| As one of the important compositions of soil,soil organic carbon plays an important role in maintaining soil fertility and soil health.In the process of vegetation restoration in Loess Hilly Region,soil organic carbon pool also changed.Different small watersheds may have different effects due to their different goals,so it needs to be further studied whether they will have different impacts on the stability of soil organic carbon pool.This study selected four small watersheds with different vegetation restoration patterns?Fangta watershed,Zhifanggou watershed,Dongzhuanggou watershed and Yangjiagou watershed?as the research objects.And the soil organic carbon pool is divided into:active soil organic carbon pool?ASOC?,slow soil carbon organic carbon pool?SSOC?and passive soil organic carbon pool?PSOC?according to the mean residence time?turnover time?.Through field sampling,indoor cultivation and experiment determination,we used the structural equation model?SEM?to analyze the changes of the contents of soil organic carbon pools,residence time,decomposition rate and their influencing factors under different vegetation restoration patterns.Exploring the characteristics of soil organic carbon pool capacity,stability and main influencing factors under different vegetation restoration patterns,we hope to provide some practical guidance and data support for evaluating the effect of vegetation restoration on soil carbon sequestration in Loess Hilly Region.The main conclusions are as follows:?1?Among the different small watersheds,the stability of soil organic carbon in Dongzhuanggou watershed with natural restoration manner were stronger than that of the other three watersheds.Under Fangta watershed with farming manner,the proportion of ASOC,SSOC and PSOC in the 0-20cm soil layer was higher than that in the 20-40cm soil layer,and the order of their proportion was PSOC>SSOC>ASOC;On the whole,the proportion of SSOC and PSOC under kinds of vegetation restoration measures was higher than the abandoned farmland,but the proportion of ASOC was lower than the abandoned farmland.In the case of Zhifanggou watersheds with vegetation restoration manner,the proportion of ASOC and SSOC in 0-20cm and 20-40cm soil layers was higher than that in abandoned farmland.The proportion of PSOC was higher than that of abandoned farmland only in the surface layer,and the proportion of three pools showed SSOC>PSOC>ASOC.Under natural and artificial restoration?Dongzhuanggou and Yangjiagou watersheds?,the proportion of three soil organic carbon pools was:PSOC>SSOC>ASOC.?2?Among different small watersheds,the turnover time of ASOC under natural recovery?Dongzhaunggou watersheds?was the slowest?3.47-20.16d?.Therefore,the activity of soil organic carbon in natural restoration pattern was lowest and the stability of it was highest.However,the turnover rate of ASOC under farming?Fangta watershed?was highest?3.98-8.40d?,and its stability was relatively poor.The dynamic characteristics of soil organic carbon decomposition showed consistent rules,that is,in the early stage of decomposition,the decomposition rate was increasing and then reached the maximum value about 5 days,after that,the decomposition rate gradually decreased until finally it was in a relatively stable state.The turnover time of ASOC in the surface layer was higher than that in the sublayer under farming?Fangta watershed?,vegetation restoration?Zhiganggou watershed?and natural restoration?Dongzhuanggou watershed?.Under the conditions of vegetation restoration?Zhiganggou watershed?,artificial and natural restoration?Dongzhuanggou and Yangjiagou watersheds?,the turnover time of SSOC in the top soil layer was shorter than sub soil layer.The turnover time of PSOC is mainly affected by local annual average temperature.Because Zhifanggou basin is near to Fangta basin and Dongzhuangdou valley was near to Yangjiagou valley,the turnover time of PSOC under farming and vegetation restoration keep similar?325.34a?but the turnover time of PSOC under natural and artificial restoration keep similar?336.81a?.?3?In different small watersheds,the variation of soil physical and chemical properties was different because of vegetation types,slope,precipitation,etc.The content of SOC was the highest(6.88-11.56 g.kg-1)in Yangjiagou watershed?artificial recovery?and the lowest in Fangta watershed?farming?.Compared with artificial restoration?Yangjiagou watershed?,the SOC of natural restoration?Dongzhuanggou watershed?was lower,and SOC under vegetation restoration?Zhifanggou watershed?is higher than that under farming?Fangta watershed?.The range of soil moisture content in Zhifanggou watershed was largest and it was lowest in Fangta watershed.The soil bulk density in the 0-20cm soil layer was lower than that in the lower 20-40cm soil layer.In terms of soil pH,the soil pH under natural restoration