Relationships Between Soil Microbial Diversity And Soil Physicochemical Properties Along The Primary Succession Of Yellow River Delta

As an important part of the terrestrial ecosystems, the microbe plays important roles in a variety of ecological processes, including soil formation and development, organic matter transformation, balance of ecosystems, soil environmental purification, and bioremediation. The diversity of microbial communities is closely related to the structure and functions of the ecosystems, and thus is vital for the maintenance of soil productivity. The microbial communities are very sensitive to environmental changes and their structure and functions can rapidly change in response to altered environmental conditions. The soil microbial communities take effect on the environment largely through the community metabolism differences. Changes in the diversity of microbial communities can reflect the changes in the soil quality at an early time, and thus are an important indicator of soil changes caused by natural and artificial interference. Therefore, it is necessary to elucidate the functions of microbial communities in different environments and the influence of their structural and functional alterations in the ecosystems. At present, the relevant research has been focused on the grassland and forest ecosystems. To answer those long-lasting questions in the ecological field and point out the future research direction, it is necessary to study the microbial communities in other ecosystems and ecological processes.The wetland ecosystem is a special type of ecosystem located in the border zone between the water and terrestrial systems. The unique ecological functions of the wetland ecosystem, including nutrient cycling, sand deposition, pollutant treatment, and soil erosion control, are essential for predicting landscape changes in the ecosystem. The Yellow River Delta wetland, a typical coastal estuarine wetland located in the west Bohai Bank, especially the estuary of the Bohai Bay and Laizhou Bay, has the most complete and broad, as well as the youngest, newborn wetland ecosystem in the warm temperate zone of China, and is one of the areas with the highest biodiversity in the world. Such a complex environment probably possesses both continental and marine microbial populations, communities, and functional and genetic resources. The microbial communities play important roles in the maintenance of wetland ecosystem stability, mineral elements cycling, and degradation of pollutants.In the present study, using the space-time substitution approach, we selectively investigated a typical wetland plant community zone in the Yellow River Estuary Nature Reserve, which can represent the vegetation primary succession process in the Yellow River Delta. Starting from the coast, the sequential plant community is as follows:bare land, Suaeda salsa (L.) Pall. community (SS), Tamarix chinensis Lour, community (TC), Lionium sienense (Girard.) O.Kuthze. community (LS), and Phragmites australis (Cav.) Trin. ex Steud. community (PA). Community investigation and soil sample collection were carried out in April2008, September2008, and September2009. By measuring the contents of water, salt, organic matter, and total nitrogen in the soil, we demonstrated the changing patterns of soil physicochemical properties at different soil depths in the spring and fall seasons during the vegetation succession process. The functional and structural studies of the microbial communities were as follows:1) using the biology system, we studied the carbon utilization function of the microbial communities;2) using the static chamber method, we calculated the soil respiratory quotient and microbial biomass carbon;3) we performed PLFA analysis to study the structure of the microbial communities;4) using the pyrosequencing method, we estimated the number of microbial species, calculated the index of genetic diversity, and analyzed the dominant flora. By these studies, we demonstrated the relationship between the microorganisms and environmental factors in different plant communities. Our study provides scientific evidence for the conservation and restoration of wetland ecosystems and also helps the improvement of the saline soil environment of the coastal areas and the development of the wetland microbial resources.The analysis of the physicochemical properties of the soil revealed that in the Yellow River Delta coastal wetlands, the conductivity of the typical plant community at different soil layers (A&B layers) decreased with the primary succession of the plant community in the spring and fall seasons. In detail, the bare land showed the highest conductivity, followed by the SS, TC, LS, and PA. These results can reflect, to some extent, the different successional stages of the plant communities. Compared to other coastal wetlands, the Yellow River Delta has a lower nutrient content in the soil, probably because its soil is newly formed and subsequently has a poor nutrient holding capacity. The soil carbon and nitrogen ratio (C/N) has a moderate degree of variation, and shows a relatively high value in the fall season plant succession. The wide range of C/N variation reflects the rapid and complex changes in the continental and marine factors during plant succession. The results of Spearman correlation analysis indicated that the plant community diversity index shows negative correlation to the soil electrical conductivity and positive correlation to the contents of organic matter and total nitrogen. These observations suggested that with the primary succession of plant communities, the plant litter was returned to the soil, which in turn increased the organic matter content and decreased the salt content in the soil, leading to improvement in the soil quality. The improvement of the soil quality can also impact the type of plant community. Hence, the succession of the plant community is a process in which the vegetation and soil interact with each other; namely, the plants become adaptive to the soil condition and further modify the soil, and different plant species compete for resources and replace each other. Therefore, soil condition is one of the important driving forces for vegetation primary succession.The analysis of the functional diversity of microbial communities suggested that with the vegetation succession, the microbial communities show correspondent changes. The microbial biomass C (Cmic), basal respiration (BAS), and AWCD of the soil all increased, reflecting an overall increase in the carbon utilization capability of the microbial communities and in the biomass. Comparing the soil environment at different depths, we found that the AWCD of microbial communities in the B layer was higher than that of the A layer in the fall season, while all other indicators (AWCD, BAS, Cmic, Cmic/Corg) of the B layer were not greater than in the A layer. These results indicated