Potential contribution by cotton roots to soil carbon stocks in irrigated Vertosols.
The well-documented decline in soil organic carbon (SOC) stocks in
Australian cotton (Gossypium hirsutum L.) growing Vertosols has been
primarily analysed in terms of inputs from above-ground crop residues,
with addition to soil C by root materials being little studied.
Potential contribution by cotton roots to soil carbon stocks was
evaluated between 2002 and 2008 in 2 ongoing long-term experiments near
Narrabri, north-western New South Wales. Experiment 1 consisted of
cotton monoculture sown either after conventional tillage or on
permanent beds, and a cotton--wheat (Triticum aestivum L.) rotation on
permanent beds; Experiment 2 consisted of 4 cotton-based rotation
systems sown on permanent beds: cotton monoculture, cotton--vetch (Vicia
villosa Roth.), cotton--wheat, and cotton--wheat--vetch.
Roundup-Ready[TM] (genetically modified) cotton varieties were sown
until 2005, and Bollgard[TM] II-Roundup Ready[TM]-Flex[TM] varieties
thereafter. Root growth in the surface 0.10m was measured with the
core-break method using 0.10-m-diameter cores. A subsample of these
cores was used to evaluate relative root length and root C
concentrations. Root growth in the 0.10-1.0m depth was measured at
0.10-m depth intervals with a 'Bartz' BTC-2 minirhizotron
video microscope and I-CAP image capture system
('minirhizotron'). The video camera was inserted into clear,
plastic acrylic minirhizotron tubes (50-mm-diameter) installed within
each plot, 30[degrees] from the vertical. Root images were captured 4-5
times each season in 2 orientations, left and right side of each tube,
adjacent to a furrow, at each time of measurement and the images
analysed to estimate selected root growth indices. 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. These measurements were used to derive root C turnover
between times of measurements, root C added to soil through
intra-seasonal root death, C in roots remaining at end of season, and
the sum of the last 2 indices: root C potentially available for addition
to soil C stocks.
Total seasonal cotton root C potentially available for addition to soil C stocks ranged between ~50 and 400 g/[m.sup.2] (0.5 and 4 t/ha), with intra-seasonal root death contributing 25-70%. These values are ~10-60% of that contributed by above-ground crop residues. As soil organic carbon in irrigated Vertosols can range between 40 and 60 t/ha, it is unlikely that cotton roots will contribute significantly to soil carbon stocks in irrigated cotton fanning systems. Seasonal root C was reduced by cotton monoculture, stress caused by high insect numbers, and sowing Bollgard II varieties; and increased by sowing non-Bollgard II varieties and wheat rotation crops. Permanent beds increased root C but leguminous rotation crops did not. Climatic factors such as cumulative day-degrees and seasonal rainfall were positively related to seasonal root C. Root C turnover was, in general, highest during later vegetative/early reproductive growth. Large variations in root C turnover and seasonal C indices occurred due to a combination of environmental, management and climatic factors.
Additional keywords: minirhizotron, Haplustert, wheat, vetch, rotation, permanent beds.
