Nitrogen addition results in Medicago sativa switching nitrogen sources

Abstract

Background Nitrogen (N) addition may have strong impacts on legume growth and their biological N fixation (BNF), but how legume N acquisition sources respond to N inputs have yet to be comprehensively assessed.Aims We quantified the effects of N addition on the growth and BNF of Medicago sativa and to assess the response of legume N acquisition to N addition.Methods We grew M. sativa in the greenhouse under gradients of added NH4NO3 and analysed the variables that were relative to growth and BNF, such as N concentration, biomass, δ15N values, nodule number, percentage of N derived from the atmosphere (Ndfa%).Results Nitrogen addition had marginal effects on plant biomass production and foliar N concentration. Foliar δ15N value increased with increasing added N, while Ndfa% decreased. The number of nodules formed also decreased with N addition while the nitrogenase (nifH)genecopies per unit nodule mass was not significantly different with N addition.Conclusions These findings indicate that increasing mineral N availability decreases symbiotic investment into BNF, mainly by reducing nodule formation; this was found to have no significant impact on plant growth because the plant changes its N source from BNF-N to mineral N derived from the soil.