Nitrous oxide emissions from grass–clover swards as influenced by sward age and biological nitrogen fixation

Grassland renovation by cultivation and reseeding has been shown to increase short-term emissions of N₂O, but there is uncertainty about long-term effects, despite the potential impacts of reseeding on sward composition and soil functions. A field experiment was therefore carried out to determine how N₂O emissions from previously renovated grasslands varied in the intermediate to long-term, compared with an undisturbed permanent grassland (PG). Plots on the PG site were renovated, either two (G2) or five (G5) years prior to the two experimental years. In each sward age and experimental year, annual N₂O-measurements were conducted on a weekly basis and compared with the undisturbed PG. Plots were either unfertilized or were fertilized with slurry (240 kg N ha^?1 year^?1). On average, annual N₂O emissions were 0.39 kg N/ha for the unfertilized swards, and 0.91 kg N/ha for slurry-fertilized swards. Sward age had no effect on N₂O emissions. With increasing sward age the proportion of legumes in the sward was reduced, but a minimum biological nitrogen fixation (BNF) of 88 kg N/ha was maintained even in the fertilized PG. Both sward age and BNF were of limited importance for the annual N₂O emissions compared with the effects of soil carbon content and nitrogen surplus levels. However, measured N₂O emissions were low in all sward age treatments, with a low risk of additional N₂O emissions when BNF is taken into account in fertilizer planning.