. Skjemstad and Dalal (1987) noted that SOC in the former was dominated by alkyl compounds which were highly susceptible to microbial degradation, whereas that in the black Vertosol was characterised by aromatic compounds resistant to microbial degradation. In a subsequent study, Skjemstad et al. (1994) suggested an interaction between the dominant pre-cropping vegetation types ([C.sub.3] v. [C.sub.4] plants) and Vertosol type (black, brown, and grey Vertosols), and hence their clay mineralogy, may influence SOC levels through a combination of physical and chemical protection from microbial decomposition. Physical and chemical protection of SOC may occur due to the chemical nature of the soil mineral fraction and the presence of multivalent cations, the presence of mineral surfaces capable of adsorbing organic materials, and the architecture of the soil matrix (Baldock and Skjemstad 2000). A model of soil organic carbon dynamics proposed by Six et al. (2002) also makes the same point, albeit more explicitly. They state that SOC decomposition is greatly reduced by having a greater proportion of the soils' organic carbon stocks in the biochemieally protected and microaggregate-protected C pools. This may be the reason why SOC levels in some Vertosols remain relatively stable at a higher level, whereas in others it continues to decline.

To summarise, while declining SOC is the norm in most cotton-based farming systems, exceptions to the general rule do occur. Understanding the underlying processes with respect to these 'exceptions' and developing suitable management systems for retaining crop stubble in situ may enable a reversal of the decline in SOC.

Rotation crops, biodiversity, and soil quality

Published reports on the effects of rotation crops in cotton-based farming systems on soil biodiversity are few (Reid et al. 2003). Even fewer reports have been published on the effects of these biota on soil quality. The little research that has been conducted is limited to soil invertebrates and microflora. With respect to soil invertebrates, Hulugalle et al. (1997) observed that compared with a cotton monoculture, ant and springtail numbers were highest when a cotton--wheat rotation was sown on permanent beds. The ants are able to change soil quality in cotton fields, particularly in areas near and adjacent to ant hills and foraging paths, by increasing soil organic matter, nitrates, and phosphates; by reducing sodicity; and by improving soil structure and deep drainage (Hulugalle 1995; Nkem et al. 2000). Nkem et al. (2000) also noted that pH of soil in foraging paths was lower than that of bulk soil. This difference may result in improving uptake of Ca, Mg, K, and phosphates in foraging paths by increasing the solubility of their respective carbonates and sulfates. In a comparison of N-fertilised and unfertilised wheat rotation crops, Nkem et al. (2002) observed that numbers of ants such as Iridomyrmex spp. and Pheidole spp., and other insects, were higher in the unfertilised wheat due to higher soil temperatures caused by less ground cover. For example, an average of 6.6 Iridomyrmex spp. and 5.5 Pheidole spp. were collected per pitfall trap under the N-fertilised wheat crop, and 12.4 Iridomyrmex spp. and 23.3 Pheidole spp. per pitfall trap under the unfertilised wheat crop during the 1997 winter.

In addition to insects, other invertebrates such as earthworms can also be found in cotton production systems in Vertosols (Hulugalle et al. 1999a). The effects of rotation crops on earthworm populations have not been studied, although lucerne (Medicago sativa L.) strips in cotton fields can increase their numbers. Native earthworms (Myall worms, Heteroporodrilus mediterreus) can modify soil structure through their burrows, which in turn results in higher infiltration, drainage, and salt leaching rates, and better subsoil aeration (Friend and Chan 1995; Hulugalle et al. 1999a).

