| Soil organic carbon（SOC）is not only an important indicator of soil quality,but also an important factor affecting the carbon cycle process of the whole ecosystem under the background of global climate change.The inhomogeneity of bare rock distribution and the diversity of underground rock structure in different karst microhabitats resulted in high soil heterogeneity,and the stability of soil organic carbon in the microhabitats has an important impact on the ecological restoration and carbon sequestration assessment in this region.However,there are still many uncertainties based on the impact mechanism of soil organic carbon accumulation and stability in karst microhabitats.The study selected six karst microhabitats including rocky soil surface,stone crevice,stone gully,stone cave,stone pit and soil surface as the research objects,and the soil organic carbon is divided into free light organic fraction（LF）,occluded-particulate organic fraction（OF）and mineral-associated organic fraction（MF）.Based on that,researches on the changes and stability characteristics of SOC,and its impact mechanism in different karst microhabitats were conducted through the combined method of field investigation and laboratory experiments,and we hope that the results of this study could provide some theoretical basis for the restoration,reconstruction and scientific management of rocky desertification ecosystem.The main findings were summarized as following:（1）Among different karst microhabitats,stone cave has the strongest C sequestration capacity,while the proportion of stable organic carbon fractions was lower than other microhabitats;soil surface was lower in the C sequestration capacity and higher in the proportion of stable organic carbon fractions than the rest of microhabitats.The average contents of soil organic carbon（SOC）ranged from 28.45to 119.41 g·kg-1 in the 0–30 cm soil profile of the microhabitats;the highest C density was observed in stone cave,with the value of 211.35 Mg C·ha-1,2.5 times that of the soil surface with the lowest C density.The C contents of soil free light organic fraction,occluded-particulate organic fraction and SOC were the largest in stone cave,the smallest in soil surface among different microhabitats,and both decreased with the increasing soil layer.The C contents of mineral-associated organic fraction has no significant change between different microhabitats and soil layers,but with the increase of soil depth,the proportion of stable organic carbon in total SOC is increasing.The proportions of free light organic fraction as a percentage of total SOC decreased with the soil layer among the six microhabitats.The occluded-particulate organic carbon fraction accounted for the largest part of the total soil organic carbon,while the proportion of mineral-associated organic carbon fraction is the lowest regardless of microhabitat type and soil layer.（2）Soil bulk density,saturation moisture content,noncapillary porosity and saturated hydraulic conductivity are the key soil physical factors that affect the stability of soil organic carbon,and the C content of occluded-particulate organic fraction was most affected by soil physical factors.Karst microhabitat types significantly affect soil bulk density,volumetric moisture content,saturation moisture content,capillary moisture capacity and saturated hydraulic conductivity,and there is a strong correlation between soil physical factors and C contents of organic fraction and its proportion in total SOC.Although the soil porosity index is not affected by the microhabitat type,the noncapillary porosity significantly changes the C contents of soil occluded-particulate organic fraction and the proportions of each carbon fraction accounting for SOC.（3）Microhabitat types have a greater impact on soil chemical factors,except for C:N and water soluble calcium,and the stability of SOC could be significantly affected by soil chemical factors.Soil total nitrogen,total phosphorus,available phosphorus and phosphorus-related stoichiometric ratios have the greatest impact on the C contents of free light organic fraction;total phosphorus,P:S ratio and exchangeable sodium and magnesium are the key factors affecting the C contents of occluded-particulate organic fraction.The C content of mineral-associated organic fraction is mainly affected by water soluble potassium,sodium,magnesium and exchangeable sodium.The percentages of free light organic fraction and occluded-particulate organic fraction C contents to SOC were significantly affected by total nitrogen,total phosphorus,P:S ratio,exchangeable calcium,exchangeable magnesium,available nitrogen and available phosphorus.The percentage of mineral-associated organic carbon content to SOC is mainly affected by total phosphorus,total sulfur,water soluble magnesium,total nitrogen and nitrogen-related stoichiometric ratios.Among the soil chemical factors,total nitrogen contributed the most to the changes of C contents of organic fraction.（4）The C:HOF,NLF,NMF and NOF are the four most critical characteristic factors affecting C contents of soil organic fractions.Different organic fractions have specific element composition characteristics,occluded-particulate organic fraction has higher nutrient contents,and mineral-associated organic fraction has the lowest nutrient contents.The variation of N,H,S contents and their stoichiometric ratios in the organic fractions among different microhabitats were also the highest in stone cave and the lowest in soil surface.The C contents of soil organic fraction were closely related to its chemical composition,especially the N and S contents.The C:H and N:H of occluded-particulate organic fraction and mineral-associated organic fraction were also positively correlated with the C contents of the occluded-particulate organic fraction.The contents of N,H,S and the C:S and N:S ratios in the free light organic fraction have the most significant influence on the proportions of each carbon fraction accounting for SOC.（5）Microhabitat characteristics are the important factors affecting the stability of SOC.On the one hand,microhabitat characteristics directly affect the carbon fractions and its proportion in the total organic carbon.On the other hand,influence factors of organic carbon stability were affected by the microhabitat characteristics,thus further altering the stability of SOC.The microhabitat characteristics of rocky soil surface and stone gully have the greatest impact on the influence factors of SOC stability.The C content of free light organic fraction is more affected by microhabitat characteristic than other fractions at stone crevices,stone cave and stone pits,but for the other microhabitats,the C content of occluded-particulate organic fraction is most susceptible to microhabitat characteristics.The C content of free light organic fraction and occluded-particulate organic fraction decreased at rocky soil surface and soil surface with soil area,while the percentage of mineral-associated organic fraction C content to SOC increased with soil area,which is conducive to soil organic carbon fixation.The medium stone cave and the medium stone pit contained higher C contents of free light organic fraction and occluded-particulate organic fraction,but the mineral-associated organic fraction C contents account for the smallest proportion of total organic carbon,so the stability is poor.The largest C contents of free light organic fraction and occluded-particulate organic fraction have been found in the wide mouth hetero-dip direction stone gullies and co-dip direction stone crevice,however,the proportion of stable organic carbon is lower.（6）The most critical factor affecting the stability of SOC in karst microhabitats is the chemical element composition of soil organic fraction.The order of interpretation of soil physicochemical factors on the variation of soil organic carbon fraction is:total nitrogen,capillary moisture capacity,total phosphorus,C:P,C:S,P:S,C:N,water soluble magnesium and volumetric moisture content.Soil physical factors have less explanation for the variation of organic fraction C contents than chemical factors,but showed a higher explanation of mineral-associated organic fraction C contents in comparison with other fractions.Chemical factors have the highest interpretation of free light organic C contents and the lowest interpretation of mineral-associated organic C contents.Except for the mineral-associated organic fraction,the variation of other fractions and total organic carbon content is mainly determined by the interaction of soil chemical and physical factors,the order of interpretation is total organic carbon>occluded-particulate organic fraction>free light organic fraction. |
