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Carbon inputs by wheat and vetch roots to an irrigated Vertosol.

Introduction

Sowing cereal crops such as wheat (Triticum aestivum L.) or leguminous crops such as vetch (Vicia villosa Roth.) or faba bean (V. faba L.) in rotation with cotton (Gossypium hirsutum L.) in irrigated Vertosols can be beneficial to soil quality in terms of improving soil structure (i.e. better soil water storage, aeration, and drainage) and soil N, although expected increases in soil carbon have not occurred (Rochester et al. 2001; Rochester and Peoples 2005; Hulugalle and Scott 2008; Hulugalle et al. 2011b). To the contrary, in most locations soil carbon stocks in cotton-based farming systems have declined or, at best, have stabilised (Hulugalle et al. 2011b). These findings are similar to those from many other row-cropped, semi-arid annual cropping systems, both irrigated and dryland (Dalal and Chan 2001; Chan et al. 2003, 2011; Eagle et al. 2011; Powlson et al. 2011). In contrast, increases in soil carbon stocks with time have been reported for land that has been under pasture or where a pasture is sown in rotation with an annual crop (Dalal and Chan 2001; Chan et al. 2003, 2011; Eagle et al. 2011; Powlson et al. 2011). Chan et al. (2011) have suggested that this may be related to the carbon added to soil through the roots of the pasture. Furthermore, some authors (Kong and Six 2010; Katterer et al. 2011) have reported that carbon derived from crop roots contributes more to soil carbon and its stability than that from aboveground residues. Few studies have, however, examined the amounts of carbon that can be added to soil by the roots of various crops in cotton farming systems on furrow-irrigated Vertosols. Such studies suggest that depending on seasonal conditions, genotype, cropping system, and crop health, the amounts of carbon added per season by cotton and rotation crops can differ: 2.3tC/ha (range 0.5-4t C/ha) by cotton, 7.7 t C/ha (range 5.5-9.3 t C/ha) by corn (Zea mays L.), and 10.9 t C/ha (range 9-12.5 t C/ha) by grain sorghum (Sorghum bicolour (L.) Moench) (Hulugalle et al. 2009, 2010a, 2010b, 2011b). Assuming that sequestration rates were of the order of 5% of total carbon inputs (Follett et al. 2005; Johnson et al. 2006; Wang and Dalai 2006; Grace et al. 2010), carbon from root materials that could be sequestered in soil would range from 0.1 to 0.6 t C/ha. Published information on the amounts of carbon added to Vertosols by root systems of the more common rotation crops such as wheat and vetch have not been reported in the literature.

Although, published data on amounts of carbon added to semi-arid or sub-humid Vertosols by wheat and vetch roots in cotton-farming systems are sparse, possible amounts can be estimated from the literature (Tables 1 and 2). With respect to wheat, across all soil types, the amount of carbon in wheat roots was [less than or equal to] 0.6 t C/ha in 68% of the studies, with 59% reporting values in the range 0.2-0.6 t C/ha (Table 1). A closer perusal of the studies listed in Table 1 indicated that most (64%) were conducted in coarse-textured soils (sandy to loamy textures), and that lower values were associated with these soils; i.e. 55% of observations [less than or equal to] 0.6t C/ha and 9% of observations >0.6 t C/ha were from coarse-textured soils. In contrast, 14% of observations [less than or equal to] 0.6 t C/ha and 23% of observations >0.6 t C/ha came from medium- and fine-textured soils (sandy clay loam to clay textures). It appears, therefore, that wheat crops grown in clayey Vertosols are more likely to produce bulky root systems and, thus, contribute more carbon to the soil. Among the studies summarised in Table 1, carbon in wheat roots in Vertosols ranged from 1.1 to 1.6 t C/ha (Izzi et al. 2008; Lopes and Reynolds 2010; Munoz-Romero et al. 2010). A similar analysis was not possible with respect to vetch roots as few studies have been conducted, and of these, only one assessed root density in the entire soil profile (Table 2). All others were restricted to depths [less than or equal to] 0.4m. Arslan and Kurdali (1996) suggested that, subject to management and water availability, the subsoil (>0.45m) could account for 10-50% of the total root mass. Hence, carbon in vetch roots at physiological maturity may well be of the order of 1-3 t C/ha. None of the studies cited in Tables 1 and 2 accounted for the addition of root material to soil through root death and decay, or rhizodeposition through root exudates during and after the crop's growing season. These amounts, may, however, be significant and are claimed to be equal to that in root biomass at maturity (Katterer et al. 1993; Bolinder et al. 1997; Steingrobe et al. 2001). Thus, carbon added to Vertosols by wheat roots may be of the order of 2-3 t C/ha and that by vetch 2-6 t C/ha. Furthermore, except for the study by Sainju et al. (1998), in which vetch was sown in rotation with cotton under rainfed, sub-humid climatic conditions, all of the other research was conducted in monocultures, or wheat-legume or wheat oilseed rotations.

