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Effect of long-fallow on soil quality and cotton lint yield in an irrigated, self-mulching, grey Vertosol in the central-west of New South Wales.

Additional keywords: cracking clay, minimum tillage, soil organic C, farming system, mychorrhiza, black root rot, long-fallow disorder.

Introduction

In eastern Australia, irrigated cotton (Gossypium hirsutum L.) is grown as a monoculture, in rotation with cereal or leguminous crops, or alternating with fallow (Cooper 1993). Fallow is used primarily as a resource conservation measure, particularly when irrigation wa-ter is limiting (Harman et al. 1989; Wiese et al. 1994). Increasing the duration of fallow can, however, result in reduced cotton growth rates, commonly referred to as `long-fallow disorder' (McGee et al. 1997). During bare fallow the population of arbuscular mycorrhizal (AM) fungi in soil declines because the fungi are obligate symbionts and cannot grow without a living host. If insufficient mycorrhizal inoculum survives the fallow then colonisation of roots is both delayed and reduced. This slow mycorrhizal development and consequent nutritional imbalances (P, Zn) are frequently reported to be the primary cause of long-fallow disorder (Thompson 1987, 1994; McGee et al. 1997). Soil physical and chemical properties associated with long-fallow cotton in irrigated Vertosols are, however, very rarely reported. More information is available with respect to soil chemical but not physical properties for cereal-based cropping systems in Vertosols. These data suggest that compared with continuous cropping, long-fallow results in higher soil nitrate-N and lower organic C when legumes are excluded from the rotation (Holford 1989, 1990). Similar published data are not available for cotton-based farming systems. The objective of this study, therefore, was to quantify soil physical and chemical properties, and soil-borne disease incidence (usually black root rot in this site), under continuous and long-fallow cotton in an irrigated, grey, self-mulching Vertosol in eastern Australia, with a view to identifying soil factors other than AM which could contribute to cotton growth rate reductions.

Materials and methods

Experimental site and treatments

The experiment was located at `Auscott-Warren', a commercial cotton farm in Warren, central-western New South Wales (147 [degrees] 46'E, 31 [degrees] 47'S), which has a semi-arid climate. The experimental site experiences a mild winter and a hot summer. The hottest month is January (mean daily maxima and minima of 33 [degrees] C and 18 [degrees] C, respectively) and the coldest is July (mean daily maxima and minima of 15 [degrees] C and 3 [degrees] C, respectively). In central-western NSW, cotton is usually sown in early October and harvested in late April of the following year. Mean annual rainfall is 479 mm. The soil at the experimental site is a deep uniform grey clay [Ug5.25 (Northcote 1979); fine, thermic, montmorillonitic Entic Haplustert (Soil Survey Staff 1996); grey, self-mulching, calcareous Vertosol (Isbell 1996)] belonging to the Mulla Grey Phase soil profile class (McKenzie 1992). It is commonly referred to as a cracking clay as it shrinks and forms deep cracks on drying. Clay mineralogy is dominated by kaolinite, illite, and smectite in a ratio of 3: 2: 3 (McKenzie 1992). Soil physical and chemical properties in the 0-0-6 m depth of the experimental site are given in Table 1.
Table 1. Soil properties in the 0-0.60 in depth in June 1993
EC, electrical conductivity

Soil property 0-0.15m 0.15-0.30m

Sand (g/kg) 300 340
Silt (g/kg) 180 140
Clay (g/kg) 520 520
Organic C (g/kg) 5.3 6.0
pH(1:5 soil:0.01 m Ca[C.sub.2]) 7.7 7.6
EC(1: 5 soil: water) (dS/m) 0.02 0.06
Air-filled porosity ([m.sup.3]/[m.sup.3]) 0.34 0.13
Plastic limit (mm) 54.5 43.2
Exchangeable cations (cmol(+)/kg)
 Ca 20.6 21.0
 Mg 12.6 12.6
 K 1.0 0.9
 Na 0.7 1.0
 Ca[CO.sub.3] (g/kg) 43 42

Soil property 0.30-0.45m 0.45-0.60m

Sand (g/kg) 340 320
Silt (g/kg) 120 150
Clay (g/kg) 540 530
Organic C (g/kg) 5.7 5.2
pH(1:5 soil:0.01 m Ca[C.sub.2]) 7.6 7.6
EC(1: 5 soil: water) (dS/m) 0.09 0.07
Air-filled porosity ([m.sup.3]/[m.sup.3]) 0.13 0.13
Plastic limit (mm) 48.4 49.1
Exchangeable cations (cmol(+)/kg)
 Ca 22.4 19.2
 Mg 14.2 13.6
 K 1.0 0.7
 Na 1.5 2.1
 Ca[CO.sub.3] (g/kg) 45 43


