Study On Soil Aggregate Structure, Soil Organic Carbon And Micro-bial Diversity After Reclamation Of Poyang Lake Wetland

Reclamation is one of the main reasons leading to the sharp reduction of wetland resources, which significantly changes the soil structure and the distribution of soil organic carbon (SOC) in aggregates. Meanwhile, SOC content in aggregates has be-come an important index of soil quality and environmental stability. Of the many factors associated with soil organic carbon pool and carbon cycling in wetlands, land-use change belongs to the key ones. Soil microbial community structure and di-versity are closely related to the evolutionary mechanism of soil quality as well as the scientific management of land use. Thus, based on the ecological principles of eco-system, impacts on wetland soil of Poyang Lake by different land-use patterns and reclamation period were studied systematically from the following aspects:changes of SOC content, aggregate structure and evolution of soil microbial diversity.Studies were carried out in Poyang Lake wetland and the area which had been reclaimed for38,48and92years. Soil samples were also collected from the0-20cm topsoil of typically used land being reclaimed for38years, such as paddy field, vege-table plot, wasteland and fishpond sediment. With the two variables of reclamation period and land-use pattern, SOC dynamic was analyzed; the structure of aggregates was discussed based on its distribution, soil MWD and GMD values; soil microbial community structure and diversity as well as the succession pattern were studied us-ing PCR-DGGE; relationships between soil environmental factors (especially the SOC content) and soil microbial community diversity were also explored with CCA analysis. The main results are as follows:92years after reclamation, farmland soil aggregate structure and the SOC con-tent have changed remarkably. A large amount of micro-aggregates are released from the macro ones due to the effect of tillage on area being reclaimed for0-48years, and aggregate stability decreases with the continuous decomposition process.48-92years during reclamation, gumming substances accumulates and stability increases with the formation of a new protection mechanism because of the self-regulation in soil sys-tem. SOC content tends to increase with reclamation period and gets stabilized48 years after reclamation, indicating a saturation value exits. It also shows a trend of increasing and stabilizing in all levels of aggregates. The greatest contribution to the rising of SOC content is made by>4.75mm and<0.075mm aggregates, while the least from1.18-0.3mm ones.Land-use patterns change the soil aggregates structure and SOC distribution dramatically after the reclamation of Poyang Lake wetland. Range of MWD and GMD values of aggregates is:fishpond sediment> natural wetland> returned farm-land> paddy field> wasteland> vegetable plot. The latter two land-use types signifi-cantly undermine the soil aggregates and SOC stability. SOC content of vegetable plot decreased most prominently to only36.37%of natural wetland, mainly because of the loss from1.18-0.3mm and0.3-0.075mm aggregates. While that of paddy field increases with the<0.075mm aggregates contributes the most. Total SOC of returned farmland restores to the natural wetland level after the breaking dikes or opening sluice for waterstore (BDOSW), but SOC content of0.3-0.075mm and<0.075mm aggregates as well as the aggregate stability is still lower than natural wetland. Fish-pond sediment shows little difference with natural wetland in the various indices, which well protects the soil aggregates and SOC distribution.Reclamation period changes the soil microbial community structure and diver-sity conspicuously. Selection of bacterial communities and enhancement of their com-petitive relationships during the homogenization process of soil resource by farmland soil system being reclaimed for0-48years has resulted in the continual decreasing of bacterial diversity index. While that of fungi increases because they can make more use of carbon source even in competitive environment.48-92years after reclamation, a new balance has formed in farmland soil, and bacterial diversity index tends to sta-bilized, while that of fungi decreases. Fungi are more sensitive to the response of rec-lamation period that the increasing fungal diversity index is always higher than that of bacteria, indicating a succession from bacteria-dominated soil microbial communities to fungi-dominated ones.Land-use patterns are also close related to the soil microbial community struc-ture and diversity. Compared with natural wetland, vegetation is the main cause of simplification of microbial community diversity in the four different land-use types. Wasteland and fishpond sediment experience smaller decrease amplitudes of micro-bial community diversity than paddy field and vegetable plot due to lack of tillage. Because of long-term use of chemical fertilizer, microbial diversity in paddy field de-creases even more sharply than vegetable plot, which utilizes organic fertilizer. Fungi are more sensitive to the response of land-use change than bacteria.Successions take place in bacterial community structure of farmland soil after reclamation. Both the richness and diversity index of soil bacteria tend to change with soil development obviously. Soil bacteria succession shows a similar pattern with soil structure (aggregate distribution) which makes a big turn48years after reclamation: the0-48years during reclamation belongs to rapid succession stage, with the overall diversity and richness of bacteria reducing significantly; while the48-92years during reclamation is called progressive succession stage, with the overall diversity and richness of bacteria increasing slightly.Different carbon constitutes and their effectiveness instead of SOC content may be the main factors limiting the bacterial community diversity during reclamation. Result from CCA analysis shows that total SOC (TOC) content is most close related to reclamation years (RY), indicating it to be the key factor of microbial community structure variations under different reclamation years. It also reveals that microbial community succession is droved by reclamation activities through altering soil chem-ical properties (especially SOC content). Collaborative feedback relationship is estab-lished between SOC dynamic and microbial community succession after reclamation.