Mitigation of postharvest nitrous oxide emissions from soybean ecosystems: A review

Abstract

In this review, the current knowledge of nitrous oxide (N2O) emissions from soybean (Glycine max (L.) Merr.) ecosystems, particularly on postharvest N2O emissions, is summarized and controlling factors of postharvest N2O emissions from soybean ecosystems are discussed. A new biological method to mitigate N2O emission is also presented. The latest (2006) guidelines of the Intergovernmental Panel on ClimateChange (IPCC) concluded that N2O emissions derived directly from biological nitrogen (N) fixation were not significant in legume crop ecosystems. The default N2O emission factor from biological N fixation was revised to zero, whereas the default N2Oemission factor for the decomposition of legume crop residues (postharvestN2Oemissions) was the same as that for nonlegume crop residues (1%). From previously measured field data, the percentage of N in soybean residue emitted as N2O after harvest was calculated to determine emission factors. Values ranged from 0.0- 10.0%(average 1.3%± 2.7%, median: 0.2%), indicating relatively low emission factors. The average emission factor calculated for N2O emissions from N in soybean residues was slightly higher than the default emission factor for N in crop residue specified in the guidelines. However, the volume of field-measured postharvest N2O emissions in soybean ecosystems is low compared with data for N2O emissions during the crop growing season. Previous field-measured data demonstrated that N2O emissions often peaked sharply immediately after the harvest. However, values for N2O fluxes in the currently available field data were determined on a weekly or monthly basis. This large time gap between samplings may have resulted in underestimation of postharvest N2O emissions in soybean ecosystems. More detailed field studies on postharvest N2O emissions from soybean ecosystems are needed. Nodule decomposition is the major source of postharvest N2O emissions in soybean ecosystems. A new microbiological method was recently developed to mitigate postharvest N2O emissions from soybean ecosystems utilizing N-fixing bacteria with increased N2O-reducing activity (increased expression of nosZ). This approach may potentially be applied to other leguminous crops and non-legume ecosystems. Ongoing research on the relationships between soil N2O emissions and nosZ may reveal a new method for N2O mitigation in the future. © 2013 Japanese Society of Soil Science and Plant Nutrition.