that the quantity and activity of soil microorganisms reduced with the increased soil depth, and their metabolic activity underwent significant changes. Comparing the diversity, we found that the H' Biolog in the B layer did not show obvious difference between the spring and fall seasons. In spring, the Gini uniform index of the bare land was significantly higher than that of the other plant communities; whereas in fall, the Gini uniform index did not show any obvious difference among different plant communities. The results of the soil environment and diversity analysis indicated that although the plant community type and soil depth can affect the microbial biomass, carbon utilization, and metabolism, they cannot significantly change the microbial community diversity index and subsequently cannot affect the functional diversity of the microbial community.The analysis of the structural diversity of the microbial communities indicated that with the vegetation succession, in spring, the total amount of bacteria and fungi in the soil underneath different plant communities all decreased first and then increased; whereas in fall, the amount of bacteria and fungi at different soil depth always increased. The peak of the content of bacteria and fungi appeared in the A layer soil of the PA in fall. In the plant rhizosphere soil, the Gram-negative bacteria showed a larger amount than the Gram-positive bacteria, and the ratios of GP/GN and bacteria/fungi did not show obvious changes during vegetation succession. Comparing the soil environment at different depths, in spring, the contents of bacteria and fungi in the B layer were always higher than in the A layer, especially in the early stage of succession; whereas in fall, the contents of bacteria and fungi in the B layer were always lower than in the A layer. These observations indicated that the total quantity of bacteria and fungi did not decrease with the increased soil depth.The pyrosequencing was applied to the bacterial community structure studies.57,684quality sequences from66,849reads were classified as Bacteria with a read length of≥200bp. The dominant phyla which represented approximately81%of all classified sequences across all samples were Proteobacteria, Bacteroidetes, Firmicutes, Actinobacteria, Acidobacteria, Chloroflexi, and Verrucomicrobia, representing40.45%,23.54%,9.01%,8.31%,1.74%,1.60%, and0.21%, respectively. The major microbial community structures were still significantly different among succession stages. The relative abundance of Firmicutes was decreased from17.2%(BL) to1.3%(PA), whereas that of Bacteroidetes was increased from1.9%(BL) to36.3%(LS) and that of Actinobacteria was increased from4.3%(BL) to16.3%(PA).Salinity may be the major factor that caused the reduced number of halophilic microbes, such as genera Halobacillus and Bacillus. The microbial types that are represented by Gram-negative bacteria, aerobic bacteria, and the non-spore producing Pseudomonas have similar composition ratios in all soils with various vegetation types. Although microbial biomass and activity were severely suppressed in saline bare land, the diversity of the microbial community was not dramatically reduced. The relative abundance of saprophytic microbes, such as the genus of Lysobacter in Gammaprotecobacteria, the genus of Arthrobacter in Actinobacteria, and the class of Flavobacteria in the phylum of Bacteroidetes, was increased. This may be caused by the improved soil fertility at later successional stages.In the present study, we applied the CLPPs, PLFAs, and DNA analysis approaches, which have been widely used to evaluate the soil biological characteristics, to study the characteristics of the microbial communities. Using the biology system, we demonstrated the carbon utilization difference in the soil microorganisms in different plant communities.The biology system focuses on the physiological state of the microbial communities. Multivariate statistical analysis revealed that the measured plant and soil physicochemical characteristics can significantly affect the microbial communities, reflecting the influence of the wetland vegetation on the physiological state of the soil microbial community during the decomposition process. The quantitative and qualitative alteration in the litters caused by plant community changes is the main force that drives the changes in the microbial communities. Microorganisms mainly utilize plant residues as the nutrient source. With the succession of plant community, the continuous de-salinization promotes the content of organic matter in the soil, which is suitable for the decomposition activities of the microbial communities. The increased amount and activity of microbial communities further promotes the plant and soil stability of the wetland ecosystems. On the other hand, plant roots might also directly or indirectly cause changes in the microbial communities.PLFAs analysis can elucidate the structural differences in soil microorganisms of different vegetation communities. This method focuses on the composition and structure of the microbial communities. Our analysis using PLFAs preliminarily demonstrated the structural characteristics and difference of the microorganisms at different soil depths of typical plant communities in the spring and fall seasons in the Yellow River Delta wetland. Due to the lack of a specific marker, the results based on phospholipid fatty acid analysis were not very accurate. Hence, we performed pyrosequencing analysis and obtained more accurate data. The acquired data of ACE, Chao, H' Pyrosequencing, and DS' Pyrosequencing did not show changes consistent with the alteration in the succession. This observation was also confirmed by the CLPPs analysis, in which the microbial diversity index did not show obvious changes during the process of vegetation succession. Analysis of the DS' Pyrosequencing data showed that compared to the bare land, the heterogenicity of the microbial community at the end stage of succession was highly increased, indicating that the increased heterogenicity of the microbial community during plant succession was probably related to the elevated H' Pyrosequencing.The analysis of the functional and structural diversity of microbial communities revealed the following procedure:the plant litters and root exudates provided inorganic carbon and resource for soil microorganisms; the decomposers degraded the organic matter and biological polymers (e.g. starch, pectin, and protein) and returned them to the soil; these nutrients could indirectly regulate plant growth and community composition. Through these steps, the plant vegetation can be closely associated with the microorganisms through the soil environment. Therefore, the combined application of multiple approaches can supplement each other and subsequently better elucidate the characteristics and influential factors of the changes in the microbial communities.