The objective of this study, therefore, was to determine the amounts of carbon added to soil by winter rotation crops [wheat and purple or Popany vetch (Vicia benghalensis L., syn. Vicia atropurpurea Desf.] sown in rotation with irrigated cotton, through root turnover during the growing season and decay of root systems thereafter. Measurements were made from 2008 to 2010 in a long-term experiment using a combination of soil cores and minirhizotron observations.

Materials and methods

Site

Wheat and vetch root growth was measured during the growing seasons (autumn-spring) of 2008, 2009, and 2010 in an experiment that commenced in 2002 at the Australian Cotton Research Institute, near Narrabri (149[degrees]47'E, 30[degrees]13'S) in New South Wales (NSW), Australia. Narrabri has a subtropical, semi-arid climate (BSh; Kottek et al. 2006) and experiences four distinct seasons, with a mild winter and a hot summer. The hottest month is January (mean daily maximum 35[degrees]C and minimum 19[degrees]C) and the coldest is July (mean daily maximum 18[degrees]C and minimum 3[degrees]C). Mean annual rainfall is 593 mm. The soil at the experimental site is an alkaline, self-mulching, grey clay, classified as a self-mulching, grey Vertosol, very-fine (Isbell 1996) or a fine, thermic, smectitic, Typic Haplustert (Soil Survey Staff 2010). Mean particle size distribution in the 0-1 m depth (per 100g) was: 64g clay, 11 g silt, and 25 g sand. Average exchangeable sodium percentage in the 0.6-1.2 m depth was 15 but did not exceed 6 in the shallower depths.

Experimental layout

The experimental treatments consisted of four cotton-based rotation systems sown on permanent beds: cotton monoculture (summer cotton-winter fallow-summer cotton); cotton vetch (summer cotton-winter vetch-summer cotton); cotton wheat (summer cotton-winter wheat-summer and winter fallow summer cotton), in which wheat stubble was incorporated into the beds after harvest with a disc-hiller; and cotton-wheat-vetch (summer cotton winter wheat-summer fallow-autumn and winter vetch-summer cotton), with wheat stubble retained as an in-situ mulch into which the following vetch crop was sown. The experiment was laid out as a randomised complete block with three replications and designed such that both cotton and rotation crop phases in the last two rotation treatments were sown every year. Individual plots were 165 m long and 20 rows wide. The rows (beds) were spaced at 1-m intervals with vehicular traffic being restricted to the furrows. In this study we compared root growth and associated indices of vetch in cotton-vetch (CV), vetch and wheat in cotton-wheat-vetch (CWV), and wheat in cotton-wheat (CW).

Crop management

Cotton

In NSW, cotton is sown in October. Roundup Ready[R] cotton was sown in the experiment from 2002 to 2005, and Bollgard II[R]--Roundup Ready Flex[R] cotton thereafter. Namoi woolly pod vetch (Vicia villosa Roth.) was sown in the experiment from 2002 to 2006 and purple or Popany vetch thereafter, because the latter does not suffer from hardseededness. Cotton in rotations which did not include a vetch component (cotton monoculture and CW) received 160-180kg N/ha before sowing until and including the 2008-09 season, and as urea after sowing thereafter. Cotton in rotations that included vetch were not fertilised before sowing but received supplementary N as urea in December or January. Application rates were dependent on N fixation by the vetch and estimated losses. Between 2008 and 2011, rates were (kg N/ha): 60 (2008-09), 70 (2009-2010), and 120 (2010-11) for CWV; and 80 (2008-09) and 120 (2009-2010, 2010-2011) for CV. All treatments were furrow-irrigated with ~100mm of water when rainfall was insufficient to meet evaporative demand. Cotton was picked during late April or early May with a two-row picker, after defoliation in early April. After picking, the cotton plants were slashed and incorporated into the beds with a disc-hiller.

Wheat and vetch

Wheat was sown on 19 May 2008, 12 May 2009, and 14 May 2010 at a rate of 60 kg/ha. It received 20 kg N/ha as urea by broadcasting at sowing, and 60 kg N/ha during late July or early August. Vetch in CWV was sown at a rate of 20 kg/ha into wheat stubble on 22 February 2008, 26 February 2009, and 26 February 2010 following summer rains, and that in CV after cotton picking and pupae-busting, at the same rate on 19 May 2008, 13 May 2009, and 17 May 2010. Nitrogen fertiliser was not applied to vetch. Phosphorus was applied only during September 2010 to all plots at a rate of 25 kg P/ha as single superphosphate. Depending on in-crop rainfall and stored soil water, wheat and vetch received up to two irrigations of 100 mm per season. Vetch, which is a prostrate, leguminous crop, was killed during or just before flowering through a combination of mowing and contact herbicides (Hulugalle et al. 2011a), and the residues were retained as in situ mulch into which the following cotton crop was sown. Wheat was harvested with a grain harvester during late November or early December.

