Study On Soil Organic Carbon And Its Ecosystem Services Function In The Process Of Rocky Desertification In Karst Area

The Karst area is a unique ecosystem that differ from non-karst area in terms of terrain and landforms due to the special geological and climatic conditions.The Karst mountainous area shows low environmental capacity,low stability and poor self-regulation.Moreover,Karst landform distribute in China widely,with low ecosystem services in its rocky desertification area and lacking of stability.Therefore,the government of the People's Republic of China has produced a series of policies and regulations aimed at controlling the rocky desertification,to realize the goal of promoting ecosystem services function.The main composition of rock in Karst area are carbonate rocks,and carbonate is the largest reservoir of carbon storage in the world.The problem of rocky desertification in Karst area is serious.In the process of Karst rocky desertification,the change in SOC content has a significant impact on the global carbon cycle,soil quality and ecosystem functions.The trend of global warming continued rising according to China climate change blue book(2020),it showed that China was the sensitive and significant area of global climate changing,and the extreme weather events were becoming more.According to the statistics on the global carbon stock,the reserve of soil organic carbon(SOC)in the soil horizon(0–100 cm)reached 1460 Pg.Which was three times greater than those of the vegetation carbon reservoir and two times greater than those of the atmospheric carbon reservoir,indicating that SOC plays an important role in the global carbon cycle.Because there are large reserves of organic carbon in soil reservoirs,small changes in SOC may lead to changes in the global climate.In addition,SOC is an important basis for soil fertility,and it plays a key role in improving soil quality and promoting the increase of agricultural output.Therefore,the conversion of SOC has a direct impact on the stores of carbon balance on land,plant growth and soil fertility,so it plays a key role in supporting the ecosystem services.Because SOC reserves in Karst rocky desertification area is huge and its high spatial heterogeneity,it has become a hot research field.In addition,the rocky desertification area in South China is facing with problems such as prominent contradiction between human and land,serious vegetation degradation,lacking of ecosystem stability,and low ecosystem service functions.In 2020,the National Development and Reform Commission and the Ministry of Natural Resources jointly issued the Master Plan for the Protection and Repairing Major Ecological System Protection and Restoration of the National Ecological System(2021-2035),which pointed out that through the implementation of ecosystem protection and restoration major projects that promote the comprehensive enhancement of ecosystem service functions.It clarified the comprehensive prevention and control of the Karst rocky desertification in Guangxi,Guizhou and Yunnan,and planned to continue to control the area of rocky desertification about 3.94×104 km~2.In 2022,the No.2 State Council File Opinions of the State Council on supporting Guizhou in the new era of western expansion clearly clarified the strategic position that Guizhou is an ecological civilization construction pilot zone.Therefore,carrying out research on Karst rocky desertification SOC and its ecosystem services,which is of great significance and contribution for Karst areas in carbon peak/neutralization,ecosystem service optimization strategy,rocky desertification control technology,characteristic industry development and rural revitalization layout.Based on the theories of geography,ecology,environmental sciences and others,these theories include time-space interchange,ecosystem services and trade-offs/synergy.Aimed at the service function of Karst rocky desertification controlling ecosystem,and how to support and adjust the ecosystem service,trade-off/coordination relationship,optimization mechanism and other scientific issues and technological requirements such as driving rural revitalization.Therefore,Bijie Salaxi area,Guanling-Zhengfeng Huajiang area,Shibing Karst area,Puding Maguan area and Libo-Huanjiang Karst area in Guizhou plateau mountainous area were chosen as study areas,where represent the overall structure of the environment economics in south China Karst.From 2018 to 2022.Based on continuous field orientation and observation,the 1496 soil samples and others information were collected by systematic method,and the SOC,Nitrogen,Phosphorus,Potassium and soil moisture etc indexes were tested in turn.Use Pearson correlation analysis,principal component analysis,redundancy analysis,multiple nonlinear regression,root mean square deviation method,ratio method and some other methods.The distribution characteristics of SOC,key impact factor,coupling mechanism,SOC support for ecosystem services,the trade-off/synergy analyses of different ecosystem services and optimization strategy were studied respectively.The results were showed as follows:(1)It was found that the spatial distribution of SOC and its impact factors are highly heterogeneous in rocky desertification,and the SOC contents increased with the increase of rocky desertification in a certain extent,breaking the inherent impression of high rocky desertification grade with low SOC:the SOC contents of different landform types ranged from 4.64-38.20 g/kg,and the SOC contents of Karst plateau mountain,plateau canyon,plateau trough valley and canyon,plateau basin and low mountain hills respectively ranged from 6.34-35.17,6.28-38.20,4.64-27.08,4.62-23.79 and 6.12-37.85 g/kg.The