Soil Organic Carbon Mineralization And Microbial Responses In Cropland Of Black Soils With Varied Organic Matter Gradients

Soil is an important part of terrestrial ecosystem,and organic carbon storage is the key function of soil.The soil organic carbon（SOC）pool in cropland is greatly affected by human beings.Frequent tillage measures and the input of organic and inorganic materials will directly affect the mineralization,turnover,and fixation of SOC.Priming effect is an extra decomposition of organic C after addition of easily available organic substances to the soil.There are many studies on the mineralization and priming effects of SOC.However,due to the difference of soil type,land use methods,exogenous substance kinds and concentration,the conclusions are also different..The northeast black soil region is an important grain production base in China,but there is still a lack of knowledge about the mineralization and priming effect of SOC in varied organic matter gradients.Therefore,in this study,cropland surface soil samples with varied organic matter gradients were collected in northeast China.Controlled mineralization experiments were conducted by adding different kinds and different concentrations of exogenous organic matter labeled with ¹³C,using ¹³C stable isotope labeling,Fourier transform infrared spectroscopy（FTIR）,¹³C-Phosphor Lipid Fatty Acid（PLFA）and other technologies.The characteristics and differences of organic carbon mineralization and priming effect in cropland black soil under different exogenous organic matter addition were distinguished.Combined with the structural characteristics of FTIR of SOC and microbial community structure,the microbial effects on mineralization and priming of SOC effect in varied organic matter gradients were revealed.The main results are as follows:（1）In this study,the SOC contents of black soil in typical cropland ranged from11.1 g·kg^-1-68.2 g·kg^-1.In the surface black soil with varied organic matter gradients,the higher the SOC content,the higher the soil total nitrogen content,microbial biomass（SMBC）,and soil organic carbon functional group liable components.The results of principal component analysis（PCA）showed that there were differences in the microbial community structure of cropland black soil with different soil carbon gradients.（2）With the addition of exogenous organic matter,the mineralization characteristics of SOC in cropland black soil with organic matter gradient are different.The soil CO₂emission rate and cumulative CO₂emission of alanine and glucose supplementation were significantly higher than that of straw supplementation.The lower SOC content,the greater the increase of cumulative CO₂emissions.The higher the concentration of exogenous organic matter,the higher soil CO₂emission rate and cumulative CO₂emission.Multivariate analysis of variance showed that the effect of exogenous concentration on soil cumulative CO₂emission was greater than that of SOC content and exogenous substance kinds.（3）The priming effect of SOC was different with varied organic matter gradients.The lower the content of soil organic matter was,the higher the relative cumulative priming effect amount of 30 day was.The priming effect caused by the addition of alanine and glucose was greater than that caused by straw addition;The concentration of exogenous additives was positively correlated with the relative priming effect amount（p<0.05）,the higher the concentration of glucose and alanine was,the greater the priming effect effect was.（4）The FTIR peak area of functional groups was used to characterize the stability of SOC.The lower the content of organic matter in the soil,the higher the relative peak area of aromatic carbon at 1630 cm^-1,that is,the lower the content of soil organic matter,the higher the stability of organic carbon.The relative peak area of the stable organic carbon functional group decreased with the addition of glucose and alanine,and the greater the concentration of glucose and alanine,the greater the decrease of the relative peak area of the stable functional group.（5）Results of ¹³C-PLFA showed that the soil with lower organic matter content had better utilization effect of exogenous organic carbon by microorganisms.By comparing the ¹³C contributions of different species of microorganisms on the 3rd day and the 30th day,it was found that the contribution of fungi and actinomyces was relatively increased on the 30th day,and the turnover of exogenous substance occurred among soil microbial.（6）Through the structural equation model,we analyzed how the amount of exogenous organic matter and SOC content regulated the biomass and community structure of soil microorganisms to affect the mineralization and priming effect of SOC.The results showed that the effect of exogenous organic carbon on the mineralization and priming effect of SOC was greater than that of SOC content.The effects of actinomycetes and G+/G-on SOC mineralization and priming effect changed from negative to positive with the incubation time（p<0.05）,indicating that soil microbial action in SOC mineralization and priming effect varied with the incubation time.To sum up,different kinds and concentration of exogenous organic matter have different effects on the SOC mineralization and priming effect of cropland black soil with varied organic matter gradient.The amount of SOC mineralization and priming effect induced by glucose and alanine was higher than that caused by straw.The higher the exogenous substances concentration,the greater the amount of SOC mineralization and priming effect.The lower the organic matter content of the soil,the priming effect was larger under the same exogenous addition condition.Among the experimental setting factors,the concentration of exogenous substances had the greatest influence on the mineralization and priming effect of SOC.The addition of exogenous substances had a significant effect on microbial biomass and community structure,thus affecting the mineralization and priming effect process of organic carbon.With the turnover of exogenous organic carbon among microorganisms,the succession process of microbial community structure took place in the incubation process.The results are expected to provide theoretical data support for clarifying the stabilization mechanism of black SOC and exploring the threshold of black SOC.