Mechanism Of Change In Phosphorus Effective Supply Driven By Microorganisms And Phosphorus Regulation Strategy In Degraded Alpine Meadow Soils

The alpine meadow on the Qinghai-Tibet Plateau is an important pastoral area in China,which plays an important role in regional ecological function maintenance and economic development.However,in the past 50 years,due to the influence of climate drying and warming,excessive utilization and unreasonable management methods of grassland resources,the alpine meadow in this area has been degraded in varying degrees.Phosphorus is a key nutrient factor for alpine meadow productivity.Meadow degradation reduces phosphorus input and availability,restricts meadow productivity and ecological function,and affects the sustainable development of animal husbandry economy.Although there have been many studies on soil phosphorus forms,phosphorus availability and its response to meadow degradation,the microbial mechanism of changes in soil phosphorus forms and availability for alpine meadows with different degrees of degradation during plant phenology is not fully understood,which restricts the scientific management of soil phosphorus in the process of ecological restoration of degraded meadows.To solve this issue,this paper takes the non-degraded meadow（NDM）,lightly degraded meadow（LDM）,moderately degraded meadow（MDM）,and heavy degraded meadow（HDM）in the surrounding area of Zoige Hua Lake as the object,to clarify the effects of different plant phenological periods and meadow degradation degrees on soil phosphorus forms,availability,composition and diversity of phosphorus-mobilizing bacterial communities,elucidate these effects of microbial driving mechanisms,and propose a soil phosphorus regulation strategy for degraded meadow using field positioning monitoring,indoor analysis,q PCR,high-throughput sequencing technology combined with mathematical statistics methods such as redundancy analysis（RDA）and S structural equation models（SEM）.The main results are as follows:（1）In plant growing season,the variation of total phosphorus（TP）in meadow soil was small with plant phenological period passing.Compared with the regreening stage（RGS）,the content of active phosphorus was significantly reduced by 26.3%-34.6%in NDM soil,and that of high stability inorganic phosphorus was significantly reduced by 17.3%-39.1%.Soil phosphorus content and effective supply of MDM and HDM were significantly changed compared with NDM.Among them,TP content in MDM and HDM soils was significantly reduced by 23.4%-27.3%and 28.2%-32.1%respectively,content and proportion of active phosphorus were significantly lower by 42.2%-65.2%and 29.0%-57.5%respectively,available phosphorus（AP）content was significantly reduced by 49.1%and 52.7%at the plant maturity stage（SMS）respectively,and phosphorus supply amount（PSA）was also significantly reduced by 46.1%-69.7%and 47.3-72.7%,respectively.It indicated that moderate and heavy degradation of meadow reduced soil active phosphorus pool and effective supply of soil phosphorus.（2）From RGS to vigorous growth stage（VGS）,soil phosphomonoesterase（PME）increased rapidly,while phosphodiesterase（PDE）decreased slightly.The PDE activity of degraded meadow soils was significantly lower than that of NDM by 50.41%-72.7%.However,the PME activity of moderate and severe degraded meadow soils increased rapidly during plant vigorous growth periods（VGS-SMS）due to the’low phosphorus stress’effect,and even soil PME activity in MDM was higher than in NDM.（3）During plant slow growth periods（RGS and WTS）,meadow degradation significantly reduced the abundance of soil pho D and pqq C phosphorus-mobilizing bacteria by 48.7%-75.6%and 47.9%-77.4%,respectively.During plant vigorous growth periods,the difference in the abundance of phosphorus-mobilizing bacteria between degraded meadow and non-degraded meadow decreased without significance.Both vigorous plant growth and meadow degradation increased the alpha diversity of phosphorus-mobilizing bacteria.Compared with NDM,Chao1 index,Shannon index and Pieluo index significantly increased by 32.5-177.7%,6.2%-43.1%and 3.4%-28.5%in MDM,and 13.1%-109.6%,5.5%-39.5%and 7.2%-26.9%in HDM.（4）Soil phosphorus-mobilizing bacterial community composition had no significant differences among phenological periods of meadow plants,but them changed significantly due to meadow degradation.Although soil phosphorus-mobilizing bacteria are dominated by proteobacteria and actinomycetes,meadow degradation promoted the emergence of rare pho D phosphorus-mobilizing bacteria,reduced the relative abundance of eutrophic proteobacteria in pqq C phosphorus-mobilizing bacteria and increased the relative abundance of oligotrophic actinomycetes.At the family level,the relative abundance of wet-tolerant saprophytic phosphorus-mobilizing bacteria（such as Pseudomonadaceae and Micrococcaceae）decreased,while the relative abundance of dry-tolerant oligotrophic phosphorus-mobilizing bacteria（such as Pseudonocardiaceae）increased under moderate and severe degradation of meadow.（5）The effective supply of phosphorus is mainly affected by the content of active phosphorus and highly stable inorganic phosphorus,the abundance of phosphorus-mobilizing bacteria,pqq C richness,and the relative abundance of Burkholderiaceae.Meadow degradation not only directly decreased soil phosphorus supply by reducing the amount of active phosphorus returned through litter.It also indirectly reduces the effective supply of phosphorus by reducing the ability of phosphorus activation,which caused by decreasing the abundance of organic phosphorus-mobilizing bacteria and the relative abundance of key inorganic phosphorus-mobilizing bacteria（Burkholderiaceae）.（6）Plant growth of alpine meadow was mainly limited by nitrogen,but with the increase of meadow degraded degrees,the risk of nitrogen and phosphorus co-limiting plant growth increases,especially in heavily degraded meadows.The threshold of soil available phosphorus deficiency in alpine meadow was estimated to be 7.64 mg kg^-1 by using the ratio of nitrogen to phosphorus in plant shoot.Based on this,the reference target yield method was used to estimate that the vegetation restoration of moderately and heavy degraded meadows requires 6.80 and 15.33 kg P hm^-2 during the plant regreening stage.In summary,meadow degradation mainly reduced the return of litter,and the content of soil carbon and nitrogen and dominant phosphorus forms,caused nutrient limitation of microbial growth and reproduction,and decreased the abundance of phosphorus-mobilizing microorganisms,which inhibited the activation of phosphorus by soil phosphorus-mobilizing microorganisms,and thus weakened the phosphorus supply.It is recommended to apply 6.80 and 15.33 kg P hm^-2 of water-soluble phosphate fertilizer at plant regreening stage when we use the prevalent grazing blocked by fencing combined with reseeding native excellent forage grass（such as Elymus nutans）to restore moderate and severe degraded meadow ecology.