Balancing fertiliser nitrogen and soil nitrogen processes for dairy pastures

Abstract

Dairy-pasture systems often receive high nitrogen (N) inputs that can disrupt soil N cycling. However, excessive N fertilisation can disrupt soil N cycling and increase environmental risks, such as nitrate leaching and greenhouse gas emissions. Despite this, N applications often exceed crop demand, with limited understanding of the impacts on soil biological processes that govern N availability to plants. This four-year field study on irrigated multi-species dairy pastures in South Africa quantified the effects of different fertiliser N rates on soil N mineralisation, related soil properties (total N, nitrate, ammonium and organic carbon contents), and pasture herbage yield. High N fertilisation rates (>350 kg N ha⁻¹ year⁻¹) significantly reduced the fraction of soil organic N being mineralised to plant-available N forms. Soils receiving >350 kg N ha⁻¹ mineralised only approximately 9% of their organic N annually compared with 13% in soils receiving <200 kg N ha⁻¹. Excessive N inputs led to accumulation of nitrate-N in the soil, which showed a negative correlation with pasture yield, whereas low N inputs (<200 kg N ha⁻¹) maintained higher soil ammonium N that was weakly positively correlated with yield. Overall, excessive fertiliser N altered the soil N cycle in ways that can limit pasture productivity, apart from the added environmental risks associated with overfertilisation. This emphasises the need for balanced N management in dairy-pasture systems.