Improving the sustainability of arable cropping systems by modifying root traits: a modelling study for winter wheat

Crop breeding to increase below-ground production and inputs of organic matter into soil has been attracting increasing attention as a potentially effective strategy to enhance soil organic matter (SOM) stocks and thus the quality of soil and sustainability of arable cropping systems. We used the new soil-crop model USSF (Uppsala model of Soil Structure and Function) to investigate the potential for increasing SOM whilst maintaining or improving yields by modifying the root system of winter wheat in terms of below-ground allocation of carbon and key root traits. USSF combines physics-based descriptions of soil water flow, water uptake and transpiration by plants, with a simple (generic) crop growth model and a model of soil structure dynamics and soil organic matter turnover that considers the effects of soil physical protection and microbial priming. 

The USSF model was first calibrated against field data on soil water contents and both above-ground and root biomass of winter wheat measured during one growing season in a clay soil in Uppsala, Sweden. Based on five acceptable calibrated parameter sets, we created four model crops (ideotypes) by modifying root-related parameters to mimic winter wheat phenotypes with improved root traits. Long-term (30-year) simulations of a conventionally tilled monoculture of winter wheat were then performed to evaluate the potential effects of cultivating these ideotypes on soil water balance, soil organic matter stocks and grain yields.

Our results suggest that exploiting winter wheat varieties that allocate more assimilate to the root system would not in itself have any positive effect on soil organic matter storage and would also decrease grain yields. In contrast, deeper root systems or root systems that are more effective for water uptake were predicted to slightly increase grain yields, as well as increasing SOM stocks in the soil profile by ca. 3 to 5%. Combining all three improved root traits showed even more promising results: compared with the baseline “business-as-usual” scenario, SOM stocks in the soil profile were predicted to increase by ca. 7% in a 30-year perspective (as an average of the five parameter sets) without negatively impacting yields.