The Response And Mechanism Of Soil Enzymes And Soil Organic Carbon Decomposition To Warming In Typical Farmland Of China

The IPCC projects that the global temperature will likely rise by 1.8?2.2? by the end of 21st century,which will lead to natural disaster happening frequently,such as sea level rise,extreme weather events,ecosystem services and biodiversity reduction etc,pose a threat to the global food safety and human health.Soil organic carbon?SOC?is the largest carbon stock in terrestrial ecosystems and plays an important role in the global carbon balance.Soil enzymes are the speed-limiting step of SOC decomposition.Therefore,it is necessary to clarify the response characteristic of SOC and soil enzymes to warming and the coupling relationship,wich are of great significance to carbon sequesteration and mitigation.Farmland ecosystem has strong capacity to reduce greenhouse gas emission.Due to the soil spatial heterogeneity,the feedback mechanism of the SOC fixation,accumulation,and turnover of farmland in different climate zones in China to warming is still unclear.Based on this,the response of SOC decomposition of farmland ecosystem in different climatic zones to warming and environmental impact factors were studied from the perspectives of soil enzyme kinetics,thermodynamics,SOC physical and chemical composition,and microbial community composition.The main conclusions are as follows:?1?Soil enzyme activity was very sensitive to warming.The temperature sensitivity(Q₁₀)values of enzymes related to carbon,nitrogen and phosphorus cycle in soil in each climate zone were greater than 1,ranging from 1.04 to 2.34.The order of extent of soil enzymes response to warming was cellobiohydrolase?CBH?,N-acetyl-?-glucosaminidase?NAG?,?-glucosidase?BG?,acid phosphomonoesterase?AcP?,and?-xylanase?BX?.The Q₁₀ declined with temperature gradually increased.The Q₁₀ of BG,CBH and C_enz were significantly negatively correlated with MAT.The enzyme stoichiometry ratios of N_enz:P_enz and C_enz:P_enzincreased with the increasing temperature.?2?The V_max for five enzymes involved in C,N,and P cycling in different climate zones increased with increasing temperature,and V_max-Q₁₀ decreased with increasing temperature.Among the five tested soil enzymes,CBH had the largest V_max-Q₁₀ values.The V_max and V_max-Q₁₀ of BG were significantly negatively correlated with MAT,and decreased linearly with the increase of MAT;K_m and K_m-Q₁₀ did not show a consistent pattern and fluctuated in the measured temperature?0?40??.Totally,K_m was smallest at 0?.The response of V_max/K_m-Q₁₀ to warming confirmed complete adaptation as predicted by physiological theory for the whole temperature range and this encouraged linking the Arrhenius and physiology theories.The V_max/K_m-Q₁₀ showed three main patterns:1)the V_max/K_m-Q₁₀ increased first and decreased afterwards;2)the V_max/K_m-Q₁₀ increased first and decreased thereafter,and then increased again;and 3)the values of V_max/K_m-Q₁₀ fell first and rose later.Structural equation modeling?SEM?showed that climate factors?MAT and MAP?indirectly affect V_max/K_m-Q₁₀ by affecting the physical and chemical properties of soil.The K_m-Q₁₀ of them was the most effect factor to V_max/K_m-Q₁₀.Clay and temperature range had greater influence on V_max-Q₁₀ by 0.453 and 0.775,respectively.V_max-Q₁₀ and pH had the greatest influence on K_m-Q₁₀,which were 0.233 and0.252.?3?The reaction rate constant k of soil enzymes increased with warming,and the k of CBH was the lowest among the five tested enzymes.V_max-Q₁₀ was significantly correlated with E_a,and increased linearly with increasing E_a.E_a was greater in cold regions than warm regions.E_a was controlled by MAT,Clay,TOC,pH,MAP and AN factors,of which MAT and Clay had the greatest impact on E_a.As the temperature rise,?H^*,?S^*,and?G^*/T decreased,?G^*changed little with the with increasing temperature.The process of enzyme-substrate complex formation was an energy-consuming process,and the disorder degree of the reaction system was related to soil sample points and soil enzyme types.The enzymatic reaction was an entropy-controlled process.?4?To study the response of SOC to warming,the two-pool model and physical fractionation method were used.The two-pool model showed that warming increased the rate constant of each carbon pool and reduced the SOC half-turnover time.The recalcitrant carbon pool was the determining pool that determined the temperature sensitivity of SOC decomposition.Based on the physical fractionation,the result showed that Min OM was the largest carbon pool and SOC existed in soil by a stable form.The cPOM was the smallest carbon pool.The order of decomposition rate for each carbon pool was:cPOM>fPOM>Min OM.The temperature sensitivity of each carbon pool showed the opposite pattern compared with