Projected climate effects on soil workability and trafficability determine the feasibility of converting permanent grassland to arable land

CONTEXT. Adapting to changes in climate and in consumer demand for commodities will force us to diversify land uses from the current status. Livestock grazing systems are dominant agricultural practices in the western regions of the British Isles. It has been suggested that grasslands in the region could be converted to other land uses, e.g. growing of cereal crops. We hypothesized that soil workability and trafficability would be important factors determining the feasibility and environmental impact of such conversion.

OBJECTIVE. Objectives were 1) to investigate the impacts of weather conditions under the current climate on agronomic management and crop yield of winter wheat using the SPACSYS model; and 2) to assess potential impacts of the land use conversion (grassland converted into arable land) on the environment under soil conditions representative of the region under baseline and future climatic conditions.

METHODS. Using simulation modelling we investigated the impacts of baseline and future climates under the Representative Concentration Pathways (RCP2.6, 4.5 and RCP8.5) on soil workability and trafficability at sowing and harvest respectively of winter wheat and its consequences for crop productivity and key indices of environmental sustainability for three major soil types of the region.

RESULTS AND CONCLUSIONS.

Under baseline and future projections, the probability of successfully sowing winter wheat on these soils was between 38 and 76%. Simulations showed that grassland conversion to arable in the region would not be sustainable in terms of carbon sequestration with a decline in soil carbon stock of 165-280 kg C ha^-1 yr^-1 on average over the simulation period. Rates of decline were greater when soil workability was taken into consideration. Although CO₂eq emissions from silage-based grassland soil were higher than those from the converted arable land, these were offset by the greater net productivity of grassland making it a larger net sink for carbon. When soil workability at sowing was considered, the NUE_crop (crop N content/N fertiliser applied) for winter wheat was lower than that for perennial ryegrass on all soil types under the baseline climate and RCP2.6, but comparable or greater under RCP4.5 and RCP8.5. In terms of carbon sequestration, grassland conversion for production of winter wheat is unsustainable under these soil-climatic conditions.

SIGNIFICANCE. Our results demonstrated that soil workability is a major factor influencing the potential impact of land-use conversion in clay soils and a wetter climate.