Response Of Soil Microorganisms And Organic Carbon In Biocrust To Short-term Warming And Nitrogen Addition In Sandy Land

Biological soil crust（biocrust）is an important component of the ecosystem in arid and semi-arid areas.Studying the effects of climate change on the microbial characteristics and the stability of soil organic carbon（SOC）in biocrust can help understand the soil carbon cycle in dryland ecosystems.It can also provide an important reference for enhancing the carbon sequestration potential of vegetation restoration and the stability of the carbon pool under the context of climate change.In this paper,we conducted a field experiment to simulate climate change scenarios on cyanobacterial and moss biocrust in Mu Us Sandland.We examined the impacts of warming and nitrogen addition(0 g m^-2 a^-1,1 g m^-2 a^-1,2 g m^-2 a^-1)on the biocrust community characteristics,soil physicochemical properties,microbial composition,SOC fractions and SOC emissions in wet and dry years.We used statistical analysis methods,such as redundancy analysis,structural equation model and first-order dynamic equation,to analyze the data.We elucidated the direction and magnitude of the effects of climate change on the microbial characteristics and the stability of SOC in biocrust.The main conclusions are as follows:（1）When the soil temperature increased by 1.13°C,the cyanobacterial biocrust coverage increased by 9.53%,and the soil carbon and nitrogen content decreased slightly;when the soil temperature increased by 1.61°C,the moss biocrust coverage decreased by 5.73%,and the soil carbon,nitrogen and phosphorus content increased to different degrees.When nitrogen addition concentration was 1 g m^-2 a^-1,cyanobacterial biocrust coverage significantly increased by 9.10%,while moss biocrust coverage significantly decreased by2.88%;when nitrogen addition concentration was 2 g m^-2 a^-1,cyanobacterial and moss biocrust coverage both significantly decreased.Moreover,warming reduced soil moisture content in biocrust,which led to a decrease in soil bacterial abundance andαdiversity.During the two-year period,the cyanobacterial biocrust coverage increased significantly by 9.53%,while the moss biocrust coverage decreased by 5.73%when the soil temperature increased by 1-2°C.Under low nitrogen concentration treatment,the cyanobacterial biocrust coverage increased by 9.10%and the moss biocrust coverage decreased by 2.88%.Under high nitrogen concentration treatment,the cyanobacterial and moss biocrust coverages decreased by 1.40%and 4.25%,respectively.During the two-year period,the soil moisture content in cyanobacterial and moss biocrust decreased by 0.52%and 1.63%,respectively,under warming treatment.Warming significantly decreased the soil carbon(0.69 g kg^-1)and nitrogen(0.06 g kg^-1)contents in cyanobacterial biocrust,while increased the soil carbon(2.89 g kg^-1),nitrogen(0.30 g kg^-1)and phosphorus(0.05 g kg^-1)contents in moss biocrust.Underhigh nitrogen concentration treatment,the soil carbon,nitrogen and phosphorus content in cyanobacterial biocrust decreased by 0.54,0.35 and 0.02 g kg^-1,significantly,while the soil carbon and nitrogen content in moss biocrust increased by 1.12 and 0.11 g kg^-1.Under warming treatment,the OTUs,Chao1 index and Shannon index decreased by 10.97,20.45 and 3.51 significantly during the two-year period,respectively.Similarly,high nitrogen concentration significantly decreased the soil bacterialαdiversity in moss biocrust.The OTUs,Chao1 index and Shannon index decreased by279.78,664.78 and 0.21,respectively.（2）When the soil temperature increased by 1.13°C,the active SOC fractions,such as particulate organic carbon（POC）(0.68 g kg^-1)and microbial necromass carbon（MNC）(0.82 g kg^-1),in cyanobacterial biocrust significantly decreased.When the soil temperature increased by 1.61°C,the active SOC fractions in moss biocrust significantly increased.Warming decreased the content of active SOC fractions in cyanobacterial biocrust.For example,the content of POC and MNC increased significantly by 0.68 and 0.82 g kg^-1,respectively.It also increased the content of active SOC fractions,such as POC(1.84 g kg^-1),light fraction organic carbon（LFOC）(1.95 g kg^-1),dissolved organic carbon（DOC）(29.42 g kg^-1)and MNC(0.54 g kg^-1),and the allocation proportions of LFOC in moss biocrust,which increased significantly by 0.69%during the two-year period.The SOC mineralization rate of moss biocrust increased significantly by 1.88mg CO₂ kg^-1 d^-1 under warming treatment.Low nitrogen concentration significantly increased the content of mineral-associated organic C（MAOC）in biocrust.It also increased the MNC content and microbial respiration rate in moss biocrust.The allocation proportions of LFOC in cyanobacterial biocrust decreased significantly by0.33%under high nitrogen addition treatment.（3）In cyanobacterial biocrust,warming（1.13°C）promoted fungal necromass carbon（FNC）,while inhibited bacterial necromass carbon（BNC）to form MAOC.Under warming treatment（1.61°C）,microbial biomass carbon（MBC）promoted the accumulation of BNC and microbial biomass nitrogen（MBN）promoted the accumulation of FNC in moss biocrust.In cyanobacterial biocrust,warming promoted FNC,while inhibited BNC to form MAOC.Low nitrogen concentration significantly inhibited the microbial phospholipid fatty acid（PLFA）content to form BNC.In moss biocrust,warming promoted microbial PLFA to form FNC,while inhibited MNC to form POC.Furthermore,under warming treatment,MBC promoted the accumulation of BNC and MBN promoted the accumulation of FNC.Low nitrogen concentration inhibited FNC,while promoted BNC to form POC.At the same time,it also inhibited the effect of MBN on the accumulation of FNC.High nitrogen concentration facilitated microbial PLFA to produce BNC and the effect of MBC on the accumulation of FNC.（4）The effects of annual precipitation on the SOC mineralization of biocrust were greater than those of warming and nitrogen addition.Nitrogen addition treatment indirectly affected the SOC mineralization of two types of biocrust through soluble carbon and nitrogen content and soil p H,respectively.There was a significant positive correlation between annual precipitation（β=1.103）and SOC mineralization in cyanobacterial biocrust,and there were significant positive correlations between annual precipitation（β=0.611）,warming treatment（β=0.301）and SOC mineralization in moss biocrust.Nitrogen treatment indirectly affected SOC mineralization by changing soluble carbon and nitrogen content（β=0.450）and soil p H（β=-0.189）.In addition,annual precipitation had the greatest effect on the SOC mineralization in biocrust,and its total effect values were 0.912 and 0.724,respectively.In general,most of the environmental factors included in the model had positive effects on the SOC mineralization in cyanobacterial biocrust and negative effects on the SOC mineralization in moss biocrust.