Impact And Mechanism Of Simulated Grazing Disturbance On Carbon Cycling Of Biocrustal Soils In The Loess Plateau Hilly Region,China

Biological soil crust?biocrusts?,as an important part of the ecosystem in arid and semi-arid regions,can fix CO₂ in the atmosphere through photosynthesis,and affect soil water and heat environment and soil carbon emissions,and significantly affect soil carbon cycling.Interference is a common natural phenomenon,can significantly affect the coverage,composition and physicochemical properties of biocrusts,which in turn may affect their soil carbon cycle.In this study,taking the mixture of cyanobacteria and moss as research object in the loess hilly region,the located monitoring of field experiment was conducted.Based on the effect of five simulated grazing disturbance intensities?disturbance?on the community composition and coverage characteristics of biocrusts,and taked no disturbance as control,including 10%-15%(1^th intensity)?15%-20%(2^th intensity)?20%-25%(3^th intensity)?25%-30%(4^th intensity)and 30%-40%(5^th intensity)by in different disturbance intensities.We researched the effects of disturbance on the physical and chemical properties,soil organic carbon?SOC?content and carbon fractions,and carbon cycle in biocrusts.By means of statistical analysis method about principal component analysis,cluster analysis and first-order dynamic equation,our study revealed the effect of disturbance on the characteristics of carbon balance and the mechanism of soil carbon cycling in theory,provided scientific basis for the study of soil carbon pool management and the research of carbon cycle in practice.Main conclusions are as follows:1.Disturbance can reduce the coverage and biomass of biocrusts,and affect their physical and chemical properties,thus lead to the degradation of biocrusts,and the impact degree are related to the disturbance intensity and species recovery rate.After disturbance,the biocrust coverage showed a linear downward trend as the intensities of disturbance increased,mainly including cyanobacteria and moss coverage.Compared with no disturbance,and the cyanobacteria coverage of the 1-5^th intensities can significantly reduce 4.8%-10.3%,and moss coverage of the 1-5^th intensities can significantly reduce 3.5%-6.9%,the biocrust coverage of the 4-5^th intensities can significantly reduce17.1%-18.5%.The disturbance reduced biocrust biomass.Compared with no disturbance,the cyanobacteria biomass of the 1-5^th intensities can reduce 8%-38%,and the moss biomass of the 1-5^th intensities can reduce 55%-92%.Among them,the reduction range of the 5^thh intensities is relative greatest.After the disturbance,the order of biocrust recovery rate was2^thh disturbance faster than 4^th disturbance,and the disturbance in rainy season was faster than that in dry season,with the order as follows:cyanobacteria biocrust>moss biocrust>lichen biocrust.Disturbance impacted physicochemical properties in biocrust soil,which focused mainly on the biocrust layer and less impact on the soil layers of 0-2 cm and 2-5 cm.Disturbance reduced bulk density and increased biocrust porosity,reached a significant level when being the 5^thh intensities compared with no disturbance.The influence of disturbance on the soil temperature and moisture of biocrust is that the biocrust layer is larger than 5 cm.The variation coefficient of soil temperature and water showed that 1-5 disturbance is hgher than no disturbance,especially in the biocrust layer.When being 2-3^thh intensities,disturbance significantly decreased carbon and nitrogen ratio compared with no disturbance.2.Disturbance significantly increased soil labile carbon fractions such as soil easily oxidized carbon,soil microbial biomass carbon and soil mineralizable carbon,and reduced the soil organic carbon content of biocrusts.The disturbance mainly affected SOC and carbon fractions of the biocrust layer and had no significant effect on the soil layers of 0-2 cm and 2-5 cm.The disturbance increased the content of soil easily oxidized carbon?SEOC?.Compared with no disturbance,the SEOC change of 1-3^th intensities have greater similarity,and 4-5^th intensities has significant difference.The 5^thh intensities notably increased SMBC by 70%compared with no disturbance.The 4^thh intensities notably increased soil mineralizable carbon?SMC?by 77%compared with no disturbance.The 1-5^thh intensities decreased SOC content by 1.90 to 2.38g·kg^-1 compared with no disturbance biocrust.After disturbance,there have a positive correlation between SEOC and SMBC.The SMBC was significantly positive correlated with SMC.There is a significant negative correlation between SOC and SEOC,SOC and SMBC.3.Disturbance significantly reduced the rate of photosynthetic carbon sequestration in biocrusts,and increased the rate of soil carbon emission,and change the daily and seasonal dynamic characteristics in biocrusts photosynthetic carbon sequestration and carbon emissions.With the increase of disturbance intensities,the photosynthetic carbon sequestration rate of biocrusts decreased a linear downward trend?R²=0.832,P<0.05?.The photosynthetic carbon sequestration rate under the 5^thh intensity reduced by 0.81?mol CO₂·m^-2·s^-1 compared to that of the control?with no disturbance?.There had no significant difference between other intensities and no disturbance biocrust.The soil carbon emission rate showed