Annual Variations Of The Active And Recalcitrant Organic Carbons And The Microbial Influences In Saline-alkali Farmland Soil

Soil organic carbon(SOC)pool is an important component of the carbon pool of terrestrial ecosystem.The small changes of SOC pool will play an important role in atmospheric CO2 concentration,thus affecting global climate change.The SOC pool includes the active carbon pool and the recalcitrant carbon pool.The current studies focus on the active carbon pool,while the studies of the recalcitrant carbon pool is relatively few.Active soil organic carbon(ASOC)is an important indicator of the SOC changes,and recalcitrant organic carbon(ROC)mainly contributes to carbon storage.Soil microbe is the important driver of carbon cycling,controlling the decomposition and accumulation of organic carbon.The annual changes of ASOC and ROC and the microbial mechanism in saline-alkali farmlands of seasonally frozen soil region are new problems of carbon cycle.Western Jilin covers one of the three largest areas of saline-alkali soil in the world.It is located in the Transect in Northeast China of global climate change,and is an important area of climate change and carbon cycle.The degree of soil salinization can change the aboveground and underground biomass and microbial activities,thus affecting the process of organic carbon cycle.The saline–alkali farmland soil in western Jilin Province are taken as the research object.This study adopted the methods of field sample collection,indoor soil analysis,molecular biology,statistical analysis and model simulation.The variations of ASOC and ROC of growing and non-growing season were observed firstly.The changes of microbial biomass and community structure in the soil were indicated then.The effects of soil microbes on ASOC and ROC were explored finally.The key results are as follows.(1)Soil dissolved organic carbon(DOC),microbial biomass carbon(MBC),light fraction organic carbon(LFOC),ASOC and ROC contents decreased with the increase of soil depth,and decreased with the increase of salinity and p H of the same layer of saline-alkali paddy and upland fields.DOC contents of saline-alkali farmlands increased from the initial frozen stage and reached maximum values during hard frost stage,then decreased at the completely unfrozen stage.However,the tendency of MBC contents was opposite to that of DOC.The contents of LFOC,ASOC,ROC of paddy fields showed no obvious changes during the initial frozen stage and hard frost stage,and increased during the completely unfrozen stage.The contents of LFOC,ASOC,ROC of upland fields increased during the initial frozen stage,changed little during hard frost stage,and increased at the completely unfrozen stage.The proportions of DOC in SOC(DOC/SOC)of saline-alkali farmlands during the non-growing season increased from the initial frozen stage and reached maximum values during hard frost stage,then decreased at the completely unfrozen stage.However,the proportions of MBC in SOC(MBC/SOC)obtained the opposite trend with DOC/SOC.The proportions of LFOC in SOC(LFOC/SOC)of paddy fields during the non-growing season changed little from the initial frozen stage to hard frost stage,and increased during the completely unfrozen stage.However,there was no consistent change law of LFOC/SOC of the upland fields,and the proportions of ASOC,ROC in SOC(ASOC/SOC,ROC/SOC)of paddy and upland fields during the non-growing season.(2)The DOC contents of paddy fields during growing season decreased during the seedling stage and the tillering stage,and increased during the filling stage and the full ripe stage.The MBC contents of paddy fields increased from the seedling stage,and reach maximum values at the filling stage,but decreased during full ripe stage.The contents of LFOC and ASOC of paddy fields decreased during the seedling and tillering stage,then increased during the filling stage,fell during the full ripe stage finally.The ROC contents of paddy fields decreased during the tillering stage and increased during the filling stage.The contents of DOC,MBC,LFOC,ASOC and ROC of upland fields increased during the seedling stage and the tillering stage,and decreased during the filling stage and the full ripe stage.DOC/SOC of paddy fields decreased during the seedling stage and the tillering stage,and increased during the filling stage and the full ripe stage.MBC/SOC of paddy fields increased during the seedling stage and the tillering stage,and decreased during the full ripe stage.But there was no consistent law of LFOC/SOC of paddy fields during the growing season.ASOC/SOC of paddy fields decreased during the seedling stage and the tillering stage,then increased during the filling stage,and decreased at the full ripe stage finally.ROC/SOC obtained the opposite trend with ASOC/SOC.DOC/SOC of upland fields increased during the seedling stage and the tillering stage,and decreased during the filling stage and the full ripe stage.MBC/SOC of upland fields increased during the seedling stage and tillering stage,and decreased during the full ripe stage.LFOC/SOC,ASOC/SOC and ROC/SOC of upland fields did not achieve a consistent law,but ROC/SOC reached maximum values at the filling stage.