The Vertical Distribution And Stability Of SOC In Marsh Before And After Reclaimation

Change in soil organic carbon (SOC) pool and climate change are closely related.SOC pool is not homogeneous, but divided into active SOC pool and stable SOC pool.The active pool is sensitive to climate change and humanactivities and decides theSOC fluxes. The stable pool controls and maintains SOC as a long-term carbon sinkof atmospheric CO2. SOC storage in deeper soil layers is large and unique to itsstability. For some soil types whose SOC storage layer is thicker, studying on SOCvertical distribution in order to accurately estimate SOC storage in deeper soil layersis more in line with the realities of their SOC pool. Generally, SOC storage layer isthicker and SOC density is higher in wetland. As China’s largest distribution area offresh water marsh, the Sanjiang Plain is the important SOC pool in North China. Inrecent decades, large area of wetland in Sanjiang Plain has been reclaimed manytimes, farmland has become to the main landscape. Wetland becoming to farmlandhas changed the carbon cycle model of wetland and affected SOC pool.In this study, the state that large area wetland had become to farmland, and landuse had changed severly were took into account. Wetland, upland fields (soybean field)and paddy field which have the same soil type peat marsh soil were chosed as sampleplots in Xingkai Lake area in which agriculture development is very early. Fieldsurvey, indoor analysis and control experiment were used to research verticaldistribution, storage and stability of SOC, and probe the effect of plant, soil microbeand vertical migration of dissolved organic carbon (DOC) on SOC content andvertical distribution. This study was with a view to enriching the studies on carboncycle in the Sanjiang Plain; and providing data for accurate estimates and projectionsof SOC pool under severe land use change in Sanjiang Plain; and providing scientific basis to identify the long-term effects of reclamation on SOC pool, and to adjust theland use structure in the balance between carbon sequestration and emission reductionand guaranting food production. Through research, main conclusions have beenreached as follows:(1)After reclamation, SOC content in0-40cm soil layers decreased significantly;SOC content in0-50cm soil layers decreased more obviously with soil depthincreased, below50cm it changed little with soil depth increased. Because ofreclamation, SOC density also decreased, and that in soybean field was higher than inpaddy field; most SOC were stored in0-50cm soil layers in wetland, soybean fieldand paddy field. Reclaiming changed the SOC content in the soil profile of wetland,but dinn’t alter the SOC content variation rule with soil depth increase; before andafter reclamation, relationships between SOC content and soil depth could bedescribed by the exponential function.(2)After reclamation, the amount of plant residues returned to soil in the end ofgrowth season decreased, and that in soybean field was more than that in paddy field.The total carbon content in plant residues also decreased, the total carbon in plantresidues in wetland, soybean field and paddy field were1694.54g/m2，299.77g/m2and240.71g/m2, respectively. It was consistent with the conclusion that SOC contentdecreased, and that in soybean field was higher than that in paddy field afterreclamation. The C:N of overground part of plant residues returned to soil in paddyfield was higher than that in wetland, that in wetland higher than that in soybean field;the C:N of belowground part of plant residues returned to soil in soybean field washigher than that in paddy field,and that in paddy field higher than that in wetland.(3) Microbial biomass decreased due to reclamation. After reclamation, in0-5cm,5-10cm,10-20cm and20-30cm soil layers Microbial Biomass Carbon (MBC)and Microbial Biomass Nitrogen (MBN) decreased, MBC and MBN in soybean fieldwere lower and higher than in paddy field respectively. At the same time MBC/MBNdecreased, microbial community structure changed, and MBC/MBN in soybean fieldwas lower than that in paddy field. Microbial activity decreased due to reclamation. Basal Respiration (BR) decreased, and that in soybean field was lower than that inpaddy field, microbial activity in paddy field was higher. In0-5cm and5-10cmtopsoil layers in May to July, Respiration Quotient-qCO2(qCO2) was higher insoybean field than that in paddy field and wetland, both were in water logging; whilein August and September logging water was drained up, qCO2in paddy fieldincreased dramatically to be higher than that in soybean field and wetland. Underwaterlogged conditions, microbial activity is subdued. In10-20cm and20-30cmsubsoil layers, qCO2were higher in wetland and paddy field than that in soybeanfiled.(4) Reduction ranges of dissolved organic carbon (DOC) in soil solution from20cm to60cm soil depth and from20cm to100cm soil depth were all higher insoybean filed than those in wetland and paddy field. That was essentially the same inwetland as that in paddy field. The retention of DOC was more obvious for soybeanfarming than wetland and rice farming, and that was roughly the same for wetland asrice farming.(5) After reclamation, content and storage of labile organic carbon componentssuch as dissolved organic carbon (DOC), microbial biomass carbon (MBC),easily-oxidized organic carbon, and easy mineralizable organic carbon decreased, andthose in soybean field were lower than in paddy field. After reclamation, ratio oflabile organic carbon storage to total SOC storage in soybean field was lower than inpaddy field. In0-5cm,5-10cm,10-20cm and20-30cm soil layers, SOC storage ofmicro-aggregates (<0.25mm) and its share in total soil SOC increased, those insoybean were higher than that in paddy field. In addition, land use patterns, season,temperature and soil water content had a significant impact on soil organic carbonmineralization. Interactions influence of seasons and land use patterns, season andtemperature, season and soil water content, land use patterns and temperature, landuse patterns and soil water content, temperature and soil water content on soil organiccarbon mineralization were also sifnificant. After reclamation, soil organic carbonmineralization rate decreased and that in soybean field was lower than in paddy field; mineralization rate rise range with temperature rise decreased under the same soilwater content. Due to reclamation, total SOC storage and labile components storagedecreased, but after reclamation SOC was stored in more stable form, soybeanfarming is more friendly for sustainable SOC residence in the soils than rice farming.