High soil microbial activity is frequently assumed to reflect better soil quality (Karlen et al. 1992; Walker and Reuter 1996; Doran 2002; Gregorich 2002), although field-based studies that conclusively demonstrate this in cotton-based farming systems have not been reported in the literature. Similarly, very few studies have compared the effects of different cropping systems on soil microbial activity. Luelf et al. (2006) reported that in comparison with cotton monoculture, cotton-vetch and cotton-wheat-vetch rotations, soil microbial biomass at 72 days after sowing was highest under a cotton-wheat rotation where wheat stubble had been incorporated into the beds (Luelf et al. 2006). This was thought to be due to the greater root activity and, hence, higher root exudation rates in this rotation. It was, however, difficult to ascertain whether this higher microbial biomass conferred any benefit to the soil or the crop. Bell et al. (2006) examined the effects of several rotations under dryland conditions, viz. cotton monoculture with long, bare fallows, cotton, cotton--sorghum, cotton--wheat double cropped, cotton--chickpea double cropped followed by wheat, and cotton--wheat with a long fallow, and concluded that microbial activity was related to the length of the fallow rather than to the rotation per se, and that it was restricted to the soil's surface layers. As their measurements were made during a period of intensive drought, their conclusions may not be applicable to periods of normal or above-average rainfall. In addition to total microbial activity in soil, several studies have documented the physiology and agronomy of arbuscular mycorrhiza (AM) in cotton crops and associated soil (Nehl et al. 1996, 1998, 1999). AM activity is reported to confer many benefits with respect to soil fertility, carbon storage, structural and hydrological properties, and crop nutrition (Hamel 1996; Degens 1997; Tisdall et al. 1997; Bearden and Petersen 2000; Bearden 2001; McGee et al. 2004). However, under conditions typical of Australian commercial cotton farming practices, there appear to be no differences among crop rotations with respect to AM populations, measured by root staining, in cotton crops (Hulugalle et al. 1999b, 2004). These papers also noted that AM numbers were above critical threshold values (25% of total root length) for cotton among all rotations studied.

Although the beneficial effects of soil biodiversity on quality of soil are claimed to be many (Reid et al. 2003), except for a few studies on soil macrofauna such as ants, conclusive field-based evidence that demonstrates this has not been forthcoming with respect to cotton rotations. This is due, firstly, to a very limited amount of field-based research on this topic, and secondly, to the virtual absence of detailed interdisciplinary research encompassing soil physics, chemistry, and biology.

Rotation crops, cotton lint yield, and profitability

Results from many cotton-rotation system experiments in New South Wales and Queensland show that cotton yield per ha was lowest or as low as with cotton monoculture, particularly if intensive tillage was practiced (Fig. 4; Hearn 1986; Constable et al. 1992; Hulugalle and Entwistle 1997; Hulugalle et al. 1997, 1998, 1999b, 2001, 2002a, 2004, 2005a, 2007; Hickman et al. 1998; Rochester et al. 1998,2001; Hulugalle and Daniells 2005; Rochester and Peoples 2005; Hulugalle and Scott 2006; Scott and Hulugalle 2007). Poor cotton lint yields under cotton monoculture also means that this system is at greater risk of financial losses in years when the cotton lint price is relatively low or costs increase significantly. Analysis of the results (Hulugalle and Scott 2006) from 3 of the above-cited sites (Hulugalle et al. 1998, 1999b, 2001,2002a) indicates that, when cotton was grown in rotation with another crop, average gross margins per ha were, in general, highest with cotton-fertilised wheat and lowest with cotton-legumes (Fig. 5). The cotton-field pea rotation at Warren was an exception in that it was only slightly less (AU$63/ha) than that of the cotton-unfertilised wheat rotation. Although average gross margins per ha were highest with cotton monoculture, this was because there were more cotton crops sown relative to cotton-rotation crop sequences and not because gross margins were higher per cotton crop. The cotton-wheat systems generally returned higher gross margins per ML of irrigation water than cotton monoculture across the experiments (Fig. 5). This indicates that on many typical cotton farms, where irrigation water rather than land is the limiting resource, cotton-wheat rotations are likely to be more profitable.

[FIGURE 4 OMITTED]

This can be demonstrated by applying a water-limiting scenario to a 1000-ha cotton farm and estimating profitability (as gross margins) for cotton monoculture and a cotton-wheat rotation (Hulugalle and Scott 2006). The farm plan for monoculture cotton (cotton-winter fallow-cotton), when full water allocations are received, involves 1000ha of cotton in summer and 1000ha of fallow in winter. For cotton--wheat (cotton--wheat--summer fallow--winter fallow--cotton), in any one financial year, the 'normal' farm plan would be 500ha pre-cotton fallow in winter and cotton in summer, and 500ha wheat in winter and wheat stubble fallow in summer (Table 1). For the purpose of this exercise, it is assumed that average spring and summer rainfall has been received, and that the farm has received only half its full water allocation. Under this scenario, the monoculture cotton plan has sufficient water only for 500 ha of cotton, but the cotton--wheat plan can still proceed as 'normal' (Table 1). The expected gross margins for this scenario can then be estimated by using values derived from actual experimental data. The results (Table 1), which were based on average crop yields and incrop costs from a site at Merah North in northern New South Wales (Fig. 4; Hulugalle et al. 2002a) and some assumed crop prices, show that cotton--wheat is more profitable. The wheat in this experiment was not irrigated, although in dry years, irrigating wheat may significantly improve its yield and profitability. Consequently, cotton--wheat rotations should be the cropping system of choice for most cotton farmers, not only because they are likely to be more profitable and efficient with respect to irrigation water use, but also because they can improve soil quality. In limited water years, this strategy is even more important.