Soils (Carbon content)
|Publication:||Name: Australian Journal of Soil Research Publisher: CSIRO Publishing Audience: Academic Format: Magazine/Journal Subject: Agricultural industry; Earth sciences Copyright: COPYRIGHT 2009 CSIRO Publishing ISSN: 0004-9573|
|Issue:||Date: May, 2009 Source Volume: 47 Source Issue: 3|
|Topic:||Event Code: 310 Science & research|
|Geographic:||Geographic Scope: Australia Geographic Code: 8AUST Australia|
Soil organic carbon stocks (SOC) in the surface 0.6m of Australian cotton (Gossypium hirsutum L.) growing soils (predominantly Vertosols; McKenzie et al. 2003) generally range from 60-100 t/ha in irrigated Vertosols to 40-60 t/ha in rainfed Vertosols (Hulugalle and Scott 2008), although lower values have been reported by some authors. Knowles and Singh (2003), for example, reported that organic carbon stocks in the surface 0.9 m of a spatially variable cotton field from northern New South Wales (NSW) sown with an irrigated cotton--cotton--wheat (Triticum aestivum L.) sequence were of the order of 49t/ha. The values reported for cotton-based farming systems are much lower than those under pasture or savannah scrubland (before cotton). Estimated (1) values of soil organic carbon in the 0 to 1 m depth under irrigated cotton fields in the Namoi valley of NSW averaged 100 t/ha, whereas under native pasture it was of the order of 139t/ha (McGarry et al. 1989). The lower SOC in cotton-growing soils is probably due to relatively low rates of return of crop residues to the soil and high rates of carbon loss caused by a combination of excessive tillage, burning of crop residues, high growing season temperatures, and wet soils as a consequence of irrigation (Hulugalle and Scott 2008). Management practices aimed at reducing this rate of decrease have included minimum tillage or permanent bed systems; cereal and leguminous crops which produce large amounts of crop residues sown in rotation with cotton; and avoiding burning of crop residues (Rochester and Constable 1996; Rochester et al. 1997, 1998, 2001; Hulugalle and Daniells 2005; Hulugalle and Scott 2008).
Much of the above-mentioned change in SOC stocks has been analysed in terms of input reductions of above-ground material. Few studies have considered it in terms of carbon additions from root material (Terry 2007; Hulugalle and Scott 2008). The primary cause of this may be the tedious nature, high spatial variability, and errors associated with measuring roots under field conditions (Smit et al. 2000; Polomski and Kuhn 2002). Addition of root material to soil carbon stocks either in the form of roots dying and decaying during and after the crop's growing season, or as root exudates during the growing season, may, however, be significant (de Kroon and Visser 2003). Estimates of carbon contained in cotton roots and, hence, potential contribution to SOC, from data in published literature, are given in Table 1. Because seasonal root turnover was not measured by any of the sources cited in Table 1, the total contribution by cotton roots to SOC is likely to be underestimated by these values. Nonetheless, they suggest that addition of carbon by cotton root systems to soil is likely to be relatively low, with higher values (~1 t/ha) being reported from clay-rich Vertisols (Vertosols) and 0.1-0.25 t/ha in sandy Ultisols (Kurosols) and Entisols (Rudosols). Direct measurements of intra- and post-seasonal contributions to SOC stocks are, however, absent from the scientific literature. The objective of this study, therefore, was to determine the contribution of cotton roots to soil carbon stocks in irrigated Vertosols, both through root turnover during the growing season and decay of root systems thereafter. Measurements were made in 2 long-term experiments using a combination of soil cores and minirhizotron observations.
Materials and methods
Cotton root growth was measured in 2 experiments at the Australian Cotton Research Institute (ACRI), near Narrabri (149[degrees]47'E, 30[degrees]13'S) in NSW. Narrabri has a subtropical semi-arid climate, BSh (Kottek et al. 2006), and experiences 4 distinct seasons with a mild winter and a hot summer. The hottest month is January (mean daily maximum 35[degrees]C, minimum 19[degrees]C) and the coldest July (mean daily maximum 18[degrees]C, minimum 3[degrees]C). Mean annual rainfall is 593 mm. The soils at both experimental sites are alkaline, self-mulching, grey clays, classified as self-mulching, grey Vertosols; very-fine (Isbell 1996) or fine, thermic, smectitic, Typic Haplusterts (Soil Survey Staff 2006). Mean particle size distributions in the 0-1m depth of both experiments were similar: 64g/100g of clay, 11 g/100 g of silt, and 25 g/100 g of sand. The soil in Expt 1 had exchangeable sodium percentage (ESP) values of the order of 10 in the 0.6-1.2 m depth, whereas in Expt 2, ESP averaged 15 in the same depth. ESP of soil did not exceed the threshold value of 6 in the shallower depths of either experiment. Subsoil structure was better in Expt 1 (mean macroporosity at 0.15-0.70m depth at field capacity, averaged over all treatments, during May 2003, 0.08[m.sup.3]/[m.sup.3]; Hulugalle et al. 2005) than in Expt 2 (mean macroporosity at the same depth and water content during September 2002, ~0.04 [m.sup.3]/[m.sup.3]). Expt 1 commenced in 1985 (Constable et al. 1992) and Expt 2 in 2002.
