Herbage nitrogen uptake in grass-only and grass-white clover swards receiving zero nitrogen

Nitrogen (N) fertilisers are major contributors to ammonia and greenhouse gas emissions through their inputs in grazed herbage systems. There is significant need to improve our understanding of more sustainable N inputs such as soil organic N mineralisation and biological N fixation to reduce our reliance on chemical N fertiliser. The objective of this study was to evaluate the herbage N uptake from grass-only and grass-white clover (Trifolium repens L.) herbage receiving zero external N inputs across two contrasting soils and climatic conditions in temperate pasture-based systems. The study consisted of zero N plots established in 2020 and 2021 within grazed paddocks of either grass-only or grass-white clover swards at two sites. The plots received no chemical or organic N and were not grazed for the duration of the study. Plots were yielded simultaneously as the paddocks were rotationally grazed by dairy cows. Nitrogen content was measured by Leco and N yield was calculated by multiplying herbage yield by N content. The grass-only sward plots yielded 7,682 kg DM/ha per year and 185 kg N/ha per year across the 2 years and sites. The grass-white clover swards yielded significantly higher herbage (10,166 kg DM/ha per year) and N (284 kg N/ha per year) than the grass-only swards (P < 0.001). The results provide an indication of background N mineralisation of 185 kg N/ha from the organic matter content of the soil and biological N fixation of 99 kg N/ha from the white clover within grass-only and grass-clover swards receiving zero N.