and artificial restoration was relatively close,while the soil pH under farming and vegetation restoration was relatively close,and the soil pH under farming and vegetation restoration was relatively high.The variation of TN showed that the content of TN under natural restoration was the highest(0.28-1.10 g.kg-1),followed by artificial restoration(0.29-0.71g.kg-1),and the lowest under vegetation restoration(0.15-0.0.32g.kg-1).There was little difference in contents of TP among different vegetation restoration patterns.?4?On the whole,the carbon content of plant organs was the highest under artificial restoration,followed by natural restoration and the lowest under farming.Except the artificial recovery,the carbon content of each organ for the other three restoration patterns was leaves>branch>root.The order of aboveground biomass was Dongzhaunggou watershed?natural restoration?>Zhifanggou watershed?vegetation restoration?>Fangta watershed?farming?.The underground biomass of the surface layer was higher than that of the bottom layer among different recovery patterns.Under artificial and natural restoration?Yangjiagou and Dongzhaunggou watersheds?,the underground biomass was higher,the vegetation restoration?Zhifanggou watershed?was the second and lowest under farming?Fangta watershed?.?5?The richness of soil microorganism in hilly-gully region was higher than the tableland gully region,while microbial biomass of hilly-gully region was lower than the tableland gully region.Among the four different small watersheds,the content of MBC and MBN were higher in Dongzhaunggou watershed and lower in Yangjiahou watershed.There was little difference in the variation of soil microbial diversity index between Fangta watershe with farming manner and Zhifanggou watershed with vegetation restoration manner,as well as between Dongzhuanggou watershed with natural restoration manner and Yangjiagou watershed with artificial restoration manner.However,there was bigger difference in the variation of soil microbial diversity index between the hilly-gully region?farming and vegetation restoration?and tableland gully region?natural and artificial restoration?,especially Shannon and Simpson index,which showed the former was higher than the latter.?6?The water-stability of soil aggregates under natural and artificial restoration?Dongzhaunggou and Yangjiagou watersheds?were higher than that of vegetation restoration and farming?Zhifanggou and Fangta watershed?.And the water-stability of soil aggregates was closely related to the content of SOC.Under farming and vegetation restoration?Fangta and Zhifanggou watershed?,the soil aggregates of all vegetation restoration measures consisted of microaggregates mainly,while under natural and artificial restoration?Dongzhaunggou and Yangjiagou watersheds?,macroaggregates were mainly parts for the soil aggregates.Compared with Yangjiagou watershed,the proportion of macroaggregates was higher in Dongzhaunggou watershed.Compared with abandoned areas,the water stability of soil aggregates under various vegetation restoration measures was improved in both the surface and the lower layers,and the improvement effect was significant under natural and artificial restoration.On the whole,the ability of soil resistance to erosion?K value is 0.1678-0.1970?under artificial restoration?Yangjiagou watershed?was strongest.Correlation analysis showed that soil moisture content,bulk density,pH and TP had different effects on the proportion of soil aggregates,MWD and K values,while SOC and TN had similar effects on the proportion of soil aggregates,MWD and K values.?7?In different small watersheds,the soil organic carbon capacity and its stability were mainly affected by the content of MBC,SOC,the proportion of macroaggregates and the richness of soil fungi and bacteria.The stability of ASOC was mainly affected by the content of MBC and SOC,the proportion of 0.25mm-0.5mm soil aggregates,the value of MWD and K;The stability of SSOC was mainly affected by the proportion of>5mm aggregates and soil moisture content,and the former played a promoting role while the latter played a moderating role.The stability of PSOC was mainly affected by the content of TN,TP and MBC,pH,the proportion of 0.5-1mm,1-2mm,2-5mm soil aggregates and BD,among which,except BD,other indicators played a promoting role.?8?The stability of soil organic carbon in Dongzhaunggou watershed under natural restoration was stronger than other three restoration patterns.Therefore,the ability of carbon sequestration under the natural restoration was strongest.Microbial diversity and the physical protection of soil aggregates on soil organic carbon had more significant effects on soil organic carbon stability compared with vegetation biomass and carbon content in various plant organs. |
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