N2O Emission Pattern in A Legume-Based Agroecosystem

Legumes provide several ecological services to agroecosystems, but there is a lack of references on services related to N flows for a wide range of legume crops. N2O emissions were measured in two field experiments using a two-year legume-cereal crop sequence. In the first year (2014 and 2016), different legume crops were grown (lupin, pea, fava bean, common bean, soybean, chickpea) and compared to fertilized cereals (barley and sorghum). Once the seeds were harvested and the residues incorporated in the soil, unfertilized wheat was sown and harvested in the second year (2015 and 2017). N2O emissions, as well as soil temperature and moisture, were measured continuously using an automated chamber method during the two years of each experiment. Daily N2O emissions were less than 10 g N-N2O ha-1 d-1, with higher values (ranging from 10 to 90 g N-N2O ha-1 d-1) being measured during exceptionally rainy conditions. Daily N2O emissions were mainly influenced by climatic conditions for field experiments and far less by inorganic N content, except for N-fertilized cereals. For both field experiments, cumulative N2O emissions during legume and cereal pre-crops + fallow period between pre-crop harvest and wheat sowing (1st year) (mean values 365.4 and 318.1 g N-N2O ha-1 for experiment I and II, respectively) were higher than during wheat crop cultivation (2nd year) (155.8 and 101.5 g N-N2O ha-1 for experiment I and II, respectively). For field experiment II, N2O emissions were slightly higher for the N fertilized cereal pre-crops (529.8 and 523.3 g N-N2O ha-1 for barley and sorghum, respectively) compared to legume pre-crops (mean values 380.6 and 417.2 g N-N2O ha-1 for legumes sown in March and May, respectively), while no significant difference was measured for field experiment I. There was no difference in N2O emissions during the cultivation of the different legume species. Furthermore, when wheat was grown after legumes or N fertilized cereals, N2O emissions were comparable for the different experimental treatments with no relation established with the amounts of N present in crop residues or their C: N ratios. Despite the small differences in emissions between N-fertilized cereals and grain legumes, introducing these leguminous species in crop rotation and in these pedoclimatic conditions makes it possible to substitute synthetic N fertilizer and mitigate the greenhouse gases emitted from these cropping systems. However, further research is still needed to clarify and quantify the value of legumes in mitigating and reducing greenhouse gas emissions from cropping systems.