Spatial Heterogeneity Of Nitrogen Components In Sand Land Biocrusts And Their Response To Climate Change

Nitrogen is the main limiting factor of ecosystem productivity in arid regions.Biocrusts,as important nitrogen fixers,regulate soil microbial communities and affect soil nitrogen status.Understanding the relationship between environment and biocrust distribution at regional scale,and the nitrogen effects of biocrusts mediated by microorganisms,is the basis for clarifying the spatial heterogeneity and mechanism of biocrust nitrogen components and assessing the nitrogen contribution of biocrusts under climate change.Most existing studies focus on plot scale,and the understanding of the relationship among environmental factors,biocrusts,microorganisms,and nitrogen components at regional scale is very limited.This thesis investigates the moss biocrusts and cyanobacterial biocrusts and their underlying 0-5cm soil layer in the Mu Us sandland using overall sampling survey,typical sampling along precipitation gradients,and 16s microbiome techniques.It systematically examines the relationship between the distribution and development of biocrusts and environmental factors at the regional scale of the Mu Us sandland.It also explores the features of nitrogen fractions and nitrogen cycle related enzyme activities of biocrusts and their underlying layer,elucidates the spatial heterogeneity of bacterial community structure and function,reveals the spatial difference mechanism of nitrogen fractions of biocrusts and their underlying layer,and assesses and predicts the biocrust cover and nitrogen storage under the current（2020）and future（2100）climate change scenarios.The main results are as follows:（1）At regional scale,precipitation,soil moisture,soil texture,and temperature are the key factors affecting biocrust distribution.The environmental factors influencing biocrusts differ significantly between under-canopy and inter-canopy areas.The average cover of biocrusts is predicted to decrease slightly under future（2100）climate change scenarios.Higher temperature（BIO10>22℃）,lower elevation（<1275m）,and lower sand content(<555 g kg^-1)are favorable for moss biocrust distribution.This is due to the fact that higher temperatures increase the physiological activity of the moss biocrusts,while finer soil texture helps to maintain the moisture content of the soil.Lower snow water equivalent(<2.75 mm month^-1)and higher potential evapotranspiration(>1040 mm year^-1)are favorable for cyanobacterial biocrust distribution.This is due to the ability of the lower moisture to inhibit competition between other plants and cyanobacterial biocrusts.The dominant factors for moss biocrust cover in inter-canopy areas are vapor pressure,while in under-canopy areas are atmospheric moisture deficit;the dominant factors for cyanobacterial biocrust cover in inter-canopy areas are snow water equivalent,while in under-canopy areas are Palmer Drought Severity Index.Currently（2020）,the total cover of biocrusts is 19.34%,with moss biocrusts and cyanobacterial biocrusts accounting for 10.44%and 8.90%,respectively.Under future（2100）climate change scenarios,the total cover of biocrusts will decrease slightly（-0.69%）,but the changes of moss biocrusts and cyanobacterial biocrusts will reach+2.27%and-2.95%,respectively.The distribution of cyanobacterial biocrusts creates conditions for moss biocrust colonization,and their decline in cover may indicate a gradual succession from cyanobacterial biocrusts to moss biocrusts.The increase in moss biocrust cover will affect the surface water and nutrient status,promoting the expansion of vegetation communities while limiting their biomass increase,which will ultimately affect the productivity of the entire ecosystem.（2）The nitrogen components and nitrogen cycling related enzyme activities of biocrusts,and their underlying layers showed obvious spatial differences from west to east;the total nitrogen content of the western biocrusts was higher than the eastern one,while the ammonium nitrogen content was lower than the eastern one.Moss biocrusts in the western region(precipitation 262 mm-324 mm a^-1)had significantly（P<0.05）higher total nitrogen(1.13±0.26g kg^-1),nitrate nitrogen(3.06±0.43 mg kg^-1),and microbial biomass nitrogen(12.45±0.58 mg kg^-1)contents and urease activity(3.17±0.54 mg g^-1 24 h^-1)than moss biocrusts and cyanobacterial biocrusts in the central(precipitation 393 mm-424 mm a^-1)and eastern regions(precipitation 424 mm-476 mm a^-1);cyanobacterial biocrusts in the western region had significantly lower ammonium nitrogen content(0.40±0.11 mg kg^-1)and nitrate reductase activity(1.48±0.27 mg g^-1 24 h^-1)and nitrite reductase activity(1.04±0.20 mg g^-1 24h^-1)than moss biocrusts and cyanobacterial biocrusts in the eastern region.Across the whole region,nitrite reductase activity of biocrusts was significantly positively correlated with ammonium nitrogen,while urease and protease activities were significantly positively correlated with total nitrogen and three nitrogen components.The lower nitrate nitrogen and higher ammonium nitrogen in the eastern part may be related to stronger leaching and microbial activity due to precipitation.