Soil Carbon And Nitrogen Coupling And Sedimentary Organic Matter Sources In A Small Karst Watershed

Since the Fifth Plenary Session of the 19th CPC Central Committee,how to scientifically promote comprehensive soil erosion management in karst areas has become an urgent practical problem.Soil nutrient loss caused by soil erosion has seriously limited the sustainable development of agricultural production,and it is important to explore the soil carbon and nitrogen coupling relationship and the source of sedimentary organic matter produced by erosion in karst areas to scientifically promote soil and water conservation.Due to the unique binary three-dimensional spatial structure of karst areas,the widespread development of above-and below-ground fissures and the complex topographic structure,soil erosion caused by rainfall leads to the redistribution of large amounts of soil and loss of organic matter,and the sedimentary organic matter in the watershed is the most direct product of soil erosion.Scientific mitigation of nutrient loss in karst areas and promotion of coordinated development of agricultural production and environmental protection have become the urgent needs for ecological development in karst areas.Soil organic matter is the main nutrient for plant growth and development.Therefore,understanding the sources of sedimentary organic matter and soil C and N characteristics caused by erosion is important for scientific soil erosion management in karst areas,as well as carbon fixation.Different land types in karst areas have an important influence on sedimentary organic matter produced by soil erosion in watersheds,and the contribution ratio of sedimentary organic matter sources can directly reflect the influence of anthropogenic activities on soil erosion in watersheds.At the same time,achieving the mitigation of watershed soil nutrient erosion with soil erosion at the source is fundamental to stone desertification control.Therefore,the quantitative identification of different sources of sedimentary organic matter in watersheds will help the development of watershed nutrient measures,as well as provide some theoretical basis for studies on land resource management and soil nutrient loss in karst areas.To further understand the dynamic process of sedimentary organic matter sources in karst small watersheds,to explore the coupling relationship between soil carbon,nitrogen and their stable isotopesδ¹³C andδ¹⁵N in areas with different stone desertification intensity,and to clarify whether soil carbon and nitrogen coupling can provide a more accurate indication for organic matter source determination.In this study,the study areas of potential-light rocky desertification in Bijie-Salaxi Karst Plateau Mountains,medium-intensity rocky desertification in Guanling-Zhenfeng Huajiang Karst Plateau Canyon and no-potential rocky desertification in Shibing Karst Plateau Trough were selected in the southwest Karst region represented by Guizhou,respectively.Long-term fixed-point monitoring,comparative analysis and Iso Source modeling and stable carbon and nitrogen isotope(δ¹³C,δ¹⁵N)tracing techniques were conducted.To explore the distribution characteristics and influencing factors of vegetation and soil organic carbon and nitrogen in areas with different stone desertification intensities,as well as the coupled carbon and nitrogen isotopes in karst sub-basins,to quantitatively and qualitatively analyze the relative contribution of different land types of soil organic matter sources to the sedimentary organic matter in sub-basins,and to elucidate the sedimentary organic matter sources and the contribution of different land types to the sedimentary organic matter sources in sub-basins.To investigate the coupling relationship between soil carbon,nitrogen and their stable isotopesδ¹³C andδ¹⁵N in areas with different stone desertification intensities,and to clarify whether soil carbon and nitrogen coupling can provide more accurate indications of organic matter sources are the key scientific questions to be addressed in this study.1.It was found that there were significant differences in soil carbon and nitrogen contents and distribution of the same vegetation type and land type in the study areas with different stone desertification intensities,with obvious geographical characteristics.There was no significant difference between the SOC contents of different vegetation types in three different stone desertification intensity areas,while the TN contents of tree and shrub forests were significant,specifically the plant N contents were significantly higher in no-potential stone desertification areas and potential-light stone desertification areas than in medium-intensity stone desertification areas.By comparing the soil SOC and TN contents of the three studies,it was found that the soil N content in the medium-intensity stone desertification area was significantly higher than that in the other two study areas.In addition,regarding the distribution characteristics of soil SOC and TN contents,the soil C and N contents of arboreal woodlands in the three study areas were significantly higher than those of other land types,except for the Zhenfeng-Huajiang study area where there was no significant difference in TN contents among land types.In terms of vertical variation,both soil SOC and TN in the no-potential stone desertification area and potential-light stone desertification area had the highest content in the top soil layer,and both showed a gradually decreasing trend with the increase of soil layer depth.In contrast,the soil SOC content in the medium-intensity stone desertification area increased with the increase of soil layer depth,and the TN decreased with the increase of soil layer depth in general.It indicates that vegetation growth in medium-intensity stone desertification areas is more C-limited and less efficient in N use,while the other two study areas may be more N-limited.The study of vegetation and soil carbon and nitrogen content and distribution in areas with different stone desertification intensities helps us to further understand the characteristics of carbon and nitrogen dynamics,and provides scientific basis for the study of carbon and nitrogen cycle in karst areas and regional land resource management.2.By comparing the plant and soilδ¹³C andδ¹⁵N values in three different stone desertification intensity areas,it was found