Soil-carbon Characteristics And Stability In Different Ecosystems On The Loess Plateau

Soil organic carbon is a mixture of complex,heterogeneous organic compounds.Increasing organic carbon stocks in soils has become an effective means of improving soils,increasing plant productivity,delaying atmospheric carbon dioxide rise and climate change,while maintaining ecosystem quality.Historical studies have focused on topsoils,but as research has progressed,more and more attention has been paid to the study of organic carbon in deep soils.In this study,different ecosystems(grassland ecosystems,farmland ecosystems and sandy ecosystems)of the Loess Plateau were selected as research objects,and the three-pool first-order dynamic theory was used to explore the organic carbon pool and fractions change characteristics of soil layers of 0-100 cm soil in the process of vegetation restoration in different ecosystems.The size and turnaround time,content and allocation ratio of soil organic carbon pools in different ecosystems were revealed,and the relationship between soil organic carbon pools and components of different ecosystems was clarified.This study has important theoretical significance and practical value for the effective management,sustainable utilization,carbon pool changes and corresponding ecological and environmental effect evaluation of different ecosystems on the Loess Plateau,and provides a scientific basis for better understanding the carbon sequestration mechanism of vegetation restoration process.The main findings and conclusions are as follows:1.The content of passive carbon pool in different ecosystems is much greater than that of slow carbon pool and active carbon pool.grassland soils,passive carbon pool(7.97 g.kg-1)> slow carbon pool(4.72 g.kg-1)> active carbon pool(1.44 g.kg-1);farmland soils,passive carbon pool(5.29 g.kg-1)> slow carbon pool(3.66 g.kg-1)>active carbon pool(0.73 g.kg-1).The organic carbon release rate decomposes rapidly in the early stage and slowly decomposes in the later stage.It shows that the activated carbon pool is a direct source of microbial utilization,which is quickly decomposed,and the inert carbon pool is not easy to be decomposed,mainly contributing to the accumulation of organic carbon in the soil.2.The proportion of active carbon pool(12.92%)and passive carbon pool(54.46%)in grassland soils was greater than that in farmland soils(7.13% and 50.50%,respectively),while the proportion of slow carbon pool(37.37%)in farmland soils was greater than that in grassland soils(31.86%).Compared with farmland,the carbon sequestration capacity of grassland is better,and the active carbon of farmland is transferred to slow carbon faster,and it is more sensitive to external interference.3.Grassland fencing can significantly improve the stability of SOC,and the turnover rate of active carbon pool in each soil layer of grassland for 1 year,10 years and 15 years is faster(0.5-1.8 days),and the turnover of active carbon pool in grasslands for 20 years,30 years and 40 years is slower(0.9-724.6 days),indicating that the increase of the fencing time can slow down the turnover time of activated carbon pool,which is conducive to the stability of SOC.Each organic carbon pool and organic carbon release rate of the farmland ecosystem are the core area> the buffer area > the experimental area,the core area of the active carbon pool turnover time(221.4 days)>the experimental area(140.7 days)> the buffer area(1.8 days),and the soil active carbon pool turnover in the buffer farmland is the fastest.4.Grassland fencing and crustal development have a positive effect on SOC increase.MAOC mainly contributes to the increase of SOC in soil with higher organic carbon content,and soil with low organic carbon content,POC mainly contributes to the increase of SOC.The development of biological soil crusts promotes the accumulation of SOC,compared with bare sand,the development of biological soil crusts increased the MAOC ratio and reduced the POC ratio.Grassland and farmland ecosystem SOC exist primarily as carbon in the form of MAOC.When the soil increased the unit amount of SOC,Organic carbon in the form of POC: sand land accounted for 45.18%,grassland accounted for 8.77%,farmland accounted for 5.26%;Organic carbon in the form of MAOC: grassland accounted for 59.85%,farmland accounted for 55.10%,and sand land accounted for 10.89%.5.The relationship between content of POC and MAOC and active carbon pool,slow carbon pool and passive carbon pool content are very significant correlation(P<0.01),in the same soil carbon pool increased by units of organic carbon,MAOC increased much more than the increase in POC.The increase of MAOC in the active carbon pool is much greater than that of the slow carbon pool and the passive carbon pool,the MAOC increase of the slow carbon pool and the passive carbon pool is similar,the POC increase of the three organic carbon pools in the farmland ecosystem is the same,and the increase of the POC in the grassland ecosystem is: the active carbon pool(1.72 g.kg-1)> the passive carbon pool(0.23 g.kg-1)> the slow carbon pool(0.22 g.kg-1).6.The relationship between POC and SOC,MAOC and SOC reached a very significant positive correlation relationship(P<0.01)in farmland,grassland and sand.when increasing the unit amount of SOC,the POC increase rate of grassland was the fastest,2.00 times and 1.69 times that of farmland and sand,respectively;the MAOC increase rate of sand was the fastest,1.73 times and 1.39 times that of farmland and grassland,respectively.the sensitivity of grassland soil POC to the response of total soil organic carbon change was 101.7% and 70.4% higher than farmland and sand.the response sensitivity of sandy soil MAOC to soil organic carbon changes was 72.4% and38.8% higher than that of farmland and grassland.In summary,grassland encapsulation and crusting development promoted the accumulation of soil organic carbon,and the stability of organic carbon in grassland soil was better;POC and MAOC were very significantly related to the soil organic carbon pool,MAOC mainly contributed to the increase of SOC in soils with high organic carbon content,and POC mainly contributed to the increase of SOC in soil with low organic carbon content.