Rotations and resilience of profitability to changes in input costs

Growers face increasing challenges due to low prices, increased costs, and limited water. Strategies that could help buffer these challenges include farming systems that increase yields, reduce costs, and maintain or improve soil quality and productivity. Practices which could do this are minimum tillage, in situ stubble retention, and complex cropping systems (i.e. increasing the number of rotation crops in a cropping sequence). In this section, recent economic results (2000-2006) from 2 experiments are presented and the consequences of increases in fuel and fertiliser prices on profitability, measured as gross margins, are discussed (Scott and Hulugalle 2007). The first experiment compared either conventional tillage (disc-ploughing, chisel-ploughing, bed construction every year) or permanent beds under cotton--winter fallow--cotton (6 cotton crops since 2000), and permanent beds under a cotton-wheat rotation (3 cotton and 3 wheat crops since 2000). The second experiment, which commenced in 2002, compares cotton--winter fallow--cotton, a cotton--vetch--cotton rotation, a cotton--wheat rotation where wheat stubble is incorporated, and a cotton--wheat--vetch rotation where vetch is sown into standing wheat stubble. All rotations were sown on permanent beds with supplementary irrigation. We compared these systems on a per field basis, e.g. assuming a field was continuously farmed under the particular system, as might be the case if there was sufficient water to irrigate all the available land, and secondly, as part of a whole farm where there is only sufficient water to fully irrigate half of the available land. The latter opens the possibility for systems which only have cotton every second year (e.g. cotton--wheat--fallow--cotton) on a per field basis, to have a cotton crop every year though in a different field following a wheat rotation each time. This system is probably more typical of many farms, especially under drought conditions with limited allocation of irrigation water.

[FIGURE 5 OMITTED]

Permanent beds v. conventional tillage--comparisons on a field basis

Using a cotton price of $450/bale, the cotton winter fallow--cotton on permanent beds returned the highest average annual gross margin ($1533/ha, Fig. 6a). This was 8% higher than the conventionally tilled cotton--winter fallow-cotton (6 cotton crops) ($1424/ha) and 27% higher than the cotton--wheat on permanent beds ($1211/ha). When cotton prices fall to $350/bale (without including input price increases), the relative ranking of average gross margins changes and is in the order cotton--winter fallow--cotton on permanent beds ($776/ha)> cotton--wheat on permanent beds ($742/ha) > conventionally tilled cotton-winter fallow--cotton ($692/ha). In the case of gross margin per ML of irrigation water and using a cotton price of $450/bale, cotton--wheat on permanent beds gave a 27% higher return ($363/ML) than conventionally tilled cotton--winter fallow--cotton ($285/ML) (Fig. 6a). Cotton--winter fallow--cotton on permanent beds was 8% higher ($307/ML) than that which was conventionally tilled.

In addition to declining prices, growers also face issues of increasing fuel and fertiliser prices due to world oil price increases. The profitability of different rotations is affected by different cotton prices relative to wheat, but rotations can also differ in terms of their resilience to changing input prices, especially as some crops such as cotton require more machinery passes for cultivation, weed control, and spraying than wheat or vetch. Generally, rotations with lower overall fuel costs will be less affected by rising fuel prices.

[FIGURE 6 OMITTED]

Price changes included using cotton lint prices of $450 and $350/bale, a cotton seed price of $175/t, and by increasing both on-farm diesel and nitrogen fertiliser prices by 25, 50, 75, and 100%. The base diesel price used was the June 2006 price of $1.52/L at the bowser (which is equivalent to $1.00/L on-farm, ex-GST and less off-road diesel rebate); bowser fuel prices used were $1.90 ($1.35 on-farm), $2.28 ($1.69 on-farm), $2.66 ($2.04 on-farm), and $3.04 ($2.38 on-farm). The base price of the major fertiliser used, anhydrous ammonia, was $900/t. All other input costs were not altered. Rising world fuel prices may also affect other inputs such as insecticides, herbicides, growth regulators, and other nutrients, but the magnitude of this is very complex and will differ between products and companies.

