Soil N2O Emissions during the Growing Phase of Legume Plants May Add Significantly to That after the Biomass Incorporation into Soil.

Computing soil N₂O emissions from green manure takes into account the amount of N in the legume biomass and the calculation of the legume-N fraction that is converted to N₂O. However, during the legume growth higher levels of mineral N in the soil have been observed compared to the same soil cropped to non-legume species, a phenomenon known as the soil N sparing effect. Differences in N accumulation patterns together with the partial N supply from biological N₂ fixation to the legume explain this effect. To evaluate how important this phenomenon would be for the soil N₂O emission, a study was performed at Embrapa Agrobiologia located in Rio de Janeiro State, Brazil. Three treatments were arranged in randomized complete blocks with five replicates. The treatments were Jack bean (Canavalia ensiformis) and dwarf sorghum (Sorghum bicolor) as green manure for elephant grass (destined for bioenergy) and the third treatment left fallow as the control. The accumulated biomass in each treatment was incorporated into the soil by ploughing. Fluxes of N₂O together with other soil variables were monitored. Significant N₂O fluxes occurred during legume growth, which were accompanied of high soil mineral N concentration indicating an N-sparing effect. These high fluxes corresponded to about 30 % of the net N₂O emission computed in the whole period, the remaining induced by the incorporation of legume biomass into the soil. Even considering the gross soil N₂O emissions, the area cropped to legume emitted 1.8 and 2.2 times that from the fallowed area and from the area cropped to sorghum, respectively. The lowest emissions during sorghum growth would be credited to an enhanced soil N exploitation. Differences between patterns of soil N accumulation by plants should be captured by mathematical models to better account to the impact of cropping systems regarded to GHG emissions.