Experiment 1 (Field C1)
A cotton monoculture (summer cotton--winter fallow--summer cotton) was sown either after conventional tillage (slashing of cotton plants after harvest, followed by disc-ploughing and incorporation of cotton stalks to 0.2 m, chisel ploughing to 0.3m followed by bed construction) or on permanent beds (slashing of cotton plants after harvest, followed by root cutting, incorporation of cotton stalks into beds, and bed renovation with a disc-hiller), and a cotton--wheat (Triticum aestivum L.) rotation (summer cotton--winter wheat--summer and winter fallow summer cotton) on permanent beds laid out in a randomised complete block design with 4 replications. The wheat stubble was retained as in-situ mulch into which the following cotton crop was sown. Individual plots were 190m long and 36-44 rows wide. The rows (beds) were spaced at 1-m intervals with vehicular traffic being restricted to the furrows. A more detailed description of this experiment is given in Constable et al. (1992) and Hulugalle et al. (2005).
Experiment 2 (Field D1)
The experimental treatments consisted of 4 cotton-based rotation systems sown on permanent beds: cotton monoculture (summer cotton--winter fallow--summer cotton), cotton--vetch (summer cotton winter vetch (Vicia villosa Roth.)--summer cotton), cotton--wheat (summer cotton--winter wheat--summer and winter fallow--summer cotton) where 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) where wheat stubble was retained as an in-situ mulch into which the following vetch crop was sown. The vetch in cotton--vetch and cotton--wheat--vetch rotations was killed before sowing cotton through a combination of mowing and contact herbicides, and the residues retained as in situ mulch into which the following cotton was sown. The experiment was laid out as a randomised complete block with 3 replications and designed such that both cotton and rotation crop phases in the last 2 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 NSW cotton is sown in October. Roundup-Ready[TM] cotton was sown at a population of 12 plants/[m.sup.2] from the 20024)3 until the 2005-06 growing season, and Bollgard[TM] [II.sup.2]-Roundup-Ready[TM]-Flex[TM] cotton thereafter. Anhydrous ammonia was applied in September before cotton sowing at a rate of 160kgN/ha in cotton monoculture or cotton-wheat plots. Urea was applied to cotton--vetch (60-100kgN/ha) and cotton--wheat--vetch (20-60 kg N/ha) rotations during December or January. Application rates depended on N fixation by the preceding vetch crop. 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 2-row picker after defoliation in early April. After cotton-picking, the cotton was slashed and incorporated into the beds with a disc-hiller.
Wheat and vetch
Wheat was sown at rates of 55-60kg/ha and vetch at 20 kg/ha. Wheat received 20 kgN/ha as urea by broadcasting at sowing during late May or early June, and 60kgN/ha during late July or early August. Vetch was not fertilised. Both wheat and vetch received 1-2 irrigations of 100mm, depending on winter rainfall, except during 2007 winter when neither crop was irrigated due to a lack of irrigation water. Wheat was harvested with a grain harvester during later November or early December.
