Soil Enzyme Stoichiometry And Microbial Community Of Degraded Meadow On Northern Tibet

Due to the impact of climate change and human activities,the large-scale degradation of grassland ecosystem has attracted wide attention of relevant scholars.Especially in the Qinghai-Tibet Plateau where environmental conditions are fragile and ecosystem structure is poor,serious grassland degradation not only endangers regional ecological security,but also affects the sustainable development of pastoral economy.Soil degradation is the core of grassland degradation.Soil degradation means the decline of grassland productivity and environmental regulation ability.Exploring the changes in soil properties is of great significance for characterizing soil quality and clarifying the mechanism of grassland degradation.Alpine meadows with different degradation degrees(non-degraded,lightly degraded,moderately degraded and severely degraded)in Dangxiong County,Tibet Autonomous Region were selected as the research objects in this study.The conventional analysis method and high-throughput sequencing technology were used to systematically analyze the changes of soil physical and chemical properties and biological properties.The characteristics of microbial biomass and enzyme metrology,microbial community structure and driving factors of nitrogen cycle function genes were clarified,which provided a scientific basis and reference value for the study of degradation mechanism and the formulation of restoration measures of alpine meadows in Northern Tibet.The main research results are as follows :1.Biomass and physicochemical properties of alpine meadows with different degradation degreesMild degradation will promote plant growth,while severe degradation will greatly reduce the productivity of meadow.Meadow degradation reduced the soil nutrient content.The contents of organic carbon,total nitrogen and total phosphorus in severely degraded soil decreased by 67.4 %,56.2 % and 36.2 %,respectively,compared with those in non-degraded soil.The contents of nitrate nitrogen,available phosphorus and available potassium in moderately degraded soil decreased significantly,and decreased by 59.2 %,23.4 % and 10.1 %,respectively,compared with those in lightly degradation.C/N,C/P and N/P were lower in mild,moderate and severe degradation than in non-degraded meadow.2.Compared with non-degraded meadow,microbial biomass carbon(MBC),microbial biomass nitrogen(MBN)and microbial biomass phosphorus(MBP)decreased by 41.4 % ? 73.9 % in severely degradation.The activities of alkaline phosphatase(ALP)activity increased with degradation,the activities of ?-1,4-glucosidase(BG)and ?-N-acetylglucosaminidase(NAG)in non-degradation were significantly lower than those of lightly degraded,moderately degraded and severely degraded,and the activities of cellulose disaccharide hydrolase(CBH)and leucine aminopeptidase(LAP)were the lowest in mild degradation.ln(BG + CBH): ln(NAG + LAP)was 0.55 ? 0.91,ln(NAG + LAP): ln(ALP)was 0.73 ? 0.97,deviated from the global average 1.41 and 0.44;The vector length and angle were the highest in moderately degraded and lightly degraded,respectively,indicating that the microbial limitation of P was stronger than that of N in meadow degradation,while the microbial limitation of C was relatively strong in moderately degraded.Microbial biomass and its metrological characteristics were jointly driven by organic carbon,total nitrogen and total phosphorus;leucine aminopeptidase(LAP)was significantly negatively correlated with aboveground / underground biomass,and ?-1,4-glucosidase(BG)was significantly negatively correlated with organic carbon.Enzymatic metrology was generally driven by p H,nitrate nitrogen and ammonium nitrogen.3.Soil microbial community structure and diversity characteristics of alpine meadow with different degradation degreesWith the increase of degradation degree,the bacterial Chao1 index increased,and the fungal Chao1 index increased first and then decreased.It was the highest in moderately degraded,and there was no significant difference in Shannon index.The ?diversity of bacteria and fungi showed significant differences among different degradation degrees.At the phylum level,the dominant bacterial communities in the degradation process were Proteobacteria(23.0%?35.7%),Actinobacteria(21.4%?27.6%),Acidobacteria(11.7%?22.3%),Bacteroidetes(2.6%?16.4%)and Chloroflexi(5.9%?7.3%),and their peaks appeared in moderately degraded,lightly degraded,non-degraded,lightly degraded and non-degraded,respectively.The dominant fungal communities were Zygomycota(54.8%?69.4%),Ascomycota(26.1%?41.7%)and Basidiomycota(1.8%?6.0%),and their peaks appeared in severely degraded,lightly degraded and non-degraded,respectively.Mantel analysis showed that available phosphorus was the main factor driving the change of soil bacterial community structure.4.Abundance and diversity of nitrogen cycling functional genes in different degraded alpine meadowsLightly degradation increases the ? diversity of nitrification genes(amoA-AOA and amoA-AOB)and denitrification genes(nirS,nirK and nosZ).The abundance of nitrogen cycle function genes showed an inverted ' V ' type change with the aggravation of degradation.Except for nosZ,the highest value appeared in moderately degraded,and the lowest value appeared in severely degraded,indicating that grassland degradation may strengthen nitrification and denitrification.Among them,the copy number of amoA-AOA is about 271.2-329 times that of amoA-AOB,and the copy number of nirS is about 41.9-111.2 times that of nirK,indicating that AOA and nirS are dominant in the study area.The diversity characteristics of nitrogen cycle functional genes are jointly driven by meadow biomass,soil nutrients and stoichiometric ratio.Among them,p H,organic carbon,total nitrogen and stoichiometric ratio are the main influencing factors of nitrification genes,and aboveground / belowground biomass,total phosphorus and available phosphorus are the main influencing factors of denitrification genes.