Patterns of diversity in plant and soil microbial communities along a productivity gradient in a Michigan old-field

The relationship between plant diversity and productivity has received much attention in ecology, but the relationship of these factors to soil microb ial communities has been little explored. The carbon resources that support soil microbial communities are primarily derived from plants, so it is lik ely that the soil microbial community should respond to changes in plant di versity or productivity, particularly if the plant community affects the qu ality or quantity of available carbon. We investigated the relationship of plant diversity and productivity to the composition of the soil microbial c ommunity along a topographic gradient in a mid-successional old-field in so uthwestern Michigan. Soil moisture, soil inorganic N, and plant biomass inc reased from the top to the base of the slope, while light at ground level d ecreased along this same gradient. We characterized the changes in resource levels along this gradient using an index of productivity that incorporate d light levels, soil N, soil moisture, and plant biomass. Average plant spe cies richness declined with this productivity index and there were associat ed compositional changes in the plant community along the gradient. The pla nt community shifted from predominantly low-growing perennial forbs at low productivities to perennial grasses at higher productivities. Although ther e was variation in the structure of the soil microbial community [as indica ted by fatty acid methyl ester (FAME) profiles], changes in the composition of the soil microbial community were not correlated with plant productivit y or diversity. However, microbial activity was indicated by Biolog average well color development and substrate-induced respiration (SIR)I was positi vely correlated with plant productivity. The similarity between patterns of plant biomass and soil microbial activity suggests that either plant produ ctivity is driving microbial productivity or that limiting resources for ea ch of these two communities co-vary.