Cotton root measurements
Root growth in the surface 0.10m was measured with the core-break method using 0.10-m-diameter cores (Drew and Saker 1980); in the 0.10-1.0m depth it was measured at 0.10m depth intervals with a 'Bartz' BTC-2 minirhizotron video microscope and 1-CAP image capture system (Bartz Technology Corporation 2007) ('minirhizotron'). 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 warm water containing a solution of 2 : 1 10% sodium hexametaphosphate :0.1 M sodium hydroxide for ~4 h. Once dispersed, root and other organic material were 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). The congo red stains the live roots a bright red colour, whereas the dead organic material remains black. The live roots were separated from the dead material and length measured using a modified Newman's line interception method with a grid 100 by 100mm (Smit et al. 2000; Polomski and Kuhn 2002). These root samples were then oven-dried and weighed. Relationships were derived between root number in the cores and root weight, and the root weight in each core estimated. Relative root length (root length/root weight) was also calculated. Carbon concentration in the oven-dried root material was measured by combustion with a LECO 2000 analyser (Rayment and Higginson 1992).
As noted previously, root growth in the 0.10-1.0 m depth was measured at 0.10-m depth intervals with a minirhizotron. The video camera part of the minirhizotron 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). Measurements were made during vegetative, flowering, boll initiation/filling, and boll filling/ opening stages between early December and late March. A summary of the tube arrangement and timing of measurements in each experiment is given in Table 2. Root images were captured in 2 orientations, left and right side of each tube, adjacent to a furrow, at each time of measurement and the images analysed with RooTracker 2.03 (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 C concentrations of samples taken from the previously described soil cores, were then used to calculate several other indices of root growth:
For individual depths and between times of measurement:
Net change in root carbon (g/[m.sup.2])
= net change in root length x relative root length x root carbon concentration
Root carbon added to soil (g/[m.sup.2])
= length of roots which died x relative root length x root carbon concentration
Seasonal changes over measured profile (in g/[m.sup.2]):
Root carbon at end of season (1)
= sum of net changes in root carbon between times of measurement in all depths
Root carbon added to the soil during season (2)
= sum of root carbon added to soil due to root death between times of measurement in all depths
Root carbon which could be potentially added to soil organic carbon (3) - root carbon at end of season (1) + root carbon added to the soil during season (2)
Root carbon turnover (net change in mass of root C due to losses caused by root death and gains caused by initiation of new roots between times of measurements; a positive value indicated a net gain, and a negative value a net loss) for given periods during individual seasons were analysed with an analysis of variance for a randomised complete block design after logo transformation. Inter-seasonal changes in root carbon (1, 2, and 3) were analysed after logo transformation with an analysis of variance for a split-plot design where treatment was designated as main plot and season as subplot treatments.
Seasonal root carbon indices for both experiments were also evaluated with multiple linear regression analysis after pooling of data. Models were selected using best subset regression (Analytical Software 2008) for a range of management and climatic variables such as tillage system (conventional tillage, permanent beds), cropping system (cotton monocuiture, rotation), rotation crops (wheat, vetch), varieties (non-Bollgard, Bollgard), insect stress (damaged, undamaged), seasonal (1 October-31 March) rainfall, and cumulative day-degrees. Significant variables were selected using Mallow's [C.sub.p], adjusted [R.sup.2], and the difference between Akaike's Information Criteria for a candidate model, MCc, and the model with the lowest value, [AIC.sub.C] (min) (Analytical Software 2008). The selected model was then analysed with least-squares linear regression.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Results and discussion
Experiment 1 (Field C1)
Mean root C turnover was, in general, highest during later vegetative/early reproductive growth in all years except 2006-07 (Fig. 1). During 2006-07 an initial period of high root C turnover was followed by a fall and a 'second peak' during boll filling (mid-February to mid-March). Overall turnover rates were also much lower in 2006-07 than 2002-03 and 2004-05. Higher root C turnover in all years was generally associated with treatments sown on permanent beds, with significantly (P < 0.05) higher values occurring with the cotton--wheat rotation. Seasonally, total root C potentially available for addition to soil C stocks, C added to soil through intra-seasonal root death, and C in root mass remaining at the end of the season were, generally, highest (P<0.05) with cotton--wheat (Fig. 2). In comparison with 2002-03 and 2004-05, seasonal values of the previously mentioned indices were significantly lower (P<0.001) during 2006-07. Total average root C potentially available for addition to soil C stocks was 400g/[m.sup.2] (4t/ha) during 2002-03, 284g/[m.sup.2] (2.8t/ha) during 2004-05 and 65g/[m.sup.2] (0.65t/ha) during 2006-07. Similarly, mean C added to soil through intraseasonal root death was 114g/[m.sup.2] (1.1t/ha) during 2002-03, 70 g/[m.sup.2] (0.7 t/ha) during 2004-05, and 16 g/[m.sup.2] (0.2 t/ha) during 2006-07. Mean C in remaining roots was of the order of 263g/[m.sup.2] (2.6t/ha) during 2002-03, 156 (1.6t/ha) g/[m.sup.2] during 2004-05, and 47g/[m.sup.2] (0.5t/ha) during 2006-07. Furthermore, relative to other years, differences among treatments were small or absent during 2006-07. These results also suggest that carbon addition to soil through intraseasonal root death was small in comparison with that remaining in roots at the end of the season: 29% in 2002-03, and 25% in both 2004-05 and 2006-07.
