Cattle diet and winter plant growth effects on nitrogen losses from cattle urine patches

Nitrogen (N) losses from agricultural land is a major environmental concern, as N leaching can cause eutrophication and nitrous oxide (N₂O) is a greenhouse gas. A lysimeter experiment was undertaken to investigate the effects of cattle diet and winter plant growth on N losses from cattle urine patches. The experiment was conducted using intact soil monolith lysimeters, beneath two different pasture or cropping systems: (i) perennial ryegrass/white clover pasture, and (ii) bare fallow after simulated grazing of fodder beet (beet fallow). Two different cow urine treatments (700 kg N ha⁻¹) were applied to each of the crop treatments in June 2017, using freshly collected urine from herds on a diet of either pasture or fodder beet (beet) (supplemented with lucerne silage). Winter plant growth was found to have a large effect on N leaching losses. There were high N leaching losses beneath the beet fallow (335–345 kg N ha⁻¹) compared to the significantly lower leaching loss beneath the pasture (55–121 kg N ha⁻¹). Urine treatment had a significant effect on N losses/transformations beneath the pasture. The pasture with beet urine treatment had 36% lower average ammonia oxidising bacteria (AOB) population abundance, 31% lower soil NO₃⁻–N concentration and subsequently 64% lower NO₃⁻–N leaching losses than the pasture with pasture urine treatment. The lower AOB population and lower average soil NO₃⁻–N suggest a biological nitrification inhibitor effect was present in the beet urine. This study shows the potential of winter plant growth and manipulation of cattle diet in reducing farm N losses