Patterns of heterotrophic microbial activity in eutrophic and oligotrophic peatlands

Abstract

Nutrient enrichment of peatlands may alter patterns of heterotrophic microbial activity (HMA) and organic matter cycling. The utilization of C-substrates by heterotrophic microbial communities in response to changes in environmental conditions may serve as sensitive indicators of changes in the trophic state of wetlands. The objective of this study was to measure the response of heterotrophic microbial communities to added C-substrates in the plant detritus layer and underlying soil (0–10 cm) for eutrophic, transitional, and oligotrophic sites along a nutrient enrichment gradient in the Florida Everglades, USA. The short-term response to C-substrates (alcohols, amino acids, carboxylic acids, and polysaccharides) was measured as CO 2 production. The nutrient gradient was characterized by decreasing P concentrations and microbial biomass from eutrophic to oligotrophic sites. Basal respiration was 73% higher at eutrophic than oligotrophic sites, and 41% higher in detritus than underlying soil. Heterotrophic microbial activity varied along the gradient with greater C-substrate utilization at the eutrophic site resulting from higher levels of microbial biomass and inorganic nutrients compared to the oligotrophic site. The C-substrates enhanced CO 2 production at all sites suggesting that labile organic C was a limiting factor to HMA in these peatlands. Substrate-induced respiration (SIR) of detritus was 25, 45, and 42% greater for polysaccharides than other C-substrates at the eutrophic, transitional, and oligotrophic sites, respectively. Likewise, SIR at the eutrophic, transitional, and oligotrophic sites was 0, 40, and 39%, respectively, greater for detritus amended with carboxylic acids than with amino acids and alcohols. Polysaccharides dominated HMA profiles at all sites along the nutrient gradient in detritus. The transitional site was characterized by carboxylic acids and alcohols, while the HMA profile at the oligotrophic site was dominated by carboxylic acids. Patterns of HMA along the nutrient gradient provided insight into the microbial response to changes in tropic status, indicating the heterotrophic microbial community was more sensitive with increasing eutrophication.