Evaluating hillslope and riparian contributions to dissolved nitrogen (N) export from a boreal forest catchment

Abstract

Catchment science has long held that the chemistry of small streams reflects the landscapes they drain. However, understanding the contribution of different landscape units to stream chemistry remains a challenge which frequently limits our understanding of export dynamics. For limiting nutrients such as nitrogen (N), an implicit assumption is that the most spatially extensive landscape units (e.g., uplands) act as the primary sources to surface waters, while near‐stream zones function more often as sinks. These assumptions, based largely on studies in high‐gradient systems or in regions with elevated inputs of anthropogenic N, may not apply to low‐gradient, nutrient‐poor, and peat‐rich catchments characteristic of many northern ecosystems. We quantified patterns of N mobilization along a hillslope transect in a northern boreal catchment to assess the extent to which organic matter‐rich riparian soils regulate the flux of N to streams. Contrary to the prevailing view of riparian functioning, we found that near‐stream, organic soils supported concentrations and fluxes of ammonium (NH4+) and dissolved organic nitrogen that were much higher than the contributing upslope forest soils. These results suggest that stream N chemistry is connected to N mobilization and mineralization within the riparian zone rather than the wider landscape. Results further suggest that water table fluctuation in near‐surface riparian soils may promote elevated rates of net N mineralization in these landscapes.

Interannual variation in seasonal diatom sedimentation reveals the importance of late winter processes and their timing for sediment signal formation

“…). In years with warmer winter air temperatures, seasonal nutrient peaks were already observed under ice (Moore et al ; Blackburn et al ), while nutrient peaks were delayed in the year with colder winter air temperature (Fig. and Supporting Information Fig.…”

Forest streams are important sources for nitrous oxide emissions

“…Additionally, the role of the riparian zone as source of N 2 O production needs to be clarified, considering the strong controls that it exerts on a wide range of biogeochemical processes in forested and agricultural catchments (Blackburn, Ledesma, Näsholm, Laudon, & Sponseller, 2017;Ledesma et al, 2018;Ranalli & Macalady, 2010).…”

“…We used spatial correlograms (function spline.correlog in the R package 'ncf'; Bjornstad, 2018) to verify the absence of spatial autocorrelation in the residuals of the models. Finally, we tested the presence of temporal autocorrelation in the mixed models by adding the correlation structure 'corAR1' from the package 'nlme'…”

Greenhouse gas emissions from boreal inland waters unchanged after forest harvesting

“…In addition, riparian zones may have acted as efficient reactors of greenhouse gases and significantly altered their concentration during transport from the hillslope to the open water (Leith et al 2015;Rasilo et al 2017, Rasilo et al 2012). This applies especially to methane which can be efficiently oxidized in the large redox gradients in riparian zones, similar to inorganic nitrogen (Blackburn et al, 2017).…”