Impact of fertilization regimes on diazotroph community compositions and N2-fixation activity in paddy soil

Nutrient status in soil is crucial for the growth and activity of resident microorganisms, which in turn regulate nitrogen transformation in the terrestrial biosphere. Biological nitrogen fixation (BNF) is an important N input in agricultural ecosystems, but the influence of fertilization on the structural and functional behavior of diazotrophs is not clear. In this study, we assessed the nutrient limitations on the abundance and N2-fixation activity of diazotrophs in a long term (20 years) fertilization experiment. Although both phosphorus (P) deficiency and potassium (K) deficiency resulted in significant decreases in nifH gene expression and N2-fixation activity, P deficiency exhibited more restrictive effects. Assessments of community structures based upon transcripts also indicated that the active diazotroph populations were more sensitive to P deficiency than K deficiency, and some diazotroph groups were detected in the P deficiency treatment. Long-term rice straw addition significantly increased diazotroph abundance, but in contrast, sharply reduced nifH gene expression and N2-fixation activity. The close relationship between N2-fixation activity and nifH gene expression rather than its copy number suggests that the nifH gene transcript level is a suitable indicator for predicting the N2-fixation activity of N2-fixing microorganisms in paddy soil of various fertility status.