Soil Extracellular Enzyme Activity And Its Stoichiometric Characteristics Of Degraded Alpine Grassland On Northern Tibet

The Qinghai-Tibet Plateau is known as the "water tower of the world",but its fragile and sensitive ecological condition has been degraded by environmental changes and human activities,and the ecological problems of northern Tibetan grasslands are particularly serious.Soil enzymes are active substances sensitive to environmental changes,and enzyme stoichiometry can reflect supply relationships and nutrient limitation in the soil nutrient cycle.The study of soil enzyme activity and stoichiometry after grassland degradation and its influencing factors can provide insight into the environmental-vegetation-soil-microbial-soil enzyme cycle and nutrient limitation during the grassland degradation process.In this study,alpine steppe and alpine meadows in six counties in Nagqu region of Tibet,including degraded grasslands and non-degraded grasslands,were surveyed and sampled in Julyto September in 2021 by field sampling,and the relationships between environmental factors,plant community characteristics,soil physical and chemical properties,microbial load and soil enzyme activity and chemometrics were analyzed to investigate the soil enzyme activity and chemometrics characteristics of degraded grasslands We also analyzed the nutrient limitation in the degradation process of two types of grasslands,aiming to provide a basic basis for vegetation restoration and scientific management of alpine grasslands in northern Tibet.The main research results are as follows:(1)The vegetation cover,above-ground biomass,water content,organic carbon,total nitrogen,total phosphorus,and nitrogen-phosphorus ratio of alpine meadows were significantly higher than those of alpine steppe,and the soil p H was significantly lower than that of alpine steppe.The vegetation cover,above-ground biomass,Shannonwinner index,Simpson index,water content,organic carbon,total nitrogen,total phosphorus,ammonia nitrogen,nitrate nitrogen,and nitrogen-phosphorus ratio were significantly higher in the non-degraded samples of both meadow types than in the degraded samples,and the soil capacitance was significantly lower than in the degraded samples.At different soil depths,water content,organic carbon,total nitrogen,total phosphorus,ammonia nitrogen,and nitrogen-phosphorus ratio of the two alpine grasslands showed a decreasing trend,and p H and soil capacity showed an increasing trend.Among them,the carbon to nitrogen ratio and carbon to phosphorus ratio of degraded alpine Steppe were the highest in 10～20 cm,followed by 0～10 cm soil layer of non-degraded alpine meadow with significantly lower soil capacity than that of nondegraded alpine steppe,and the opposite of ammoniacal nitrogen and fast-acting phosphorus;10～20 cm soil layer with significantly lower soil capacity than that of non-degraded alpine steppe,and the opposite of nitrate-to-phosphorus ratio;20～30 cm soil layer with significantly higher ammoniacal nitrogen than that of non-degraded alpine Steppe.Ammoniacal nitrogen in the 20～30 cm soil layer was significantly higher than that in the non-degraded alpine steppe.(2)Microbiomass carbon,microbiomass nitrogen,β-1,4-glucosidase,N-acetyl-β-D-aminoglucosidase activity and enzyme N/P were significantly higher in alpine meadows than in alpine steppe,and polyphenol oxidase activity and enzyme C/N were significantly lower than in alpine steppe.Among them,exo-β-1,4-glucanase activity and enzyme C/P were significantly lower in the 20-30 cm soil layer of degraded alpine meadows than in degraded alpine steppe;alkaline phosphatase activity was significantly lower in the 20-30 cm soil layer of non-degraded alpine meadows than in non-degraded alpine steppe,and significantly higher in the 10-20 cm and 20-30 cm soil layers of degraded alpine meadows than in degraded alpine steppe.Compared with the non-degraded sample,microbial quantity carbon,microbial quantity nitrogen,carbon cycle enzyme activity,nitrogen cycle enzyme activity,enzyme C/P and enzyme N/P decreased significantly and enzyme C/N increased significantly in the degraded alpine meadow;phosphorus cycle enzyme activity decreased significantly and enzyme C/P increased significantly in the degraded alpine steppe.At different soil depths,the soil microbial load and enzyme activity of both types of grassland were in decreasing trend,and the enzyme stoichiometry ratio was in increasing trend.(3)Alpine meadow and alpine steppe polyphenol oxidase activity was significantly and negatively correlated with plant Shannon-winner diversity index;peroxidase activity was significantly and positively correlated with ammonia nitrogen;β-1,4glucosidase activity was significantly and positively correlated with plant Shannonwinner diversity index,water content,Simpson index,and significantly and negatively correlated with p H;N-acetyl-β-D-aminoglucosidase activity was significantly and positively correlated with Shannon-winner diversity index,Simpson index,vegetation cover,above-ground biomass,total nitrogen,nitrate nitrogen,and negatively correlated with soil capacity;leucine aminopeptidase activity was significantly and positively correlated with growing season precipitation;alkaline phosphatase activity was significantly and positively correlated with water content,p H,microbial load carbon,and with The enzyme C/P was significantly and negatively correlated with the growing season mean temperature and soil capacity.The enzyme activity and stoichiometric ratios in alpine meadows were driven by ammonia nitrogen and plant cover in alpine steppe.(4)The vector length and vector angle were larger in both types of meadows in the study area,and carbon limitation and phosphorus limitation existed in soil microorganisms.Among them,microbial carbon limitation was stronger in nondegraded alpine meadows than in non-degraded alpine steppe,and phosphorus limitation was weaker than in non-degraded alpine steppe.The degraded alpine meadow microbes were mainly phosphorus-limited,and the degraded alpine steppe microbes were mainly carbon-limited.At different soil depths,the number of microorganisms in the deeper layers of alpine meadows was lower and phosphorus limitation was weaker;in the 20-30 cm soil layer of alpine steppe,the organic carbon content was low and the soil phosphorus content was sufficient,so the microorganisms were strongly limited by carbon and weakened by phosphorus limitation.