Effects of soil heterogeneity on the uncertainty in modelling the fate of urinary nitrogen deposited during winter forage grazing

Abstract

Despite the importance of soil physical properties on loss pathways from high nitrogen (N) inputs quantitative information on the effect of soil heterogeneity on the fate of N is lacking. To quantify how changes in the physical description of soil layers, used as model inputs, affect various outputs of the biophysical APSIM model regarding the fate of urinary nitrogen (N) following winter grazing, a modelling sensitivity analysis was performed. APSIM was set up according to a field experiment on a poorly drained Flaxton soil in the Canterbury region of New Zealand. Soil profile descriptions were based on various measured datasets, with and without compacted soil surface layers due to treading during grazing of the winter forage crop. The effect of the inclusion of more permeable sandy layers within the subsoil, a common occurrence in the study region, was also examined. By systematically replacing soil layers with different physical properties, the effect of these layers on the N loss pathways was evaluated. The analysis identified that the bottom layer (1–1.5m depth) had the greatest impact. With a low-permeability bottom layer nitrate leaching was 20% lower, whereas pasture N uptake, denitrification and nitrous oxide emissions were greater (by 5%, 58% and 43%). This is due to the effect of the bottom layer on water movement and storage within the soil profile. This description of the bottom layer had a substantially greater effect than compaction of the top soil layers due to treading. These results highlight the need for accurate soil physical descriptions for adequate model parameterisation, when such models are used for assessing management practices aimed at reducing environmental impacts.