The experimental treatments, which commenced in October 1993 following 3 years of continuous cotton with intensive tillage practices (disc and chisel ploughing, and ridging every year; deep ripping every 2-5 years) and burning of crop residues, were continuous cotton and long-fallow cotton (cotton, alternating with a bare fallow, was sown every other year). Cotton was sown in continuous cotton plots during every growing season from 1993 to 1997. Long-fallow plots were fallowed during the growing seasons of 1993-94 and 1995-96 and cotton was sown during the growing seasons of 1994-95 and 1996-97. Following harvest (in April), the crops were slashed and stalk-pulled, and all residues were retained in situ. From 1993, minimum tillage was used at the site and consisted of re-forming old ridges (1 m spacing, 0.15 m high) and deepening furrows by direct listing without any prior tillage, followed by ridge cultivation to a depth of 0.075 m with a Lilliston cultivator. Traffic was controlled by restricting it to specific furrows and avoiding trafficking on ridges. The plots, when sown, were furrow irrigated with approximately 100 mm of water during each irrigation event (4-6 times per growing season). The experimental design used was a randomised complete block with 3 replications. Individual plots consisted of 40 rows (ridges), 700 m long, spaced at 1-m intervals. Prior to sowing cotton in 1993, fertiliser was applied in July as anhydrous ammonia at a rate of 110 kg N/ha and mono-ammonium phosphate at a rate of 45 kg/ha. In all subsequent years, anhydrous ammonia was applied in July at a rate of 116 kg N/ha and mono-ammonium phosphate at a rate of 90 kg/ha, and in December water-run urea was applied at a rate of 84 kg N/ha.

Soil physical and chemical properties

Soil was sampled with a spade from the 0-0.15, 0.15-0.30, 0.30-0.45, and 0.45-0.60 m depths, using a stratified random sampling pattern (Webster and Oliver 1990), in rows adjacent to non wheel-tracked furrows in June 1993, early March 1994, 1995, and 1997, and mid July 1996. This corresponded to 0, 8, 20, 44, and 36.5 months after commencing the experiment. Sampling was done approximately 1 week after either rainfall or irrigation when profile soil water content approximated that of field capacity and the soil was traffickable. The sampling zone was restricted to a 100-m-wide zone which centred on the spot that was 300 m from the head-ditch. Measurements based on pre-trial grid sampling and analyses of particle size distribution to a depth of 1.5 m and soil strength to a depth of 0 - 45 m had indicated that soil variability was least in this zone.

Air-dried soil ([is less than] 2 mm diameter) was used to determine plastic limit with a drop-cone penetrometer (Campbell 1976) on 25-g samples pre-wetted to different water contents and equilibrated over a 2-day period without manual mixing in sealed plastic containers (25 [cm.sup.3]), nitrate-N (by steam distillation after extraction with 1 m KCl), pH (1: 5 soil: 0.01 m Ca[C.sub.2]), and Ca[CO.sub.3] equivalent and exchangeable Ca, Mg, K, and Na (after washing with aqueous alcohol and aqueous glycerol to remove soluble salts and extraction with alcoholic 1 m [NH.sub.4]Cl at a pH of 8.5) (Rayment and Higginson 1992). Nitrate-N was determined only on samples taken in 1994, 1995, and 1997. Total soil organic carbon was determined by the wet oxidation method of Walkley and Black on soil which had been passed through a sieve with aperture diameters of 0.5 mm (Rayment and Higginson 1992). Soil bulk density was determined only on samples taken in June 1993, March 1994, July 1996, and March 1997. Oven-dried bulk density of the samples taken from the 0-0.15 m depth was determined on air-dried soil aggregates (1-10 mm diameter) with the kerosene saturation method of McIntyre and Stirk (1954), which was then converted to an oven-dried equivalent using an air-dry water content determined on subsamples (soil shrinkage curves from this site had indicated that volume change between air-dried and oven-dried soil was insignificant), and on oven-dried soil clods with the `saran' resin method (Blake and Hartge 1986; McGarry 1990). The results were expressed as a weighted mean of the two methods with a one-third weighting allocated to values derived from the clods and two-thirds to those from the aggregates. This was done on the basis of a visual assessment of the distribution of clods and loose aggregates in the 0-0.15 m depth (which included soil from the ridges). Bulk densities of soil from the 0.15-0.60 m depth were determined with the `saran' resin method only on oven-dried soil clods (mean oven-dried volume of 178x[10.sup.-6] [m.sup.3]). Coating of soil clods from all depths with the resin was done when they were air-dry, after which they were oven-dried at 110 [degrees] C for 96 h and weighed in air and water to evaluate oven-dried weight and volume (Blake and Hartge 1986). [When compared with soil cores, bulk density measured with the clod method in irrigated cotton fields tends to overestimate values by 10-20% (N. R. Hulugalle, unpubl. data).] The data were converted to air-filled porosity [1-(bulk density /particle density)] where particle density is taken to be equal to 2.65 Mg/[m.sup.3]. Exchangeable Ca, Mg, K, and Na, organic C, Ca[CO.sub.3] equivalent, and nitrate-N were expressed in kg/[m.sup.2] by multiplying the values for each depth interval by its bulk density and the depth increment, followed by summing up all depth intervals. In situ soil strength to a depth of 0.45 m was measured at 0.015-m depth increments with a `Rimik' recording penetrometer during the seasons when cotton war, sown in both treatments on 27 October 1994 and 17 August 1996. Measurements were made from the top of ridges with 3 profiles being measured from the 4 central rows of each plot. Concurrently, soil water content was measured gravimetrically from the same locations.

