Sward composition and soil moisture conditions affect nitrous oxide emissions and soil nitrogen dynamics following urea-nitrogen application.

Increased emissions of NO, a potent greenhouse gas (GHG), from agricultural soils is a major concern for the sustainability of grassland agriculture. Emissions of NO are closely associated with the rates and forms of N fertilisers applied as well as prevailing weather and soil conditions. Evidence suggests that multispecies swards require less fertiliser N input, and may cycle N differently, thus reducing N loss to the environment. This study used a restricted simplex-centroid experimental design to investigate NO emissions and soil N cycling following application of urea-N (40 kg N ha) to eight experimental swards (7.8 m) with differing proportions of three plant functional groups (grass, legume, herb) represented by perennial ryegrass (PRG, Lolium perenne), white clover (WC, Trifolium repens) and ribwort plantain (PLAN, Plantago lanceolata), respectively. Swards were maintained under two contrasting soil moisture conditions to examine the balance between nitrification and denitrification. Two NO peaks coincided with fertiliser application and heavy rainfall events; 13.4 and 17.7 g NO-N ha day (ambient soil moisture) and 39.8 and 86.9 g NO-N ha day (wet soil moisture). Overall, cumulative NO emissions post-fertiliser application were higher under wet soil conditions. Increasing legume (WC) proportions from 0% to 60% in multispecies swards resulted in model predicted NO emissions increasing from 22.3 to 96.2 g NO-N ha (ambient soil conditions) and from 59.0 to 219.3 g NO-N ha (wet soil conditions), after a uniform N application rate. Soil N dynamics support denitrification as the dominant source of NO especially under wet soil conditions. Significant interactions of PRG or WC with PLAN on soil mineral N concentrations indicated that multispecies swards containing PLAN potentially inhibit nitrification and could be a useful mitigation strategy for N loss to the environment from grassland agriculture.