Nitrogen cycling in a forested Minnesota bog

Abstract

URBAN, N. R., and EISENREICH, S. J. 1988. Nitrogen cycling in a forested Minnesota bog. Can. J. Bot. 66: 435-449.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 for 4 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-I . year-') is a minor input to the bog relative to the input from atmospheric deposition (10.4 kg. hg-' . year-I). The bog is a large sink for N with approximately 65 % of inputs retained. Annual turnover of N (66 kg, ha-I) is much larger than the total input (14.6 kg. ha-I). 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-I . year-I is buried in anaerobic peat and rendered unavailable to the biota.

Headwater Mires Constitute a Major Source of Nitrogen (N) to Surface Waters in the Boreal Landscape

“…Adding DON to the fluvial loss term (another 1.82 kg N ha -1 y -1 ) generates a considerably lower estimate of annual (total) N retention (ca. 60%), yet still within the range published for other mires (Urban and Eisenreich 1988). However, our results emphasize that the biogeochemical role of these systems in the broader landscape is not directly coupled to how the acrotelm responds to N inputs at annual time scales-but is instead driven by processes occurring at greater depths, including the development of preferential flow through deep peat layers.…”

“…N limitation of terrestrial vegetation is widely observed in the boreal biome (Hyvö nen and others 2008), and plant-soil systems at the surface of northern mires are notably efficient at retaining and recycling this nutrient (Aerts and others 1992;Moore and others 2004). Despite this efficiency, the long-term development of mires has resulted in a landscape mosaic that includes vast stores of peat, where reduced N (DON and NH 4 + ) can accumulate at depths below the reach of most plants (Urban and Eisenreich 1988). The juxtaposition of these deep resource pools with subsurface hydrologic flowpaths sustains the export of readily usable forms of N at markedly higher rates than would be expected from the nutrient status of overlying vegetation.…”

“…While peat accumulation is widely studied from a carbon (C) balance perspective (for example, Nilsson and others 2008), N bound in organic matter is also known to accumulate at relatively high rates (Loisel and others 2014). Once N reaches sufficient depth in the catotelm, it is essentially lost from the most biologically active compartment of these systems (Urban and Eisenreich 1988). Where such peatforming ecosystems are spatially extensive and hydrologically connected with the broader landscape, exports of N from watersheds may be sustained by deep storage zones-even when overlying and surrounding vegetation is strongly N limited.…”

Nitrogen and Phosphorus Mineralization in Fens and Bogs

“…More data, especially from a comparative study, using standardized methods in bogs subject to different levels ofN deposition, are now needed if a valid comparison is to be made. This study* This study Urban & Eisenreich (1988) Hemond (1983) Martin & Holding (1978) …”

Fate of N in a peatland, Whim bog: immobilisation in the vegetation and peat, leakage into pore water and losses as N<sub>2</sub>O depend on the form of N

“…Changes in water table through drainage, climate or species composition, the latter often observed in response to N eutrophication, (Berendse et al, 2001) are particularly likely to influence C exchange, both via the uptake of carbon dioxide (CO 2 ) and the release of methane (CH 4 ). Increases in anthropogenic N deposition can increase productivity and C assimilation but also release C if decomposition rates are accelerated through changes in N availability or species composition (van Breeman, 1995a). The overall consequences of enhanced N deposition for vegetation, function and sustainability and its fate in peat bogs are not fully understood.…”