Disease and arbuscular mycorrhizal incidence

The tap roots of 5 seedlings were sampled from each of 10 randomly selected sites per plot in late November 1994 and 1996 (approximately 6 weeks after sowing). Sites were selected using a step-point method while traversing plots at an angle to the rows. Roots were washed free of soil and the severity of black root rot caused by Thielaviopsis basirola (Berk. & Br.) was assessed using the scale of Tabachnik et at. (1979) where 0 is healthy, 1 is [is greater than] 0-25% of tap root blackened, 2 is [is greater than] 25-50% of tap root blackened, 3 is [is greater than] 50-75% of tap root and few lateral roots blackened, and 4 is [is greater than] 75-100%,of tap root blackened and no lateral roots remaining.

Soil and roots from 5 randomly selected seedlings were Also collected from each plot in late November 1994 and 1996 for assessing mycorrhizal development. Roots were immediately washed from the soil and stored in 50% alcohol before being cleared and stained (Koske and Gemma 1989) and assessed for percent mycorrhizal colonisation (Giovanetti and Mosse 1980).

Cotton growth, nutrient uptake, and yield

Cotton growth was monitored by measuring dry matter in 3 randomly selected sites of 2 [m.sup.2] in each plot at intervals of 4-6 weeks between 2 and 5 months, and 2 and 6 months after sowing during the 1994-95 and 1996-97 growing seasons, respectively. Dry matter from each plot was bulked to give a single sample. Total boll production was monitored in 3 randomly selected sites of 2 [m.sup.2] in each plot at intervals of 4-6 weeks between 2 and 5 months after sowing during the 1994-95 growing season, and at intervals of 1-2 weeks between 2 and 6 months after sowing during the 1996-97 growing season.

Following Constable and Gleeson (1977) the dry matter data (DM, g/[m.sup2]) were fitted to an equation of the form:

[log.sub.e]DM = a + b[exp(1/T)]

and total boll production (TB, bolls/[m.sup.2]) to an equation of the form:

TB = a + b/[1 + exp(T)]

where T is time (months after sowing). The resulting slopes, intercepts, and total regressions for the cropping systems were compared within growing seasons using analysis of variance. Plants sampled in early March 1997 were used to evaluate nutrient uptake. Nitrogen in plant tissues was measured with a near-infrared protein analyser which had been pre-calibrated with the Kjeldahl method for tissue N (Handson and Shelley 1993). Plant uptake of S, Mo, Zn, B, Mn, Cu, Mg, Ca, K, and Na was evaluated by determining nutrient concentration in plant dry matter with an inductively coupled plasma-atomic emission spectrometer after microwave digestion with concentrated nitric acid (Handson and Shelley 1993). Soil water content in the 0.10-1.10 in depth interval was measured at regular intervals in all plots from November until April during the 1996-97 cotton growing season with a neutron moisture meter (CPN 503DR Hydroprobe). Cotton lint yield was evaluated by mechanical harvesting at crop maturity in late April. All data were analysed using the MSTAT-C data analysis software package (Michigan State University 1993).

Results and discussion

Soil organic C

Soil organic C did not differ significantly (P = 0.05) between cropping systems at any time during the trial. Mean soil organic C in the 0-0.60 m depth averaged over times of sampling in continuous and long-fallow plots was 6.9 and 6.9 kg/[m.sup.2], respectively (s.e. = [+ or -] 0.09, n.s.). Significant differences (P = 0.01) did occur, however, between times of sampling. Mean values of soil organic carbon in the 0-0.60 m depth at 0 (June 1993), 8 (March 1994), 20 (March 1995), 36.5 (July 1996), and 44 (March 1997) months after commencing the experiment were 7.2, 8.3, 5.9, 7.8, and 5.2 kg/[m.sup.2], respectively (s.e. = [+ or -] 0.48, P = 0.01). These results may be caused by a combination of several factors.

Firstly, the C/N ratios of cotton residues are relatively high, ranging from 40 to 80, by comparison with those of cereal crops such as wheat (25-30) (Ghidey and Alberts 1993; I. J. Rochester, pers. comm. 1997). Cotton above-ground residue (C/N = 40) decomposition curves obtained in the laboratory over 18 months [Res = 86.6347exp(-0.0405 T), r = -0.84***, where Res is the amount of remaining residues as a percentage of the original amount and T is time in months] showed that after 18 months, 41% of the original residues remained in a relatively undecomposed state (N. R. Hulugalle and P. C. Entwistle, unpubl. data 1996). Although data were not collected from taproot materials (C/N 80-180), we suggest that their decomposition would be very much slower. These results suggest that there is a `store' of partially decomposed residues from cotton crops sown from up to 10 years previously remaining in the soil, and that organic C from these residues may be added continuously to the soil by their slow decomposition and mineralisation. The rapid loss of soil organic C reported from long-fallow in cereal- and legume-based cropping systems does not, therefore, occur in these soils, and any significant decline in soil organic C may require much longer periods of time than in this study (44 months). Furthermore, application of carbon balances based on crop residues measured over a 2-3-year period would result in significant errors.