Crop root measurements

Root growth in the surface 0.10 m was measured with the core-break method using 0.10-m-diameter cores (Drew and Saker 1980). Soil cores were used for the surface 0.10m because minirhizotron measurements underestimated root growth in this depth, presumably due to light leakage and temperature effects (Smit et al. 2000). A subsample of the cores taken from the surface 0.10 m in each plot at each time of sampling was transported to the laboratory in labelled and sealed plastic bags and stored in a cold room (4[degrees]C) for root washing and separation. The root samples were soaked in a warm water solution containing 2 : 1 10% sodium hexametaphosphate : 0.1 M sodium hydroxide for ~4h. Once dispersed, root and other organic material was separated from soil by flotation and decantation and by washing through a 0.212-mm sieve. The organic material obtained (including roots) was then stained with a 0.1% congo red solution for 4-8 h (depending on age of crop), followed by washing in absolute alcohol (Ward et al. 1978; Polomski and Kuhn 2002). Congo red stains the live roots a bright red colour, whereas the dead organic material remains black. Live roots were separated from the dead material under a bright light using forceps after spreading the sample in a shallow, white plastic tray filled with ~5 mm of water. The live roots were then stored in a 25% alcohol solution until their length was measured using a modified Newman's line interception method with a 100mm by 100mm grid during 2008 (Smit et al. 2000; Polomski and Kuhn 2002), or scanned and measured with WINRHIZO[R] software (www.regent.qc.ca/products/rhizo/WinRHIZO.html) during 2009 and 2010. The root samples were then oven-dried and weighed. Relationships were derived between root number, root length, and root weight, and the root length and weight in each core were estimated. Relative root length (root weight/root length) was also calculated. Carbon concentration in the oven-dried root material was measured by combustion with a LECO CHN 2000[R] analyser (www.leco.com/ resources/application_notes/pdf/CHN2000_PLANT_TISSUE_ 203-821-160.pdt).

Root growth in the 0.10-1.0 m depth was measured at depth intervals of 0.10m with a Bartz BTC-2 Minirhizotron Video Microscope[R] camera system (http://bartztechnology.com/btc2. html) and BTC I-CAP Image Capture System[R] (http:// bartztechnology.com/icapsystem.html). The video camera was inserted into clear, plastic acrylic minirhizotron tubes (50-mm-diameter) installed within each plot, 30[degrees] from the vertical. The operating and measurement procedures used were those described by Johnson et al. (2001). Depending on crop growth stage and environmental conditions, measurements were taken at intervals of ~3-5 weeks. Root images were captured in two orientations, left and right side of each tube, at each time of measurement and analysed with RooTracker 2.03[R] (Duke University 2001) to estimate selected root growth indices. The results for each orientation at each depth and over the entire measured profile were summed to provide an assessment of root growth over a 360[degrees] plane of vision. The indices evaluated were the length and number of live roots at each time of measurement, number of roots which changed length, number and length of roots which died (i.e. disappeared between times of measurement), new roots initiated between times of measurement, and net change in root numbers and length. The above, together with relative root lengths and root carbon concentrations of samples taken from the previously described soil cores, were then used to calculate several other indices of root growth:

(1) Root length density, [L.sub.V], for individual depth intervals, and root length per unit area to a depth of 1 m, [L.sub.A];

(2) Root carbon at end of season, [C.sub.root] = sum of net changes in root carbon between times of measurement in all depths, where, for individual depths and between times of measurement, the net change in root carbon was calculated as: net change in root length x relative root length x root carbon concentration;

(3) Root carbon added to the soil during season, [C.sub.lost] = sum of root carbon added to soil due to root death between times of measurement in all depths, where, for individual depths and between times of measurement, root carbon added to soil was calculated as: length of roots which died x relative root length x root carbon concentration;

(4) Root carbon which could be potentially added to soil, [C.sub.total] = [C.sub.root] (2) + [C.sub.lost] (3).

Details of the previously mentioned calculations are reported in Hulugalle et al. (2009).

Statistical analyses

Data were analysed after logs transformation with analysis of variance using a randomised complete block design. During the 2010 winter, vetch in replicate 1 of the CWV rotation was severely damaged by cotton aphids (Aphis gossypii). The aphid-damaged plot was analysed as a separate treatment using a mixed models approach to quantify the effect of the aphids on root growth.

Results and discussion

Vetch root length

Peak [L.sub.A] of vetch (early flowering) in CWV (early-sown vetch) exceeded that of CV (late-sown vetch) during 2009 and 2010 by an average of 2.5 times, whereas during 2008, values in both treatments were generally similar, with that of early-sown vetch being marginally higher (by 12%) (Fig. 1). These interseasonal differences may be due to differences in the length of growing season (5-6 months for the early-sown vetch and 3-4 months for the late-sown vetch), in-crop rainfall and soil-water storage (Hulugalle et al. 2011b), and increasing winter weed numbers in the late-sown vetch with increasing duration of the experiment. Due to high populations of winter weeds such as dead nettle (Lamium amplexicaule L.) and, to a lesser extent, milk thistle (Sonchus oleraceus L.) in the late-sown vetch, control measures included application of herbicides (glyphosate, diuron). Although relatively minor damage occurred to aboveground parts of vetch, vegetative growth recovered. It appears, however, that inhibition to root growth may have been greater and more long-lasting. The effect of winter weeds on the early-sown vetch was limited because it was able to establish during later autumn before the abovementioned winter weeds germinated and, thus, outcompete them.

