Negative impacts of excessive nitrogen fertilization on the abundance and diversity of diazotrophs in black soil under maize monocropping

Abstract

Excessive nitrogen (N) fertilization (ENF) and low utilization efficiency of fertilizer-derived N in high-input, high-yield cropping systems are serious ecological and economic problems in China. To examine the effects of long-term ENF on soil diazotrophs in relation to maize growth, we performed a 5-year field trial whereby 0–300 kg N ha−1 were added to black soil under a maize monocropping system in Northeast China. At the end of the 5-year field trial, the abundance and community structure of diazotrophs in the bulk soil and rhizosphere were investigated at the maize jointing stage by real-time quantitative polymerase chain reaction (qRT-PCR) and high-throughput sequencing. The results showed that: 1) ENF (N300) limited maize growth in the early stages and did not increase grain yield compared with moderate N supplementation (N180); 2) ENF significantly reduced nifH copy number, species richness, and Shannon index values of soil diazotrophs by 53.7%–79.7%, 37.2%–47.6%, and 20.0%–31.6%, respectively, reduced the relative abundance of Burkholderia, and increased the relative abundance of Sphingobium; 3) ENF decreased soil pH and increased electrical conductivity, which were the main factors affecting soil diazotrophic community structure as determined by Mantel test; and 4) diazotrophs were 1.3–3.0 times more abundant in the rhizosphere than in bulk soil, but no differences in α-diversity and community composition of diazotrophs were observed between rhizosphere and bulk soil. Thus, long-term ENF has negative impacts on the soil diazotrophic community, inhibiting maize growth in the early stage. Split N fertilization or slow/controlled-released N fertilizer should be used to avoid the negative effects of ENF on the soil diazotrophic community structure and early maize growth.