| Soil is one of the most important carbon pools on the Earth,small change of soil organic carbon(SOC)may have huge impact on the ecological environment of the world.Under the effects of high intense human activities and frequently extreme climate events,the intensity and range of accelerated erosion are being increased.Not only lots of soil and water are lost,but also SOC is lost due to transportation and mineralization.As the green house effect has been more obvious,the lost carbon caused by water erosion has been attracted more and more attention from all over the world.Despite a lot of studies were conducted,the role of erosion on the global carbon cycle is controversy.The fundamental cause is that the SOC dynamics under water erosion is not fully understood,especially in the dynamic processes with interaction and variation of space and time.Therefore,a small watershed in the red soil hilly region was selected as the study object,SOC dynamics during erosion processes were monitored and studied,and the results may provide theory basis for promoting the declaring the effects of erosion on global carbon cycles.Combined with GIS,radioactive element tracer technique and thermal analysis technique and so on,field simulated rainfall experiments,watershed sampling experiments and indoor analysis experiments were conducted to explore the migration,redistribution and deposition of SOC during erosion processes and the stability of SOC post erosion.Based on these targets,five interrelated experiments were designed and performed,the specific research work and the main results are showed as follows:(1)Field simulated rainfall experiments were conducted to monitor runoff,sediment and SOC loss under different erosion conditions,and the results elucidated the regulations and mechanisms of SOC loss.The average amount of lost SOC in the form of dissolved organic carbon less than 10%,and there is no exactly regulation of its loss rate can be found.Whereas the amount of lost SOC in form of sediment-bound organic carbon accounts for more than 70%of the total lost SOC,and the sediment-bound organic carbon was the main form of the lost SOC.The variation of SOC loss rate is the same as that of runoff and sediment,showing the characteristics of first increasing and then stable.The results showed that the total amount of the lost SOC is significantly correlated with sediment loss(P<0.05),and the correlation coefficients decreased with the increasing of sediment loss.Rainfall intensity and tillage can affect the loss of SOC by acting on sediment transport.Compared with no tillage,traditional tillage reduced the amount of lost SOC by about 70%.Although this result only appeared in low intensity rainfall event,it had important practical significance to soil management in red soil hilly region.While the lost SOC increased with the increasing of rainfall intensity,however,these two parameters were not linearly related.This is because that the SOC content in the sediment varied with rainfall duration and the amount of lost sediment;2,Based on the continuous monitoring of SOC concentration in sediment under complex erosion conditions,the selective migration processes and mechanisms for SOC were analyzed and explained.The results showed that the enrichment ratio of organic carbon(ERoc)in the sediment is 0.61～2.13 under different erosion conditions.Three different variation trends were observed:Gauss distribution(y=a*exp(b((t-c)/d)^2)),exponential descending trend(y=a+b*exp(ct))and disorderly trend.The variation of the ERoc corresponding to these trends indicated the high selectivity,medium selectivity and non-selective migration of organic carbon,respectively.Correlation analysis showed that the ERoc was negatively correlated with runoff volume and sediment yield,and only the sediment concentration in the moderate intensity erosion was significantly correlated with sediment ERoc(P<0.05).The study results also showed that the general migration of organic matter during the erosion process following the order:small plant residues,fine sediment particles,coarse sediment particles.But the migration process is associated with erosion intensity and does not strictly follow such a sequence.Therefore,the selective migration of SOC is due to the non-uniform distribution of organic carbon with "quality" and "quantity" in original soils and presents different trends under the effects of erosion intensity and rainfall time.3 Based on the analysis of SOC distribution on watershed,long-term effects of water erosion and the ecological and environmental factors on the redistribution of organic carbon were studied.It is found that the topography and vegetation type are important factors affecting erosion intensity and SOC distribution.Soil erosion rates ranged from 307.13 to 2471.25 t km-2 a-1 in the broad-leaved forest(LI,ES,CC and SM),and the values in pine forest is 72.34～3455.1 5 t Km-2 