Secondly, minimum tillage, controlled traffic, and residue retention replaced intensive tillage and residue burning in 1993. Reduction of tillage intensity and retention of crop residues are known to reduce carbon losses from soil (Loch and Coughlan 1984; Dalal 1989; Hulugalle and Entwistle 1997; Lal 1997; Potter et al. 1997). Minimum tillage and residue retention may, therefore, have dominated decomposition processes at this site such that it minimised any effect of cropping system on soil organic C.

Thirdly, movement of crop residues, and soil organic C with dry soil and as dissolved organic C in irrigation and rain water from the topsoil to the subsoil via soil cracks, and differential swelling and shrinking in different soil horizons cause redistribution of soil organic carbon from surface to subsoil horizons, i.e. `soil inversion' (Dalal 1989; Skjemstad et al. 1997).

Fourthly, continuous cotton plots were irrigated every growing season during this experiment, whereas long-fallow plots were irrigated only every other year. Although dry matter additions with continuous cotton would have been higher than with long-fallow cotton, mineralisation of soil organic matter would also have been higher due to the continuous cotton being wetter in years when long-fallow plots were fallowed.

Sequestration of organic carbon in soil as an option to reduce carbon-based greenhouse gas emissions has been suggested by some authors (Lal 1997; Potter et al. 1997). Evaluation of published data for cereal-based cropping systems in Vertosols suggests that long-fallow has a detrimental effect on carbon sequestration due to the decline in soil organic carbon which occurs with time (Holford 1989, 1990). The results of this trial suggest, however, that long-fallow had no effect on carbon sequestration in this soil. However, regression analyses indicated that organic C in both treatments was negatively correlated (r = -0.53***, n = 30) with time since commencement of the trial. This suggests that a net decline in soil organic C occurred with time, and is probably related to the intensive irrigation practised at this site (N. R. Hulugalle, unpubl. data).

pH, exchangeable cations, and calcium carbonate

Soil pH at any depth was not affected by cropping system (Table 2). Both depth and time of sampling did, however, significantly affect soil pH such that, by comparison with values observed in 1993 (Table 1), pH towards the end of the cotton growing season in 1997 had decreased. A significant (P = 0.001) interaction was also observed between depth and time of sampling. The decrease in pH is probably due to the substitution of minimum tillage and residue retention for intensive tillage and residue burning from 1993 onwards (Dalal 1989; Hulugalle and Entwistle 1997).

Table 2. Effect of long fallow on pH (in 0.01 m calcium chloride) during March 1994, March 1995, July 1996, and March 1997
 Depth (m) Continuous Long-fallow Mean
 cotton cotton
1994 0-0.15 7.3 7.2 7.3
 0.15-0.30 7.0 7.1 7.0
 0.30-0.45 6.8 6.7 6.8
 0.45-0.60 7.0 7.1 7.1
 Mean 7.0 7.1 7.0
1995 0-0.15 7.6 7.0 7.5
 0.15-0.30 7.6 7.4 7.5
 0.30-0.45 7.5 7.5 7.5
 0.45-0.60 7.7 7.6 7.7
 Mean 7.6 7.5 7.5
1996 0-0.15 6.7 6.7 6.7
 0.15-0.30 7.3 7.3 7.3
 0.30-0.45 7.4 7.5 7.4
 0.45-0.60 7.5 7.5 7.5
 Mean 7.2 7.3 7.2
1997 0-0.15 7.5 7.5 7.5
 0.15-0.30 7.4 7.4 7.4
 0.30-0.45 7.4 7.4 7.4
 0.45-0.60 7.5 7.5 7.5
 Mean 7.4 7.5 7.5
ANOVA P s. e.
 Year (Y) 0.001 0.04
 Depth (D) 0.001 0.03
 Cropping system (CS) (n.s.) 0.03
 YxD 0.001 0.06
 YxCS (n.s.) 0.05
 CSxD (n.s.) 0.04
 YxCS x D (n.s.) 0.08


(n.s.), not significant.

Exchangeable Ca (P = 0.001), Mg (P = 0.001), and K (P = 0.05) were significantly affected by time of sampling (Fig. 1). In addition, there was a significant interaction between time of sampling and cropping system (P = 0.05) such that exchangeable Ca and Mg in continuous cotton plots were higher than those in long-fallow plots in March 1994, although significant differences did not occur at other times. Exchangeable K was not similarly affected. These results may be related to the combined effects of burning of crop residues in May 1993 and the presence of a living crop in continuous cotton plots. The high values of exchangeable Ca, Mg, and K in all plots during March 1994 may be caused by the nutrients released into the soil by burning of cotton residues (Loch and Coughlan 1984) after the 1993 harvest. The higher exchangeable Ca and Mg in continuous cotton plots (Fig. 1) may be due to root exudates from cotton (sown only in continuous cotton plots during the 1993-94 growing season) keeping more Ca and Mg in an available form (Strom et al. 1994). The increases in exchangeable Ca, Mg, and K between March 1995 and July 1996, however, may be related to crop residue and soil organic matter mineralisation (see later discussion). Exchangeable Na was not affected by either time of sampling or cropping -system (Fig. 1). A significant net decline (r = -0.62***, n = 30) in exchangeable Mg levels also occurred with time since commencement of the trial. Exchangeable Ca and K did not, however, decline significantly over time. Soil carbonates, measured as the Ca[CO.sub.3] equivalent, were not significantly affected by cropping system, but decreased significantly between June 1993 and July 1996 from 5.6 to 3.2 kg/[m.sup.2] in the 0-0.6 m depth (s.e. = [+ or -] 45, P = 0.05).

