Seasonal effects reveal potential mitigation strategies to reduce N2O emission and N leaching from grassland swards of differing composition (grass monoculture, grass/clover and multispecies)

Abstract

Multispecies pastures containing grasses, N-fixing legumes and forage herbs can maintain high herbage dry matter (DM) yield, while reducing the need for fertiliser N inputs thus potentially reducing harmful N losses such as nitrous oxide (N2O) emission and nitrate (NO3−) leaching to the wider environment. However, our understanding of these processes in grassland swards of contrasting composition is still very limited. A lysimeter experiment was carried out over 12 months to test the hypotheses that (1) cumulative and (2) seasonal N2O emission and N leaching would be highest from the Perennial Ryegrass only (PRG, 250 kg N ha-1 yr-1) treatment compared to three other treatments; Perennial Ryegrass and Low White Clover (PRG+LWC, 90 kg N ha-1 yr-1), Perennial Ryegrass and High White Clover (PRG+HWC, 0 kg N ha-1 yr-1) and Perennial Ryegrass, White Clover and Ribwort Plantain (PRG+WC+P, 45 kg N ha-1 yr-1) and (3) that the soil N cycling pathways (nitrification/denitrification) linked to these N losses would be different between treatments. Cumulative N2O emissions (2.74–3.25 kg N2O-N ha-1), leached N (10.91–13.70 kg N ha-1), DM yields (5083–5493 kg DM ha-1 yr-1) and N uptake (122–169 kg N ha-1 yr-1) were not significantly different between treatments. Nitrogen yield scaled N2O emissions were significantly lower from all other treatments compared to the high fertiliser N input PRG monoculture. A significant interaction between treatment and season showed that in Spring, N2O emission was significantly higher from PRG (1.39 kg N2O-N ha-1) and PRG+LWC (1.19 kg N2O-N ha-1) than PRG+HWC (0.81 kg N2O-N ha-1) and PRG+WC+P (0.85 kg N2O-N ha-1). This result was linked to a numerically lower fraction of nitrification (FN) for PRG+WC+P potentially indicating that biological nitrification inhibition associated with ribwort plantain could lessen N2O emissions. This study demonstrates some of the environmental benefits of grassland management systems that require less intensive chemical fertiliser N input.