Effects of grazing exclusion on the abundance of functional genes involved in soil nitrogen cycling and nitrogen storage in semiarid grassland

Abstract

We investigated the effects of grazing exclusion on the abundance of functional genes (nifH, amoA-AOA, amoA-AOB, narG, nirK, nirS, and nosZ) involved in soil nitrogen cycling in soil profiles (0-10, 10-20, 20-40 and 40-60 cm) from a chronosequence of grazing exclusion (0, 7, 18, 27 and 35 years) in the semiarid grasslands of the Loess Plateau. The relationship between abundance of functional genes and soil nitrogen storage was evaluated. The results showed that 35 years exclusion increased the abundance of nifH and amoA-AOB genes by 67.8% and 17.6% compared with the grazed grassland, respectively, and decreased that of nirK genes. The abundance of nifH, narG, and nirS genes in surface soil (0-10 cm) were significantly higher than that in deep soil (20-40 and 40-60 cm), indicating that those genes had surface accumulation effects. Grazing exclusion increased soil nitrogen storage. Soil nitrogen storage in 0-60 cm layer was the highest at 27 years (20.96 mg·hm-2), indicating that 27 years might be the optimum for grazing exclusion. The abundance of nifH, amoA-AOA and amoA-AOB had a significant linear relationship with nitrogen storage, suggesting that microbes harboring these genes played an important role in soil nitrogen accumulation. Total nitrogen, bulk density, and available phosphorus content were the dominant factors affecting the abundance of functional genes involved in soil nitrogen cycling. Our results provided a scientific reference for understanding soil nitrogen cycling and restoration of degraded grassland.