Biological nitrogen fixation and soil N2O emissions from legume residues in an Acrisol in SE Brazil

Abstract

Information is lacking concerning the contribution of green manure legumes to N2O emissions from tropical soils. In a three-year field study, we measured the N accumulation and the N2 fixation by three green manure legumes and the effects of their residues on N2O production from a tropical soil (Haplic Acrisol) under an organic farming system. The legumes were jack bean (Canavalia ensiformis L.), velvet bean [Mucuna pruriens (L.) DC] and sunn hemp (Crotalaria juncea L.), which were mown at the late pod-filling stage. The N accumulation in residues varied substantially among the legume types and the growing seasons (78–422 kg ha−1). The contribution of N2 fixation to legume residues, expressed as the proportion of N in plant derived from air (%Ndfa), was relatively low in the first year (23–36%) and increased significantly in the following two years (46–85%). The highest amount of N2 fixed was in the residues of jack bean, reaching 360 kg N ha−1. The residue of this legume and that of velvet bean caused the highest mean annual soil N2O emissions (3.78 and 3.29 kg N2O-N ha−1, respectively). The mean percentage of N lost as N2O derived from residues was not significantly affected by legume type, ranging from 0.58% (sunn hemp) to 0.72% (velvet bean). These percentages were below the default IPCC emission factor of 1% for N additions from plant residue or from synthetic fertilizers. We also showed that N in legume residues derived from BNF offers an important environmental advantage over synthetic N fertilizers owing to the avoidance of GHG emissions (360–944 CO2-equivalent ha−1 yr−1) during Haber-Bosch process and also during transportation and application in the fields. The three green manure legumes provided significant higher cash crop (squash) yield compared to maize, indicating the direct economic benefit of BNF for organic farming. More field studies are needed to assess the dynamics of gaseous N losses from plant residues in other tropical biomes.