Effect of grazing on the abundance of functional genes associated with N cycling in three types of grassland in Inner Mongolia

Abstract

Purpose: The aim of the study was to investigate the patterns of soil nitrogen (N)-cycling functional gene abundance along a precipitation gradient on the Mongolian Plateau, and the effects of grazing on the population size of microbial functional group under different precipitation regimes.

Materials and methods: Soil samples were taken from grazing and non-grazing plots of meadow steppe, typical steppe, and desert steppe plots on the Mongolian Plateau for soil gravimetric moisture content, pH, and soil organic carbon (SOC), total N, and inorganic N (NH4 +-N and NO3 −-N) concentrations, and the abundance of functional genes associated with N2 fixation (nifH gene), nitrification (AOA and AOB genes), and denitrification (narG, nirS, nirK, and nosZ genes) was studied. The relationships between environmental variables, soil physicochemical properties, and functional microbial abundance were examined.

Results and discussion: Soil properties (soil moisture, pH, soil organic carbon, total nitrogen, NH4 +-N, and NO3 −-N content) and abundance of N-cycling groups all varied with precipitation. Compared with desert steppe, precipitation significantly decreased the abundance of nifH gene by 1 order of magnitude, but markedly increased the abundance of AOA and AOB genes by 1.32 to 4.72 times and denitrifying genes narG, nirS, nirK, and nosZ by 0.66 to 9.02 times in meadow steppe. Grazing significantly decreased the abundance of functional groups in desert steppe and typical steppe (p < 0.001), while there was no difference between grazing and non-grazing treatments in meadow steppe which had the highest precipitation level. Soil pH was the main factor affecting the abundance of nifH gene according to simple linear regression (R 2 = 0.934, p < 0.001), while moisture was positively related with population sizes of nitrifier and denitrifier groups, explaining 53.8–92.34 % of the variation in the abundance of AOA, narG, nirS, and nosZ genes in all three steppes.

Conclusions: Soil pH was the major factor that significantly affected the gene abundance of nitrogen fixation process, and soil moisture was the dominant factor controlling the gene abundance of nitrification and denitrification process along the precipitation gradient. Grazing had no effect on the gene abundance of N-cycling process in meadow steppe but decreased it in desert and typical steppe. Our results suggest that grazing may not necessarily be associated with a reduction in microbial functional potentials when soil moisture was relatively good but will decrease the soil microbial functional potentials in a more arid environment in northern China grasslands.