Soil Organic Carbon Properties Of Subsidence Wetland In The Coal Mining Areas With High Ground Water Table

China has surpassed the United States as the biggest GHGs emission country,with large pressure of energy conservation and emission reduction.Coal is one of the most important energy resources,and its contribution to the global GHGs emission is about 19%.In China,coal is the main energy resource,and its dominant position will not change in the near future,therefore,we need to raise our attention to the carbon cycle in the coal mining areas.Despite the GHGs emissions from the coal industry,the land use change and eco-environment impact caused by coal mining activities largely influence the carbon cycle in the mining areas.In the coal mining areas with high underground water table,there are large scale of land subsidence and waterlogging areas caused by coal mining activities,and is the key focus in the ecological restoration in the region.The carbon storage per unit is highest in the wetland system,and could be effectively stored for a long time,which plays a key role in the global climate change.The rational utilization and restoration of the subsidence wetland has already been focused,however,the following issued haven’t been discussed:(1)how the formation and utilization of the subsidence wetland impact on the soil carbon,and what is the main impact factors?(2)what is the difference of the soil carbon compared with the former farmland system and the natural wetland system?(3)what is the difference among the variety utilization types of subsidence wetland impact on the soil carbon? In this study,subsidence wetland in the coal mining areas with high ground water table has been defined,categorized and its formation has been analyzed.Control farmland,a typical,and the main utilization types of wetlands under the same soil type has been chosen in the coal mining areas with high ground water table.Based on the field sampling,laboratory test and statistic analysis,the spatial and vertical distribution of soil organic carbon in the typical subsidence wetland has been stated,furthermore,the liable organic carbon fraction has also been studied to better understand the features of the soil organic carbon and try to find out the main impact factors of its formation and distribution.In addition,subsidence wetland with different utilization types has been chosen to study the human impact on the organic carbon.The research could provide a basic data for the accurate estimation of the soil carbon storage in the coal mining areas to precisely assess the impact of soil carbon change on the carbon cycle in the region,and also provide a reference to the subsidence wetland restoration.The main results including:(1)The formation and classification of subsidence wetlandBased on the coal mine subsidence theory,analyzed the reason and process of the subsidence wetland,including the topography,hydrology,soil and vegetation establishment process.On the basis of the existed wetland classification system analysis,to classify the subsidence wetland according to the formation and utilization types,including the natural successionand artificial restoration subsidence wetland.(2)The organic carbon in the typical subsidence wetlandTOC content in the topsoil of the typical subsidence wetland is 7.82±3.40g/kg,which is obviously lower than the control farmland(10.18 ± 1.30g/kg)and it is significantly decrease in the topsoil with the increase of waterlogging time.The spatial distribution of TOC is higher in the middle and lower in the margin,with obvious variation with topography.The organic carbon plays a significant role in the carbon in the typical subsidence wetland topsoil,TOC/TC is 82.02%,and TOC is significantly correlated with TC in the typical subsidence wetland.The main impact factors are including topography,hydrology and the waterlogging time.The research results in 0-50 cm soil under different vegetation indicate that the TOC content in the typical subsidence land under different vegetation are lower than it is in the control farmland(5.03±0.91g/kg),and Typha angustifolia(4.68±0.20g/kg)> submerged plants(4.54±1.32g/kg)> Nelumbo SP.(4.37 ± 2.05g/kg),which is different with the research results in the natural wetland systems where the TOC content is higher in the emergent aquatic plant and floating-leaved plant,the reason is probably the short formation time of the subsidence wetland and thus mainly impact by the carbon redistribution by the transformation of the topography.The TOC content decrease with soil depth under all the vegetation types.The vertical distribution of soil properties and its relationship with TOC is also different from the natural wetland,the TOC in the typical subsidence wetland is significantly positively related with the coarse sand,C/N is lower than the natural wetland and do not significantly related with TOC,and pH have no evident changes.(3)The distribution of liable organic carbon in the typical subsidence wetlandThe spatial distribution of liable organic carbon in the typical subsidence wetland is higher in the middle and lower in the margin.The trend is very similar between LFOC and POC,however,POC is more relative with the water depth,and the content difference is larger between the middle and margin.DOC do not significantly relative with water depth,and the change is more moderate from the middle to the margin.CC is more random in the spatial distribution and do not significantly related with TOC,liable organic carbon and water depth.The POC account for 34.89% of the TOC,which is the highest among all the liable organic carbon,LFOC and DOC account for 8.91% and 0.32% respectively.All the liable organic carbon content in the 0-50 cm soil is higher in the control farmland than in the subsidence wetland,except for the POC.DOC is obviously lower in the subsidence wetland,and the order of the content is Nelumbo SP.>submerged plants > Typha angustifolia.In the control farmland and Typha angustifolia,DOC significantly decrease with soil depth,the LFOC and POC content are obviously largest in the topsoil,LFOC is highest in control farmland and POC is highest in the submerged plants.This difference results in the more important impact of topography in the soil carbon of the typical subsidence wetland,and also the influence of vegetation to the soil carbon could be found out.(4)The vertical distribution of TOC in the typical subsidence wetlandIn comparison with the control farmland,the TOC content in the subsidence fen(4.82±1.74g/kg),lake(3.77±1.64g/kg)and aquaculture pond(1.11±1.15g/kg)declined 4.13%,25.03% and 77.91%,respectively.And TOC content significantly decrease with depth in subsidence lake,fen and control farmland,however,in the aquaculture pond the difference is not significantly.The source of TOC in the subsidence lake is mainly the soil carbon from the soil around the lake and located in the basin,the residues from the origin farmland crops and aquatic vegetation.In the subsidence fen,the soil carbon mainly from the origin farmland crops and aquatic vegetation,and the Phragmites australias Trin.is the dominant vegetation with high biomass,therefore,the TOC density is highest in the subsidence fen.The TOC content and density of aquaculture pond is far less than other types,because of the few vegetation cover,the engineer measures in the beginning,and the lack of external sources.(5)The vertical distribution of liable organic carbon in different types of subsidence wetlandIn the control farmland,TOC significantly related to liable organic carbon with high correlation coefficient.In the subsidence lake and fen,TOC also significantly related to all the liable organic carbon,however,with lower correlation coefficient.The correlation coefficient among LFOC,POC and TOC is higher in the subsidence fen than in the lake.In the aquaculture pond,TOC is only significantly relative with POC,due to the few vegetation cover,and the engineer measures in the beginning and the management after its establishment.CC has more influence to the control farmland and subsidence lake than the aquaculture pond and fen,mainly effected by the location and human disturbance.CC decrease with depth,except the slight increase in the 40-50 cm soil in the aquaculture pond,might because of the plough before the subsidence and the disturbance of the soil layers during the land reclamation.(6)Suggestions for the human intervention of the subsidence wetland formation and utilizationIt could be concluded that the subsidence waterlogging time and its induced change of topography,hydrology are the main factors of the TOC spatial distribution,besides,the vegetation has positive effect on the TOC accumulation.It is suggested that proper vegetation measures could be taken to mitigate the TOC loss by soil erosion and improve the TOC accumulation and it is better to minimize the disturbance to the original soil and vegetation in the process of subsidence wetland restoration,which is helpful for the TOC accumulation.Additionally,proper human intervention of the subsidence wetland is useful for the regulation of the soil property,and make the transformation of farmland to wetland eco-system a more healthy process,and benefit for the accumulation of soil carbon.