Driving Mechanism of Soil Organic Carbon Response to Increase Hydrothermal Conditions in Different Climatic Regimes
The Mollisol in north-eastern China is rich in organic matter,which supports high crop yields and agricultural production.Soil organic carbon(SOC)is the largest carbon pool in terrestrial ecosystems,which responds to climate change directly affects the global carbon cycle.Understanding the response of SOC to increased hydrothermal conditions is important for soil conservation in the context of global climate change.[Objective]This study aimed to analyze the response of SOC to long-term increasing hydrothermal conditions and their driving factors.[Method]Based on a soil transplantation experiment,large transects of Mollisol in a cold temperate region(Hailun,HL)were translocated to warm temperate(Fengqiu,FQ)and mid-subtropical(Yingtan,YT)regions to simulate the increasing conditions of MAT and MAP.The experiment was started in 2005,soil samples were collected in August 2013.The SOC and microbial necromass C(MNC)were measured to investigate the changes and drivers of SOC after 8 years of increasing hydrothermal conditions.[Results]The results showed that the increased hydrothermal conditions increased plant biomass,litter C/N,and potential activities of hydrolytic enzymes while decreasing the content of SOC,MNC and soil activity minerals.Moreover,the contribution of MNC to SOC also decreased as hydrothermal conditions increased.The ratio of ΔMNC and ΔSOC was calculated to characterize the change in microbial necromass C per unit decrease in SOC,which could be considered a quantitative representation of necromass loss efficiency.ΔMNC/ΔSOC increased with increasing hydrothermal activity,with values of 72.50%±9.35%in FQ and 82.67%±2.37%in YT.The correlation and Random forest analysis showed that ΔMNC/ΔSOC positively correlated with the changes in MAT,MAP,α-D-Glucosidase,β-N-Acetyl Glucosaminidase,plant biomass and Straw C/N,while negatively correlated with the changes in pH,poorly crystalline Fe and Al oxyhydroxides,organically complexed Fe and Al,and exchangeable Ca.Structural equation modeling(SEM)indicated that ΔMNC/ΔSOC increased with increasing hydrothermal conditions,while decreased with soil mineral protection,with standardized coefficients of 0.64 and-0.24,respectively.It was confirmed in the variance partitioning analysis(VPA),which showed that hydrothermal conditions together with changes in soil mineral protection explained 83.69%of the variance in ΔMNC/ΔSOC.The above results indicate that hydrothermal conditions and soil mineral protection play decisive role in regulating ΔMNC/ΔSOC.[Conclusion]In conclusion,long-term increases in hydrothermal conditions reduce the protection of MNC by soil minerals and/or stimulate the utilization of MNC by soil microorganisms.As evidenced by increased soil N limitation that allowed microbes to decompose more MNC and weakened the conservation capacity of minerals for MNC,contributing to the significant loss of SOC by reducing MNC.
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