Biological Nitrogen Fixation Response to Soil Fertility Is Species-Dependent in Annual Legumes

Abstract

In low-input farming systems, the availability of nutrients can limit yields as well as biological nitrogen fixation (BNF). The adaptability of legume species to soil fertility conditions may be relevant for designing crop rotations. The aim of this study was to determine the effect of soil fertility on biological N fixation of two annual legumes (chickpea Cicer arietinum and bitter vetch Vicia ervilia) adapted to dry conditions growing in organically amended soils, and whether, this effect was species-dependent. Two legume species were sown in two fields with contrasting soil fertility, amended or non-amended with cow manure. We measured soil N and P pools and availability and estimated N derived from the atmosphere (%Ndfa) by 15N natural abundance. In low-fertility soils, both legume species showed high BNF (17–22 mg N2 fixed per kg of dry matter) that correlated with plant production. While chickpea BNF responded negatively to soil fertility and manure application, bitter vetch BNF remained virtually unchanged, irrespective of soil fertility, and did not correlate with soil N or P available forms. Bitter vetch productivity and BNF, both in low- and high-fertility scenarios, was not responsive to a single manure application event, while chickpea showed a strong response to added manure. Bitter vetch %Ndfa and BNF were always higher than chickpea and consequently this former species could be more appropriate for increasing soil N pools in low-input cropping systems.