Changes in vetch [L.sub.V] with depth (Figs 2, 3, 4) reflected the abovementioned variations in [L.sub.A] with respect to root distribution in the soil profile. During 2008 at early flowering, 22% of the late-sown and 21% of the early-sown vetch roots occurred at depths [greater than or equal to] 0.6 m (Fig. 2). Average [L.sub.V] in the 0.6-0.9 m depth of the early- and late-sown vetch was 12.8 and 11.8 km/[m.sup.3], respectively. During 2009 and 2010, however, at the same growth stage, an average of 46% of the late-sown (undamaged by aphids) and 1% of the early-sown vetch roots were observed at depths [greater than or equal to] 0.6m (Figs 3 and 4). Average [L.sub.V] in the 0.6-0.9m depth of the early- and late-sown vetch was 0.1 and 31.4 km/[m.sup.3], respectively, during 2009, and 0.5 and 42.5 km/[m.sup.3], respectively, during 2010. Consequently, the late-sown vetch had a sparser and shallower root system than its earlier sown counterpart. Aphid-damage during 2010 to the early-sown vetch resulted in a significant reduction in LA (Fig. 1, Table 3). Average [L.sub.A] during late vegetative growth and early flowering in June 2010 was of the order of 54 km/[m.sup.2] in undamaged vetch and 21 km/[m.sup.2] in aphid-damaged vetch, i.e. a reduction of 61%. Reduction of root biomass due to aboveground herbivory has been reported for many plant species, ranging from aphid damage in annual grasses such as Poa annua (Sinka et al. 2009) to damage caused by pine sawflies (Neodiprion spp.) in woody perennials such as Pinus ponderosa (Sanchez-Martinez and Wagner 1999). Van Dam and Bezemer (2006) suggested that this response is mediated through an alteration in the internal plant hormone balance. Insect damage to aboveground plant organs may, therefore, inhibit root functions such as water and nutrient extraction and, ultimately, amounts of carbon in crop roots, with the intensity of the changes varying between tap and feeder roots (de Kroon and Visser 2003).

Wheat root length

Wheat [L.sub.A] was comparable to that of the early-sown vetch but was generally higher than that of late-sown vetch (Fig. 1). Consistent differences in [L.sub.A] were absent between wheat treatments. Significant differences were present on only three occasions. During grain filling/ripening in 2008 and flowering (anthesis)/milking in 2010, [L.sub.A] of wheat in CW was greater than of wheat in CWV by 2.6 and 1.7 times, respectively (Fig. 1, Table 3). This was mainly due to a high concentration of roots in the beds (0-0.1 m) of the CW treatment (Figs 2 and 5). During stem elongation in 2009, [L.sub.A] of wheat in CWV was greater than that in CW by 3.4 times. Again, this was due to a higher concentration of roots in the surface 0.2m in the CWV rotation (Fig. 3). Overall, wheat tended to have higher concentrations of roots in the surface 0.1 m than either of the vetch crops. The differences between the two wheat treatments, and between vetch and wheat crops, may be related to differences in soil water storage, drainage, and soil evaporation (Hulugalle et al. 2011b), soil compaction (Hulugalle et al. 2011a), and the application of N fertiliser to wheat. Root proliferation in soil patches enriched with water and nutrients is a strategy that annual crops employ to improve their survivability and competitiveness (Hodge 2004).

Root carbon indices of vetch and wheat

Over the 3 years, average total root carbon potentially available for addition to soil ([C.sub.total]) was in the order: vetch in CWV (510 g/[m.sup.2]) > vetch in CV (192 g/[m.sup.2]) > wheat in CW (160 g/[m.sup.2]) = wheat in CWV (173 g/[m.sup.2]) (Table 4). The [C.sub.total] was, thus, more with vetch than with wheat; vetch in CWV was 3.33 times greater and that in CV 1.2 times greater. Consequently, wheat was less effective in returning carbon to the soil. Since the [L.sub.A] of both wheat crops was comparable to that of early-sown vetch and greater than that of late-sown vetch, this was probably related to the finer root system of wheat. Averaged over the 3 years, relative root weight of vetch was 29.8 mg/m and that of wheat 9.7 mg/m (t = 3.15, d.f. = 35, P < 0.01). Assuming an average soil carbon sequestration rate of 5% (the literature proposes values ranging between 3 and 15% of plant inputs from fertilised, minimum, or no-tilled crop residues; Follett et al. 2005; Johnson et al. 2006; Wang and Dalai 2006; Grace et al. 2010), carbon sequestered by root inputs of the rotation crops are estimated to be of the order of 0.34 t C/ha.year for vetch and wheat (combined) in CWV, 0.10 t C/ha.year for vetch in CV, and 0.08 t C/ha.year for wheat in CW. As the C/N ratio of vetch roots was ~18 (Hulugalle et al. 2011b), and thus, readily decomposable, the values for rotations that contained vetch may well be lower. By comparison, average C/N ratio of mature, pre-senescent wheat roots was ~49. Katterer et al. (2011) reported that carbon sequestration by root inputs was 2.3 times more than that by aboveground inputs in a cold temperate, sub-humid environment. Assuming a carbon concentration of 40% and a 5% sequestration rate (Follett et al. 2005; Johnson et al. 2006; Wang and Dalai 2006; Grace et al. 2010), carbon sequestered by aboveground crop residues in our study were estimated to be of the order of 0.14 t C/ha.year for vetch and wheat (combined) in CWV, 0.06 t C/ha.year for vetch in CV, and 0.04 t C/ha.year for wheat in CW (Hulugalle et al. 2011b). Thus, the corresponding root/aboveground input ratios for C sequestration were 2.4 for vetch and wheat (combined) in CWV, 1.7 for vetch in CV, and 2.0 for wheat in CW. In comparison with Katterer et al. (2011), the ratio for vetch and wheat (combined) in CWV was similar but that for vetch in CV was lower by 26% and wheat in CW by 13%. Total carbon inputs in above- and below-ground crop residues reported by Katterer et al. (2011) were similar to those of the two latter rotations but approximately three times less than that for vetch and wheat (combined) in CWV. The relatively lower carbon sequestration in the irrigated cotton farming environment of northern NSW (warm to hot temperatures, high N inputs, and frequent wet/dry cycles) was probably related to enhanced microbial activity and, thus, higher soil respiration rates (Dalal and Chan 2001; Chan et al. 2003).

