The Influence Mechanism Of Biological Crusts On Carbon Fixation Function Of Patchy Degraded Alpine Meadow In The Source Zone Of The Yellow River

Biological soil crusts（BSCs）,known as ecological engineers,play an important role in carbon cycle process in dryland ecosystems.Although numerous individual studies had been conducted,the contributions of biocrust to the carbon function in degraded grassland ecosystem remain unclear.In order to know the effect of BSCs on carbon fixation function of the degraded alpine meadow,we investigated the vegetation characteristics,soil chemical properties,cbb L bacterial community diversity,soil carbon-hydrolase activity,photosynthesis and respiration of vegetation and BSCs community,as well as the variation characteristics of soil labile organic carbon（LOC）components of different patches at different altitudes,so as to identify the underlying mechanisms with constructing the structural equation model（SEM）.The results showed that:1.The coverages of grass,sedge and BSCs coverage and thickness at different altitudes increased significantly with the restoration succession of degraded patches（P< 0.05）.The soil nutrients in 0-2 cm soil layer such as total nitrogen（TN）,total phosphorus（TP）and soil organic matter（SOM）increased during the restoration succession at different altitudes.But nutrient content in BSCs soil was basically higher than that in degraded patches.2.The dominant phylum of carbon-fixing bacteria（cbb L bacteria）in various patches and alpine meadow soil was Proteobacteria,with relative abundance > 95% in different patches.The dominant genera of cbb L bacteria at different altitudes were Sulfurifustis,Cupriavidus,Hydrogenophaga and Alkalispirillum.The coverage,height and aboveground biomass of forb at 3570 m were positively correlated with Proteobacteria.Proteobacteria at the altitude of 4013 m was positively correlated with BSCs thickness and aboveground biomass of Gramineae.Proteobacteria at the altitude of 4224 m were positively correlated with BSCs and Cyperaceae coverage,and negatively correlated with the coverage of forbs grass.BSCs were correlated with cbb L diversity index at different altitudes.3.The soil carbon-hydrolase activity of BSCs was higher than that of degraded patches,but lower than that of alpine meadow.The deepening of soil layer reduced the soil carbon-hydrolase activity at different altitudes.In each patch at different altitudes,the soil hydrolase activity at the early regreening stage was basically significantly higher than that at the peak growth stage（P < 0.05）.BSCs plaque had the highest hydrolytic enzyme activity at the early stage of regreening among different patches.Nutrient phosphorus was the main factor affecting soil α-1,4-glucosidase（AG）and β-1,4-xylanase（BXYL）activities at the altitudes of 3570 m and 4224 m.4.Communities and soil CO₂ fluxes of BSCs were significantly lower than that of alpine meadows and degraded patches（P < 0.05）.The community CO₂ fluxes of lichens and moss BSCs were 8.18 μmol/m2/s and 7.90 μmo/m2/s,respectively,which were significantly higher than those of algal crusts（4.01 μmol/m2/s,P < 0.05）.However,the soil CO₂ flux of algae crust was the largest,which was about 2 times of lichen and moss.The BSCs coverage had a strong correlation with the vegetation community and soil CO₂ flux in the middle and low altitude areas.But in the high-altitude areas,the vegetation community and soil CO₂ flux were extremely significantly correlated with the total coverage,Gramineae and Cyperaceae coverage（P < 0.01）.5.Minimal fluorescence（F0）,maximal fluorescence（Fm）and optimal/maximal photochemical efficiency of PSⅡin the dark（Fv /Fm）of alpine meadow were the largest,followed by BSCs.Fv/Fm gradually increased during the restoration succession of degraded patches.The chlorophyll fluorescence parameters of moss crust were the largest.With the increase of altitude,F0 and Fm of moss crust showed a significant trend of increasing.The anthocyanin reflectance index 1（ARI1）of moss crusts was 1.12,significantly higher than that of algae crusts and lichen crusts（P < 0.05）.Gramineae height and coverage,total nitrogen,available nitrogen content and p H were important factors affecting chlorophyll fluorescence of BSCs.Available nitrogen,total phosphorus and total potassium had strong correlation with light use efficiency.6.Compared with active patches,biocrust patches increased dissolved organic carbon（DOC）by 292.66 %,easily oxidized organic carbon（EOC）by 72.98 % and microbial biomass carbon（MBC）by 35.65 % in 0 2 cm soil layer at the altitude of3570 m.The increase in altitude reduced the MBC content of active patches,but increased MBC/MBN.DOC and EOC contents in different soil layers of different types of patches and alpine meadows in the early regreening period was basically higher than that in the growth peak period.SEM results showed that TN had a strong positive direct relationship with DOC,and the influence coefficient was 0.677 at the altitude of 3570 m.There was a direct negative relationship between SOM and DOC（influence coefficient =-0.278）.And LOC was also affected by SOM throwing regulating BG and CBH activities.The p H affected LOC indirectly by affecting BG and CBH at the altitude of 4224 m.The contribution of biological crusts to soil nutrients and LOC should be confirmed.Increased altitude reduces the number and intensity of direct pathways among environmental factors and labile organic carbon,but increases the complexity of the network relationship among soil properties,Carbon-hydrolase activity and labile organic carbon.In summary,BSCs mainly promote the recovery of soil carbon sequestration function in degraded alpine meadow by affecting soil chemical properties,especially by increasing soil AP content.Therefore,the inoculation of BSCs in the restoration of carbon sequestration function of degraded alpine meadow may be an efficient recovery measure,and the application of phosphorus can shorten the recovery time and achieve the purpose of rapid recovery of carbon sequestration function of degraded alpine meadow.