[FIGURE 3 OMITTED]
Experiment 2 (Field D1)
Mean root C turnover was highest during late vegetative/early reproductive growth in all years except 2005-06 (Fig. 3), when high Helicoverpa numbers occurred (Fig. 4). Threshold values for significant crop damage, which are of the order of 2 larvae/[m.sup.2] (Deustcher et al. 2007), were exceeded on several occasions during the 2005-06 season, resulting in herbivory of apical meristems, which, in turn, caused excessive branching and increased vegetative growth (Hulugalle et al. 2008) and may have decreased root growth. Sadras (1996a, 1996b) reported that decreases in root mass were associated with increased vegetative growth due to removal of vegetative buds at a plant population similar to that used in our study. In other seasons, higher mean turnover rates were usually associated with rotations which included a wheat crop, particularly cotton--wheat rotations (Fig. 3). Seasonally and except for 2005-06, total root C potentially available for addition to soil C stocks, C added to soil through intra-seasonal root death, and C in root mass remaining at the end of the season were in the order cotton--wheat [greater than or equal to] cotton--wheat--vetch > cotton--vetch [greater than or equal to] cotton--winter fallow, although differences were small during 2007-08 (Fig. 5). Between 2004 and 2008, total root C potentially available for addition to soil C stocks ranged from 85 to 300g/[m.sup.2] (0.85 to 3 t/ha). These values are similar to those previously reported in the literature (Table 1).
During 2005-06 a large proportion of total root C was derived from intra-seasonal root death (averaged among all treatments it was ~70% in comparison with other years when it ranged from 44-50%), and may be related to the damage caused by the large numbers of Helicoverpa (Fig. 4). Reduction in root mass of damaged plants relative to undamaged plants was also reported by Sadras (1996a) in cotton crops with plant populations similar to the present study where Helicoverpa damage was simulated by removal of vegetative buds. Van Dam and Bezemer (2006) have suggested that this response is mediated through an alteration in the internal plant hormone balance. Insect damage may, therefore, influence root functions such as water and nutrient extraction, with the intensity of the changes varying between tap and feeder roots (de Kroon and Visser 2003).
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Overall, the values of intra-seasonal additions to soil C through root death were very much greater than those in Expt 1. The higher root mortality rate in Expt 2 may be related to the poorer structure and higher ESP in the subsoil. Poor structure and high ESP can increase root mortality primarily through a combination of high soil strength and low oxygen concentration (Atwell 1993; de Kroon and Visser 2003).
