Abstract
The capture of subsoil water by wheat roots can make a valuable contribution to grain yield on deep soils. More extensive root systems can capture more water, but leave the soil in a drier state, potentially limiting water availability to subsequent crops. To evaluate the importance of these legacy effects, a long-term simulation analysis at eight sites in the semi-arid environment of Australia compared the yield of standard wheat cultivars with cultivars that were (i) modified to have root systems which extract more water at depth and/or (ii) sown earlier to increase the duration of the vegetative period and hence rooting depth. We compared simulations with and without annual resetting of soil water to investigate the legacy effects of drier subsoils related to modified root systems. Simulated mean yield benefits from modified root systems declined from 0.1-0.6 t ha(-1) when annually reset, to 0-0.2 t ha(-1) in the continuous simulation due to a legacy of drier soils (mean 0-32mm) at subsequent crop sowing. For continuous simulations, predicted yield benefits of >0.2 t ha(-1) from more extensive root systems were rare (3-10% of years) at sites with shallow soils (<1.0 m), but occurred in 14-44% of years at sites with deeper soils (1.6-2.5 m). Earlier sowing had a larger impact than modified root systems on water uptake (14-31 vs 2-17mm) and mean yield increase (up to 0.7 vs 0-0.2 t ha(-1)) and the benefits occurred on deep and shallow soils and in more years (9-79 vs 3-44%). Increasing the proportion of crops in the sequence which dry the subsoil extensively has implications for the farming system productivity, and the crop sequence must be managed tactically to optimize overall system benefits.
Highlights
In simulations where roots were modified, the mean benefit to rooting depth on deep soils was smaller in continuous simulations (−0.06–0.26 m) than in the reset simulations (0.23–0.34 m; Table 4)
Variability was greater in the continuous simulation (Fig. 3) as root depth was more frequently restricted due to soil drying by the previous crop
On soils with a depth constraint (Harden, Paskeville, Esperance and Birchip) there was no effect of modified root systems on final root depth, and roots reached the bottom of the accessible profile sooner
Summary
Several authors have proposed root traits which improve yield in water-limited environments, including increased root elongation rate and depth of rooting (Cooper et al, 1987; Lopes and Reynolds, 2010), root distribution at depth (Hurd, 1968, 1974; O’Brien, 1979), xylem vessel diameter (Richards and Passioura, 1989), angle of seminal roots (Nakamoto and Oyanagi, 1994; Manschadi et al, 2008), and the ratio of root:shoot dry matter (Siddique et al, 1990). Experiments and simulation studies have shown that the capture of subsoil water by deeper wheat roots can make a valuable contribution to yield on a range of deep soil types (Kirkegaard et al, 2007; Lilley and Kirkegaard 2007; Christopher et al, 2008). Tennant and Hall (2001) reviewed root depth and water uptake of 20 annual crop and pasture species included in ten different field studies in Western Australia. They concluded that rooting depth was strongly affected by soil type, where limiting conditions occurred and that amelioration of chemical or physical constraints increased root depth. They concluded that rooting depth was strongly affected by soil type, where limiting conditions occurred and that amelioration of chemical or physical constraints increased root depth. Gregory et al (1984) and Kirkegaard and Lilley (2007) showed that even on potentially deep soils, the depth of soil wetting
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