The Characteristics Of Vegetation And Soil Microbial Nutrient Utilization In Degraded Grasslands In The Qinghai Lake Watershed

Grassland degradation in the Qinghai Lake watershed will seriously affect the stability of terrestrial and aquatic ecosystems in the watershed,and also have an important impact on the ecological security of the Qinghai-Tibet Plateau and the global carbon cycle.In order to further understand the effects and mechanisms of grassland degradation on plants,soil nutrients,and their interactions in the Qinghai Lake watershed,based on ecological stoichiometry,we explored the effects of grassland degradation on plant community structure and its nutrient characteristics,soil physicochemical and microbial nutrient characteristics,and the nutrient interactions among plants,soils,and microbial communities in the Buha River Basin,the largest tributary of Qinghai Lake.In addition,we investigated the characteristics of nutrient limitation of soil microorganisms,and further revealed the influence process of grassland degradation on the characteristics of nutrient stoichiometry imbalances between soil microorganisms and their resources.Simultaneously,we elucidated how soil microorganisms coped with the nutrient stoichiometric imbalance by regulating the extracellular enzyme stoichiometric ratios,nutrient use efficiency,and community structure.The objectives of this study were to deeply understand the impact of alpine grassland degradation on the nutrient interactions of plant-soil-microbial system,advance ecological stoichiometry theory,and provide a scientific basis for the protection and ecological evaluation of fragile ecosystems in Qinghai Lake watershed and even the whole Qinghai-Tibet Plateau.The main results are as follows:1.With aggravation of grassland degradation,the total coverage of plant community and the relative coverage of dominant functional groups decreased,while the relative coverage of forbs increased.Plant community productivity,species and function richness significantly decreased with grassland degradation.Carbon(C),nitrogen(N),and phosphorus(P)storage in aboveground and belowground compartments of plant communities significantly decreased,while the nutrient stoichiometric ratios did not significantly change.Changes in nutrient storage of aboveground and belowground compartments of plant communities were associated with their biomass and was directly or indirectly affected by plant community coverage,species richness,and functional diversity.2.With aggravation of grassland degradation,soil organic carbon(SOC),total nitrogen(TN)and dissolved organic carbon(DOC)and nitrogen(DON)significantly decreased,while soil total phosphorus(TP)and dissolved organic phosphorus(DOP)did not significantly change.SOC:TP and TN:TP significantly decreased,whereas SOC:TN did not significantly change at three phenophases(green grass stage,grassy stage,and hay stage).Soil dissolved organic nutrient stoichiometric ratios only changed significantly in the grassy stage,including a significant increase in DOC:DON and decrease in DOC:DOP and DON:DOP.Soil microbial biomass carbon(MBC),nitrogen(MBN),and phosphorus(MBP)contents were not significantly affected by grassland degradation,and only MBC:MBN ratio in the hay stage significantly decreased with aggravation of grassland degradation.The changes of plant community coverage,productivity and nutrient storage of aboveground and belowground induced by grassland degradation impacted on soil nutrients to some extent,especially the plant belowground nutrient storage.Meanwhile,effects of changes in plant community and nutrient characteristics on soil nutrients and their stoichiometric ratios were stronger than that of microbial nutrient content and its stoichiometric ratios.3.With aggravation of grassland degradation,the activities of ?-1,4-glucosidase(BG),?-1,4-N-acetylglucosaminidase(NAG),leucine aminopeptidase(LAP),and alkaline phosphatase(AP)significantly decreased in the grassy stage,but did not significantly change in the hay and green grass stages.BG:(NAG+LAP)and BG:AP at three phenophases did not significantly change,whereas(NAG+AP):AP significantly increased in the grassy and hay stages.The changes of soil extracellular enzyme activities and its stoichiometric ratios were regulated by soil physical,chemical,and microbial properties,and their relative importance was affected by seasons.Soil microorganisms in the three phenophases were limited by C and P,but not by N.At the same time,with aggravation of grassland degradation,the degree of soil microbial C limitation did not significantly change,while the degree of soil microbial P limitation gradually weakened.4.With aggravation of grassland degradation,C:N imbalance between soil microorganisms and their available resources significantly increased,whereas C:P and N:P imbalances significantly decreased in the grassy stage;by contrast,there were no significant changes in the hay and green grass stages.Soil microorganisms could regulate nitrogen mineralization rate and carbon use efficiency(CUE)to cope with C:N imbalance,and regulate BG:AP and CUE to cope with C:P imbalance,and regulate(NAG+LAP):AP,nitrogen mineralization rate,and CUE to cope with N:P imbalance.Simultaneously,the change of soil bacterial community structure rather than fungi was another important strategy to cope with the nutrient stoichiometry imbalance.