The potential effectiveness of four different options to reduce environmental impacts of grazed pastures. A model-based assessment

Abstract Pastoral agriculture can have negative environmental impacts. These include greenhouse gas emissions (such as enteric CH4 and soil N2O emissions), NH3 volatilisation, and leaching of nitrogen into waterways. We used a modelling approach to assess the effect of three plant traits and one management practice on N2O emissions, nitrogen (N) losses via leaching and NH3 volatilisation, pasture production and soil organic carbon (SOC) changes (as applicable). The aim was to identify traits/management practices that could potentially be environmentally beneficial and could then inform future research to find or breed plants with those traits. The traits and practice investigated were: 1) N content in animal feed; 2) plant-excreted nitrification inhibitors; 3) deep rooting; and 4) frequency of pasture renewal. Of these, the N content in animal feed provided the most promising results, with low N content resulting in lower urine N excretion, and consequently reduced leaching losses and emissions of N2O and NH3. Modelling the effect of plant-excreted nitrification inhibitors showed mixed results, with reduced leaching rates but increased NH3 emissions. N2O emissions could be reduced if nitrification rates were strongly reduced. However, at lower rates of inhibition, there was little effect on N2O emissions. In the deep rooting scenarios, we found that N leaching losses were minimised if roots grew predominantly in upper soil layers where any mineral N was likely to spend more time before being leached. Nitrogen was, therefore, more effectively intercepted and prevented from leaching by greater root proliferation in the upper rather than deeper layers. For pasture renewal, we unexpectedly found that SOC could be increased by more frequent pasture renewal. However, this would come at the cost of reduced pasture production as renewal events reduced grazing off-takes proportionately more than the reduction in carbon gain by pasture plants. A renewal period of about 10 years was found to be optimal for pasture production and could be maintained with minimal SOC changes.