(3)The contents of bacterial PLFA,fungal PLFA,actinomycetic PLFA and total microbial PLFA of saline-alkali farmlands during the non-growing season decreased from the initial frozen stage,and reached minimum value during the hard frost stage,then increased at the completely unfrozen stage.The proportions of bacterial PLFA in total microbial PLFA(bacterial PLFA/total PLFA)of paddy and upland fields increased during the initial frozen stage,and decreased during the completely unfrozen stage.The proportions of fungal PLFA in total microbial PLFA(fungal PLFA/total PLFA)of paddy and upland fields decreased during the initial frozen stage and hard frost stage,and increased at the completely unfrozen stage.However,there was no consistent change law of the proportions of actinomycetic PLFA in total microbial PLFA(actinomycetic PLFA/total PLFA)of paddy and upland fields during the non-growing season.Fungal PLFA/bacterial PLFA of paddy and upland fields decreased at the initial frozen stage,and reached minimum values during the hard frost stage,increased at the completely unfrozen stage finally.(4)The contents of bacterial PLFA,fungal PLFA,actinomycetic PLFA and total microbial PLFA of saline-alkali paddy fields during the growing season increased from the seedling stage,and reached maximum values during the filling stage,decreased at the full ripe stage finally.The contents of bacterial PLFA,fungal PLFA,actinomycetic PLFA and total microbial PLFA of saline-alkali upland fields during the growing season increased during the seedling stage and the tillering stage,while decreased during the filling stage and the full ripe stage.Bacterial PLFA/total microbial PLFA of paddy fields decreased during the seedling stage,the tillering stage and the filling stage,then increased during the full ripe stage.Fungal PLFA/total microbial PLFA of paddy fields did not achieve a consistent law.Actinomycetic PLFA/total PLFA of paddy fields had an upward tendency from the seedling stage to the filling stage,and reached maximum values at the tillering stage or the filling stage,but decreased at the full ripe stage.There was no consistent law of bacterial PLFA/total PLFA and fungal PLFA/total PLFA of upland fields during the growing season.Actinomycetic PLFA/total PLFA of upland fields increased during the seedling stage and the tillering stage,and decreased during the filling stage.Fungal PLFA/bacterial PLFA of paddy and upland fields did not achieve a consistent law during the growing season.(5)Saline-alkali stress restraines the mineralization of SOC and the decomposition of soil organic matter by inhibiting microbial activities.Higer saline-alkali stress contributes to lower turnover rates of SOC.Small changes of ASOC,ROC,active organic carbon fractions and microbial PLFAs were observed in the soil with high salinity and deep depth of the growing and non-growing season.(6)There was no significant correlation between DOC and the PLFAs of bacteria,fungi,actinomycetes,total microbes of paddy soil during non-growing season.While MBC,LFOC,ASOC and ROC of paddy soil were significantly positive correlated with the PLFAs of bacteria,fungi,actinomycetes,total microbes during non-growing season,respectively.DOC,MBC,LFOC,ASOC and ROC of paddy fields were significantly positively correlated with the PLFAs of bacteria,fungi,actinomycetes and total microbes during growing season.DOC,MBC,LFOC,ASOC,ROC of upland fields were significantly positively correlated with the PLFAs of bacteria,fungi,actinomycetes,total microbes of the growing season and non-growing season,respectively.(7)The soil microbes were important drivers of ASOC and ROC during growing season.The soil microbial activities during the non-growing season not only affected the changes of ASOC and ROC during non-growing season,but also had potential impacts on ASOC and ROC in the next growing season.