Phosphorus and Nitrogen Dynamics Across an Ombrotrophic-Minerotrophic Peatland Gradient in Upper Peninsula of Michigan, USA

The ombrotrophic-minerotrophic gradient in peatlands, a diverse assemblage of wetlands, has led to the assumption that nutrient availability is significantly lower in bogs than fens. This dissertation focuses on the availability and movement of phosphorus (P) and nitrogen (N) across the ombrotrophic-minerotrophic peatland gradient using both radioactive and stable isotopes. In initial studies, available, microbial, and root P at 10-20 cm depth did not differ across the gradient, although total soil and aboveground vegetation P content increased from bog to rich fen. This study showed that P retention was greatest in the more ombrotrophic sites, but the proportion found in the vegetation and microbial pools in the rich fens were in the same range as the intermediate fens and bogs. In a subsequent study, we saw major differences in both retention and distribution of P among the pools across the gradient. The P pool size in the vegetation and recalcitrant soil fractions increased from bog to rich fen. This time uptake of radioactive P into microbial biomass was more important in bogs than fens, while roots were more effective at uptake in fens. Despite overall P retention being greater in more minerotrophic peatlands, the distribution and uptake of P into individual pools were determined by complex interactions that varied over time and among peatland types. The overall N pool and individual N pools increased from bogs to rich fens. Retention of the added N tracer was greater in rich fens and within the soil profile. Microbial biomass was not a significant sink for N. Our results indicate that mosses and vascular plants have spatially segregated competition for N, with mosses being more competitive for atmospherically deposited N and vascular plants being more competitive for N mineralized from within the soil profile. Uptake of N into the residual soil fraction dominated retention in most cases indicating a potential role for abiotic transformation of N. In comparing four methods of gross P mineralization, we suggest the Kirkham and Bartholomew method, because the assumptions were generally met and does not require sterilized samples, representing a conservative estimate of P mineralization.