Soil Organic Carbon Dynamic And Its Microbiological Mechanism Affected By Water Erosion In Plot Scale

There has been a drastic increase in soil erosion and its adverse impacts on environment because of the increased human activities and frequent appearance of extreme rainfall events,with that the dynamics of soil organic carbon induced by erosion have received significant concern due to its direct relationships with the pattern of global carbon cycle.Despite such situation,however,considerable uncertainties still exist in the understanding of how SOC pool and dynamics are affected by soil erosion.Researches on the contribution of soil erosion on pedosphere to atmospheric CO2 have been conducted to explore either the effect of soil organic carbon(SOC)mineralization during transport as a CO2 source or the effect of the deep burial of SOC in landscape as a CO2 sink,even though these effects were controversial.A key uncertainty is the pattern of SOC mineralization induced by erosion and its implication on global carbon cycle.The fate of the eroded and in situ SOC after erosion as well as to what extend those SOC could be mineralized is unknown and remains highly debated.However,most of the researches about soil carbon erosion,which was considered to be a complex process in natural open systems,focused on SOC budget by directly analyzing the variation of SOC pools based on black-box theory,which might be the main reason why the global carbon budget still could not be balanced nowadays.Based on the analysis of previous studies,it has been shown that further researches related to carbon budget should take more concern in exploring the microscopic mechanism of soil carbon erosion relative to merely macroscopic estimation,which is considered to be an effective way to accurately estimate the dynamics of soil carbon pool under the influence of soil erosion and finally solve the current uncertainties.Therefore,this study takes soil microorganisms,which is the main mediator of soil carbon turnover,as the original focus to go on the investigation and research.Through plot-scaled field simulated rainfall,feld-positioning observations and indoor qualitative simulation,the dynamics of soil microbial abundance and community structure as well as SOC induced by water erosion within the slope scale were observed,and the patterns of SOC mineralization under the influence of water erosion were detected simultaneously.In further,as the main purpose of this study,the SOC mineralization and loss mechanisms conducted by soil microorganisms under the influence of water erosion were thoroughly discussed based on the above observations,and the effects of water erosion on the input mechanism of SOC were also analyzed.On the basis of the present research topics,this study carried out five interrelated research works.1.Through observing the dynamics of microbial abundance and its community structure as well as SOC in an eroded slope after soil rewetting,the relationships and interactions between microbial community composition and soil carbon dynamics in response to rewetting at three sites under varying erosion or deposit characteristics within a typical sloping cropland were investigated.The results showed that bacterial abundance in the middle position of the eroded slope(MID-SLOPE)increased significantly by 14.2%(P<0.05)during the initial 24 hours after rewetting,while fungal abundance decreased immediately in the deposition area of slope bottom position(DOW-SLOPE)during this period,and decreased by 20.7%(P<0.001)finally.There wasn't any significant changes of the microbial abundance in other positions of the eroded slope.In general,fungal abundance(FCA)showed more dynamics than bacterial abundance(BCA)in response to soil rewetting.Bivariate correlation analysis indicates that only bacterial populations decreased and correlated negatively with DOC content in the erosional area of up-slope position(UP-SLOPE)and MID-SLOPE within the initial 60 hours after rewetting,while there was no significant correlation between fungal gene abundance and soil carbon content at any tempo-spatial scales in this study.Redundancy analysis(RDA)revealed that SOC and DOC could explain up to 46.0%of the variation in bacterial community structure(Monte Carlo permutation test with 499 permutation,P ?0.002),which contributed to 69.4%of the explanation by all the measured soil physicochemical parameters;this carbon data set together could explain up to 21.9%of the variance in fungal community structure without sharing effects with other environmental data(Monte Carlo permutation test with 499 permutations,P = 0.04),which contributed to 41.2%of the explanation by all the measured soil physicochemical parameters.In conclusion,site erosion or deposit characteristics may affect the drying/rewetting(D/R)susceptibility of soil biogeochemical carbon cycles by inducing shifts in functional microbial communities with differing responses to rewetting;2.On the basis of the above researches of soil rewetting,the further effects of the transport processes by water erosion on soil microbial communities and SOC within a sloping land was examined in this paper through simulating one-hour rainfall and analyzing the soil samples collected from three plots along the sloping land during and for 132 hours after the rainfall,and the relationships and interactions between microbial community composition and soil carbon dynamics in response to water erosion within the typical sloping cropland were investigated.During the rainfall,water erosion significantly reduced BCA and FCA by 17.24%and 28.42%(P<0.05)in MID-SLOPE where suffered severe erosion,while no significant changes of microbial diversity was detected at any tempo-spatial scales in this study.During the subsequent 132 hours after the rainfall,BCA decreased by 4.7%within the entire studied land(P<0.05),and fungal Shannon diversity index(FH)of the entire land increased by 21.2%(P<0.05).RDA results illustrate that there was a positive correlations between BCA,FCA and SOC.The forward selection illustrated that BCA was best related to erosion and SOC,which together contributed to 59.5%(P<0.01)of its variance.SOC and moisture were found to be the significant properties which together contributed to 38.8%of the variance in FCA.However,no definite explanations for the effects of SOC on FH and bacterial Shannon diversity index(BH)were generated.Together,our results suggest that