Effects Of Desertification On The C:N:P Stoichiometry Of Soil-microbial-extracellular Enzyme In Desert Grassland

Anthropogenic disturbance and global climate change had led to serious deterioration of ecological environment in arid and agro-pasture transition zone,and grassland degradation has intensified.Land desertification is one of the typical phenomena of land degradation process.The desert grassland in the middle and north region of Ningxia is located on farming-pastoral transition zone,which is seriously disturbed by human factors and its ecological environment is extremely fragile.The overuse of grassland resources and the impact of climate change caused the long-term,extensive degradation and desertification of desert grasslands.In order to discuss the underlying mechanism of desertification effect on the ecological stoichiometry of soil,microbes and extracellular enzymes,we used a space-for-time method,selected four desertification stages(grasslands,fixed dunes,semi-fixed dunes and mobile dunes)in desert grassland as the research object.The change characteristics of soil available nutrients,soil and soil microbial biomass carbon,nitrogen and phosphorus contents,and extracellular enzyme activity in the process of desertification of desert grassland have analyzed to discuss the effect mechnasim of desertification on soil-microbial-extracellular enzyme C:N:P stoichiometry in desert grassland.The main results are as followed:(1)Soil available nitrogen,soil ammonium nitrogen and soil nitrate nitrogen decreased significantly with increasing desertification of desert grassland,but soil available phosphorusdecreased but was not significant.Soil available nitrogen was more sensitive to the desertification of desert grassland.Compared with grasslands stage,soil available nitrogen in the fixed dunes,semi-fixed dunes and mobile dunes was decreased by 12.0%,50.1%and 54.4%.respectively,Soil C,N,P contents and soil C:P,N:P significantly decreased with increasing desertification,but soil C:N gradually increased.Soil available nitrogen,ammonium nitrogen were significantly correlated with soil carbon,nitrogen,phosphorus contents,and soil C:N,C:P,N:P.Soil available phosphorus was significantly positively correlated with soil carbon,nitrogen and phosphorus contents,but was not significant correlated with soil C:N,C:P and N:P.(2)Soil microbial biomass carbon,nitrogen and phosphorus reduced by 46.1%,80.8%and 30.0%from grassland to mobile dunes,respectively.During the desertification process of desert grassland,the rank of soil ?-1,4-glucosidase(BG),?-1,4-N-acetylglucosaminidase(NAG)and alkaline phosphatase(AP)were grasslands>fixed dunes>semi-fixed dunes>mobile dunes.Soil microbial biomass C:soil microbial biomass P(MBC:MBP),soil microbial biomass N:soil microbial biomass P(MBN:MBP)and soil ?-1,4-glucosidase:?-1,4-N-acetylglucosaminidase(BG:NAG,C:N)gradually decreased,while BG:AP(C:P)and NAG:AP(N:P)generally increased with increasing desertification.(3)The soil microbial quotient(qMBC,qMBN and qMBP)decreased with the development of grassland desertification,while soil-microbial stoichiometry imbalance(C:Nimb,C:Pimb and N:Pimb)generally increased with the development of grassland desertification.There were significant positively relationship between soil microbial biomass nitrogen and C:Nimb,soil microbial biomass phosphorus and C:Pimb,while negatively relationship between soil microbial biomass nitrogen and N:Pimb.The RDA analysis showed:that the first RDA axis explained 35.4%of the variations in soil microbial quotient,and the first and second RDA explained 43.0%of soil microbial quotient together,and soil ecological stoichiometry(C:N,C:P)has the most negative effect on soil microbial quotient carbon(qMBC).Soil microbial biomass and microbial quotient were significantly influenced by desertification in desert grassland.(4)Soil microbial carbon use efficiency(CUEC:N and CUEC:P)gradually increased with the development of grassland desertification,while soil microbial nitrogen use efficiency(NUEN:C)and soil microbial phosphorus use efficiency(PUEP:C)generally decreased.Soil and soil microbial biomass C,N,P contents,and extracellular enzyme activities(BG,NAG,AP)were significantly negatively correlated with the soil C:N and extracellular enzyme NAG:AP(N:P)stoichiometry,but were significantly positively correlated with the soil(C:P,N:P),microbial biomass(C:P,N:P)and extracellular enzyme BG:NAG(C:N)in the desertification process of desert grassland.Soil,soil microbial and soil extracellular enzyme C:N stoichiometry(C:N,MBC:MBN,BG:NAG)were significantly negatively correlated with the soil,soil microbial and extracellular enzyme N:P stoichiometry(N:P,MBN:MBP,NAG:AP).Soil and extracellular enzymes C:N(C:N,BG:NAG)were significantly positively correlated with soil and extracellular enzymes C:P(C:P,BG:AP).Soil N:P was significantly positively correlated with the soil microbial biomass N:P(MBN:MBP),but significantly negatively correlated with extracellular enzymes N:P(NAG:AP).The analysis demonstrated that soil microbial biomass and extracellular enzyme activity changed with soil nutrient during the desertification process in the desert grassland.The covariation relationship between soil nutrient and microbial-extracellular enzyme C:N:P stoichiometry,which can provide theoretical basis for understanding the underlying mechanism of C,N,P cycling in the soil-microbial system in desert grasslands.