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Comparisons of soil nitrogen mass balances for an ombrotrophic bog and a minerotrophic fen in northern Minnesota
Citation:
Hill, B., T. Jicha, L. Lehto, C. Elonen, S. Sebestyen, AND R. Kolka. Comparisons of soil nitrogen mass balances for an ombrotrophic bog and a minerotrophic fen in northern Minnesota. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, Netherlands, 550:880-892, (2016).
Impact/Purpose:
The role of wetlands, especially peatlands, in the global carbon budget has been widely discussed, but much less attention has been paid to the corresponding role of peatlands in nitrogen cycling. Northern peatlands are particularly sensitive to N additions, owing to their unique hydrological and biogeochemical properties. Peatlands are divided into two broad classes on the basis of pH and hydrology. Peatlands accumulate C, as is evident from their global significance in soil C budgets, but they also store large quantities of N and P, with 56% of total N (80-90% of labile N) and 76% of total P inputs to a bog being retained rather than exported. While N and P stores in peatlands are large, the available fractions were much smaller and tightly cycled, leading to relative N and P limitations on productivity. The goals of our project were to compare the N budget of an ombrotrophic bog with that of a minerotrophic fen with an eye towards the relative importance of denitrification to the overall N budget. We demonstrated significant differences in denitrification between the bog and the fen, despite the paucity of nitrate in the bog. Denitrification in both the bog and fen, and their uplands, was significant relative to N inputs. Our results highlighted differences between the bog and fen, between the upland watersheds and the downslope peatlands, and the importance of biogeochemical hotspots within the peatlands. Our results also point out the importance of organic N storage, as a source of N for denitrification, and also as a source of N to support microbial biomass production. We provide a plausible link between organic N storage, denitrification, and N export from peatland watersheds. Finally, we considered the nuanced interactions of microbial metabolism with nutrient availability and stoichiometry, and how N dynamics might be affected by climate change in peatland ecosystems.
Description:
We compared the N budgets of an ombrotrophic bog and a minerotrophic fen to quantify the importance of denitrification in peatlands and their watersheds. We also compared the watershed upland mineral soils to bog/fen peat; lagg and transition zone peat to central bog/fen peat; and surface, mid-layer and deep soil and peat horizons. Bog and fen area were derived from a wetland boundary GIS data layer, and bog and fen volumes were calculated as the interpolated product of area and depth of peat. Atmospheric N deposition to the bog and fen were based on measurements from a station located 2km north of the bog watershed and 0.5km from the fen watershed. Precipitation was analyzed for nitrate (NO3-), ammonium (NH4+), and total N (TN), and aggregated to annual values. Outflow water samples from the bog and fen were collected as surface grab samples on each of the May-October sampling dates over the 2010-2013 study, and were analyzed and aggregated annually as for atmospheric N. Soil and peat samples were analyzed for N content, and for net ammonification (AM), nitrification (NT), and ambient (DN) and potential (DEA) denitrification rates. Nitrogen mass balances are based on mean annual atmospheric deposition and outflow; soil and peat standing stocks of N, and mean annual estimates of DN, weighted for contributions of the uplands, lagg or transition zone, and bog or fen hollows and hummocks, and accounting for soil depth effects. Annual deposition of N species was: NH4+ 0.88-3.07 kg N ha-1 y-1; NO3- 1.37- 1.42 kg N ha-1 y-1; TN 2.79-4.69 kg N ha-1 y-1. Annual N yields were: bogNH4+ 0.01-0.04 kg ha-1 y-1, NO3- 0.01-0.06 kg ha-1 y-1, and TN 0.11-0.69 kg ha-1 y-1; fenNH4+ 0.01-0.16 kg ha-1 y-1, NO3- 0.29-0.48 kg ha-1 y-1, and TN 1.14-1.61 kg ha-1 y-1. Outflow N was positively correlated with atmospheric N deposition. Soil N storage depended on location within the bog or fen, and on soil depth. Atmospheric N deposition was correlated with soil N pools. AM, NT, DN and DEA rates were low throughout the uplands and peatlands of both watersheds, and were variously correlated with atmospheric N deposition, soil N storage, or N outflow. DEA was significantly greater than DN indicating C or N limitation of the denitrification process. N inputs to the bog include atmospheric N (3.3 kg N ha-1 y-1) and N fixation (0.5 kg N ha-1 y-1), and these inputs were more than countered by DN losses and stream outflows from the watershed. In addition to atmospheric N deposition and N fixation, N inputs to the fen included 1.51 kg N ha-1 y-1 from the regional water table. There were negligible DN from the fen. In both the bog and fen watersheds, the magnitude of N fluxes is dwarfed by N storage in watershed soils. DN in both the bog and fen watersheds was significant relative to N inputs. Our results highlighted differences between the bog and fen, between the upland watersheds and the downslope peatlands, and the importance of biogeochemical hotspots within the peatlands. Our results also point out the importance of organic N storage, as a source of N for denitrification, and also as a source of N to support microbial biomass production. We describe a plausible link between organic N storage, denitrification, and N export from peatland watersheds. Finally, we considered the nuanced interactions of microbial metabolism with nutrient availability and stoichiometry, and how N dynamics might be affected by climate change in peatland ecosystems.