a linearly increase trend?R²=0.796,P<0.05?with the increase of disturbance intensities.The soil carbon emission rates under 1-5^th intensities were significantly greater than that with no disturbance and the increase rate was 0.82 to1.43?mol CO₂·m^-2·s^-1.After disturbance,the photosynthetic carbon fixation rate of biocrust was positively correlated with cyanobacteria coverage,and the carbon emission rate was negatively correlated with cyanobacteria coverage.Photosynthetic carbon sequestration and carbon emissions of biocrusts have obvious diurnal and seasonal dynamic characteristics after disturbance.In terms of daily dynamics,biocrust photosynthetic carbon fixation showed a distinct unimodal curve,and the peak appeared at around 8:00.The disturbance reduced the photosynthetic carbon fixation rate from biocrust,and the 3^th intensities reduced by 28%compared with no disturbance,averagely.Biocrust carbon emissions showed a unimodal curve,and the peak appeared between 12:00 and 14:00.The disturbance increased the carbon emission rate from biocrust soil,and the 3^th intensities reduced by 17.4%compared with no disturbance,significantly.In terms of seasonal dynamics,the photosynthetic carbon fixation rate of biocrust after disturbance showed that,with the order as follows:before rainy season>during the rainy season>after the rainy season.Disturbance reduced the photosynthetic carbon fixation rate of biocrust,which showed significant difference mainly in before rainy season,and during the rainy season,but not after the rainy season.The carbon emission rate from biocrust soil after disturbance was the highest in the rainy season,which was 7%-52%higher than that before the rainy season,and 84%-188%higher than that after the rainy season.After the disturbance,Soil carbon flux mainly related to disturbance intensity and disturbance time,which is was higher in summer than in autumn,night than daytime,and the 5^th intensity was significantly higher than no disturbance.4.The response process of biocrust soil carbon cycle to disturbance is related to the SOC stability and SOC transformation after disturbance.Biocrust SOC stability of is related to the biocrust types.With the development and succession of biocrusts,the moss crusts at advanced stage increased SOC stability compared to light cyanobacteria biocrust.The moss crust reduces the ratio of SEOC to SOC,which is12%lower than that of the light cyanobacteria crust,and the ratio of SMC to SOC in moss crusts are 23%lower than light cyanobacteria crust.The disturbance can reduce the SOC stability by increasing the proportion of SEOC to SOC and SMC to SOC.Disturbance significantly accelerates soil labile carbon transformation,mainly in reducing the ratio of SMBC to SMBN,and increasing the number of soil fungi,and reducing the number of soil bacteria,and improving the SOC activity.Several important influencing factors,such as SEOC,SMBC,soil temperature,soil air conductivity,and soil bacteria numbers,C/N,can account for 83.5%of SOC transformation.According to the indexes affecting SOC transformation after disturbance,non-metric multidimensional scal analysis was carried out.The disturbance levels were divided into the three categories:no disturbance,1-3^th intensities and 4-5^th intensities.Compared with SOC transformation about no disturbance biocrusts,the disturbance 1-3^thh intensities have greater similarities and the 4-5^thh intensities had significant differences.5.In the loess hilly region,soil carbon emissions showed an increasing trend after the biocrusts were disturbed by simulated grazing disturbance.The direct effects of the number of fungi,SEOC,C/N,SOC,bulk density,SMBC,cyanobacteria coverage,and SMC on soil carbon emissions from biocrust were larger.The effects of disturbance on soil carbon emissions from biocrusts are mainly reflected in the direct and indirect effects of biocrust community composition and soil physical and chemical indicators.The number of soil fungi,SEOC,C/N,SOC,bulk density,SMBC,cyanobacteria coverage,and SMC had larger direct effects on carbon emission from biocrust soil.Soil moisture has a greater indirect effect on soil carbon emissions by affecting SEOC.After 1 year of disturbance in the study area,the 1-5^thh intensities increased carbon emissions by 2.6 t·ha^-1,3.1 t·ha^-1,3.2 t·ha^-1,3.9 t·ha^-1,5.6 t·ha^-1,respectively,compared with no disturbance biocrusts.Biocrusts SOC stability coefficient(K_os)and organic carbon mineralization potential?C_P?had a greater correlation with soil carbon emissions.Disturbance significantly reduces the biocrust SOC stability coefficient(K_os),1-4^thh intensities were significantly lower than that of biocrusts with no disturbance.The 5^thh intensity has the largest reduction compared to biocrusts with no disturbance.The first order kinetics equation was preferably used to descript the soil carbon mineralization potential under the effects of different disturbance intensities.The SOC mineralization potential of the 4^th intensity was significantly different from that of biocrusts with no disturbance.The soil carbon emissions changes of biocrust under the influence of disturbances can be divided into four categories,which no disturbance biocrust can be classified as one,disturbance 1-3^thh intensities can be classified as one,and disturbance 4^thh intensities and 5^thh intensities can be classified as a single category.