Effects of alpine meadow degradation on soil microbial biomass and nitrogen mineralization rate in the Qilian Mountains

Soil microbial nitrogen and carbon content, soil nitrogen mineralization, and enzymes related to soil microbes were studied in order to better understand the variation characteristics of degrading alpine meadows. At the Qilian Mountains, alpine meadows with four different degradation degrees(no degradation, ND; light degradation, LD; moderate degradation, MD; and severe degradation, SD) were selected and soil samples from the uppermost layer(0-10 cm in depth) were used to study several plant factors, soil physicochemical properties, soil ammoniation, nitrification, and soil net nitrogen mineralization rates, as well as nitrogen-related enzymes and microorganisms. Results showed that(1) degradation of alpine meadows reduced both the ammoniation and the net nitrogen mineralization rates in soils while increased their nitrification rate;(2) degradation of alpine meadows also reduced the activity of nitrogen-related enzymes such as soil protease, urease, and leucine aminopeptidase(LAP), while the activity of β-1,4-N-acetylglucosaminidase(NAG) decreased first and then increased, showing its highest activity under SD conditions;(3) along with the increase in degradation, the amount of microbial biomass carbon(MBC) and nitrogen(MBN) in the soil was reduced. At the same time, soil basic respiration(SBR), soil microbial entropy, and metabolic entropy indices were reduced as well. Results of an RDA analysis showed that both the ammoniation rate and the soil net nitrogen mineralization rate significantly positively correlated with MBC, MBN, SBR, plant coverage, plant height, aboveground biomass, and protease, urease, and LAP activities, while the nitrification rate showed a negative correlation. The degradation of alpine meadows showed an important impact on soil microbial properties and nitrogen transformation and cycling.