In both experiments, total root C potentially available for addition to soil C stocks, C added to soil through intra-seasonal root death, and C in root mass remaining at the end of the season was significantly (P< 0.001) higher in years when non-Bollgard II cotton varieties were sown (2002-03 and 2004-05 in Expt 1; 2003-04, 2004-05, 2005-06 in Expt 2) than in years when Bollgard II varieties were sown (2006-07 in Expt 1; 2006-07, 2007-08 in Expt 2). Although these differences were undoubtedly due in part to variation in climatic factors, these results do suggest that the root mass and turnover of Bollgard II varieties are less than those of non-Bollgard II varieties, implying in turn that a smaller proportion of photosynthesised carbon is translocated into the roots of the former. The significantly higher yields achieved with Bollgard 1I varieties (Constable and Bange 2006) may mean that there is a larger sink for carbon in the above-ground organs such as the bolls of these varieties relative to non-Bollgard II varieties. This may cause a greater proportion of photosynthates to be translocated to above-ground organs rather than the roots.
Regression analysis of pooled results
Multiple linear regression analyses of pooled data indicated that by sowing either a non-Bollgard II variety or a wheat rotation crop, seasonal root C indices such as [C.sub.total] (total root C potentially available for addition to soil C stocks) and [C.sub.lost] (C added to soil through intra-seasonal root death) were increased, but were decreased by either insect damage and sowing a Bollgard II variety (Table 3). These same root indices were also positively related to climatic variables such as cumulative day-degrees and seasonal rainfall. [C.sub.root] (C in root mass remaining at the end of the season) was increased by permanent beds but decreased by conventional tillage. Leguminous rotation crops such as vetch did not significantly affect seasonal root carbon indices.
Although Mallow's [C.sub.p] and [AIC.sub.C]-[AIC.sub.C] (min) indicated a good fit of the results to the selected models, the adjusted [R.sup.2] values suggested that they explained only 30-45% of the observed variation (Table 3). Other variables not measured during this study such as root diseases of cotton viz. black root rot (Nehl and Allen 2002; Hulugalle et al. 2004), transient waterlogging during irrigation (Tisdall and Hodgson 1990), soil structure, and subsoil sodicity may have also influenced root C dynamics during these experiments.
Total seasonal cotton root C potentially available for addition to soil C stocks ranged between ~50 and 400 g/[m.sup.2] (0.5 and 4 t/ha), with intra-seasonal root death contributing 25-70%. These values are ~10-60% of that contributed by above-ground crop residues. Given that soil organic carbon in irrigated Vertosols can range between 40 and 60 t/ha (see Introduction), it is unlikely that cotton roots will contribute significantly to soil carbon stocks in irrigated cotton farming systems. Lower values of seasonal root C were due primarily to management practices such as cotton monoculture and Bollgard II varieties, and higher values to non-Bollgard II varieties and wheat rotation crops. Tillage system had less of an effect on root C than expected, and leguminous rotation crops had no effect. Seasonal factors such as stress caused by high insect numbers reduced root C whereas climatic factors such as cumulative day-degrees and seasonal rainfall were positively related to seasonal root C. Root C turnover was, in general, highest during later vegetative/early reproductive growth. Large variations in root C turnover and seasonal C indices occurred due to a combination of environmental, management, and climatic factors.
Funding for this study was provided by the Cotton Research and Development Corporation of Australia, the Australian Cotton Cooperative Research Centre and the Cotton Catchment Communities Cooperative Research Centres through grants CRC 32C, 45C, and 86C. N Luelf gratefully acknowledges the receipt of a summer scholarship (CRC grant 4.1.06SS18) from the Australian Cotton Co-operative Research Centre. Dr Stephen Milroy of CSIRO, Floreat Park, Perth, is thanked for his comments during manuscript preparation.
Manuscript received 5 August 2008, accepted 12 January 2009
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N.R. Hulugalle (A,B,E), T.B. Weaver (A,B), L.A. Finlay (A,B), N.W. Luelf (B,C,D), and D.K.Y. Tan (B,C)
(A) New South Wales Department of Primary Industries, Australian Cotton Research Institute, Locked Bag 1000, Narrabri, NSW 2390, Australia.