（3）The bacterial species composition and diversity,functional gene abundance,co-occurrence network characteristics of biocrusts and their underlying layers showed obvious spatial heterogeneity at regional scale,and these structural and functional differences were the main causes of spatial variation of nitrogen components.Biocrusts in the western region had high bacterial Alpha diversity,high relative abundance of Cyanobacteria,high abundance of carbon and nitrogen fixation genes,high network heterogeneity,low relative abundance of Bacteroidota,Acidobacteriota,Proteobacteria,low abundance of denitrification and nitrate reduction genes,which resulted in high total nitrogen,microbial biomass nitrogen,nitrate nitrogen contents in the western region.Biocrusts in the eastern region had high bacterial Beta diversity distance,high relative abundance of heterotrophic microorganisms such as Chloroflexi and Crenarchaeota,high abundance of nitrification,denitrification,nitrate reduction and nitrite reduction genes,low relative abundance of Cyanobacteria,low abundance of nitrogen fixation genes,which resulted in low total nitrogen content and high ammonium nitrogen content in biocrusts and their underlying layers in the eastern region.Cyanobacteria,as important nitrogen-fixing microorganisms,increase the soil nitrogen content,while other microorganisms such as actinobacteria transform nitrogen into other forms.The lower precipitation in the western part implies less nitrate-N loss and also inhibits microbial nitrogen transformation,resulting in higher nitrogen fraction content than in the eastern part.Future precipitation increase will change the composition and function of biocrust microorganisms,and thus affect the soil nitrogen cycle.（4）Biocrusts in Mu Us sandland have huge nitrogen storage,and their nitrogen density is comparable to that of higher plants such as Artemisia ordosica;the total nitrogen storage of biocrusts will change significantly under future（2100）climate change scenarios and will increase under low CO2 emission scenarios and decrease under high emission scenarios.The total nitrogen density of biocrusts and their underlying 0-5cm soil layer was as high as 64.61 g m^-2(higher plants such as Artemisia ordosica were 50-100 g m^-2).The total nitrogen storage of biocrusts in the whole region was 532.30 thousand tons,and the microbial biomass nitrogen,nitrate nitrogen,and ammonium nitrogen storage were6589.10 tons,2656.90 tons,and 1844.10 tons,respectively,among which the nitrogen storage of moss biocrusts（71.93%）was significantly higher than that of cyanobacterial biocrusts（28.07%）.Under future（2100）climate change scenarios,the total nitrogen storage of biocrusts will increase by 156.6 thousand tons and 58.7 thousand tons under ssp126 and ssp245scenarios with low CO₂ emission,respectively,and decrease by 1.3 thousand tons and 9.5thousand tons under ssp370 and ssp585 scenarios with high emission,respectively.Future climate change will affect the nitrogen fixation and transformation of biocrusts.Increased precipitation may enhance nitrogen loss,alter soil nitrogen availability and thus affect the structure and function of vegetation communities.The above results reveal the dominant role of temperature and moisture in the spatial distribution of biocrusts,and the influence mechanism of bacterial community characteristics on the spatial heterogeneity of nitrogen fractions of biocrusts and their underlying layer.Future climate change will cause significant changes in the cover of moss biocrusts and cyanobacterial biocrusts,but little change in the total nitrogen content of biocrusts.Due to the changes in precipitation and temperature,the microbial community structure and function of biocrusts and their underlying layer will change,and the response of biocrust distribution and nitrogen fractions to climate change will be complex.Therefore,it is necessary to further assess the impact of biocrust nitrogen effect changes on regional and global ecosystems,in order to fully utilize the role of biocrusts in arid land ecosystem management.