Dynamic Changes Of Soil Organic Carbon And Its Effect On Soil Inorganic Carbon In Erosion—deposition Conditions

Soil erosion is one of the main environmental problems that endanger human survival and development.The dynamic changes of soil carbon caused by erosion and deposition has an important impact on the global carbon cycle and climate change.In the past few decades,scholars at home and abroad have extensively discussed the dynamic changes of soil organic carbon（SOC）during the erosion process,but there is still a lack of in-depth understanding of the changing characteristics of soil inorganic carbon（SIC）pools and the transformation regulation.The Loess Plateau has serious soil erosion,and a large number of check dams have been widely built,which provide a good carrier for the study of the erosion-deposition environment.At the same time,the region is located in an arid and semi-arid region,and the storage and carbon sequestration potential of SIC are higher than SOC.Therefore,clarifying the characteristics of SIC components in the erosion and deposition environment on the Loess Plateau,and further studying dynamic changes of SOC and its impact on SIC driven by erosion and deposition is of great significance for revealing the role of soil erosion in the global carbon cycle and increasing soil carbon sink.This study selected the Loess Plateau and three typical small watersheds（Changliang,Luoyugou,and Shayangou watersheds）as the research objects.Combined with field investigation,sample collection,in-situ monitoring,and stable carbon isotopes,using multivariate stepwise regression,path analysis,partial least squares regression,and other methods,aiming at the impact mechanism of the erosional and depositional environment on SOC and SIC dynamic changes.We analyzed the distribution characteristics of SOC and SIC in erosion and deposition environments at different scales（loess Plateau-typical watershed-soil aggregates）,explored the influence of erosion and deposition on SOC mineralization and its temperature sensitivity(Q₁₀),and revealed the composition characteristics and influencing factors of SIC driven by erosion and deposition.The main results are as follows:（1）The distribution characteristics of SOC and SIC in erosion and deposition environments were significantly different,and the relationship between SOC and SIC converted.The results showed that,compared with erosional sites,the average SOC and SIC contents of soil（0-25 cm）in the deposition area of the Loess Plateau increased by 24.4%and 15.4%,respectively.Both SOC and SIC contents in depositional sites exhibited a decreasing tendency with depth（0～600 cm）,however,the SIC in the deeper soil showed an increasing trend（below 1000 cm）.The transport and deposition of sediment significantly increased the content of<0.053 mm aggregates and decreased the content of macroaggregates.At the depth below 400 cm in the deposited profile,the content of clay particles decreased,while the proportion of large aggregates increased.The concentrations of SOC and SIC in aggregates showed a decreasing trend in the order of macroaggregates,<0.053 mm aggregates and microaggregates.The contents of SOC/SIC in aggregates of different particle sizes depended more on the mass fraction of aggregates of the particle size.In addition,the relationship between SOC and SIC showed that there was a significant negative correlation（R²=0.25,P<0.01）between SOC and SIC in the erosional surface soil（0～25 cm）.However,SIC was significantly positively correlated with SOC in all depositional layers beneath 100 cm（P<0.05）.Nevertheless,when the deposition depth reaches 0～1600 cm,the positive correlation between SOC and SIC was not significant（P>0.05）.Sediment transport and deposition converted the negative relationship between SOC and SIC in the erosional sites to a positive relationship in deep deposited soil.The PLSR analysis suggested that SOC had an important（VIP>1）positive impact on predicting SIC in depositional sites,which indicated that SOC increase contributed to enhancing SIC accumulation under the depositional conditions in the semi-arid region.（2）The regulation mechanisms of organic carbon mineralization and its temperature sensitivity were different in erosion and deposition conditions.The study found that sediment deposition caused SOC,total nitrogen,heavy fraction organic carbon,and dissolved organic nitrogen to decrease by 20.0%,26.7%,20.1%,and 49.1%,respectively.However,due to the accelerated loss of dissolved nitrogen and the selective transport of light-fraction organic matter,a higher carbon-to-nitrogen ratio and the proportion of labile organic carbon in soil organic carbon were observed in the depositional sites.Soil temperature was the main factor explaining the change in the temporal variation in organic carbon mineralization.The average organic carbon mineralization rates were 2.18μmol m^-2 s^-1 and 2.89μmol m^-2 s^-1 in the eroded and deposited sites,respectively,and the average Q₁₀ is 2.49 and 3.38,respectively.Multiple stepwise regression analysis showed that soil organic carbon:total nitrogen and light fraction organic carbon explained 48.9%of the organic carbon mineralization variation,and the main control factors for Q₁₀ were dissolved organic carbon:dissolved organic nitrogen（34.6%）,soil temperature（17.0%）and dissolved organic carbon（10.0%）.The results suggested that the ratios of carbon-to-nitrogen and labile carbon-to-organic carbon play an important role in regulating the mineralization of organic carbon and its temperature sensitivity.The increase in carbon-to-nitrogen ratios induced by sediment deposition can contribute to increasing the mineralization of SOC and its temperature sensitivity in the topsoil.（3）The composition and influence mechanisms of SIC were different under the driving of erosion and deposition.The proportion of soil pedogenic carbonate in both erosional and depositional sites was greater than 45%,and there was a very significant positive linear relationship between the content of pedogenic carbonate and SIC（R²=0.78,P<0.01）.The contents of SIC and pedogenic carbonates decreased first（0～500 cm）and then increased（500～1600 cm）with depth in the deposited profile.The proportion of pedogenic carbonates origin from atmosphere source CO₂was larger than that from biological source CO₂.The proportion of soil biogenic CO₂ pedogenic carbonate in the depositional sites was significantly higher than that in the erosional sites,while the proportion of atmospheric CO₂ pedogenic carbonate was significantly lower than that in all erosional sites（P<0.05）.The results of multivariate stepwise regression and path analysis showed that calcium ions had the strongest direct positive effect on various forms of pedogenic carbonates.The main controlling factors of biogenic CO₂ pedogenic carbonate were calcium ions（43.4%）,soil organic carbon（11.9%）,and clay（3.4%）,which to a certain extent could reflect the regulation of the transfer and transformation from SOC to SIC.In summary,the increase of the carbon-nitrogen ratio in the depositional sites can increase the mineralization rate of SOC in surface soil,but the CO₂ generated from the mineralization of SOC in deep deposited soil is the main carbon source for the formation of pedogenic carbonate.The dynamic change of SOC and SIC in the erosion-deposition environment converted the relationship between SOC and SIC from a negative relationship in the erosional sites to a positive relationship in deep deposited soils.This indicates that increasing SOC content under depositional conditions in arid and semi-arid regions can increase the storage of SIC pools by promoting the formation of pedogenic carbonates.The results are of great significance for understanding the carbon sink/source issues related to soil erosion,and provide theoretical support for achieving carbon neutrality.