Impact of biochar amendment on the abundance and structure of diazotrophic community in an alkaline soil.

Biological nitrogen (N) fixation contributes to the pool of plant-available N in soil and helps to minimize the use of inorganic N fertilizer in agricultural ecosystems. Although diazotrophs play an important role in the biological fixation of atmospheric N2 in a range of soil types, the knowledge of their response to biochar amendment is still limited. Here, using the nifH gene as a molecular marker, we investigated the short-term effect of biochar application on the abundance, community composition and activity of diazotroph in an alkaline soil. A field trial was established before soybean sowing in 2017 and five treatments were included: inorganic NPK fertilizer (CK); inorganic NPK fertilizer + wheat straw (CS); inorganic NPK fertilizer + low rate of biochar (B4); inorganic NPK fertilizer + high rate of biochar (B20); biochar compound fertilizer (BCF). The field trial was lasted for one crop season and samples were collected by soybean harvest. The results showed that biochar addition generally increased the concentration of soil organic carbon (SOC) and available phosphorous (AP), while B20 treatment significantly increased the total nitrogen (TN) and available potassium (AK). Biochar addition treatments increased the nifH gene abundance and altered the community structure of soil diazotrophs. The abundance of nifH gene was positively correlated with SOC, indicating that increasing SOC potentially affected diazotrophic population in the alkaline soil. Community structure of diazotrophs in the CS treatment was similar with the CK treatment; thus, there was no effect of crop straw on diazotroph community structure. In contrast, the application of biochar and biochar compound fertilizer altered the diazotroph community structure with shifts in the dominant genus, with higher Sinorhizobium in the biochar-amended treatments. SOC, C/N and AP were the key factors correlated with change in diazotroph community structure. Overall, our results suggest that the addition of biochar or biochar compound fertilizer could increase the abundance and alter the community structure of diazotrophs, which may benefit N fixation in alkaline agricultural soil. Conversely, the direct straw return had no effect on the abundance and community structure of diazotrophs.