decomposition rate.In addition,the effect of re-wetting on SOC decomposition was studied.Compared with fresh soil,the SOC decomposition of air-dried soil was significantly increased after re-wetting,and the temperature sensitivity of SOC decomposition was decreased.The temperature sensitivity of SOC decomposition was related to SOC quality,which had conformed to the theory of“carbon quality-temperature”.?5?The response of SOC chemical components in different climatic zones to warming was studied.The results showed that the content of alkyl-C decreased with increasing temperature,and the content of O-alkyl-C increased with warming.The ratio of alkyl-C to O-alkyl-C?A/O-A?and the hydrophobicity index increased with warming,indicating that the increase of temperature was conducive to the SOC decomposition and made soil organic carbon more stable.The decomposition rates and degree of SOC were closely related to the SOC functional group.The relative content of O-alkyl-C had a very significant negative correlation with the cumulative decomposition of SOC,and the A/O-A had a very significant positive correlation with the cumulative decomposition of SOC.Carbohydrate was an important functional group that affected the cumulative decomposition of SOC and the rate constant k.O-alkyl-C was significantly positively correlated with clay and silt,and negatively correlated with sand and pH;carbonyl carbon was significantly negatively correlated with clay and silt,and positively correlated with sand.?6?Warming significantly affected the microbial community compositions and network structures.Response ratio of OTU richness and?diversity response to temperature showed a regional distribution property.There were two types:first increase and then decrease?low MAT region?and constant increase?high MAT region?.Dominated taxas in bacterial communities were Proteobacteria,Actinobacteria,Acidobacteria,and Gemmatimonadetes.Of the total bacterial communities the proportion of dominated taxas was 73.47%.The response of different microbial communities to temperature varied with soil sample points,which were an important factor in the response of microorganisms to warming.In addition,the links between species and key species in the microbial network structure changed with temperature.The microbial network of HLJ,XZ,LN,Hu N and SX,HN have the samall-word networks properties at 20? and 40?,respectively.Bacterial communities were significantly related with soil physical and chemical properties and climate factors.The main factors affecting bacterial community composition were SOC,TN,pH,MAT,and MAP.?7?The network structure of bacterial community composition and SOC chemical components indicated that the bacterial community that affected SOC chemical components differed by the SOC chemical functional group.There were 13 bacterial species that had functioned on the carbohydrate group,and they were all negative connections.The number of connection of COO,NC=O and Alkyl-C were the least,both were 1,and the taxa were Caulobacteraceae?positive connection?and Streptomycetaceae?negative connection?,respectively.Streptomycetaceae which belonged to Actinomycete phylum was a key species in the network structure.?8?Structural equation modeling?SEM?was used to study the relationship between temperature,bacterial community and SOC chemical components and its influence on SOC decomposition.Temperature,bacterial community,and SOC chemical components can explain 84.6%of the variation of SOC decomposition.Temperature not only directly affected SOC decomposition,but also indirectly affected SOC decomposition by affecting microbial communities and SOC chemical components.Microorganisms and SOC chemical components interacted on each other and affected SOC decomposition.?9?The activity of BG increased as the availability of the substrate decreased,and the response of its temperature sensitivity to glucose addition varied with temperature range,incubation time,and soil samples.The CO₂ efflux rate and cumulative emission had increased after glucose added.The early stage of this process was mainly affected by the soil microbial biomass?MBC?,and the later stage was mainly affected by the BG activity.The response of CO₂ efflux rate for glucose addition was significantly negatively correlated with pH,and positively correlated with MAP.In addition,the addition of exogenous glucose reduced the temperature sensitivity of the CO₂ efflux rate.This study revealed the kinetic and thermodynamic mechanism of soil enzyme,the biological-molecular mechanism of SOC decomposition,and the coupling relationship between SOC decomposition and soil enzyme under the background of warming.It provides a theoretical basis for accurately assessing the carbon emission from agro-ecosystem,increasing soil carbon sequestration and reducing carbon loss.