(B) Cotton Catchment Communities Co-operative Research Centre.
(C) Faculty of Agriculture, Food and Natural Resources, The University of Sydney, Sydney, NSW 2006, Australia.
(D) present address: Agrisearch Services Pty Ltd, 50 Leewood Drive, Orange, NSW 2800, Australia.
(E) Corresponding author. Email: email@example.com
(1) Bulk densities used in these estimates were calculated using the pedotransfer functions described by Vervoort et al. (2006).
(2) Genetically-modified varieties which carry an insecticidal gene from Bacillus thuringiensis and are resistant to attack by Helicoverpa spp.
Table 1. Carbon contained in field-grown cotton roots DAS, days after sowing Irrigated/ Source location dryland Soil type (A) Constable et al. Irrigated Vertisol (Vertosol) (1992), Australia Hodgson et al. (1990), Irrigated Vertisol (Vertosol) Australia Zamora et al. (2007), Dryland Ultisol (Kwosol) SE USA Li Zhen et al. (2005), Dryland ? (loam) (C) China Al-Khafaf et al. Irrigated Entisol (Rudosol) (1977), SW USA Carmi et al. (1993), Irrigated Entisol (Rudosol) Israel Schwab et al. (2000) Dryland Ultisol (Kurosol) Root C Depth (kg/ha) Source location Treatment (m) (B) Constable et al. Cotton monoculture/ 0-1.2 966 (1992), Australia conventional tillage Cotton monoculture/ 0-1.2 1066 permanent beds Cotton-wheat rotation/ 0-1.2 970 permanent beds Cotton monoculture/ 0-1.2 512 conventional tillage Cotton monoculture/ 0-1.2 516 permanent beds Cotton-wheat rotation 0-1.2 806 permanent beds Hodgson et al. (1990), Buried drip 0-1.2 1216 Australia Furrow 0-1.2 1169 Surface drip 0-1.2 1052 Zamora et al. (2007), Cotton monoculture 0-0.9 95 SE USA Cotton monoculture 0-0.9 87 Cotton monoculture 0-0.9 100 Li Zhen et al. (2005), Flowering/boll filling 0-1.0 99 China Boll opening 0-1.0 86 Picking 0-1.0 61 Al-Khafaf et al. 96 DAS 0-0.68 666 (1977), SW USA Carmi et al. (1993), No deep soil wetting/ 0-0.9 194 Israel continued irrigation, 85 DAS No deep soil wetting/ 0-0.9 197 continued irrigation, 106 DAS Deep soil wetting/ 0-0.9 194 continued irrigation, 85 DAS Deep soil wetting/ 0-0.9 281 continued irrigation, 106 DAS Deep soil wetting/no 0-0.9 194 irrigation, 85 DAS Deep soil wetting/no 0-0.9 5 irrigation, 106 DAS Schwab et al. (2000) 37 DAS 0-0.6 7 49 DAS 0-0.6 26 64 DAS 0-0.6 55 87 DAS 0-0.6 108 99 DAS 0-0.6 214 112 DAS 0-0.6 243 122 DAS 0-0.6 246 134 DAS 0-0.6 195 151 DAS 0-0.6 194 (A) Taxonomic classification without parentheses is that of Soil Survey Staff (2006) and within parentheses Isbell (1996). (B) Root weight/length ratio (specific root length) was assumed to be 20 g/m of root when not provided by the source. Root carbon concentration was assumed to be 40% of root dry weight. These values are based on observations taken over a 6-year period by the authors. (C) Only texture of soil provided by source. Table 2. Numbers of tubes and frequency of measurements in each experiment Cotton Replications Tubes/ Experiment season sampled plot Vegetative 1 (Field C1) 2002-03 4 1 06/12/2002, 16/12/2002 2004-05 3 3 22/12/2004 2006-07 2 (A) 3 