Biological nitrogen fixation in field pea and vetch: Contribution from above and belowground structures to the partial nitrogen balance

Abstract

Context: Including winter legumes during fallow periods in crop rotations may add nitrogen (N) to the agroecosystem through biological N fixation (BNF). This gain will depend on management practices and whether the crop is planted for grain production or as a cover crop. Despite many studies quantified the partial N balance of different legume crops, very few considered the contribution of BNF from belowground structures, which is necessary for more accurate estimations. Objective: To estimate biomass, N uptake, and BNF in field-grown field pea and vetch, and study the impact of including belowground structures in the partial N balance of both crops. Methods: A two-year field study evaluating two genotypes of field pea (semi-leafless and conventional-leaved) and vetch was performed under field conditions in the Pampean Region (Argentina). Above and belowground biomass (measured in field monoliths to 0.3 m depth), N uptake and 15N abundance were quantified for determining BNF, and the partial N balance. Results: The partial N balance of field pea as grain crop was negative in most cases (average − 7 kg N ha− 1 ), but, was positive when vetch was evaluated as cover crop (average 51 kg N ha− 1 ). Including belowground components, BNF improved the partial N balance by 1 and 6 kg N ha− 1 in field pea and vetch, respectively. In field pea, the proportion of N uptake derived from BNF (%Ndfa) averaged 43 %, and was not affected by genotype or year, whereas for vetch represented 40 and 18 % of total N uptake in 2018 and 2019, respectively. In field pea, belowground BNF averaged 1.2 kg N ha− 1 , and represented 0.9 % of the total N uptake at maturity. In vetch, belowground BNF averaged 6.3 kg N ha− 1 and accounted for 3.8 % of total N uptake at full bloom. Conclusions: Biomass and the proportion of N derived from BNF were not affected by the different genotypes in field pea, while in vetch, the environmental conditions that increased %Ndfa were more important than those that affected biomass production for increasing the partial N balance. Implications: Although field pea as a grain crop did not provide a consistent N input to the agrosystem, it arises as a convenient crop in terms of N balance compared to other cash crops. Vetch stands as an attractive cover crop, contributing 46–57 kg N ha− 1 and 6–9.5 Mg ha − 1 of residues to the agrosystem.