Mechanism Of Soil Organic Carbon Mineralization During Depositing Process Of Check-dams In A Amall Watershed

The dynamic changes of soil organic carbon(SOC)bank and its driving mechanisms are the focus and hotspot of carbon cycle research of land ecosystem.The Loess Plateau is one of the areas with the most serious soil erosion in the world.In order to control it,large-scale of check dams had been built in the gully and formed a widely distributed dam land.Therefore,it is particularly important to deeply explore the driving mechanism of SOC mineralization in the process of dam land formation.Through indoor mineralization experiments,high-throughput sequencing,isotope tracing and other technologies,the small watershed of Zhenggou on the Loess Plateau was taken as the research object.This research compared and analyzed the physical and chemical properties of soil,bacterial abundance,species diversity,microbial community composition and structure.The response relationship between SOC mineralization and soil physical,chemical and biological properties was clarified to reveal the main influencing factors and mechanisms of SOC mineralization in the process of dam land formation.This research will ultimately provide theoretical support for the estimation of potential SOC capacity of ecosystem in the Loess Plateau.The main conclusions are as follows:(1)The measures to return farmland to forest(grass)on slope increased soil bioactivity,improved capacity of microbial community metabolism to carbon sources,and thus promoted SOC mineralization.The conversions of farmland to other types can not only significantly ncreased the SOC mineralization,but also significantly increased the soil nutrient content and soil enzyme activity.Although soil microbes in different erosion source regions(forest land,grassland,shrub land and cultivated land)mainly use the carbon sources of amino acids,acids and saccharides,there were few specific carbon source types of microbial utilization in different erosion source regions,reflecting the diversity of carbon source metabolism in the microbial community.Microorganisms in forest land,grassland and shrub land are always restricted by carbon(C)and phosphorus(P).But that in cultivated land is only C restricted.The peculiar bacterial community dominated in the top soil of cultivated land and grassland,while that in the forest land and shrub land increased with soil depth.Therefore,the bacterial community diversity in forest land and shrub land increased with soil depth.Choanoflagellata,amino acids,bacteroidetes and planctomycetacia were the main explanatory factors for the changes of SOC mineralization in cultivated land,grassland,shrub land and forest land,respectively.(2)The SOC mineralization amount in the cyclic wet-dry conditions was between that of deficient and saturation water treatment.The changes in the characteristics of the fungal/bacterial community in the water-deficient state were the main regulating factors limiting mineralization.The activity of the microbial community increased under the water-saturated state,leading an increase in the SOC mineralization.The cyclic wet-dry conditions accelerated the SOC mineralization by augmenting the unique microbial community.Moisture has an instantaneous priming effect on the SOC mineralization.The cyclic wet-dry conditions significantly affected the content of SOC and total nitrogen(N),and significantly stimulated the activities of related C-,N-and phosphate enzymes.But the response of phosphate enzymes to water was significantly lower than that of C-and N enzymes.The SOC mineralization was always limited by C and P under the cyclic wet-dry conditions,and the more dry-wet alternating cycle,the more intense the carbon limitation,while that under constant dry and constant humidity treatment was mainly limited by P.With the increase in the number of wet and dry cycles,the utilization of carbon sources by soil microorganisms gradually decreased.And the common dominance of saccharides,amines and acids at the beginning had gradually evolved into a single acid,lipids or alcohols.Bacteroides and ascomycetes,C-N ratio and average well color development,SOC and microbial dominance were the main explanatory factors for the changes in SOC mineralization under constant dryness,constant humidity and cyclic wet-dry conditions,respectively.When the soil were chronically short of water(constant dry),fungi/bacteria account for the amount of SOC mineralization degree was significantly higher than other factors.Obviously,the change of fungal/bacterial community characteristics under water shortage was the main regulatory factor causing the change of SOC mineralization.Soil nutrients and microbial community activity became the key "regulators" of SOC mineralization under constant humidity.When the soil underwent repeated drying and wetting processes,soil aggregates were separated,polymerized and separated repeatedly,resulting in the loss of the protective effect of aggregates on SOC.The decomposition of SOC by specific microbial communities resulted in the "regulator" of SOC mineralization turned into SOC and microbial dominance.