Contract costs are a large part of cotton operations, so it was assumed that there would be a 0.5% increase in contract charges for every 1% increase in the price of fuel. Even though in practice some contract rates are quoted 'plus fuel' (i.e. the grower pays a base rate per ha and pays for fuel on top of that), for ease of calculation, contract rates used were calculated to include the cost of fuel. The relative rate increases were estimated by calculating the estimated contract rates for a sample tractor (using variable costs, including fuel and oil, and overhead costs per ha plus a 20% profit margin). The average increase in estimated contract rates was in the order of 50% when fuel prices increased by 100%. Using this assumption, for example, an aerial spraying charge of baseline $14/ha would increase to $21/ha if fuel prices rose by 100%.

[FIGURE 7 OMITTED]

The cotton--wheat rotation on permanent beds was less affected than either of the cotton monoculture systems by the increase in diesel and fertiliser costs and so appears to be more resilient to such increases, especially at low cotton prices (Fig. 7). This is due to a lower frequency of cotton crops in the cotton--wheat sequence compared with cotton monoculture, and therefore lower overall input costs. The relative profitability of the rotations also changes as cotton price increases, due to the increase in the price of cotton relative to wheat.

Rotations on permanent beds--comparisons on a field basis

At a cotton price of $450/bale, average annual gross margins were in the order cotton--winter fallow-cotton ($1419/ha)>cotton--vetch--cotton ($1288/ha) > cotton--wheat--vetch ($1227/ha)>cotton--wheat ($1077/ha) (Fig. 6b). In terms of gross margin per ML of irrigation water, profitability was in the order of cotton-wheat-vetch ($368/ML) > cotton--wheat ($359/ML) > cotton--winter fallow --cotton ($304/ML) > cotton vetch-cotton ($227/ML) (Fig. 6b).

When cotton prices fall to $350/bale (without including input price increases), the relative ranking of average gross margins changes and is in the order cotton--wheat--vetch ($751/ ha) > cotton--winter fallow--cotton ($673/ha) > cotton--wheat ($633/ha) > cotton--vetch--cotton ($567/ha) (Fig. 8).

On the basis of the results so far, the inclusion of vetch between cotton crops has not been profitable. This is because cotton yield in the cotton--vetch--cotton rotation was lower than that in cotton winter fallow--cotton. This contrasts with previous research which found that cotton--vetch--cotton was more profitable than cotton--winter fallow--cotton (Williams et al. 2005). However, sowing vetch after wheat in a cotton--wheat rotation has increased profitability relative to cotton--wheat alone, similar to that reported by Williams et al. (2005). As discussed in a previous section, cotton-wheat rotations (with or without vetch) are likely to be more profitable, but can also improve soil quality.

[FIGURE 8 OMITTED]

When the fuel and fertiliser price changes mentioned previously were applied, the relative profitability of the rotations changed similarly (Fig. 8). The rotations with a higher frequency of cotton crops (cotton--winter fallow--cotton, cotton--vetch) gave better returns with a higher cotton price, but with a lower cotton price relative to wheat, the cotton--wheat and cotton--wheat--vetch become more profitable per ha (Scott and Hulugalle 2007). The cotton-wheat and cotton--wheat--vetch rotations were also less sensitive to falling cotton prices due to lower overall costs.

Whole-farm impact--rotations and reduced water availability

In recent times water has been the limiting factor on total area on-farm under irrigation, rather than land. The average gross margin results reported thus far here show the relative profitabilities in the experiments where the same system was used in the same field over time, but growers need to consider the whole--farm impacts of these rotations also, since cotton--winter fallow--cotton is a 1-year 'cycle' of cotton winter fallow, whereas cotton--wheat is in fact a 2-year rotation of cotton--wheat--summer fallow winter fallow. If water is limited, growers may have fields out of synchrony, hence allowing a cotton crop following a wheat--fallow every year, although in a different field.

