Soil Carbon Responses To Simulated Precipitation Increase During Plant Growing Season In A Desert Ecosystem

Climate change is a major global challenge facing humanity today.Soil is the largest carbon（C）pool amongst terrestrial ecosystems,it is vital to investigate the formation and driving mechanisms of soil carbon cycles,sources and sinks in response to changing precipitation patterns as the climate gradually warms and global precipitation patterns change.Soils are constantly subjected to anthropogenic and environmental perturbations affecting their C stocks in different ecosystems.The preliminary experiment carried out at an alpine meadow site at Ma qu county,a typical steppe site in Huan Xian county,and a desert site in Ping shan hu county,supported by a literature review,identified that precipitation change is the most critical environmental influencer on soil C stock and aggregate C pools.Further experiments were therefore carried out to investigate the precipitation increase impact on soil C stock,soil aggregate C pools and the factors driving C changes in a desert site at Ping shan hu county.The desert ecosystem was chosen as it was the most sensitive to precipitation and climate changes.The average annual precipitation at the experimental site was 113 mm and the annual average air temperature was 8.3℃.The soil type was Brown desert soil and the Salsola passerine and Kalidium foliatum were the dominant shrub species.Through an artificial precipitation simulation system based on the average monthly precipitation of Zhangye City from May to September during 1981-2010,four precipitation treatments were applied.The four treatments included Control（natural precipitation;0%increase）,Low（30%increase of average monthly precipitation）,Middle（60%increase of average monthly precipitation）,and High（100%increase of average monthly precipitation）precipitation.The precipitation simulation was carried out during the plant growing season（May to September）in 2019,2020 and 2021.Monthly soil C measurements up to 40 cm（0～10,10～20,20～40 cm）soil depth were carried out every year.Soil C stock measurements up to 100 cm（0～10,10～20,20～40,40～60,60～80,80～100 cm）soil depth were measured in October every year.The responses of dominant shrubs,soil properties and microorganisms,and their relationships to soil C and to precipitation increase in desert ecosystem were systematically studied.The main observations and conclusions are as follows:Deserts are more alpine meadows and have smaller organic carbon stocks in the soil,but the carbon exists in a more stable form due to chemical protection.There were different microbial phyla associated with different C protection pools.Bacterial phyla;Acidobacteria,β-proteobacteria,γ-proteobacteria,δ-proteobacteria are significantly positively correlated with unprotected carbon and physically protected carbon（P<0.05）,Chloroflexi,Actinobacteria,Bacteroidetes were significantly positively correlated with chemically protected carbon（P<0.05）.Basidiomycota and Ascomycota were significantly positively correlated with unprotected carbon and physically protected carbon（P<0.05）;Zygomycota were significantly positively correlated with biochemically protected carbon（P<0.05）.Among microbial functional groups,desert and typical grassland soils were dominated by chemo-heterotrophic,aerobic chemo-heterotrophic functional bacteria and endophytic fungi;while alpine meadow soils were dominated by nitrogen-fixing,aerobic nitrite-oxidizing,nitrifying functional bacteria,and saprophytic fungi and symbiotic fungi.Structural equation modelling showed that,compared with vegetation biomass or microbial diversity,precipitation was the decisive environmental factor for the changes to grassland carbon protection mechanism and the increase of organic carbon stock.Precipitation simulation impact on C accumulation in plant biomass:Precipitation simulation significantly impacted the C accumulation in plant biomass in dominant shrubs at the site;the Middle（+60%）treatment resulted in the largest biomass and carbon accumulation of Kalidium foliatum,in contrast,Low（+30%）treatment resulted the largest biomass and carbon accumulation of Caroxylon passerinum.The decomposition rate of litter in both species increased with the increased levels of precipitation.Precipitation simulation impact on C pools up to 40 cm soil depth:Three years of simulated precipitation during the plant growing season at the desert site significantly increased soil organic C stock up to 40 cm,and showed a trend of C accumulation in soil year by year.Organic C storage increased significantly with increasing levels of precipitation.The C response in different aggregates to precipitation treatments varied.Simulated precipitation significantly increased organic and inorganic C stock up to 40cm in macroaggregates and in silt+clay.In contrast,simulated precipitation did not significantly affect the total,organic or inorganic C stock up to 40 cm in microaggregates.The experiment using ¹³C natural abundance in soil aggregates revealed that at high precipitation（+100%）,carbon fixation ability was highest in micro aggregates,while under low precipitation（+30%）,silt+clay had the strongest carbon fixation ability.Microbial biomass C increased significantly by the simulated precipitation,but the response varied in different months during May to September.Simulated precipitation significantly increased soil CO₂flux,where soil showed net CO₂absorption in September,and net CO₂release in May-August.Simulated precipitation had significant effects on soil bacterial community structure.The bacterial richness and diversity decreased significantly with the increased level of precipitation.The relative abundance of bacterial phyla Proteobacteria,Planctomycetes,and Firmicutes increased significantly whereas Actinobacteria significantly decreased.The bacterial community complexity and stability increased with the increased level of precipitation.Increasing levels of precipitation significantly increased the key microbial functional groups involved in carbon cycle in desert soil,such as methylotrophic,aromatic compound degrading,hydrocarbon degrading,excreta saprophytic bacteria,plant saprophytic bacteria.Precipitation simulation impact on C pools up to 1 m soil depth:three years of simulated precipitation during the plant growing season at the desert site significantly increased soil organic C stock up to 1 m.The average soil C stocks were 30.84,35.16,41.53,38.04 g·m^-2in control,low,middle and high precipitation treatments.The soil C stocks in precipitation treatments on average increased by 24%compared to the control.Simulated precipitation significantly increased organic and inorganic C stocks up to 1 m soil depth in macroaggregates and in silt+clay.In contrast,simulated precipitation did not significantly affect the total,organic or inorganic C stock up to 1m soil depth in microaggregates.Dissolved organic C contents in precipitation simulated treatments were significantly higher than in the control treatment.The highest dissolved organic C was found at 10-20 cm depths in the control and low precipitation treatments,whereas at 40-60 cm and 60-80 cm soil depths,the highest dissolved organic C was found in mid and high precipitation treatments.Soil inorganic C in precipitation simulated treatments significantly decreased in 60-100 cm soil depths compared to 0-40 cm soil depths.Structural equation modelling:The direct and indirect pathways of precipitation simulation effect on different C pools were examined by structural equation modelling.The SEM identified that precipitation simulation can have direct effects on microbial biomass C,aggregate organic C,aggregate inorganic C,and soil C stock.In addition,precipitation simulation indirectly impacted soil C pools through soil properties,plant litter decomposition,aggregate structure and soil organic and inorganic C contents.In summary,this Ph D study establishes that the precipitation is the most critical environmental factor driving the structural difference of soil carbon pools in the desert ecosystem.Different aggregate structures have different forms of carbon protection,resulting in different soil carbon pool activity levels,which in turn lead to different carbon cycle mechanisms.Increased precipitation directly changes the soil structure and carbon content distribution in different structural components in desert ecosystems.Increased precipitation promotes vegetation growth,enhances soil aggregation,increases organic carbon content,and improves soil moisture effectiveness.The soil structure changes to macro aggregates,which is conducive to the survival of carbon metabolism microorganisms,activates the carbon turnover rate in desert ecosystems,and then stimulates the potential of material cycle and energy flow in desert ecosystems.