Soils are the main contributor of N2O emissions to the atmosphere. Despite intensive measurement, the high spatial and temporal variability of N2O emissions from the soil surface has made it difficult to predict patterns of N2O flux at the ecosystem scale, leading to imbalances in N budgets at all scales. Our research sought to identify and quantify topographic controls on the sources or sinks of N2O in surface and deeper soils to improve our ability to predict fluxes of N2O from upland soils to the atmosphere. We further sought to construct a watershed N budget in order to assess the contribution of soil surface N2O fluxes in upland topographic positions to total N outputs.
We monitored surface N2O fluxes and collected ancillary N cycling measurements along two contrasting hillslopes (swale versus planar) and three landscape positions (ridge top, mid-slope, and valley floor) for two years in the acidic, shale-dominated Susquehanna Shale Hills Critical Zone Observatory (CZO) in central Pennsylvania. An estimate of the N budget for the watershed revealed that despite a long-term record of high depositional N inputs, the Shale Hills CZO is still far from N-saturated, with inputs of N exceeding outputs. Soil-atmosphere fluxes of N2O (0.7 kg N ha-1 yr-1) from the watershed were similar to those measured in the N-limited Hubbard Brook Experimental Forest (~0.9 kg N ha-1 yr-1), but higher than those measured in the N-saturated Fernow Experimental Forest (~0.5 kg N ha-1 yr-1), both which also receive high atmospheric N inputs. This suggests that the magnitude of N2O fluxes are not driven by new N inputs, nor the current N status of the ecosystem. Rather, soil characteristics, like clay content, may be a better predictor of N2O fluxes, with higher fluxes seen in soils with medium clay contents (15-20%), like in the Shale Hills CZO and Hubbard Brook Experimental Forest.
Weitzman, J.N. and Kaye, J.P. (2016): N Budget and Topographic Controls on N2O in a Shale Watershed with High Atmospheric N Deposition. 2016 Fall Meeting, American Geophysical Union, San Francisco, CA, 12-16 Dec..
This Paper/Book acknowledges NSF CZO grant support.