Autumn fluxes of nitrogen oxides from Oregon coastal forest soils

Background/Question/Methods Moist coastal forests of the Pacific Northwest often have high soil nitrogen (N) availability compared with more inland forests.  High soil N, when combined with adequately moist soils and warm temperatures, may create ideal conditions for the production and release of N oxide gases from soil.  In the fall of 2007, we began measuring soil fluxes of nitrous oxide (N₂O), a powerful greenhouse gas, and nitric oxide (NO), important for regional atmospheric chemistry, from three, 24-30 year old Douglas-fir plantations on sedimentary parent material.  Douglas-fir plantations cover wide areas in the region.  Given the predicted increases in temperature, it is important to understand the factors controlling N oxide fluxes from these forests and how their contributions to global budgets may change.

Results/Conclusions Covering a 3 month period that transitioned from the dry to wet season, mean N₂O fluxes ranged from a low of 1.0 μg m^-2 h^-1 in September (warm, dry) to a mean of 1.9 μg m^-2 h^-1  for the months of August (warm, wet) and October (cold, wet), indicating the influence of soil moisture on N₂O fluxes.  On the other hand, mean NO fluxes, measured during the latter two months, ranged from a high of 19.5 μg m^-2 h^-1 (September) to a low of 2.0 μg m^-2 h^-1 (October).  Averaged by site and date, total N oxide fluxes, largely dominated by NO, were strongly driven by soil temperature (r = 0.93, P = 0.008). Only NO fluxes were related to soil nitrate (r = 0.79, P = 0.06). Taken together, these findings suggest N₂O emissions from these forests may be less sensitive to moderate changes in soil N availability and soil temperature than NO emissions.