a-1,and the soil deposition rate in the corresponding depositional site is 2301.63 t km-2 a-1.The range of SOC concentrations in the broad-leaved forest was 12.10～4.38 g kg-1,and there was no significant difference in the content of organic carbon in different soils,but these values were significantly higher than that of Pine forest(8.95 g kg-1)(P<0.05).The spatial distribution of SOC in the watershed is not consistent with the distribution of soil erosion intensity,indicated that SOC is significantly affected by vegetation.The range of organic carbon pool in the upper 60 cm soil of broad-leaved forest was 50.08～62.21 Mg C ha-1,which was significantly higher than that of pine forest(45.90 Mg C ha-1).Compared to eroded areas with non-vegetation,the presence of vegetation can maintain a stable level of organic carbon stocks,and broad-leaved forests can significantly increase the SOC pool.The results emphasized the decisive effect of vegetation on soil carbon content under the influence water erosion.4 Based on the thermal analysis of SOC in soil aggregates at the typical eroding and depositional sites,the study clearly declared that the applicability of thermal analysis on carbon stability,and indicated that the soil aggregation are the main mechanisms for the difference of SOC stability under the effect of erosion and deposition.The results showed that the proportions of 0.25-0.002 mm aggregate in eroding and depositional soils are above 70%,while the proportion of<0.002 mm aggregates is less than 2%and is almost negligible.In addition to<0.002 mm aggregates,the variation of organic carbon contents in aggregates for erosion and depositional sites ranged from 1.56-10.09 g kg-1 and 1.32-21.77 g kg-1,respectively.The ranges of TG-T50 of different size aggregates in eroding and depositional sites were 394.84～494.81 ℃ and 378.30～464.35 ℃,respectively.Significant differences of soil properties and the thermal analysis index of aggregates between the two sites were only found at the surface layer 0-10cm.The thermal stable indicators of TG-T50 and DSC-T50 indicated that the eroded soil had higher stability,and EXOl/EXOtol indicated that the depositional soil can promote the long-live of labile organic carbon.The results showed that the soil aggregation and the stability of organic carbon in the two sites are consistent.Good soil aggregation condition can promote the stability of labile organic carbon.5 Based on the thermal analysis,effects of erosion and deposition on the structural characteristics of organic carbon were explored,and the effects of environmental factors on the stability of organic carbon were discussed.It was clearly that soil depth was the most important factor for organic carbon stability under the effect of erosion and deposition.The results showed that there were almost no differences in the soil carbon structures between the eroding and depositional sites,but the aromatic compounds and molecular weight between the eroding and depositional sites were significantly different.The results indicated that the SOC composition in the two sites was similar,but the decomposition rate of SOC at the depositional site was significantly lower than that at the eroding site.As important factors affecting the stability of organic carbon,sand,silt and 0.25-0.053 mm aggregates were positively correlated to SOC stability,while the clay content,>0.25 mm and<0.002 mm aggregates,mean weight diameter,the content of free iron oxide,complex iron and active iron in soil were negatively correlated to the stability of organic carbon.The environmental factors account for 81.2%of the thermal stability of organic carbon,and the individual interpretation of soil depth is 56.9%,which is the most important factor affecting the thermal stability of organic carbon.In summary,the study indicated that most of the eroded organic carbon was lost in the form of sediment-bound organic carbon,and most of the eroded carbon in the erosion process was redistributed in the watershed,and only a small part of the organic carbon with high quality and light bulk density was transported and deposited into the sedimentary area of the basin by selective migration.Long-term analysis found that the organic carbon in eroded slope soils increased rather than decreased by the replacement processes.Effect of erosion on organic carbon redistribution almost can be neglected.Further,the replacement and stability of of organic carbon in the eroding and depositional sites are the most important factors deciding the role of soil erosion on atmosphere carbon.Therefore,in combination with the existing studies,it is believed that erosion is carbon sink for atmospheric carbon.But the specific carbon sinks are determined by a quantitative description of the replacement and stabilization of organic carbon in the eroding and depositional sites. |
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