[Figure 1 ILLUSTRATION OMITTED]

The relationship between soil organic C, exchangeable cations, and soil carbonates (primarily calcium carbonates) in alkaline soils has been described previously (Dubey and Mondal 1993; Chorom and Rengasamy 1997). To summarise, the process involves solubilisation of native carbonates (primarily calcium carbonates) by H+ released by (a) crop residue and soil organic matter mineralisation, and (b) ionisation of carbonic acid formed by dissolution of carbon dioxide, a product of microbial respiration; and release of [Ca.sup.2+], which may either become adsorbed onto the clay minerals or remain in soil solution. As calcium has a beneficial effect on soil physical properties (Little et al. 1992; Chorom and Rengasamy 1997), Ca[CO.sub.3] solubilisation should improve soil physical properties such as aggregate stability and aggregate size and increase exchangeable calcium. In view of the changes which occurred with respect to exchangeable Mg and K, it is likely that a similar solubilisation of magnesium and potassium carbonates also occurred at this site.

Air-filled porosity, plastic limit, and soil strength

Air-filled porosity of oven-dried soil was significantly affected by depth (Table 3). Significant interactions also occurred between times of sampling and depths, and times of sampling, depths, and cropping systems. Significant increases occurred with time in the 0-0.15 m, 0.30-0.45 m, and 0.45-0.60 m depths. This may be a consequence of the minimum tillage and controlled traffic (Yule and Tullberg 1995) which was practised at this site since 1993. In comparison with long-fallow cotton, continuous cotton in 1994 had a higher air-filled porosity in the 0-0.15 m depth, and in 1996 in the 0.15-0.30 m depth. This may be due to trafficking combined with the absence of a growing crop in long-fallow plots, whereas the continuous cotton plots had irrigated crops growing therein during the 6 months prior to sampling. The frequent wetting-drying cycles to which the latter treatment is subjected can maintain or improve soil structure (Pillai-McGarry and McGarry 1996). It is notable that once a crop was sown in long-fallow plots in October 1996 and subjected to frequent wetting-drying cycles, significant increases in air-filled porosity occurred (Table 2).

Table 3. Effect of long fallow on air-filled porosity ([m.sup.3]/[m.sup.3]) of oven-dry soil during March 1994, 1996, and 1997
 Depth (m) Continuous cotton Long-fallow cotton Mean

1994 0-0.15 0.351 0.302 0.326
 0.15-0.30 0.130 0.132 0.131
 0.30-0.45 0.132 0.132 0.132
 0.45-0.60 0.132 0.132 0.132

1996 0-0.15 0.365 0.367 0.366
 0.15-0.30 0.112 0.077 0.094
 0.30-0.45 0.147 0.151 0.149
 0.45-0.60 0.164 0.172 0.168
1997 0-0.15 0.342 0.361 0.352
 0.15-0.30 0.121 0.128 0.125
 0.30-0.45 0.155 0.142 0.148
 0.45-0.60 0.153 0.156 0.155

ANOVA P s.e.
 Year (Y) n.s. 0.0045
 Depth (D) 0.001 0.0045
 YxD 0.001 0.0077
 Cropping system (CS)xD n.s. 0.0063
 YxCSxD 0.05 0.0109


n.s., not significant.

Plastic limit was significantly affected by time of sampling, depth, and cropping system (Table 4). Significant interactions also occurred between times of sampling and depths, and times of sampling and cropping systems. In general, continuous cotton had a higher plastic limit than did long-fallow cotton. The major determinants of plastic limit (PL, mm. water) in each individual depth interval were exchangeable Ca and Mg (in kg/[m.sup.2]), thus:

PL = 24.14 + 13.44Mg + 3.88Ca ([R.sup.2]=0.53***,n=96)
Table 4. Effect of long fallow on plastic limit (mm water)
during March 1994, March 1995, July 1996, and March,1997

 Depth (m) Continuous cotton Long-fallow cotton Mean

1994 0-0.15 50.8 43.0 46.9
 0.15-0.30 51.9 43.1 47.5
 0.30-0.45 61.0 51.8 56.4
 Mean 56.9 47.7 52.3
1995 0-0.15 35.2 33.1 34.1
 0.15-0.30 62.4 62.4 62.4
 0.30-0.45 46.9 47.2 47.1
 0.45-0.60 48.8 46.0 47.4
 Mean 48.3 47.2 47.8
1996 0-0.15 40.8 39.8 40.3
 0.15-0.30 57.7 55.7 56.7
 0.30-0.45 53.2 51.2 52.2
 0.45-0.60 49.9 46.5 48.2
 Mean 50.4 48.3 49.4
1997 0-0.15 36.2 35.1 35.7
 0.15-0.30 48.5 50.2 49.3
 0.30-0.45 43.7 47.2 45.4
 0.45-0.60 44.9 47.5 46.2
 Mean 43.3 45.1 44.2

ANOVA P s. e.