Carbon in root mass remaining at the end of the season ([C.sub.root]) followed a similar trend to [C.sub.total] such that vetch in CWV > vetch in CV > wheat in CW = wheat in CWV. Carbon added to soil through intra-seasonal root death ([C.sub.lost], however, differed, and was in the order of wheat in CW = wheat in CWV > vetch in CWV > vetch in CV. Although there was much variation across seasons, average [C.sub.root] in vetch crops accounted for 88% of [C.sub.total] and [C.sub.lost] accounted for 12%, whereas in wheat crops average [C.sub.root] and [C.sub.lost] accounted for 64% and 36%, respectively of [C.sub.total]. The difference between crop species with respect to these indices is probably largely related to their management; i.e. wheat was allowed to mature until grain ripening, whereas vetch was terminated at flowering, although a species effect cannot be excluded. Values of [C.sub.lost] for wheat are comparable to those reported for temperate zone wheat (~40-60%) and unstressed cotton (25-49%) but are lower than reported for corn (11%) (Steingrobe et al. 2001; Hulugalle et al. 2010a, 2010b).

It was previously noted that the weed control measures implemented in the cotton-vetch rotation appeared to have inhibited vetch root growth. This was also reflected in the vetch's root carbon indices. Compared with values for 2008, average [C.sub.root] during 2009 was 66% lower and during 2010 was 72% lower. At the same time, compared with with vetch in CWV, average [C.sub.root] of vetch in CV was 68% lower, [C.sub.lost] 78% lower, and [C.sub.total] 62% lower, with greatest differences occurring during 2009 and 2010.

Compared with undamaged vetch, damage by aphids to vetch during 2010 resulted in [C.sub.total] decreasing by 73% (Table 4). The proportion of root mass at termination during 2010 (relative to [C.sub.total]) decreased from 94% in undamaged plants to 80% in aphid-damaged plants. Proportional root mortality, by contrast, increased from 6% when undamaged to 20% in aphid-damaged vetch. Aphid damage to vetch, therefore, resulted in a root system that contributed less carbon to soil due to an overall reduction in biomass (see discussion of [L.sub.A] results) and that suffered from greater level of intra-season root mortality.

Conclusions

Root length per unit area of early-sown vetch and wheat were comparable, although the latter tended to have a higher concentration of roots in surface 0.10 m. Root growth of the later sown vetch was sparse. Root growth was inhibited by aphid damage to aboveground parts of vetch. Root carbon available for addition to soil was greater with vetch than with wheat and was in the order: vetch in CWV (5.1 t C/ha/ year) > vetch in CV (l.9 t C/ha/year) > wheat in CW (1.6 t C/ ha/year) = wheat in CWV (1.7 t C/ha/year). Intra-seasonal root mortality accounted for 12% of total root carbon in vetch and 36% in wheat. The remaining fraction consisted of carbon in the root mass at the end of the growing season. Carbon sequestered by root inputs of the rotation crops was estimated to be of the order: 0.34 t C/ha.year for CWV, 0.10 t C/ha for CV, and 0.08 t C/ha for CW. The rotation CWV was, therefore, the most effective in sequestering carbon from roots.

Acknowledgments

Funding for this research was provided by the Cotton Catchment Communities Co-operative Research Centre and Cotton Research and Development Corporation of Australia.

