Effects Of Water Erosion On Soil Organic Carbon Molecular Composition And Stability In Hilly Red Soil Region

Soil erosion is the most widespread form of soil degradation.It not only destroys soil structure and alters the living environment of microorganisms,but also causes in-situ loss of soil organic nutrients,and interferes the carbon cycles between soil and atmosphere.The soil carbon pool is three that of atmospheric carbon pool.Therefore,subtle changes in soil organic carbon（SOC）pool may cause large fluctuations in atmospheric CO₂ concentration,and then affect global climate.Although studies have shown that soil erosion can cause the dynamic changes of SOC,the specific affecting mechanism of soil erosion on SOC is still not clear.Based on this,a typical watershed in the Southern Red Soil Hilly Region was studied and soils from Quaternary red clay were analyzed.Spectroscopic and¹³C solid-state NMR techniques were used to characterize the molecular composition of both SOC and DOM.Mineralization incubation was used to investigate SOC stability.The relationship between the molecular composition of soil organic carbon and its stability was further explored The main research results are as follows:（1）Under the effect of long-term water erosion,molecular activity of the SOC in deposited topsoil are significantly higher than those in the eroded topsoil.¹³C NMR analysis showed that the molecular groups of SOC in both eroded and deposited soils were mainly alkoxy carbon（about 70%）.Because of the density of organic carbon in depositional site is significantly higher than that in eroding site（P<0.05）,more alkoxy carbon is contained in the deposited topsoil.Furthermore,the dissolved organic matter（DOM）content in deposited topsoil is significantly higher than that in eroded topsoil（P<0.05）,and the hydrophobicity,aromaticity and molecular weight of DOM in deposited topsoil were significantly lower than that in eroded topsoil（P<0.05）.In addition,the humus-like substance in DOM from eroded topsoil is significantly higher than that from deposited topsoil（P<0.05）,and correspondingly the organic matter humification degree is also higher in eroded topsoil than in deposited soil.（2）Under the effect of long-term water erosion,the stability of SOC in eroded topsoil is significantly higher than that in the deposited tops（P<0.05）.Results of mineralization incubation show that the cumulative CO₂ of SOC in both eroding and depositional sites decreased by 60%and 86%from soil 0-5 cm to 120-150 cm,respectively,which indicated that the stability of OC in deep soil Ian generally higher than that in topsoil.Furthermore,the cumulative CO₂ of SOC in deposited and eroded topsoil is 1909.6 mg CO₂-C kg-1 soil and 624.80 mg CO₂-C kg-1 soil,respectively,which is about three times as much as that of the later,indicating that the stability of SOC in deposited topsoil is significantly lower than that in eroded topsoil,under the effect of water erosion.With the increase of soil depth,the difference of SOC stability between eroded and deposited soil decreases gradually.When the soil depth reaches 120-150 cm,the difference of SOC cumulative CO₂ between the two sites is only 18 mg CO₂-C kg^-1soil,indicating that water erosion has little effect on the stability of deep soil organic carbon.（3）Under the effect of water erosion,SOC stability is negatively correlated with the O-Alkyl-C and DOM contents,but positively correlated with the aromaticity,hydrophobicity,molecular weight and humus-like components of organic matter.The correlation coefficients between cumulative CO₂ and O-Alkyl-C in eroding and depositional sites are as high as 0.98 and 0.99,which indicates that the content of O-Alkyl-C determines the stability of SOC to a certain extent.Furthermore,the correlation coefficients R² of cumulative CO₂ and DOM content in eroding and depositional sites are 0.92 and 0.99,respectively.The correlation coefficients R²of cumulative CO₂ and DOM content vary from 0.74 to 0.98.The results show that the process of spatial differentiation of DOM content and molecular attributes induced by hydraulic erosion is likely to be a process of formation.These results indicate that the spatial differentiation of DOM content and molecular attributes induced bywater erosion is likely to be the root cause of the difference in SOC stability between eroded and deposited soils.In conclusion,water erosion has a significant impact on the composition and stability of topsoil OC.Changes in the molecular composition of SOC are key factors for the differentiation of SOC stability in eroding and depositional sites.