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 with NH4NO3 added at rates of 0, 2, 5, 10, 20, 50 g N m−2 yr−1, and analysed the variables that were relative to growth and N fixation, such as N concentration, biomass, δ15N values, nodule number, the percentage of plant N derived from symbiotic fixation of atmospheric N2 (Ndfa%). Results Nitrogen addition had marginal effects on plant biomass production (mean total biomass was 20.4 ± 1.6 and 22.3 ± 2.1 g pot−1 for the 0 and 50 g N m−2 yr−1 treatments, respectively) and foliar N concentration (4.6 ± 0.2% and 4.9 ± 0.1% for the same treatments). The δ15N value of the leaves increased with increasing added N, while Ndfa% decreased. The number of nodules formed also decreased with N addition while the number of nitrogenase (nifH) gene copies 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.