This can be demonstrated by applying the average yield results for the experiment on rotations on permanent beds to a farm of 1000 ha. Assuming the annual water allocation for the farm of 6000 ML (enough for 1000 ha of cotton at an average of 6 ML/ha and assuming that average summer rainfall is received) the comparative farm plans would be for cotton--winter fallow--cotton, 1000 ha of cotton in summer and 1000 ha of fallow in winter (total water required 6000 ML/year), and for cotton--vetch--cotton, 1000 ha of cotton in summer and 1000 ha of vetch in winter. For cotton--wheat, the 'normal' farm plan would be 500 ha cotton and 500 ha wheat stubble fallow in summer, and 500 ha wheat and 500 ha pre-cotton fallow in winter (water required 3625 ML/year with wheat using 1.25 ML/ha). For cotton--wheat--vetch, the 'normal' farm plan would have 500 ha pre-cotton vetch in winter instead of fallow (requiring 4155 ML/year if the vetch received 1 ML/ha as in the experiment).

Assuming the annual water allocation is cut by 50% for whatever reason to 3000 ML/year. This leaves the cotton--winter fallow--cotton and cotton--vetch--cotton plans with only enough water for 500 ha cotton (50% of the area). For the cotton--wheat plan, allowing 6 ML/ha for the cotton and 1.25 ML/ha for the wheat (these were the average amounts used during the experiment so far), 3000 ML is enough for 416 ha of cotton and 400 ha of wheat, so the cotton--wheat or cotton--wheat--vetch plan can still carry on with some reduced areas. The average crop yields and in-crop costs from the experiment have been used to calculate the example, with $43/ha winter fallow costs and $191/ha summer fallow costs.

Using the average yields and costs from the trial and $400/ bale for cotton and $150/t for wheat, cotton--wheat has the advantage over both continuous cotton systems (cotton--winter fallow-cotton and cotton--vetch--cotton) in this water-limited situation, but cotton--wheat--vetch in turn has an advantage over cotton-wheat (Table 2). Cotton--vetch--cotton is, however, less profitable than cotton--winter fallow--cotton. In the experiment, the vetch received 1 ML/ha irrigation, but for this reduced allocation example we have assumed that the water is allocated between the cotton and wheat component only.

When water is not limiting, in absolute financial terms, cotton--winter fallow--cotton will generate the higher total farm gross margin. Over the longer term, however, this higher profitability is accompanied by declining soil fertility, crop health, and cotton yields/ha. Also, a cotton--wheat rotation uses less water over the same area than a cotton--winter fallow--cotton 'rotation', freeing up water to be carried over to following seasons or able to be sold under temporary seasonal transfers, where such arrangements are possible.

However, financial comparison of rotations can be a complicated issue, especially as it is likely that machinery needs, labour needs, and infrastructure maintenance and therefore overhead costs will be different. Gross margins cannot indicate these differences and whole farm budgets for the individual circumstances are more useful when comparing different rotations that will involve overhead costs.

Future areas for research

This review has highlighted several issues with respect to cotton--rotation crop sequences where knowledge is lacking or very limited. We suggest that they should be investigated using a combination of long-term experiments and simulation models which focus on soil quality, hydrology, and system profitability (Probert et al. 2005; Causarano et al. 2007). The research issues summarised below are not mutually exclusive but are complementary.

1. Designing 'new' crop rotations

(a) Crop rotations which include 2 or more rotation crops sown after cotton: Results from other soil types, farming systems, and climatic zones indicates that increasing complexity of crop rotations ensures improvements in soil quality and agricultural sustainability (Greenland and Szabolcs 1994; Doran 2002). Some possible combinations are: cotton--cereal--legume, cotton--cereal--legume--cereal, and cotton--oilseed--cereal--legume.

(b) Incorporation of high value crops into irrigated cotton-based cropping systems: Examples of high value crops are selected vegetable crops such as rock melon (Cucumis melo), onion types (Allium spp.), and sweet corn, and 'energy' crops for biofuel production.

2. Comparative soil quality effects of managing rotation crop stubble: This literature review has indicated that there is no comparative information published on frequently used management practices such high- and low-temperature stubble burning, shallow and deep incorporation, and in situ mulching of cereal, legume, and oilseed rotation crop stubble, although somewhat more information is available with respect to cotton stubble. Information is also lacking on the interactions which may occur between irrigation systems such a sprinkler, furrow, and drip with the abovementioned stubble management systems, and their consequences on soil quality.

3. Machinery attachments for managing rotation crop stubble in situ in permanent bed systems: A limited amount of research has been conducted into these management systems but large knowledge gaps remain (Hulugalle and Daniells 2005). Economic, energy and ergonomic analyses are also required with respect to any such attachments.

