| As a sensitive ecological environment system,karst area has been widely concerned because of its unique dual structure.It is an active zone of material and energy transfer and transformation of water and terrestrial ecosystems and is sensitive to external changes.Karst area in south China have the largest area of exposed carbonate rocks among the three contiguous karst area in the world.Due to its extremely fragile ecological environment,a series of ecological environmental problems,such as soil erosion,soil degradation and rapid reduction of species diversity,are derived,among which the rocky desertification is the most prominent.Rocky desertification is the primary problem restricting the sustainable development in southwest China.Efficient utilization of limited available nutrients in rocky desertification areas to achieve a virtuous cycle of nutrients is of great significance for promoting the control of rocky desertification and predicting the future change of nutrient distribution across landscape due to changes in land management and hydrodynamic processes.According to the theories of geography,ecology,karstology,environmental science and other disciplines,it is necessary to explore the spatial distribution of dissolved organic matter and the mechanism of nutrient biogeochemical processes in karst areas.The Bijie Salaxi,Guanling-Zhenfeng Huajiang and Shibing karst research areas were selected in the Guizhou Plateau Mountains,which represent the overall structure of the southern karst ecological environment.Through field research,relevant experiments were designed in a targeted manner.Using qualitative and quantitative techniques such as spectroscopy and isotopes,and using EEMs-PARAFC and Mix SIAR models,the research mainly focuses on the differences in dissolved organic matter between different levels of rocky desertification,different vegetation restoration（covering）patterns,and different topographic differences.Focus on clarifying the distribution characteristics of dissolved organic matter in small drainages of rocky desertification of different grades,and analyze the sources of dissolved organic matter in soil,water and sediment in small drainages of rocky desertification of different grades.This study contributes to the scientific assessment of rocky desertification control ecosystems,and the scientific understanding of rocky desertification soil quality and nutrient cycling.（1）The heterogeneity of soil organic matter content and structural characteristics in karst small drainage was found,and the distribution characteristics of soil organic matter in different vegetation restoration（cover）modes and ground potential difference were clarified.The maximum mean values of SUVA254 in Chaeryan drainage and Heichong Drainage were 1.03±0.20 L mg C-1m-1and 1.07±0.23 L mg C-1m-1in abandoned land,respectively.The mean SUVA254 of abandoned land in Salaxi Drainage was 1.21±0.31 L mg C-1m-1.FI mostly concentrated between 1.4-1.9;HIX mainly distributed in 1.5-2.9.The SOC and DOC of site A1 in Salaxi and Heichong were higher than those of site A3 and site A2.In karst areas,SOC content is high,DOM content is low,soil microbial activity is low,low molecular weight organic matter lacks microbial action,soil aroma is weak,soil humification is weak,and the quality of energy matrix is inferior to that of other ecosystems.Therefore,the migration mechanism of effective energy matrix in karst areas must be clarified.（2）The characteristics of soil DOM migration to water body in karst area are revealed,and the structural characteristics of soil and water DOM in karst area are analyzed.Three kinds of fluorescence peaks were identified in soil and water in Chailyan Drainage,Salaxi drainage and Heichong Drainage,and the fluorescence spectral characteristics of the fluorescence components were highly similar.The mixed peaks of humus-like component peaks M,A and C and protein-like peaks all appeared.Two of them are hum-like peaks and one is protein-like peak.The relative abundances of DOM fluorescence components in soil and water of three small drainages with different grades of rocky desertification were humus-like>proteinoid.Therefore,we can judge that DOM in the water of the three small drainages has strong exogenous characteristics,and DOM migrates into the water.It is helpful to explain the mechanism of water-soil nutrient migration in karst area（3）The DOM sources of each small drainage under different intensity of rocky desertification were analyzed.In the Chareryan drainage（medium-intensity rocky desertification）,the contribution rates of prickly ash and peanut lands to sediment at the outlet of the drainage reached 46.7%,in the Salaxi drainage（potential-mild rocky desertification）and Heichong drainage（no-potential rocky desertification）,the sum of the contribution rates of tree forest and corn land reached 47.9%and 58.7%,respectively that is the main source of lost soil.The DOM of sediments at the outlet of karst small drainage is highly exogenous and the humification degree is very weak.Among the three components identified by the EEMs-PAEAFAC model,two components belong to difficult-to-degrade humus-like substances,representing the exogenous input of DOM in sediments.It provides qualitative and quantitative evidence for the migration of dissolved organic matter in small drainage.（4）It is pointed out that dissolved organic matter is highly sensitive to the grade of rocky desertification,which can be used as a potential index for the evaluation of the process of rocky desertification control,and can provide an indication for the selection of the model of rocky desertification control.The variation patterns of Uv-vis spectral parameters and fluorescence spectral parameters are very consistent with the landscape distribution of each drainage.Salaxi drainage（potential-mild rocky desertification）and Heichong drainage（no-potential rocky desertification）presented a decreasing（or increasing）trend from site A1 to site A2 and site A2 to site A3.In the small drainage of medium-intensity rocky desertification area,the export sediment DOM is the most exogenous,the humification degree is the weakest,and the DOM humus composition in water is the largest,followed by the small drainage with potential-slight rocky desertification,and the soil and water nutrient loss degree is the least.Soil and water nutrient loss occurs to a certain extent in karst areas with different intensity of rocky desertification,and the soil and water nutrient loss may be more serious in medium-intensity rocky desertification small drainage,followed by potentially mild rocky desertification small drainage,and the soil and water nutrient loss degree is the least in non-potential rocky desertification.Dissolved organic matter is highly sensitive to the grade of rocky desertification,which can be used as an evaluation index for the process of rocky desertification control,and can provide an indication for the selection of the model of rocky desertification control. |
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