Broughton, L. C. 2001. Linking plant communities to soil microbial communities and processes in old-fields. PhD Dissertation, Michigan State University, East Lansing, Michigan, USA.

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The resources that support soil microbial communities are primarily derived from plants, so the soil microbial community should respond to changes in plant diversity or productivity, particularly if changes in the plant community affects the quality or quantity of available resources. I investigated the role of soil and plants on the structure and function of the soil microbial community by conducting observational and experimental studies and two manipulative greenhouse experiments.

I examined the relationship between plant diversity and productivity and soil microbial community structure and function along a topographic gradient in a successional old-field in Michigan. Variation in plant productivity was confounded by changes in plant community diversity and edaphic characteristics, so I could not determine which of these variables caused the observed changes in the soil microbial community.

To further investigate the relationship between the soil microbial community and plant species diversity, I sampled soils from a set of experimental grassland plant communities established as part of the BIODEPTH experiment at Silwood Park, England. Plant species diversity, functional group diversity, and species composition varied across treatments. I found that plant diversity significantly affected soil microbial community structure. However, N-mineralization rates and microbial respiration responded to variation in plant community composition, but not diversity.

In a greenhouse experiment I examined how variation in soil fertility influenced the soil microbial community. I found that soil origin had strong effects on the structure and function of the soil microbial community. Higher fertility soils had higher organic nitrogen pools and microbial activities and more eukaryotes in the microbial community. In addition, the presence of Andropogon gerardi also affected the structure and function of the soil microbial community. However, the magnitude of the plant effect on soil microbial respiration was inversely related to soil fertility.

In a second greenhouse experiment I further explored the plant species effect on the soil microbial community. I found strong effects of both plant species identity and soil origin on the structure and function of the soil microbial community. In particular, the presence of a legume (Trifolium pratense) increased soil nitrogen cycling processes. Plant species identity had a small effect on soil microbial community structure, but it was dwarfed by the soil origin effect.

Results from these studies indicate that several aspects of the plant community, including diversity, composition and individual plant species identities, can strongly influence the structure and function of the soil microbial community. However, other environmental factors that affect soil quality can have strong and persistent effects on the soil microbial community.

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