corresponding coefficients of variation were 0.23,0.37,0.19,0.26 and0.17,all of which reached a medium-high level of variation.Among them,the SOC contents of Karst plateau canyon medium-strong rocky desertification were greater than others Karst landform and its variation was the largest.Due to the influences of the“aggregation effect”of the micromorphic landforms(such as stone basin,stone tank,stone trough and stone ditch,etc.),that result in SOC had the spatial distribution pattern of aggregation in surface soil layer,and the higher the degree of rocky desertification was,the more prominent this distribution characteristic was.The comparison revealed that there was high content and high variation of SOC in medium-intensity rock desertification and low content and low variation in no-potential rocky desertification,the key impact factors were highly different,breaking the notion that SOC was poor when the rocky desertification grade was high,which is helpful to reveal the distribution and migration mechanism of SOC in rocky desertification area.(2)The spatial distribution remodeling mechanism of the SOC density in southern China Karst was different from its content,with the characteristics of high content and low density.They show a reciprocal development process with the increase of rocky desertification,and the higher the degree of rocky desertification was,the greater the contribution of the SOC density in upper soil layer:The SOC content has a tendency to increase with the degree of rocky desertification under the aggregation effect of micro-topography;The increase of rocky desertification and the thinning of soil layers had weakened the storage capacity of SOC,resulting in a generally smaller SOC profile density value(7.06-10.52 kg/m~2)than the national average(10.53 kg/m~2),which reflected the reciprocal trend of its content and density.At the same time,the SOC density of each soil layer in low-level rocky desertification was relatively uniform,while the main contribution of SOC density in high-level rocky desertification came from the 0-40 cm soil layer,and the contribution of SOC density in the topsoil layer of its higher rocky desertification degree was relatively larger,up to 87%.The comparison revealed that highly rocky desertification soils had weak storage capacity for SOC,low uniformity of surface-bottom distribution,and lacking of stability due to easy disturbance.This was helpful to reveal the difference and connection between SOC content changes and soil carbon sequestration benefits during rocky desertification process.(3)The driving mechanisms of SOC ecosystem services in south China Karst rocky desertification area was revealed,and the ecosystem service functions of SOC in agroforestry was generally higher than other ecosystem types,and all decreased with the increase of rocky desertification:SOC had a significant promote effect on total nitrogen(TN)and total phosphorus(TP),and it was more significant in agroforestry ecosystems,with each 10%increase in SOC corresponding to 12.50%-48.32%and 17.62%-26.10%increase in TN and TP,respectively.However,with the increase of rocky desertification,the increase in TN and TP respectively decreased by 35.82%and 8.48%.There was no significant direct positive correlation between SOC and total potassium(TK)of the soil,which was the mechanism that influenced the migration of TK through TN;In addition,SOC content was highly significantly negatively correlated(r=-0.99,p<0.01)with K value of soil erosion factor.Fitting curves with soil water:y=-0.00064x2+0.5021x+17.016(R~2=0.527,p<0.05),too high or too low SOC content was not conducive to soil moisture retention,the relative suitable SOC content were between 26.02-32.35 g/kg;Meanwhile,the agroforestry SOC content and its light group/active fraction were both relatively high.Overall,the ecosystem service functions and synergism of SOC in rocky desertification area showed that agroforestry>forest>grassland,and no-potential>potential-light>medium-intensity rocky desertification,which laid the foundation for optimizing and enhancing the ecosystem service functions of rocky desertification SOC.(4)The agroforestry SOC high content and high synergistic ecosystem service model for potential-light rocky desertification prevention and controlling in Karst plateau mountains,the agroforestry SOC high content and low synergistic ecosystem service model for medium-intensity rocky desertification controlling in plateau canyon,and the agroforestry SOC low content and high synergistic ecosystem service model for no-potential rocky desertification prevention and controlling in plateau trough valley were created respectively,and the scalable areas in rocky desertification control was analyzed.SOC and the associated project synergy were the basis of rocky desertification management to enhance ecosystem service functions.By analyzing the D_RMS values of0.116,0.127 and 0.109 for Bijie,Guanling-Zhenfeng and Shibin models,respectively,SOC had good synergy with various ecosystem service functions.According to the natural and social differences of the study area,five sub-models of“forest-grain”,“forest-vegetable”,“forest-medicine”,“forest-grass”and“forest-fruit”were constructed respectively.According to the distribution of Karst rocky desertification in southern China,these three main models account for 36.48%,9.86%and 67.29%of the total Karst area in eight southern provinces/regions,respectively.The analysis of SOC ecosystem service synergy and optimization technologies and their application contexts need to be strengthened in rocky desertification controlling process to promote sustainable development of rural revitalization ecological construction industrialization.