4. SOC dynamics in irrigated cotton systems

(a) What is the minimum amount of crop stubble needed to be retained to increase soil organic carbon levels from present values? A single laboratory and field study suggests that the amounts of stubble required to reverse SOC decline in irrigated Vertosols are of the order of 2-3 kg/[m.sup.2] per cropping cycle. Long-term field studies in different Vertosols and climatic zones are, however, needed to confirm these values. An approach similar to that proposed by Blanco-Canqui and Lal (2007), who removed corn stubble from a field at various rates to identify the amount which resulted in the least degradation, may be appropriate. Alternatively, rather than removing the entire stubble amount from a single crop, smaller amounts may also be removed from a sequence which includes 2 cereal crops such as a winter and summer cereal grown in rotation with cotton. Given the current interest in biofuel production from cellulose-rich material, such a study would also provide information on the optimum amounts of vegetative material which can be removed with negligible soil degradation.

(b) Relative efficacy of [C.sub.3] and [C.sub.4] rotation crops in soil carbon sequestration: Skjemstad and Dalal (1987) and Skjemstad et al. (1994) noted that soil organic carbon sequestration by [C.sub.4] plant species in dryland grain and pasture systems in Vertosols was more efficient than that by [C.sub.3] species due to chemical and physical protection. Similar research has not been conducted for irrigated cotton rotations.

(c) The processes related to long-term carbon sequestration in irrigated Vertosols and their interactions with rotation crop type, and soil physical and chemical properties such as clay mineralogy and aggregation.

5. Effects of sowing rotation crops after cotton on-farm and cotton industry economic indicators such as the economic incentives for adopting new cotton rotations, and farm level and industry- and community/catchment-wide economic modelling of the impact of research and extension activities.

Conclusions

Wheat rotation crops can improve soil quality indices such as structure and water-holding capacity, particularly where sodicity and salinity are present in the subsoil, recycle N leached below cotton's root-zone, and facilitate leaching of excess salts. Legumes improve soil N balance and surface structural stability. Legumes may reduce growth and yield of the following cotton crop, primarily because they are alternative hosts to seedling diseases of cotton and their seed residues can be allelopathic. Good management (fertiliser, irrigation, weed control) of rotation crops can benefit the following cotton crop in terms of crop nutrition. Soil organic carbon in most locations has not been increased merely by sowing any rotation crop, but by sowing crop sequences which return about 2 kg/[m.sup.2]/crop cycle of residues to the soil, minimising tillage and optimising N inputs.

Lowest average lint yields per ha were with cotton monoculture. In the longer term, under the scenario of increasing input costs and variable prices, relatively low-yielding cotton crops are at greater risk of poor profitability. The cotton--wheat systems also generally returned higher average gross margins per ML of irrigation water than cotton monoculture and other rotation crops. This indicates that where irrigation water, rather than land, is the limiting resource, cotton--wheat systems would be a more profitable option. As land is not a limiting factor on many cotton farms, whereas water is, cotton--wheat rotations should be the cropping system of choice for most cotton farmers. In limited water years, this strategy is even more important. At the same time, when cotton prices fall and fuel and nitrogen fertiliser costs increase, gross margins per ha and gross margins per ML of irrigation water are higher in rotation systems which include a wheat crop, especially the cotton--wheat--vetch systems. In other words, the profitability of cotton--rotation crop sequences such as cotton--wheat, where cotton is not sown in the same field every year, are more resilient to fluctuations in the price of cotton lint, fuel, and nitrogen fertiliser. The addition of vetch to the cotton--wheat system has improved average cotton yields and profitability in the experiment.

Research gaps remain with respect to: research into 'new' crop rotations; comparative soil quality effects of managing rotation crop stubble; machinery attachments for managing rotation crop stubble in situ in permanent bed systems; the minimum amount of crop stubble which needs to be returned per cropping cycle to increase SOC levels from present values; the relative efficacy of [C.sub.3] and [C.sub.4] rotation crops, and their interactions with soil physical and chemical properties such as clay mineralogy and aggregation in relation to carbon sequestration in irrigated Vertosols; the interactions between soil biodiversity and soil physical and chemical quality indicators, and cotton yields; and the effects of sowing rotation crops after cotton on farm and cotton industry economic indicators such as the economic incentives for adopting new cotton rotations, farm level impacts of research and extension investments, and industry- and community/ catchment-wide economic modelling of the impact of cotton research and extension activities.