Year (Y) 0.001 0.49
Depth (D) 0.001 0.53
Cropping system (CS) 0.001 0.34
YxD 0.001 1.05
YxCS 0.001 0.69
CSxD n.s. 0.75
YxCSxD n.s. 1.49


n.s., not significant.

The relatively low coefficient of determination suggests that other factors such as controlled traffic (Yule and Tullberg 1995), C/N ratios of crop residues and short-term variations in soil microflora (Rawitz et al. 1994), and direct effects of organic matter on soil microstructure (Koppi 1995) may also have been involved. Comparison with values observed in 1993 and 1994 also suggests that a general reduction in plastic limit has occurred. This may be linked to the previously discussed decrease in exchangeable Mg (Fig. 2).

[Figure 2 ILLUSTRATION OMITTED]

By comparison with continuous cotton, soil strength measured on 28 October 1994 was significantly lower (P = 0.05) in the 0.18-0.27 m depth, and that measured on 17 August 1996 was significantly lower (P = 0.05) in the 0.12-0.225 m depth with long-fallow cotton, whereas soil water content did not differ significantly (Fig. 2).

Soil nitrate-N

Significant differences between cropping systems occurred only in 1994 during the fallow phase when values in the 0.15-0.60 depth of long-fallow plots were significantly greater than those in continuous cotton plots (Table 5), and may be due to mineralisation of soil organic matter in the former (Hulugalle and Entwistle 1996). In 1995 a `bulge' of nitrate-N was observed in the 0.45-0.60 m depth of both cropping systems, and may be due to leaching caused by the unusually heavy rainfall in January 1995 (191.5 mm; long-term mean is 58 mm). By comparison with long-fallow cotton, soil nitrate-N in the 0.30-0.60 m depth in 1995 was higher with continuous cotton, and may be caused by a reduction in nitrogen uptake in the latter due to the greater severity of black root rot disease (Table 6). There were no significant differences between treatments or cropping systems in 1997, and this perhaps indicates the increasing severity of black root rot in long-fallow cotton (Table 6). An increase in soil nitrate-N also occurred over time. Soil nitrate-N (g/[m.sup.2]) in the 0-0.60 m depth varied with time, t (months after commencing trial), such that:

Nitrate-N = 2.28 + 0.31t (r=0.88***, n=18)

Table 5. Effect of long fallow on soil nitrate-N (g/[m.sup.2]) during March 1994, 1995, and 1997 Standard errors are those of [log.sub.e]-transformed values of 10 x nitrate-N

 Depth (m) Continuous cotton Long-fallow cotton Mean

1994 0.00-0-15 1-80 1.15 1.44
 0.15-0-30 0-40 1.46 0.76
 0-30-0-45 0.38 1.65 0.79
 0-45-0-60 0.53 2.22 1.08
 Mean 0.62 1-58
1995 0.00-0-15 0.89 0.84 0.86
 0-15-0-30 0.95 0-90 0.92
 0.30-0-45 1.79 0.70 1.12
 0.45-0-60 4.79 2.55 3.49
 Mean 1.64 1.08
1997 0.00-0-15 4-85 6.19 5.48
 0.15-0-30 2.97 3.39 3.17
 0.30-0-45 2-97 3.66 3.30
 0.45-0-60 3-29 2.90 3.13
 Mean 3.44 3.89

ANOVA P s.e.
 Year (Y) 0.01 0.169
 Depth (D) 0.01 0.110
 Cropping system (CS) n.s. 0.138
 YxD 0-001 0.190
 YXCS n.s. 0.234
 CSxD n.s. 0.155
 YxCSxD 0.05 0.268


n.s., not significant.

Table 6. Effect of long-fallow on severity of black root rot and mycorrhizal colonisation during December 1994 and 1996 in cotton

Black root rot severity: 0, healthy; 1, >0-25% of tap root blackened; 2, >26-50% of tap root blackened; 3, >50-76% of tap root and few lateral roots blackened; and 4, >75-100% of tap root blackened and few or no lateral roots remaining
 Black root rot severity
 1994 1996 Mean
Cropping system
 Long-fallow cotton 0.8 2.4 1.6
 Continuous cotton 3.2 3.3 3.3
ANOVA
 Cropping systems s. e. P
 Yearsxcropping systems 0-22 0.01
 0-31 0.05

 Mycorrhizal colonization
 1994 1996 Mean
Cropping system
 Long-fallow cotton 41.0 64.3 52.7
 Continuous cotton 40.0 48.3 44.2
ANOVA s. e, P
 Cropping systems 2.79 n.s.
 Yearsxcropping systems 3.95 n.s.


n.s., not significant.

