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

Thorsten Reinsch,Friedhelm Taube,Ralf Loges,Carsten Malisch

doi:10.1111/gfs.12496

Abstract

AbstractGrassland 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.

Highlights

Agricultural production systems affect the emissions of all major greenhouse gases (GHG), namely carbon dioxide (CO2) (Freibauer, Rounsevell, Smith, & Verhagen, 2004), nitrous oxide (N2O) (Reay et al, 2012) and methane (CH4) (Haque, 2018)
Sward age of grassland does not have a direct effect on elevated N2O emissions
High N2O emissions can be expected if high N surpluses coincide with high C stocks, and the latter are more likely in older grassland swards

Summary

| INTRODUCTION

Agricultural production systems affect the emissions of all major greenhouse gases (GHG), namely carbon dioxide (CO2) (Freibauer, Rounsevell, Smith, & Verhagen, 2004), nitrous oxide (N2O) (Reay et al, 2012) and methane (CH4) (Haque, 2018). A further problem that frequently arises is deterioration of grass–clover swards with age, as ingress of weed species and decreasing proportions of clover reduce the agricultural value of the ley mixture This may be addressed by oversowing, but if that fails, the deterioration might require a complete sward renewal (i.e. by cultivation and complete reseeding) to maintain productivity and forage quality (Conijn, 2004). These methods should be limited to severely degraded swards, as grassland renovation results in substantial CO2 emissions: recent studies have shown soil organic C losses of 32.2 t/ ha within two years following grassland renovation on poorly drained gleys (Necpálová et al, 2013b) and high N2O emissions (Mori & Hojito, 2007; Reinsch, Loges, Kluß, & Taube, 2018; Velthof et al, 2010). To compare N2O-emissions over time from fertilized treatments, receiving organic fertilizer, and from unfertilized treatments with only BNF as N input

| MATERIALS AND METHODS

| DISCUSSION

Findings

| CONCLUSIONS