Acknowledgments

Funding support from the Co-operative Research Centre (CRC) for Sustainable Cotton Production, Australian Cotton CRC, Cotton Catchment Communities CRC, and the Cotton Research and Development Corporation of Australia is gratefully acknowledged. Our thanks to Dr Judy Tisdall of LaTrobe University and Dr Ian Taylor of the Cotton Research and Development Corporation for their helpful comments during manuscript preparation.

Manuscript received 5 June 2007, accepted 21 January 2008

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N.R. Hulugalle (A,C) and F. Scott (B)

(A) New South Wales Department of Primary Industries and Cotton Catchment Communities Co-operative Research Centre, Australian Cotton Research Institute, Locked Bag 1000, Narrabri, NSW 2390, Australia.

(B) New South Wales Department of Primary Industries and Cotton Catchment Communities Co-operative Research Centre, Tamworth Agricultural Institute, 4 Marsden Park Road, Calala, NSW 2340, Australia.

(C) Corresponding author. Email: nilanthah@csiro.au
Table 1. Comparison of water limited farm plans (Hulugalle and
Scott 2006)

1 bale = 227 kg lint

Assumptions Cotton-wheat Cotton-cotton

Average cotton lint 6.9 5.9
 yield (bales)
Cotton price/bale 400 400
 (AU$)
Cotton seed (t/ha) 2.48 2.12
Cotton seed price 200 200
 ($/t)
Income/ha ($) from 3257 2785
 cotton
Costs/ha ($) 2123 1898
Cotton gross 1134 887
 margin/ha ($)
Average wheat yield 1.94
 (t/ha)
Average wheat price 150
 ($/t)
Income/ha ($) from 291
 wheat
Costs/ha ($) 177
Wheat gross 114
 margin/ha ($)

 Gross Gross
 Area margin Cotton-cotton Area margin
Cotton wheat plan (ha) ($) plan (ha) ($)

Cotton 500 566 900 Cotton 500 443 400
Wheat 500 57 000 Summer fallow 500 -17 500
Summer fallow after 500 -17 500 Winter fallow 1000 -35 000
 wheat
Winter fallow 500 -17 500
 before cotton
Farm gross margin 588 900 Farm gross 390 900
 margin
 Advantage of 198 000
 over
 cotton-wheat
 cotton-cotton

Table 2. Comparison of water limited farm plans for 4 cotton
rotation systems (Scott and Hulugalle 2007)

1 bale = 227 kg lint

 Water
 Gross use Gross
Cropping system margins/ha Area (ha) (ML) margin

Cotton monoculture plan (A)

Cotton $1256 500 3000 $62 750
Summer fallow -$191 500 0 -$95 500
Winter fallow -$443 1000 0 -$43 000
ML water used and farm 3000 $489 250
 gross margin

Cotton-vetch-cotton plan (B)

Cotton $1239 500 3000 $619 366
Summer fallow -$191 500 0 -$95 500
Vetch -$200 500 0 -$100 000
Winter fallow -$43 500 0 -$21 500
ML water used and farm 3000 $402 366
 gross margin

Cotton-wheat plan (C)

Cotton $1938 416 2496 $806 042
Summer fallow -$191 584 0 -$111 544
Wheat $199 400 500 $79 400
Winter fallow -$43 600 0 -$25 800
ML water used and farm 2996 $748 098
 gross margin

Cotton-wheat-vetch plan (D)

Cotton $2308 416 2496 $960 003
Summer fallow -$191 584 0 -$111 544
Wheat $239 400 500 $95 600
Vetch -$200 600 0 -$120 000
ML water used and farm 2996 $824 059
 gross margin
Difference between -$86 884
 cotton-vetch-cotton and
 cotton monoculture
 (B--A)
Difference between $258 849
 cotton-wheat and cotton
 monoculture (C--A)
Difference between $334 809
 cotton-wheat-vetch and
 cotton monoculture
 (D--A)
Difference between $75 962
 cotton-wheat-vetch and
 cotton wheat (D--C)
Difference between $345 732
 cotton-wheat and
 cotton-vetch-cotton
 (C--B)
Difference between $421 693
 cotton-wheat-vetch and
 cotton-vetch-cotton
 (D--B)
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Author:Hulugalle, N.R.; Scott, F.
Publication:Australian Journal of Soil Research
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Date:Mar 1, 2008
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