Nitrogen cycling in a forested Minnesota bog

The nitrogen cycle of a small, forested, Sphagnum peatland in northern Minnesota was studied for 4 years. Hydrologic inputs and outputs (atmospheric deposition, upland runoff, streamflow) were monitored for4 years, and annual uptake of N by vegetation was measured over a 3-year period. Microbe-mediated processes of nitrogen fixation and mineralization were measured in the laboratory and field, and accumulation rates of N within the peatland were measured in dated peat cores. Aerobic heterotrophs appear to be the dominant agents of N fixation at this site. Rates of N fixation decrease rapidly below the surface. Perhaps limited by moisture and low pH, N fixation (0.5–0.7 kg∙ha −1 ∙year −1 ) is a minor input to the bog relative to the input from atmospheric deposition (10.4 kg∙hg −1 ∙year −1 ). The bog is a large sink for N with approximately 65% of inputs retained. Annual turnover of N (66 kg∙ha −1 ) is much larger than the total input (14.6 kg∙ha −1 ). This large turnover is achieved by rapidly cycling a relatively small pool of N in the aerobic layers of peat. Plant uptake is closely coupled to mineralization such that losses from the system in runoff are small. However, 7 to 12 kg N∙ha −1 ∙year −1 is buried in anaerobic peat and rendered unavailable to the biota..