(3)The concentration of oxygen in the sediment profile of the dam land could change the community structure of bacteria and fungi,restricted the related biological activities,and limited the SOC mineralization.The SOC mineralization amount at any stage under aerobic treatment is higher than that with anaerobic treatment.Soil enzyme activities were higher in aerobic condition than that in anaerobic condition,while soil carbon restriction was greater in anaerobic condition.Soil microorganisms were always restricted by P and mainly used saccharides as carbon source both in aerobic and anaerobic conditions.Microorganisms with high carbon metabolism in anaerobic conditions were weak in aerobic conditions,and vice versa.When the soil changed from aerobic state to anaerobic state,nitrocellulose,corynebacterium and thaumarchaeota disappeared gradually,and replaced by acidobacteria,cyanobacteria and bacteroidetes.As the soil changes from anaerobic to aerobic,basidiomycetes disappeared and were replaced by dinoflagellates.Compared with the anaerobic state,the richness and diversity of soil bacterial community in aerobic state were significantly higher,which on the other hand verified that anaerobic environment could delay the development of soil bacterial community,slow down the SOC mineralization,and played a protective role in SOC storage.(4)The variations of microbial carbon source utilization types in different siltation stages were studied,and the characteristics of SOC mineralization in different siltation stages and the key influencing factors were identified.The amount of SOC mineralization gradually decreased in the ST-4 stage during the 70d culture process at the bottom of the sedimentary profile.With the upward increase of soil depth,the amount of SOC mineralization first increased or tended to be stable for a period of time,and then gradually decreased.The contents of SOC,total N and total P in the ST-1 deposition stages were significantly higher than those of other stages in each culture period,and were relatively stable as a whole.From ST-1 to ST-4(increasing deposition depth),the response of SOC mineralization to carbon-,N-and P enzymes gradually decreased.The carbon limitation is most significant in ST-1 stage,and gradually decreases with the increase of deposition depth.Throughout the deposition profile,soil microorganisms were restricted by P.With the advance of the sedimentation stage(the sedimentation depth deepened),the utilization of carbon sources by soil microorganisms gradually changed from carbohydrates in ST-1 stage to acids(ST-2,3,4).The unique bacteria and fungi communities gradually dominated the soil,and some special communities gradually disappeared due to the change of environment(water,heat,air).The C-P ratio of enzyme and nitrocellulose,SOC and β-xylosidase,actinomycetes and amines,C-N ratio of enzyme and saccharides were the main factors to explain the change of SOC mineralization in ST-1,ST-2,ST-3 and ST-4 stage,respectively.(5)The deposition rates of 137Cs in different periods were analyzed,the erosion modulus and its variations were determined.Based on the sources of sediment and organic carbon determined by the composite fingerprint method,the amount of SOC sequestration and mineralization at watershed scale were further defined.From 1960 to 2017,the sedimentation rates of 137Cs generally showed a downward trend.Soil erosion was serious in the 1960s and mid-1970s.After soil conservation measures were introduced in 1979,the deposition rates gradually decreased.It was not until the late 1990s that large-scale conversions of cultivated land to forest(grassland)reduced slope erosion and led to a rapid decline in the settlement rate.The minimum sedimentation rate occurred at the top of the sedimentation profile,while the sedimentation rate was larger at the bottom of the profile.In the early and middle period(before 1990s),cultivated lands on slope were the main sources of erosion SOC,while ditch wall became the main source after 1990s.On the whole,the main source of SOC was ditch wall,accounting for 48.4%of the total,followed by slope land(24.8%).The change of slope erosion source area can significantly increase organic carbon storage,but a lso increase the amount of organic carbon mineralization so that the net organic carbon storage i s still increased.The implementation of the measure of siltation of dam land by check dam cons truction compared with the slope surface fallowing(grass)can effectively increase the storage o f organic carbon and reduce the mineralization of organic carbon.The storage of organic carbon in the whole watershed reached 2377.04t,and the mineralization of organic carbon was 695.38.