Responses Of Soil Organic Carbon,microbial Communities,and Enzyme Activities To Different Restoration Measures In Desertified Alpine Meadow

Grasslands are a key component of global terrestrial ecosystems.Due to human disturbances and climate changes,grassland desertification is widespread and accelerating in the past decades,resulting in significant loss of biodiversity and soil carbon stocks.Ecological restoration is an effective measure to reverse land degradation,conserve biodiversity and mitigate global warming,and is one of the effective ways to achieve“carbon neutrality”.Ecological restoration measures can affect soil carbon storage and the key processes of carbon cycle,and also change the microbial community involved in regulating the transformation of soil organic carbon（SOC）.Studying the response and mechanisms of SOC,microbial communities,and enzyme activities to different vegetation restoration in desertificated grasslands,and exploring the relationships between SOC,microbial communities and enzyme acitivties,are important for understanding the soil C cycle in vulnerable ecological environments.It can also provide an important scientific reference for improving the C sink effect of vegetation areas.In this study,three restoration measures（i.e.,planting mixed grasses（MG）,planting shrub with Salix cupularis alone（SA）,and planting shrub with Salix cupularis plus planting mixed grasses（SG））that have been successfully established after 14 years on th Northwest Sichuan were selected as the research objects,and the desertified alpine meadow was use as the control（CK）.By field sampling,artificial incubating and High-throughput sequencing techniques,we studied the influences of different restoration measures on vegetation and soil physicochemical properties,clarified the response of the stock,fractions（labile,chemical,and physical fractions）and mineralization of SOC to different restoration measures,demonstrated the response and mechanism of microbial communities and extracellular enzyme activities to different restoration measures,and synthesized soil-microbe-enzyme factors and used random forest,multiple regression,and structural equation modeling to reveal the relationships between SOC and microbial communities and soil enzyme activities.Our findings enriched the theoretical basis for soil carbon cycling in desertified alpine meadow,and provided a certain scientific basis for the management of restoration on the Northwest Sichuan.The following is a summary of the primary results:（1）Restoration measures significantly changed the numner species and coverage of vegetation,plant biomass,and vegetation carbon stock,and there were large differences among different restoration measures.The order of vegetation carbon stock in each treatment was:MG>SG>SA>CK.Treatments MG and SA decreased soil p H.Compared with the CK,three measures increased the macro-aggregate（Mac）content.Ecological restoration had significant effect on soil nitrogen and phosphorus.Restoration measure had stronger positive effect on nitrogen than phosphorus.The ratio of carbon to phosphorus（C:P）and the ratio of nitrogen to phosphorus（N:P）in all three restoration measures were higher than in the CK.（2）Restoration measures significantly increased SOC content and stock.The order of SOC stock in 0-60 cm depth was:MG(5.27 kg m^-2)>SG(4.82 kg m^-2)>SA(2.51 kg m^-2)>CK(1.53 kg m^-2).Ecological restoration significantly affected soil C fractions（labile,chemical,and physical fractions）,and this effect was strongly dependent on restoration measure.For labile SOC fractions,MG and SG can improve the content of each fraction of labile SOC better relative to the SA.For chemical SOC fractions,treatments SA and SG increased the decomposition resistance of SOC in 0-20 and 40-60 soil depths.For physical SOC fractions,three restoration measures increased the contribution of Mac to SOC.Response of the cumulative SOC mineralization（Cmin）to restoration measures was significantly varied with restoration measure and soil depth.The SOC mineralization efficiency（Cmin-SOC）under both MG and SG were lower than in the CK,indicating that MG and SG enhanced the ability of soil C sequestration.SOC stock was positively correlated with plant C stock and number species.Random forest modeling indicated that SOC,RCP,C:P,and TN were important indicators that predicting SOC stock,and labile C fractions（HWEOC,MBC,and DOC）were critical indicators that predicting the cumulative SOC mineralization.