microbial abundance rather than its community structure may play a major role in the process of carbon turnover under the influence of water erosion,which is opposite to the pattern induced by soil rewetting.It is thus necessary to reveal the soil microbial dynamics and then give insight into the dynamics of a certain functional group or specie which is sensitive to erosion when trying to evaluate the eroded carbon dynamics,instead of merely analyzing the variation of SOC pools content.3.Through plot-scaled field simulated rainfall experiments and CO2-flux positioning observation,the effects of water erosion induced by a single rainfall event on SOC mineralization in the eroded slope were investigated.In further,the mechanism controling the SOC mineralization induced by water erosion was analyzed.Results showed that soil loss rate during the simulated rainfall were 0.72× 103 and 0.05 × 103 gm-2 in no-tillage and conventional tillage plots,respectively;the corresponding SOC-loss rate associated with sediment and runoff were 10.83 and 0.84 gm-2· During the 420 minutes of Co2-flux positioning observation,the CO2-evolution rates of different slope positions in the conventional tillage plot were at least 61.9%lower than their control,while the CO2-evolution rates of different slope positions in the no-tillage plot were at most 61.9%lower than their control;there was no significant difference of the CO2-evolution rates among the different positions in neither no-tillage nor conventional tillage plots,and no significant correlation between soil physicochemical parameters(including SOC)and CO2-evolution was found.The results of this experiment suggeste that conventional tillage could slow the SOC mineralization rate in eroded sloping land,and there is no isolated interaction between the pattern of SOC redistribution induced by water erosion and SOC mineralization after erosion,which indicates the effects of water erosion on SOC mineralization should be more complex.4.Based on the different soil texture of erosional and depositional areas which were shaped by selective transport process of water erosion,the indoor qualitative simulation experiment was applied in this study to detect the dynamics of soil microorganisms and SOC mineralization in different sized aggregates after rewetting,and the feature of SOC loss by mineralization in erosional and depositional areas were analyzed in further.The results showed that SOC mineralization rates during the initial period of incubation were significant different among different sized aggregates,1-2 mm aggregates had the highest SOC mineralization rates,the value is 1.07 and 18.94 times higher than in<0.5mm and 2-3mm aggregates,respectively.Soil bacterial community structure in different sized aggregates showed remarkablely variation during the incubation period,and the dynamics of bacterial species diversity and relative abundance in<0.5mm were almost opposite to that in 2-3mm aggregates during incubation.No significant correlation between bacterial community structure and SOC mineralization was found in this experiment.To conclude,the patterns of SOC mineralization in erosional and depositional environments can be affected by soil aggregates distribution in these areas,and SOC quality and carbon source availability in different sized aggregates might be responsible for the different patterns of SOC mineralization.5?Through field planting experiments,the effects of water erosion(including long-term historical erosion and single erosion event)on soil properties and productivity in different farming systems were investigated,and the relationships and interactions between soil productivity and SOC dynamics induced by water erosion within the typical sloping cropland were discussed thoroughly,which could provide theoretical basis for the reserch related to soil organic carbon input affected by water erosion.RDA on soil productivity data limited by historical erosion data accounted for 61.4%(P = 0.002)of the productivity variation under same tillage practices but different rainfall-intensity treatments(STDR),and the same RDA model statistically accounted for 53.8%(P = 0.002)of the productivity variation under the same rainfall-intensity but different tillage practices treatments(SRDT). This finding indicated that the effects of long-term water erosion should have a major function in inducing soil heterogeneity and variations in soil productivity relative to single rainfall erosion event.In no-tillage systems,no significant explanation was detected when the RDA model was solely limited by rainfall-intensity data(P = 0.076).In SRDT,the resuts showed that soil productivity in no-tillage land was 45.27%higher than that in tiallge land when suffered them with same intensity of rainfall.The 'forward selection' procedure indicated that the soil variables explaining the largest statistically significant amount of the productivity variation were TP(P = 0.002),SOC(P = 0.006),and TN(P = 0.01)in SRDT.The shared explanations developed by SOC and historical erosion data reached to 49.1%and 41.3%in STDR and SRDT,respectively.In general,this study provided strong evidence that single erosion event could also impose significant constraints on soil productivity and SOC input by integrating with tillage operation,although single erosion is not the dominant effect relative to the long-term historical erosion.The effect of single erosion event on soil productivity is its indirect action by changing the dynamics of soil nutrient elements,especially the dynamic of SOC.In summary,the effects of water erosion on soil organic carbon pool were related to each process of SOC dynamics,including SOC input,migration,redistribution and mineralization.The SOC mineralization pattern induced by water erosion mainly includes rewetting stimulation,soil aggregate-crushing stimulation and induction after water erosion due to selective migration and enrichment,which was considered to be closely controled by soil tillage methord,soil erosional or depositional characteristics,microbial community composition and the quantity and quality of SOC.Obviously,soil microorganisms play a dominant role in this complex process mechanism.Therefore,this study suggest that taking soil microorganisms as the original focus to go on the investigation of SOC mineralization induced by water erosion will help to verify the role of erosion in soil carbon cycle at mechanism level.