Nitrous oxide emissions from streams within the US Corn Belt scale with stream order

Abstract:

Nitrous oxide (N₂O) is an important greenhouse gas and the primary stratospheric ozone depleting substance. Agriculture is the main source of N₂O emissions as a result of high nitrogen (N) fertilizer inputs. The Intergovernmental Panel on Climate Change (IPCC) uses bottom-up emission factors (EFs) to estimate annual emissions from direct and indirect sources. Top-down N₂O emission estimates based on independent tall tower observations indicate that bottom-up approaches severely underestimate the annual budget at the regional scale. It is well established that indirect N₂O emissions from streams are poorly characterized due to a scarcity of observations, poorly constrained piston velocity relationships, and high variability among surface water types. Indirect emissions from streams represent a large source of uncertainty in the global N₂O budget. Here we examine the extent to which indirect emissions from streams contribute to the regional budget and propose a strategy for scaling up indirect emissions based on stream order relations. Two years of floating chamber-based flux measurements showed that N₂O emissions from headwater streams (drainage ditches) in an agricultural landscape often exceeded 45 nmol N₂O m^-2 s^-1 and decreased exponentially as a function of stream order. This scaling function was used to estimate indirect emissions for the region and indicated that the IPCC EF_5r significantly underestimated the observed emissions within a representative watershed of the US Corn Belt in the Midwestern United States. Headwater streams and rivers contributed ~75% and <1% respectively to the watershed indirect N₂O budget, highlighting the need to better constrain low order stream system emissions. Furthermore, instantaneous fluxes from high order streams were well constrained and significantly smaller than low order streams. Consequently, a single EF for all streams and rivers does not appear to be appropriate. Our findings suggest that bottom-up models likely underestimate N₂O emissions in the US Corn Belt in part because low order stream emissions are underestimated.