Nitrous oxide emissions from grain legumes as affected by wetting/drying cycles and crop residues

Grain legume production with rhizobial inoculation has drawn attention not only because of the economic value of nitrogen (N) fixation by grain legumes, but also because of the concern that N-2 fixation by grain legumes may enhance emissions of nitrous oxide (N2O), a powerful greenhouse gas. However, the relationship between N2O emissions and biological N-2 fixation by grain legumes is not well understood. The objective of this study was to quantify N2O emissions associated with N-2 fixation by grain legumes as affected by wetting/drying cycles and crop residues. Two grain legumes, lentil (Lens esculenta Moench) and pea (Pisum sativum L.), either inoculated with two Rhizobium leguminosarum biovar viciae strains, 99A1 and RGP2, respectively, or fertilized with N-15-labeled fertilizer were grown in a controlled environment under three wetting/drying cycles. Profile N2O concentrations and surface N2O emissions were measured from the soil-plant systems, which were compared with those from a cereal, spring wheat (Triticum aestivum L. ac. Barrie). After harvest, crop residues were incorporated into soils that were seeded to spring wheat to evaluate the effect of crop residues on N2O emissions. Results indicated that: (1) inoculating grain legumes with non-denitrifying rhizobia did not enhance N2O emissions and the presence of grain legumes did not increase N2O emissions compared with the cereal crop, and (2) profile N2O accumulation and surface emissions were not related to the type of crop residues added to the soil, but related to the residual N applied previously as N fertilizer. This suggests that N2O emissions are not directly related to biological N-2 fixation by grain legumes, and on a short time scale, N rich residues of N-2-fixing crops have a limited impact on N2O emissions compared with N fertilization.