that both plant and soilδ¹³C andδ¹⁵N in the medium-intensity stone desertification study area were more positive than in the potential-light stone desertification area and the no-potential stone desertification area,and the soilδ¹³C andδ¹⁵N showed a general trend of vertical variation of increasing first and then decreasing with increasing soil layer depth.Theδ¹³C of vegetation in the medium-intensity stone desertification area（-27.15‰）was relatively positive compared with the potential-light stone desertification area（-27.27‰）and the no-potential stone desertification area（-28.33‰）.Similarly,soilδ¹³C（-21.79‰）andδ¹⁵N（9.77‰）were significantly higher（P<0.05）in the medium-intensity rock-deserted areas than in the potential-light rock-deserted areas（-24.10‰and 7.33‰）and the no-potential rock-deserted areas（-24.27‰and 7.79‰）.It indicates that vegetation in medium-intensity rock-deserted areas consumes less water and has higher water use efficiency based on the same amount of carbon sequestration.In addition,the ecosystem N saturation is higher in the medium-intensity stone desertification area,which makes its soil N effectiveness also relatively higher,and there is a compensation mechanism that water use efficiency and nitrogen use phase rate are mutually constrained.The vertical variation of soilδ¹³C andδ¹⁵N showed a general trend of increasing and then decreasing with increasing depth of soil layer.δ¹⁵N maximum value mainly appeared at 20-40 cm of soil layer,and the main reason for the appearance ofδ¹⁵N maximum value in the middle layer of soil in karst area may be due to the strong denitrification of soil in the middle layer,which caused theδ¹⁵N depleted NO₃^-to be expelled as gas,thus leading to the maximum value ofδ¹⁵N in the middle layer of soil.The maximum value ofδ¹⁵N in the middle layer of the soil may be caused by the strong denitrification of the soil in the middle layer,which causes the maximum value ofδ¹⁵N.In addition,the soil organic matterδ¹³C in shrublands in areas of no-potential stone desertification and in shrublands,grasslands and croplands in areas of medium-intensity stone desertification varied considerably（5.5%-5.98%）with depth of soil layer,while usually the soil organic matterδ¹³C increased 1%-3%with depth,so it can be inferred that this land type was mainly dominated by C₄ plants in the past.This study provides scientific and theoretical guidance for effectively avoiding and overcoming water stress on vegetation in areas with different stone desertification intensities and for the study of land disturbance history.3.It was found that the variation between SOC and TN of soils of different land types in karst sub-basins and their stable isotopes(δ¹³C andδ¹⁵N)between different soil layers were not synergistic,making the coupling relationship between them not consistent.In this paper,traditional correlation analysis and regression analysis are used to analyze the correlation and linear relationship between soil SOC and TN andδ¹³C andδ¹⁵N in different land types in three study areas.The study shows that there are two cases of coupling relationship between soil SOC and TN andδ¹³C andδ¹⁵N in karst sub-basins:the coupling relationship is not consistent;and the coupling relationship has opposite characteristics,respectively.Since the purely natural correlation between soil C and N was considered,an attempt was made to analyze the coupling between C and N and their stable isotopes in the surface,intermediate and deep layers of soils of different land types,and to analyze the coupling between soil C and N in the whole soil layer of different land types.The results show that there are differences in soil C and N coupling among different land types,and there are also significant differences between different soil layers of the same land type.There is a significant coupling relationship between soil C and N in the surface and intermediate layers,while soil C and N in the deep layers show decoupling（change in opposite directions）.Overall,soil C and N coupling is not consistent with its stable isotope coupling,and the coupling relationship is influenced by vegetation type,soil layer depth,and climatic factors such as temperature and precipitation.It indicates that the interference of various factors needs to be considered when using soilδ¹³C andδ¹⁵N coupling as indicative indicators to determine the conversion of soil organic matter and the origin of the elements.4.The qualitative and quantitative analyses showed that there were differences in the sources of soil deposited organic matter among the study areas.The qualitative analysis showed that the main sources of soil deposited organic matter in the study areas of Shibing and Bijie-Salaxi were soil surface organic matter and C₃ plant debris,with less contribution from C₄ plants,while the source of deposited organic matter in the study area of Zhengfeng-Huajiang was probably C₄ plant debris and soil surface organic matter.In addition,the quantitative analysis using soilδ¹³C andδ¹⁵N combined with the Iso Sourece model showed that the range ofδ¹³C values of soil sedimentary organic matter in the study area of Shibing Karst was-26.66‰to-26.17‰,and its main source was the rice field soil,with a contribution of 79.3%.It was followed by maize land with a contribution of 7.2%,peach forest with 5.2%,roasted tobacco with 4.3%,and arboreal woodland with the lowest contribution（4%）.The sedimentary organic matterδ¹³C values in the Bijie-Saraki karst study area ranged from-26.38‰to-25.91‰,with the highest contribution from maize land and grassland with 48.65%and 40.54%,respectively,and the contribution from tree woodland,shrubland,and potato land were less than 10%with 7.71%,2.7%,and 0.4%,respectively.The sedimentary organic matterδ¹³C values in the catchment area of the Zhengfeng-Huajiang karst study area ranged from-21.57‰to-20.26‰,and its main contributing source was the topsoil of pepper land,which contributed 88.2%,while the contributions of the remaining peanut land,corn land,grassland,and tree woodland were 7.3%,2.1%,1.9%,and 0.5%,respectively.The main sources of deposited organic matter in each study area were the main land types in the watershed,and the contribution of their sources was related to the area of distribution of that land type in the watershed.This study provides theoretical references for soil nutrient loss in karst areas and regional land resource management.