（3）Compare with the CK,all three restoration measures significantly increased bacterial Chao1 index in 0-20 cm depth,but had no significant effect on bacterial Shannon index.The highest value of bacterial Shannon and bacterial Chao1 index in 0-20 cm soil depth were observed in the SG.Restoration measures had no significant effect on fungal alpha diversity.The alpha diversity of fungi in the SA was significantly lower than in the CK in 20-40 and 40-60 cm soil depths.The result of PCo A analysis indicated that restoration measures had significant effect on bacterial and fungal community composition.Compare with the CK,measures MG and SG decreased soil bacterial beta-diversity.Compared with the CK,all three restoration measures increased the relative abundance of Proteobacteria,Acidobacteria,and Bacteroidetes,but decreased the relative abundance of Actinobacteria,Thaumarchaeota,and Chloroflexi.For fungal phyla,MG and SG increased the relative abundance of Ascomycota,but decreased the relative abundance of Basidiomycota,relative to the CK.Additionally,three restoration measures enhanced the abundance of C-fixation and C-degradation functional genes.The alpha diversity of bacteria and fungi were positively correlated with plant number species.Soil p H and TN were critical factors that affecting bacterial community structure,SOC,C:P,and N:P were important factos that affecting fungal community structure.（4）Restoration measures had significant effect on the activities ofβ-1,4-glucosidase（BG）,β-D-cellobiosidase（CBH）,β-1,4-N-acetylglucosaminidase（NAG）and acid phosphatase activity（ACP）,but had no significant effect on the activies of Peroxidase（POD）,Polyphenol oxidase（PPO）,and Leucine aminopeptidase（LAP）.The activies of BG,CBH,NAG and ACP in three restoration measures were higher than in the CK,and the highest value of those enzymes was observed in the SG.Three restoration measures significantly decreased the ratio of ligninase to cellulase,indicating that restoration measures were beneficial to the accumulation of soil lignin,namely recalcitrant carbon.All restoration measures decreased Micorbial C use efficiency（CUE）and microbial C limitation,but enhanced microbial P limitation.Microbial CUE was negatively correlated with microbial P limitation,indicating that trade-off between the two.Microbial P limitation was positively associated with BG,NAG,and ACP,but all these enzyme activities were negatively correlated with microbail C use efficiency.These fingdings suggested that to balance microbial P limitation with restoration,microorganisms transfer more energy from the microbial C use efficiency to extracellular enzyme production.The result of random forest anlaysis indicated that soil physiochemical properties and microbial parameters can explained 64.8%of the variation in extracellular enzyme activities.SOC,TN,N:P,C:P,and microbial community composition were important factor that affecting extracellular enzyme activities.（5）SOC were tightly linked to soil microbial communities and soil enzymes activities.Random forest modeling indicated that the ratio of ligninase to cellulase was a critical factor that affecting SOC stock and SOC mineralization.Structural equation modeling（SEM）sh that the composition of bacterial and fungal community can regulate SOC stock via impacting the ratio of ligninase to cellulase.The diversity of bacteria and fungi were significantly and positively correlated with SOC stock and the Chao1 index of bacteria and fungi were critical factors that predicting SOC stock.Labile SOC fractions were critical factors that affecting the community composition of soil bacteria and fungi.Both SOC stock and SOC mineralization were tightly associated with the abundance of C-cycling functional genes.SEM indicated that bacterial diversity and community composition can indirectly control SOC mineralization via direct affecting soil enzyme activities and microbail biomass.In summary,different restoration measures can alter enzyme activities and microbial communities by changing vegetation and soil physichemical properties,thereby affecting the stock,factions and mineralization of SOC.MG and SG can better improve vegetation,soil feritility,enzyme activities and microbial diversity,and increase SOC stock and C sequestration capacity in northwestern Sichuan.Our study analyzed the effects of three restoration measures on SOC from four levels combining vegetation,soil properties,enzyme activity and microorganisms.Our findings provided new insights into the interaction between soil carbon cycling and microbial properties.