The increase in soil nitrate-N appears to be `Caused by the combination of N fertiliser application, minimum tillage, and residue retention (Dalal 1989; Constable et al. 1992; Rochester et al. 1993, 1997; N. R. Hulugalle and P. C. Entwistle 1997). Similar increases have been reported for other irrigated Vertosols, although the increases in this site are 1 .5-2 times higher than the highest values observed at the end of a growing season in other sites (Constable et al. 1992; Rochester et al. 1993, 1997; Hulugalle and Entwistle 1996, 1997, unpubl. data). The magnitude of the increase may also be due to a reduction in nitrogen uptake in both cropping systems due to an increasing severity of black root rot (Table 6).

Disease and mycorrhizal incidence

Severity of black root rot in continuous cotton plots was much higher in 1994 than that in long-fallow cotton plots (Table 5). This is because continuously cropping with a susceptible host such as cotton results in a more rapid build-up of Thielaviopsis basicola (Berk. & Br.) propagules than would occur when cotton is alternated with a fallow or a non-host crop. Between 1994 and 1996 this difference was reduced and the severity of the disease in plants in long-fallow cotton plots increased substantially. Black root rot does not usually cause seedling death but can reduce early season crop growth. Furthermore, depending on seasonal conditions, infected plants may compensate later in the season and lint production may be delayed but not reduced. This does not appear to have occurred in this instance.

Mycorrhizal colonisation did not differ significantly between long-fallow plots and continuous cotton plots (Table 6) and was comparable to that previously reported for healthy cotton in Vertosols (Nehl et al. 1996; McGee et al. 1997). The long-fallow treatment did not, therefore, cause a decline in mycorrhizal inoculum that was large enough to affect colonisation of the subsequent cotton crop. Cycles of wetting and drying of soil during fallow periods, particularly when combined with soil disturbance such as with tillage, can greatly decrease the survival of mycorrhizal fungi in fallowed soils (Pattinson and McGee 1997). Although changes in soil quality indicators such as organic C and air-filled porosity were not monitored by Pattinson and McGee (1997), it appears that climatic factors such as rainfall frequency may have contributed to the discrepancy between our observations and those of others (Thompson 1987, 1994; Hickman and Eveleigh 1995; McGee et al. 1997).

Cotton growth, nutrient uptake, profile water content, and lint yield

Neither dry matter production nor total boll production differed significantly between cropping systems during the 1994-95 growing season. Mean plant dry matter at 2, 3.5, and 4.7 months after sowing was 12-6, 366.6, and 637.5 g/[m.sub.2], respectively, and mean total bolls produced were 0, 59.2, and 72.9 bolls/[m.sup.2], respectively. Using pooled data from both cropping systems for the 1994-95 growing season, variation of dry matter production, DM (g/[m.sup.2]), with time, T (months after sowing), was described by:

[log.sub.e]DM = 18.48 - 9.66exp (1/T) (r= -0.99***, n=18)

whereas total boll production, TB (bolls/[m.sub.2]), was described by:

TB = 78.75 - [660-63/(1 + expT)] (r= -0.91***, n=18)

Both dry matter and boll production in long-fallow cotton plots were, however, greater than those in continuous cotton plots during the 1996-97 growing season (Fig. 3, Table 7). These differences in crop growth were presumably caused by a greater severity of black root rot (Table 6) and higher soil strength (Fig. 2) in continuous cotton plots in combination with declining organic C and nutrient availability (Fig. 1). Although black root rot is primarily a seedling disease, damage to root anatomy and function during early crop growth may have long-term consequences on water extraction and nutrient uptake, particularly when accompanied by other soil stresses such as high strength and declining organic C and nutrient availability (Smucker 1993). The end result would be a reduction in crop growth and yield. Severity of other soil-borne diseases of cotton such as verticillium and fusarium wilts was also evaluated in this study and their incidence was found to be very low and effects negligible (S. J. Allen, unpubl. data 1997).

[Figure 3 ILLUSTRATION OMITTED]

[TABULAR DATA 7 NOT REPRODUCIBLE IN ASCII]

Significant differences in tissue nutrient concentrations at crop maturity (early March) occurred only with respect to Ca and S. Concentrations of Ca in long-fallow and continuous cotton were 26.3 and 24.7 g/kg, respectively (s.e. =[+ or -]0.24, P = 0 .05), and those of S were 6.9 and 5.5 g/kg, respectively (s.e. =[+ or -]0.24, P = 0.01). These differences may be related to the greater severity of black root rot (Table 6) and higher soil strength (Fig. 2) in continuous cotton plots, and associated reductions in root function and nutrient uptake as discussed earlier (see previous paragraph). Mean values of N, P, K, Na, and Mg were 18.3, 1.1, 12.4, 5.4, and 5.4 g/kg, respectively; and those of B, Cu, Fe, Mn, and Zn were 53.5, 9.5, 143.3, 62.3, and 16.7 mg/kg.

