Effects of applying maize stover on soil diazotroph community diminish with depth

Abstract

Biological nitrogen fixation (BNF) is a vital approach to replenishing soil nitrogen (N) pools by converting atmospheric N2. Agronomic practice of growing legumes frequently includes stover return, which generally induces the growth of N2-fixers (i.e., diazotrophs) and stimulates BNF; however, there is uncertainty regarding the stover return effects along soil depth. Here, soil samples were collected from 0 to 10, 10–20 and 20–30 cm in the peanut (Arachis hypogaea) field experiment (established in 2013) comprising treatments with no fertilizer (CK), maize (Zea mays) stover (MS), and MS + chemical fertilizers (SNPK). The abundance of 16S rRNA and nifH genes was determined by real-time PCR, the rate of N2 fixation in soil (RNfix) was measured by acetylene reduction assay, and diazotroph community structure was explored by high-throughput sequencing. Results showed that maize stover application alone increased nifH gene abundance in the 0–10 cm layer, and combined with chemical fertilizer application increased RNfix in the 0–20 cm layers, affecting diazotroph community structure succession in the 0–20 cm layers. This is consistent with the effects of stover on soil properties such as dissolved organic carbon and nitrogen, soil organic matter (SOM) and carbon-to‑nitrogen ratio, with SOM playing a dominant role in governing the abundance, structure and activity of diazotroph communities in the soil. In addition, total nitrogen (TN) was the key factor in shaping the vertical stratification of diazotroph community structure, the abundance of diazotrophs and bacteria as well as the structure and RNfix of the diazotroph community were highly correlated. This study demonstrated that maize stover incorporation caused significant positive changes in the vertical stratification of soil diazotroph communities; although these effects decreased with soil depth and were minor when stover was combined with chemical fertilizers, they still highlighted the crucial role of stover return in enhancing BNF in soil where legumes were present.