4/12/2006 2 (Field D1) 2003-04 (B) 3 3 12/12/2003 2004-O5 2 6 21/12/2004 2005-06 3 3 02/12/2005, 20/12/2005 2006-07 3 3 06/12/2006 2007-08 3 3 13/12/2007 Time of measurement Boll Boll Cotton initiation/ filling/ Experiment season Flowering filling opening 1 (Field C1) 2002-03 09/01/2003 28/01/2003 19/03/2003 2004-05 06/01/2005 15/02/2005 10/03/2005, 18/04/2005 2006-07 02/01/2007 22/01/2007 13/03/2007 2 (Field D1) 2003-04 (B) 05/01/2004 29/01/2004 01/03/2004 2004-O5 07/01/2005 02/02/2005 22/02/2005 2005-06 l0/01/2006 13/02/2006 16/03/2006 2006-07 09/01/2007 06/02/2007 12/03/2007, 27/03/2007 2007-08 04/01/2008 30/01/2008 20/01/2008, 18/03/2008 (A) The experiment was split into a low irrigation frequency and normal' irrigation frequency treatments in the 2005-06 season. Measurements were made only from the 'normal' irrigation frequency treatment. (B) Only cotton monoculture and cotton-vetch were sampled during 2003-04 as the cotton phases of the other two rotations had not been sown as yet. Table 3. Multiple linear regression analysis and best-fit predictors for seasonal total root C potentially available for addition to soil C stocks ([C.sub.total]), C added to soil through infra-seasonal root death ([C.sub.lost]), and C in root mass remaining at the end of the season ([C.sub.root]) Number of observations = 185. Cumulative day-degrees and rainfall were those from 1 October and 31 March Mallow's [AI.sub.C]- Seasonal root Model [C.sub.p] Adjusted [AIC.sub.C] C index parameters (A) [R.sub.2] (min) (B) [Log.sub.e] 6 4.8 0.43 *** 0 ([C.sub.total]) [Log.sub.e] 6 4.2 0.45 *** 0 ([C.sub.lost]) [Log.sub.e] 7 7.0 0.30 *** 0 ([C.sub.root]) Seasonal root C index Variable Coefficient [Log.sub.e] Constant -9.25 *** ([C.sub.total]) Variety -0.59 *** Wheat rotation crop 0.61 *** Insect damage -1.25 *** Cumulative day-degrees 3.75E-03 *** Seasonal rainfall 4.67E-03 *** [Log.sub.e] Constant -13.18 *** ([C.sub.lost]) Variety -0.37 ** Wheat rotation crop 0.47 *** Insect damage -1.31 *** Cumulative day-degrees 4.79E-03 *** Seasonal rainfall 6.13E-03 *** [Log.sub.e] Constant -6.32 ** ([C.sub.root]) Variety -0.59 ** Wheat rotation crop 0.83 *** Insect damage -1.67 *** Cumulative day-degrees 2.73E-03 ** Seasonal rainfall 3.07E-03 ** Tillage system -0.46 * Seasonal root Total C index Variable s.e. regression [Log.sub.e] Constant 1.45 P < 0.001 ([C.sub.total]) Variety 0.12 Wheat rotation crop 0.11 Insect damage 0.22 Cumulative day-degrees 5.19E-04 Seasonal rainfall 6.60E-04 [Log.sub.e] Constant 1.54 P < 0.001 ([C.sub.lost]) Variety 0.13 Wheat rotation crop 0.12 Insect damage 0.23 Cumulative day-degrees 5.50E-04 Seasonal rainfall 7.06E-04 [Log.sub.e] Constant 2.38 P < 0.01 ([C.sub.root]) Variety 0.19 Wheat rotation crop 0.18 Insect damage 0.35 Cumulative day-degrees 8.22E-04 Seasonal rainfall 1.05E-03 Tillage system 0.15 (A) Values near to or less than the number of parameters in the model indicate a good fit to the data. (B) Values of 0-2 indicate a very good fit of the model to the data.
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