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N. R. Hulugalle (A,B), T. B. Weaver (A), and L. A. Finlay (A)

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

(B) Corresponding author. Email: nilantha.hulugalle@dpi.nsw.gov.au

http://dx.doi.org/10.1071/SR11281

Table 1. Carbon in field-grown wheat (Triticum aestivum L.)
roots at crop maturity (anthesis or later) under semi-arid
or sub-humid climatic conditions, 1975-2011

Only studies that sampled to a depth that included 90% or
more of the root system were selected. Values were
averaged among treatments and years but expressed
according to soil texture. Carbon concentration in
roots was assumed to be 40% of root dry matter. Where only
root length density was reported, root
dry matter was estimated using a relative root length of
10 mg/m (N. R. Hulugalle, unpubl. data)

Source                   Soil texture              Location

Asseng et al. (1998)     Sand overlying clay;      Western Australia
                           5-cm-thick gravel
                           layer between
                           the two zones
Block and Van Rees       Sandy loam to loam        Saskatchewan, Canada
  (2006)
Box and Ramseur          Sandy loam                Georgia, USA
  (1993)
Caires et al. (2008)     Sandy clay loam           Parana, Brazil
Campbell and de Jong     Loam                      Saskatchewan, Canada
  (2001)
Dracup et al. (1992)     Sand overlying clay       Western Australia
Entz et al. (1992)       Clay loam; fine sandy     Saskatchewan, Canada
                           loam to loam

Gan et al. (2009a)       Silty loam                Saskatchewan, Canada
Gan et al. (20096)       Silty loam                Saskatchewan, Canada
Gregory (1998)           Sand overlying clay;      Western Australia
                           5-cm-thick gravel
                           layer between
                           the two zones.
Hamblin and Tennant      Loamy sand; coarse        Western Australia
  (1987)                   loamy sand overlying
                           lateritic sandy clay;
                           sandy loam overlying
                           fine sandy
                           clay
Incerti and O'Leary      Sandy clay loam           Victoria, Australia
  (1990)                   overlying
                           sandy clay
Izzi et al. (2008)       Fine clay                 Aleppo, Syria
Lopes and Reynolds       Clay                      Sonora, Mexico
  (2010)
Meyer and Batrs          Clay loam overlying       NSW, Australia
  (1991)                   clay
Munoz-Romero et al.      Clay                      Cordoba, Spain
  (2010)
Proffitt et al. (1985)   Sandy clay loam           Gauteng, South Africa
Rickman and Klepper      Silty loam                Oregon, USA
  (1980)
Taylor and Terrell       Loam                      Saskatchewan, Canada
  (1982)

Source                     Irrigated/rainfed       Depth (m)

Asseng et al. (1998)       Rainfed                 0-0.75
Block and Van Rees         Rainfed                 0-0.9
  (2006)
Box and Ramseur            Rainfed                 0-0.75
  (1993)
Caires et al. (2008)       Rainfed                 0-0.6
Campbell and de Jong       Irrigated and rainfed   0-1.2
  (2001)
Dracup et al. (1992)       Rainfed                 0-0.50 to 0-1.75
Entz et al. (1992)         Rainfed                 0-1.3
Gan et al. (2009a)         Irrigated and rainfed   0-1.0
Gan et al. (20096)         Irrigated and rainfed   0-1.0
Gregory (1998)             Rainfed                 0.75
Hamblin and Tennant        Rainfed                 0-1.0 to 0-2.5
  (1987)
Incerti and O'Leary        Rainfed                 0-1.25
  (1990)
Izzi et al. (2008)         Irrigated               0-0.75
Lopes and Reynolds         Irrigated and rainfed   0-1.2
  (2010)
Meyer and Batrs            Irrigated               0-1.05
  (1991)
Munoz-Romero et al.        Rainfed                 0-0.85
  (2010)
Proffitt et al. (1985)     Irrigated               1.6
Rickman and Klepper        Rainfed                 1.2
  (1980)
Taylor and Terrell         Rainfed                 0-1.65
  (1982)

Source                         Root C (t/ha)

Asseng et al. (1998)               0.28

Block and Van Rees                 0.02
  (2006)
Box and Ramseur                    0.77
  (1993)
Caires et al. (2008)               0.36
Campbell and de Jong               0.57
  (2001)
Dracup et al. (1992)               0.26
Entz et al. (1992)           Clay loam, 0.25;
                              fine sandy loam
                               to loam, 0.18
Gan et al. (2009a)                 0.52
Gan et al. (20096)                 0.47
Gregory (1998)                     0.35
Hamblin and Tennant             Loamy sand,
  (1987)                    0.21; coarse loamy
                             sand, 0.30; sandy
                                loam, 0.48
Incerti and O'Leary                1.01
  (1990)
Izzi et al. (2008)                 1.08
Lopes and Reynolds                 1.14
  (2010)
Meyer and Batrs                    0.78
  (1991)
Munoz-Romero et al.                1.56
  (2010)
Proffitt et al. (1985)             0.52
Rickman and Klepper                0.37
  (1980)
Taylor and Terrell                  2.7
  (1982)

Table 2. Carbon in field-grown cover crops of vetch (Vicia spp.)
roots during flowering or later growth stages under semi-arid
or sub-humid climatic conditions, 1975-2011

All crops were rainfed. Values were averaged among
treatments and years but expressed according to soil
texture. Carbon concentration in roots was
assumed to be 40% of root dry matter