Profile water content during the 1996-97 growing season suggests that, in comparison with continuous cotton, water extraction was greater with long-fallow cotton (Fig. 4). These differences were probably caused by a reduction in root activity of plants in continuous cotton plots due to the greater severity of black root rot and higher soil strength therein. The mechanism may be via damage to root cortical tissues of young cotton plants by black root rot, which in turn reduces root hydraulic conductivity of cotton, and consequently long-term water and nutrient uptake (Brar et al. 1991). Furthermore, new roots developed by continuous cotton following recovery from black root may also have lower hydraulic conductivity by comparison with the more mature roots in long-fallow cotton (Brar et al. 1991). The lower water extraction also appears to have resulted in an excess of water remaining in the soil profile and, consequently, waterlogging of continuous cotton plots between 65 and 92 days after sowing (Fig. 4). The waterlogging in turn may have further reduced crop growth and exacerbated the effects of black root rot disease in continuous cotton.

[Figure 4 ILLUSTRATION OMITTED]

Cotton lint yield was higher with long-fallow cotton. Lint production with continuous cotton and long-fallow cotton during the 1994-95 season was 179 and 205 g/[m.sub.2], respectively (s.e. =[+ or -]6.6, P = 0.01); and during the 1996-97 season was 129 and 186 g/[m.sup.2], respectively (s.e. = [+ or -] 10.3, P = 0.001). These differences were probably caused by the greater severity of black root rot disease and higher soil strength in continuous cotton plots in combination with declining organic C and nutrient availability.

There was no evidence for the existence of `long-fallow disorder' caused by low numbers of AM in long-fallow cotton plots. When compared with continuous cotton, indicators said to be characteristic of `long-fallow disorder' such as reduced seedling and crop growth, and yield, and reduced uptake of P and Zn were absent in long-fallow cotton. We suggest that the occurrence of `long-fallow disorder' due to low numbers of AM in any given field site may be caused more by an interaction between climatic factors such as rainfall frequency, poor soil quality as indicated by high soil compaction, salinity, and sodicity, and survival and root colonisation ability of mychorrhizal spores in poor quality soil, rather than the absence of cotton or other host crops per se (Douds et al. 1995; Hickman and Eveleigh 1995; Nadian et al. 1996; McGee et al. 1997; Pattinson and McGee 1997). The absence of `long-fallow' disorder in our study is probably due to relatively `good' quality soil, whereas its presence and associated low numbers of AM in cotton grown during the same season in a study reported by Hickman and Eveleigh (1995) were probably related to the high sodicity, waterlogging, and compaction in their site.

Significant differences in soil quality indicators due to long-fallow cotton occurred only with respect to exchangeable Ca and Mg in 1994, soil strength, and plastic limit. Short-term differences in air-filled porosity also occurred. Overall, however, these differences were small. By comparison with 1993, net decreases in the values of organic C, exchangeable Mg, and plastic limit occurred with both cropping systems, by 1997, whereas net increases occurred with respect to subsoil air-filled porosity and soil nitrate-N. The absence of major differences in soil quality between cropping systems, and improvements in some soil quality indicators in both cropping systems, suggest that soil processes at this site were dominated by crop characteristics such as C/N ratio and management practices such as minimum tillage, crop residue retention, and controlled traffic, which were imposed in 1993 at the commencement of this study. The absence of any differences between cropping systems with respect to soil organic C suggests that long-fallow cotton, unlike long-fallow in cereal- and legume-based cropping systems, does not have any short-term detrimental effect on carbon sequestration in soil (to reduce carbon-based greenhouse gas emissions).

Conclusions

`Long-fallow disorder' due to low numbers of arbuscular mychorrhiza, which is characterised by reduced crop growth rates and yields and reduced uptake of P and Zn, was not observed in long-fallow cotton in this study. Soil quality indicators which were affected by long-fallow cotton were exchangeable Ca and Mg in 1994, soil strength, and plastic limit, all of which were lower than with continuous cotton. Short-term differences in air-filled porosity also occurred. Severity of black root rot disease was lower with long-fallow cotton. By comparison with continuous cotton, crop growth, nutrient uptake, water uptake, and lint yield of cotton were better with long-fallow cotton, probably due to a lesser severity of black root rot disease and lower soil strength.

Acknowledgments

Funding for this project was provided by the Co-operative Research Centre for Sustainable Cotton Production and Cotton Research and Development Corporation as CRDC Project Nos DAN 83C, 100C,' DAN 95C, and DAN 108C. The manager of `Auscott-Warren', Mr Chris Hogendyke, is thanked for provision of land to conduct the trial and his continuing support and interest in the project. The staff of `Auscott-Warren' are thanked for their support in routine maintenance and operation of the field site. The technical assistance of Mr L. A. Finlay is gratefully appreciated.

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Manuscript received 24 November 1997, accepted 25 February 1998

N. R. Hulugalle(AC), P. C. Entwistle(AC), J. L. Cooper(B), S. J. Allen(AC), and D. B. Nehl(AC)

(A) Australian Cotton Research Institute, NSW Agriculture, Myatt Vale Mail Run, Narrabri, NSW 2390, Australia.

(B) Agricultural Research Centre, NSW Agriculture, Trangie, NSW 2823, Australia.

(C) Co-operative Research Centre for Sustainable Cotton Production, Myall Vale Mail Run, Narrabri, NSW 2390, Australia.
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Publication:Australian Journal of Soil Research
Date:Jul 1, 1998
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