Nitrous Oxide and Dinitrogen: The Missing Flux in Nitrogen Budgets of Forested Catchments?

Most forest nitrogen budgets are imbalanced, with nitrogen inputs exceeding nitrogen outputs. The denitrification products nitrous oxide (N2O) and dinitrogen (N2) represent often-unmeasured fluxes that may close the gap between explained nitrogen inputs and outputs. Gaseous N2O and N2 effluxes, dissolved N2O flux, and traditionally measured dissolved nitrogen species (i.e., nitrate, ammonium, and dissolved organic nitrogen) were estimated to account for the annual nitrogen output along hillslope gradients from two catchments in a temperate forest. Adding N2O and N2 effluxes to catchment nitrogen output not only reduced the discrepancy between nitrogen inputs and outputs (9.9 kg ha-1 yr-1 and 6.5 or 6.3 kg ha-1 yr-1, respectively), but also between nitrogen outputs from two catchments with different topographies (6.5 kg ha-1 yr-1 for the catchment with a large wetland, 6.3 kg ha-1 yr-1 for the catchment with a very small wetland). Dissolved N2O comprised a very small portion of the annual nitrogen outputs. Nitrogen inputs exceeded nitrogen outputs throughout the year except during spring runoff, and also during autumn storms in the catchment with the large wetland. Failing to account for denitrification products, especially during summer rainfall events, may lead to underestimation of annual nitrogen losses.

Soil denitrification fluxes from three northeastern North American forests across a range of nitrogen deposition.

In northern forests, large amounts of missing N that dominate N balances at scales ranging from small watersheds to large regional drainage basins may be related to N-gas production by soil microbes. We measured denitrification rates in forest soils in northeastern North America along a N deposition gradient to determine whether N-gas fluxes were a significant fate for atmospheric N inputs and whether denitrification rates were correlated with N availability, soil O2 status, or forest type. We quantified N2 and N2O fluxes in the laboratory with an intact-core method and monitored soil O2, temperature and moisture in three forests differing in natural and anthropogenic N enrichment: Turkey Lakes Watershed, Ontario; Hubbard Brook Experimental Forest, New Hampshire; and Bear Brook Watershed, Maine (fertilized and reference plots in hardwood and softwood stands). Total N-gas flux estimates ranged from <1 in fertilized hardwood uplands at Bear Brook to >100 kg N ha(-1) year(-1) in hardwood wetlands at Turkey Lakes. N-gas flux increased systematically with natural N enrichment from soils with high nitrification rates (Bear Brook < Hubbard Brook < Turkey Lakes) but did not increase in the site where N fertilizer has been added since 1989 (Bear Brook). Our results show that denitrification is an important and underestimated term (1-24% of atmospheric N inputs) in N budgets of upland forests in northeastern North America, but it does not appear to be an important sink for elevated anthropogenic atmospheric N deposition in this region.