Spatial structures of soil microbial communities and their controlling factors

The goal of this research was to quantify the relationship between soil microbial community structures and candidate controlling factors, such as plant species and soil nutrients. A meadow at the Blandy Experimental Farm was prepared with different grass-species diversity. Different environmental dimensions were employed for the study: temporal changes (succession), and vertical and horizontal spatial gradient analyses. The succession study was conducted using the entire field with samples collected for 2 years. For the vertical and horizontal dimensional studies, a single 5 m x 5 m experimental field was prepared at the border of an area in the field distinct vegetation patterns. Vertical profiles were studied in two 25-cm deep trenches with different plant cover. A combination of multi-scale sampling (0.05 m--1 m) and the regional sampling was conducted for the horizontal dimension study, and geostatistics were applied for statistical inference.; A systematic comparison of results from several sample sizes of soil indicated that the abundance and community structures of soil microorganisms obtained varied with the sample size. Based on these results, it is recommended that a reconnaissance survey be done to determine the best sized sample to use at the onset of new projects. The vertical profiles of soil characteristics were highly correlated between the trenches, while the profiles of microbial community structure were not well correlated. Given that the difference between the trenches was largely in the overlying plant community, the clear difference in the community profiles indicated that plant cover was the dominant factor influencing the vertical profiles of soil microbial community structures. Furthermore, the successional changes in the structure of the soil bacterial community were developed in parallel with changes in the plant communities for the first 2 years of secondary succession.; Soil characteristics and total microbial abundance were well spatially structured, but the soil microbial community structures were autocorrelated with less spatial dependence than were the soil characteristics. The clear separation of both bacterial and fungal communities between different plant settings suggests the dominant influence of plant composition. Among soil characteristics, the best spatial correlation with both bacterial and fungal community structures was found with soil nutrients (total carbon and nitrogen content). Causal modeling indicated that distance (space) had an important role in the differentiation of microbial community structures. Soil characteristics also had a causal relationship with bacterial communities, but not for fungal communities. Thus, plant identity is the dominant controlling factor on bacterial and fungal community structure in the soil of the experimental field for all aspects examined.