Carbon substrates exert a stronger role than mineral nitrogen application in structuring soil diazotroph communities during Chinese milk vetch growth

Carbon (C) quality and quantity and nitrogen (N) availability are known to play a crucial role in influencing diazotroph community structure in soils and they are commonly affected by crop residue management and fertilizer application. However, a full understanding of how C and N interactions contribute to shaping soil diazotroph communities remains elusive. An experiment comparing two different C substrates (rice straw vs. glucose) plus low to high rates of mineral N application was conducted in two paddy soils with contrasting pH (pH = 6.05 and 7.85) with Chinese milk vetch (CMV) growth. Soils were sampled during the full blooming stage of CMV and diazotroph community structure was characterized using the nifH marker gene. The results showed that the diazotroph community responded differently to C substrates (straw and glucose) depending upon the C availability. In both soils, mineral N addition decreased nifH gene copy numbers in the straw-included treatments, but not in the glucose-included soils. Compared to the straw-included soils, glucose addition resulted in less α-diversity of diazotroph. Meanwhile, diazotroph community structure was clustered into different groups by the C sources, and marginally affected by N levels. These results suggested that the responses of the diazotroph community to N supply were regulated by C availability. Glucose addition decreased the relative abundance of Bradyrhizobium compared to straw incorporation (16.0%–25.9% vs. 19.5%–38.2%), but significantly increased the abundance of the second most dominant genus Geobacter (12.8%–23.7% vs. 4.1%–8.7%). In addition, the diazotroph diversity and community structure were less responsive to straw, glucose and mineral N addition in the lower vs. high pH soils. Overall, the results suggest that the responses of diazotrophs to N availability rely on C availability in paddy soils, and that C substrates exert a stronger influence than mineral N application in structuring diazotroph communities.