Source                        Texture                Location

Sainju et al. (1998)          Fine sandy loam        Georgia, USA
Ozpinar and Baytekin (2006)   Clay loam              NW Turkey
Sidiras et al. (1999)         Clay loam              Greece
Arslan and Kurdali (1996)     Silt to 0.15m, silty   Jillin, SW Syria
                                clay thereafter

Source                        Species                Depth (m)

Sainju et al. (1998)          V. villosa Roth.       0-0.30
Ozpinar and Baytekin (2006)   V. sativa L.           0.40
Sidiras et al. (1999)         V. sativa L.           0-0.15
Arslan and Kurdali (1996)     V. sativa L.           0-1.20

Source                        Root C (t/ha)

Sainju et al. (1998)          0.98
Ozpinar and Baytekin (2006)   1.05
Sidiras et al. (1999)         1.49
Arslan and Kurdali (1996)     0.50

Table 3. Analyses of variance for root growth parameters (F-values)

[L.sub.v], Root length density; [L.sub.A], root length per unit area.
Back-transformed values are shown in Figs 1-5.
* P<0.05; ** P<0.01; *** P<0.001

                                Ln ([L.sup.v]+10)

                             0.05m       0.10m      0.20m

                                       2008

27 June (d.f.=3,30)        14.57 ***  12.47 ***      16.28 ***
5 August (d.f.=3,30)       67.01 ***   4.94 **        2.71
11 September (d.f=2,22)    41.05 ***   0.39           0.66
26 September (d.f.=2,22)   62.28 ***   0.94           4.04 *
30 September (d.f=1,14)    79.93 ***   1.09           0.51

                                      2009

6 July (d.f=3,30)          27.92 ***   4.35 *         17.49 ***
4 August (d.f=3,30)        78.14 ***   8.60 ***        9.46 ***
31 August (d.f=2,22)       29.49 ***   0.64           6.82 **
7 October (d.f =1,14)       1.52       6.59 *          7.38 *

                                      2010

14 April (d.f=1,7)          3.72       0.05           0.03
5 May (d.f.=1,7)            0.05       0.24           0.01
10 June (d.f=1,7)           5.59 *     0.30           0.05
23 June (d.f.=2,29)        21.56 ***   2.11           8.79 ***
16 August (d.f=2,22)       22.91 ***   5.04 *         7.04 ***
21 September (d.f=2,22)    40.96 ***   0.61           0.60
27 October (d.f.=1,14)      1.53       0.47           0.00
25 November (d.f.=1,14)     0.01       0.64           0.19

                                Ln ([L.sup.v]+10)

                           0.30m         0.40m        0.50m

                                       2008

27 June (d.f.=3,30)        16.93 ***      8.05 ***     21.45 ***
5 August (d.f.=3,30)        3.72 *        0.23         0.88
11 September (d.f=2,22)     4.66 *        1.47         1.82
26 September (d.f.=2,22)    6.92 **       1.88         3.40 *
30 September (d.f=1,14)     0.33         0.01          2.57

                                      2009

6 July (d.f=3,30)          10.58 ***    74.68 ***     48.64 ***
4 August (d.f=3,30)         2.58         7.70 ***     17.30 ***
31 August (d.f=2,22)        0.88        22.12 ***      8.38 **
7 October (d.f =1,14)       0.24         0.00         0.61

                                      2010

14 April (d.f=1,7)          1.33         0.05         0.00
5 May (d.f.=1,7)            0.93         0.01         1.10
10 June (d.f=1,7)           0.13         0.22         3.96
23 June (d.f.=2,29)        23.80 ***    30.60 ***     39.26 ***
16 August (d.f=2,22)        4.87 *       2.28         5.85 **
21 September (d.f=2,22)     2.58         0.03         0.66
27 October (d.f.=1,14)      0.79         0.18         0.21
25 November (d.f.=1,14)     0.45         0.02         0.10

                                Ln ([L.sup.v]+10)

                             0.60m          0.70m      0.80m

                                          2008

27 June (d.f.=3,30)        5.25 **     5.52 **         1.31
5 August (d.f.=3,30)        0.77       1.68           0.80
11 September (d.f=2,22)     1.19       2.41           1.94
26 September (d.f.=2,22)    1.30       0.64           1.46
30 September (d.f=1,14)     0.02       0.96           0.24

                                      2009

6 July (d.f=3,30)          21.83 ***  15.14 ***       10.59 ***
4 August (d.f=3,30)        13.61 ***  20.77 ***       6.87 **
31 August (d.f=2,22)        8.62 **    5.89 **        1.17
7 October (d.f =1,14)       3.12       0.00           0.67

                                      2010

14 April (d.f=1,7)          0.47       0.38           -
5 May (d.f.=1,7)            0.13       0.47           -
10 June (d.f=1,7)           1.83       3.17           9.92 *
23 June (d.f.=2,29)        19.39 ***  12.78 ***      10.35 ***
16 August (d.f=2,22)        8.25 **    1.32           1.00
21 September (d.f=2,22)     0.67       5.94 **        1.31
27 October (d.f.=1,14)      0.20       0.06           0.00
25 November (d.f.=1,14)     0.18       0.00           0.02

