Applying a process-based livestock model to predict spatial variation in agricultural nutrient flows in Scotland

Achieving an optimal nutrient balance is one of the main targets of sustainable agriculture. The aim of this study was to identify “hotspots” of agricultural nutrient imbalance. This was done by developing a modelling framework and using it to analyse the spatial distribution of agricultural nitrogen and phosphorus flows. The nutrient flows for the main livestock species in Scotland (namely cattle, sheep, pigs and chickens), were quantified using the Scottish Agricultural Emission Model (SAEM). The model was used in connection with agricultural census data for Scotland, which provided spatial distribution of the livestock production, including numbers of animals in certain livestock categories. The average fertiliser application rates for the main crops in Scotland were obtained from the British Survey of Fertiliser Practice and were used as an indicator of the nutrient demand for these crops. The Scottish agricultural census data were used to determine the spatial distribution of different arable crops and grassland in Scotland. This spatial information was combined with the nutrient excretion results to determine the nutrient balance. The results show that the areas with highest nitrogen fertiliser application rate are mainly located in central and eastern Scotland, including the areas where cereal production mainly occurs, while areas where the availability of excreted nutrients is highest are located in southern and north eastern Scotland, i.e. in the main cattle production areas. The higher stability of excreted phosphorus compared to nitrogen allowed a more detailed analysis of a regional phosphorus balance. In major parts of the agricultural production areas in eastern Scotland, the fertiliser application rate (crop nutrient demand) exceeds the amount of phosphorus excreted by the livestock. There are also areas where high phosphorus fertiliser application rate (crop production) and organic phosphorus output (from animal production) overlap, indicating that in these areas there is a high potential to re-use the phosphorus excreted by livestock. Compared to other spatial models for nutrient flows, the modelling framework utilizing a process-based livestock model provides scope to analyse the effects of livestock management and potential changes in livestock systems on regional nutrient balances in more detail.