N₂O Emission Pattern in A Legume-Based Agroecosystem

Abstract

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. N<sub>2</sub>O 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). N<sub>2</sub>O emissions, as well as soil temperature and moisture, were measured continuously using an automated chamber method during the two years of each experiment. Daily N<sub>2</sub>O emissions were less than 10 g N-N<sub>2</sub>O ha<sup>-1</sup> d<sup>-1</sup>, with higher values (ranging from 10 to 90 g N-N<sub>2</sub>O ha<sup>-1</sup> d<sup>-1</sup>) being measured during exceptionally rainy conditions. Daily N<sub>2</sub>O 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 N<sub>2</sub>O emissions during legume and cereal pre-crops + fallow period between pre-crop harvest and wheat sowing (1<sup>st</sup> year) (mean values 365.4 and 318.1 g N-N<sub>2</sub>O ha<sup>-1</sup> for experiment I and II, respectively) were higher than during wheat crop cultivation (2<sup>nd</sup> year) (155.8 and 101.5 g N-N<sub>2</sub>O ha<sup>-1</sup> for experiment I and II, respectively). For field experiment II, N<sub>2</sub>O emissions were slightly higher for the N fertilized cereal pre-crops (529.8 and 523.3 g N-N<sub>2</sub>O ha<sup>-1</sup> for barley and sorghum, respectively) compared to legume pre-crops (mean values 380.6 and 417.2 g N-N<sub>2</sub>O ha<sup>-1</sup> for legumes sown in March and May, respectively), while no significant difference was measured for field experiment I. There was no difference in N<sub>2</sub>O emissions during the cultivation of the different legume species. Furthermore, when wheat was grown after legumes or N fertilized cereals, N<sub>2</sub>O 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.