Droughts and Nitrous Oxide Emissions in Agricultural and Forested Watersheds.

Abstract:

Climate change is expected to increase extreme events in most parts of the world. Droughts and floods may combine in a nonlinear manner with other anthropogenic effects, such as the release of known and new contaminants, possibly magnifying their impact on freshwater ecosystems. Among solutes, dissolved reactive nitrogen, especially nitrate, could play an important role because it can fuel an important source of the potent greenhouse gas nitrous oxide (N₂O) via the process of microbially-mediated denitrification. Under a changing climate, droughts may last longer and become more frequent than present, and positive feedback or amplification may occur as potentially more N₂O is released from streams and rivers compared to baseflow conditions, especially in agricultural watersheds where nitrate runoff is readily available and may increase to feed a growing population. Estimates of watershed-scale N₂O emissions are highly uncertain because of the difficulties to extend local measurements to the river network. With the objective of developing scaling laws to be used in the estimate of global N₂O emissions from river networks, we analyze the seasonal and spatial variability of N₂O emissions along two Midwestern US river networks with contrasting land use: the Manistee R. (MI; ~83% forested) and the Tippecanoe R (IN; ~82% agricultural). We combine numerical modeling and geostatistical analysis to upscale network-scale empirical data of N₂O emissions collected over four seasonal synoptic sampling campaigns, with the focus of the analysis being emissions during droughts. We investigate the spatial distribution of dissolved inorganic nitrogen and N₂O emissions along the two stream networks by comparing the behaviors of streams of different sizes, the role of land use, and contrast surface and subsurface (benthic-hyporheic zone) stream habitats among different seasons and thermal regimes. For example, during a simulated drought, we found increased N₂O emissions in both watersheds where a combination of low flow, increased temperatures, and elevated nitrate concentrations increased stream denitrification, and headwaters remained hotspots for N₂O emission. Nevertheless, during drought, agricultural land use amplify N₂O emissions relative to forested watershed.