                         Ln ([L.sup.v]+10)

                           0.90m             Ln [L.sub.A]

                                      2008

27 June (d.f.=3,30)        1.76                18.75 ***
5 August (d.f.=3,30)       5.44 **              4.21
11 September (d.f=2,22)    1.49                 0.44
26 September (d.f.=2,22)   1.62                 0.63
30 September (d.f=1,14)    1.01                 9.09 **

                                      2009

6 July (d.f=3,30)          7.21 ***            72.66 ***
4 August (d.f=3,30)        6.68 **             51.95 ***
31 August (d.f=2,22)       1.91                27.91 ***
7 October (d.f =1,14)      1.82                 0.60

                                      2010

14 April (d.f=1,7)         -                    0.38
5 May (d.f.=1,7)           -                    0.43
10 June (d.f=1,7)          0.15                 5.79 *
23 June (d.f.=2,29)        3.58 *              53.66 ***
16 August (d.f=2,22)       1.00                28.12 ***
21 September (d.f=2,22)    0.98                13.61 ***
27 October (d.f.=1,14)     0.02                 0.58
25 November (d.f.=1,14)    0.13                 0.03

Table 4. Effect of crop rotation on root carbon indices (g/[m.sup.2])

[C.sub.root], Carbon in crop roots at end of season; [C.sub.root],
carbon in dead roots; [C.sub.total], total carbon potentially
available for addition to soil.
Values in parentheses are means and standard errors of [log.sub.e]
transformed values. CV, Cotton-vetch rotation; CW, cotton wheat;
CWV, cotton-wheat-vetch

                                             [C.sub.root]

2008    Vetch in CV                  280 (5.63 [+ or -] 0.23)
        Wheat in CW                  31 (3.43 [+ or -] 0.73)
        Vetch in CWV                 383 (5.95 [+ or -] 0.17)
        Wheat in CWV                 4 (1.46 [+ or -] 1.21)
        P <                                   0.001
2009    Vetch in CV                  89 (4.49 [+ or -] 0.19)
        Wheat in CW                  110 (4.70 [+ or -] 0.27)
        Vetch in CWV                 625 (6.44 [+ or -] 0.17)
        Wheat in CWV                 120 (4.79 [+ or -] 0.11)
        P <                                   0.001
2010    Vetch in CV                  103 (4.63 [+ or -] 0.22)
        Wheat in CW                  71 (4.26 [+ or -] 0.51)
        Vetch in CWV                 376 (5.93 [+ or -] 0.33)
        Wheat in CWV                 42 (3.74 [+ or -] 0.84)
        Aphid-damaged vetch in CWV   111 (4.71 [+ or -] 0.20)
        P <                                   0.05

                                            [C.sub.lost]

2008    Vetch in CV                  15 (2.74 [+ or -] 0.82)
        Wheat in CW                  36 (3.57 [+ or -] 0.38)
        Vetch in CWV                 34 (3.53 [+ or -] 0.15)
        Wheat in CWV                 135 (4.91 [+ or -] 0.18)
        P <                                     0.05
2009    Vetch in CV                  9 (2.22 [+ or -] 0.27)
        Wheat in CW                  54 (3.99 [+ or -] 0.11)
        Vetch in CWV                 66 (4.19 [+ or -] 0.24)
        Wheat in CWV                 32 (3.46 [+ or -] 0.26)
        P <                                     0.001
2010    Vetch in CV                  0.8 (-0.21 [+ or -] 0.67)
        Wheat in CW                  86 (4.45 [+ or -] 0.21)
        Vetch in CWV                 13 (2.57 [+ or -] 0.44)
        Wheat in CWV                 76 (4.33 [+ or -] 0.30)
        Aphid-damaged vetch in CWV   22 (3.10 [+ or -] 0.45)
        P <                                     0.001

                                         [C.sub.total]

2008    Vetch in CV                  369 (5.91 [+ or -] 0.27)
        Wheat in CW                  110 (4.7 [+ or -] 0.16)
        Vetch in CWV                 425 (6.05 [+ or -] 0.15)
        Wheat in CWV                 191 (5.25 [+ or -] 0.16)
        P <                                     0.001
2009    Vetch in CV                  102 (4.62 [+ or -] 0.18)
        Wheat in CW                  176 (5.17 [+ or -] 0.19)
        Vetch in CWV                 708 (6.56 [+ or -] 0.16)
        Wheat in CWV                 157 (5.05 [+ or -] 0.11)
        P <                                     0.001
2010    Vetch in CV                  106 (4.67 [+ or -] 0.21)
        Wheat in CW                  194 (5.27 [+ or -] 0.22)
        Vetch in CWV                 395 (5.98 [+ or -] 0.30)
        Wheat in CWV                 172 (4.15 [+ or -] 0.31)
        Aphid-damaged vetch in CWV   138 (4.93 [+ or -] 0.19)
        P <                                     0.05
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Author:Hulugalle, N.R.; Weaver, T.B.; Finlay, L.A.
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Geographic Code:8AUST
Date:May 1, 2012
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