Summer storms trigger soil N₂O efflux episodes in forested catchments

Climate change and climate-driven feedbacks on catchment hydrology and biogeochemistry have the potential to alter the aquatic versus atmospheric fate of nitrogen (N) in forests. This study investigated the hypothesis that during the forest growth season, topography redistributes water and water-soluble precursors (i.e., dissolved organic carbon and nitrate) for the formation of gaseous N species. Soil nitrous oxide (N₂O) and nitrogen (N₂) efflux and soil physical and chemical properties were measured in a temperate forest in Central Ontario, Canada from 2005 to 2010. Hotspots and hot moments of soil N₂O and N₂ efflux were observed in topographic positions that accumulate precipitation, which likely triggered the formation of redox conditions and in turn intercepted the conversion of nitrate N flowing to the stream by transforming it to N₂O and N₂. There was a strong relationship between precipitation and N₂O efflux (y = 0.44x1.22, r² = 0.618, p<0.001 in the inner wetland; y = 1.30x^{1.16} r² = 0.72, p<0.001 in the outer wetland) and significantly different N₂:N₂O ratios in different areas of the wetland (19.6 in the inner wetland and 10.1 in the outer wetland). Soil N₂O+N₂ efflux in response to precipitation events accounted for 16.1% of the annual N input. A consequence of the higher frequency of extreme precipitation events predicted under climate change scenarios is the shift from an aquatic to atmospheric fate for N, resulting in a significant forest N efflux. This in turn creates feedbacks for even warmer conditions due to increased effluxes of potent greenhouse gases."This research was funded by an NSERC Discovery grant to IFC (217053‐2009 RGPIN)."https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JG00302

Snow covered soils produce N₂O that is lost from forested catchments prior to snowmelt

The magnitude of net soil nitrous oxide (N₂O) production from a snow‐covered catchment in a northern temperate forest was investigated. There was considerable net soil N₂O‐N production and consumption through the snowpack, ranging from −6.6 to 26.2 g‐N ha⁻¹ d⁻¹. There was no difference in net N₂O production among topographic positions despite significant variation in soil moisture, reduction‐oxidation conditions, and pore water dissolved organic carbon and nitrate. Soil temperatures did not vary among topographic positions, suggesting that temperatures at or above the freezing point allow N₂O production to proceed under the snowpack. Redox conditions were lower at wetland positions compared to lowlands and uplands, suggesting that the biogeochemical pathway of N₂O production varies with topography. Over the entire nongrowing season, 1.5 kg of N₂O‐N was exported to the atmosphere from the 6.33 ha catchment, representing 31% of the growing season N₂O‐N production. These results suggest that winter is an active time for gaseous N production in these forests and that N₂O production under the snowpack represents an often unmonitored flux of N from catchments."This research was funded by an NSERC Discovery grant to I.F. Creed (217053-2009 RGPIN)."https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016JG00341

The role of wetland coverage within the near-stream zone in predicting of seasonal stream export chemistry from forested headwater catchments

Stream chemistry is often used to infer catchment-scale biogeochemical processes. However, biogeochemical cycling in the near-stream zone or hydrologically connected areas may exert a stronger influence on stream chemistry compared with cycling processes occurring in more distal parts of the catchment, particularly in dry seasons and in dry years. In this study, we tested the hypotheses that near-stream wetland proportion is a better predictor of seasonal (winter, spring, summer, and fall) stream chemistry compared with whole-catchment averages and that these relationships are stronger in dryer periods with lower hydrologic connectivity. We evaluated relationships between catchment wetland proportion and 16-year average seasonal flow-weighted concentrations of both biogeochemically active nutrients, dissolved organic carbon (DOC), nitrate (NO 3 -N), total phosphorus (TP), as well as weathering products, calcium (Ca), magnesium (Mg), at ten headwater (<200 ha) forested catchments in south-central Ontario, Canada. Wetland proportion across the entire catchment was the best predictor of DOC and TP in all seasons and years, whereas predictions of NO 3 -N concentrations improved when only the proportion of wetland within the near-stream zone was considered. This was particularly the case during dry years and dry seasons such as summer. In contrast, Ca and Mg showed no relationship with catchment wetland proportion at any scale or in any season. In forested headwater catchments, variable hydro

Snow, ponds, trees, and frogs: how environmental processes mediate climate change impacts on four subarctic terrestrial and freshwater ecosystems

Amplified warming in subarctic regions is having measurable impacts on terrestrial and freshwater ecosystem processes. At the boundary of the discontinuous and continuous permafrost zones, and at the northern extent of the boreal forest, the Hudson Bay Lowlands has experienced, and is projected to continue to experience dramatic rates of climate change in the coming decades. In this review, we explore the impacts of climate change on terrestrial and freshwater ecosystems in the Hudson Bay Lowlands and other environmental processes that mediate these impacts. We surveyed published literature from the region to identify climate indicators associated with impacts on snowpacks, ponds, vegetation, and wood frogs. These climate indicators were calculated using statistically downscaled climate projections, and the potential impacts on ecosystem processes are discussed. While there is a strong trend towards longer and warmer summers, associated changes in the vegetation community mean that snowpacks are not necessarily decreasing, which is important for freshwater ponds dependent on snowmelt recharge. A clear throughline is that the impacts on these ecosystem processes are complex, interconnected, and nonlinear. This review provides a framework for understanding the ways in which climate change has and will affect subarctic regions