Summer storms trigger soil N₂O efflux episodes in forested catchments

Abstract

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 (N 2 O) and nitrogen (N 2 ) 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 N 2 O and N 2 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 N 2 O and N 2 . There was a strong relationship between precipitation and N 2 O efflux (y = 0.44x 1.22 , r 2 = 0.618, p < 0.001 in the inner wetland; y = 1.30x 1.16 r 2 = 0.72, p < 0.001 in the outer wetland) and significantly different N 2 :N 2 O ratios in different areas of the wetland (19.6 in the inner wetland and 10.1 in the outer wetland). Soil N 2 O + N 2 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.

Lorenz Curve and Gini Coefficient Reveal Hot Spots and Hot Moments for Nitrous Oxide Emissions

“…Topography interacts with landscape hydrology to influence carbon (C) and N biogeochemical cycling. Hot spots of N 2 O emissions have been observed in convergent hydrological flow paths due to high moisture and low O 2 concentration in the soil, favoring N 2 O production via denitrification (Enanga et al, 2016;Izaurralde et al, 2004;Pennock et al, 1992;Saha et al, 2017;Vilain et al, 2010). The hot moments are brief temporal windows with emissions that are substantially higher than the background level emission (Groffman et al, 2009;McClain et al, 2003;Molodovskaya et al, 2012).…”

Snow‐covered soils produce N2O that is lost from forested catchments

“…The soil pore water DOC and NO 3 − concentrations varied among the topographic positions, which provides further insight into the processes generating N 2 O. The DOC and NO 3 − accumulated at the UP position, which may suggest that denitrification was limited by high redox conditions, as is observed during the growing season [ Enanga et al ., ; Kralova et al ., ], but that the NO 3 − pools were maintained through nitrification. In contrast, DOC and NO 3 − did not accumulate in the wetland positions, suggesting that denitrification may be limited by nutrient conditions beneath a snowpack in the wetland.…”

“…Previous work investigated topographic controls on dissolved N export, including NO 3 − , ammonium, and dissolved organic nitrogen [ Creed and Beall , ; Creed and Band , ; Creed and Band , ]. A related study investigated the physical and chemical controls on biological N transformations within forested catchments during the growing season and observed that precipitation triggered changes in redox conditions in wetland soils that resulted in net soil N 2 O production events [ Enanga et al ., ]. This study builds on this work by providing new insights into gaseous N exports during the nongrowing season including the snow period.…”

Forest soil CO2 efflux models improved by incorporating topographic controls on carbon content and sorption capacity of soils

“…For example, the IW position may have been influenced by a hydrological ''decoupling'' between surface and subsurface processes during a rain event, especially in late summer/early fall when water table depths typically drop well below the surface. It is possible that the rain triggers soil CO 2 efflux events by creating optimal conditions that include delivery of fresh substrate from rain passing through the canopy to sedentary soil microbes, in addition to providing optimal temperature and moisture conditions for microbial respiration (e.g., Enanga et al 2016). Further, labile carbon-laden water in the uplands would tend to flow rapidly downslope through surface and shallow subsurface flowpaths during a rain event, increasing carbon pools in the FS and TS positions especially on steeper transects (Riveros-Iregui and McGlynn 2009).…”