Seasonality of dissolved nitrogen from spring melt to fall freezeup in Alaskan Arctic tundra and mountain streams

Abstract

Predicting the response of dissolved nitrogen export from Arctic watersheds to climate change requires an improved understanding of seasonal nitrogen dynamics. Recent studies of Arctic rivers emphasize the importance of spring thaw as a time when large fluxes of nitrogen are exported from Arctic watersheds, but studies capturing the entire hydrologic year are rare. We examined the temporal variability of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) concentrations in six streams/rivers in Arctic Alaska from spring melt to fall freezeup (May through October) in 2009 and 2010. DON concentrations were generally high during snowmelt and declined as runoff decreased. DIN concentrations were low through the spring and summer and increased markedly during the late summer and fall, primarily due to an increase in nitrate. The high DIN concentrations were observed to occur when seasonal soil thaw depths were near maximum extents. Concurrent increases in DIN and DIN‐to‐chloride ratios suggest that net increases from nitrogen sources contributed to these elevated DIN concentrations. Our stream chemistry data, combined with soil thermistor data, suggest that downward penetration of water into seasonally thawed mineral soils, and reduction in biological nitrogen assimilation relative to remineralization, may increase DIN export from Arctic watersheds during the late summer and fall. While this is part of a natural cycle, improved understanding of seasonal nitrogen dynamics is particularly important now because warmer temperatures in the Arctic are causing earlier spring snowmelt and later fall freezeup in many regions.

Quantifying DOC and Its Controlling Factors in Major Arctic Rivers during Ice-Free Conditions using Sentinel-2 Data

“…This phenomenon was also reported in previous studies on the dynamic variation of riverine DOC in the Arctic and other rivers in high latitude areas [2,43,62,63]. Previous studies pointed out that the river discharge, as a primary carrier of DOC, was a critical means of DOC transportation [13,16,24,60]. The surge of precipitation and extensive snowmelt generate unique spring freshets across Arctic regions.…”

“…DOC transportation from land to ocean through river systems is mainly influenced by natural and artificial processes. Many factors, such as the topography, hydrology, soil, land cover, eutrophication, and human activities, play critical roles in regulating the DOC concentration in water bodies [38,47,60]. To obtain a better understanding of the role of DOC in the ecological environment, we quantitatively analyzed the effects of different potential drivers on the DOC concentration using field measured data and remote sensing data.…”

“…For permafrost regions, however, tundra could serve as an impermeable barrier and prevent water from penetrating deeper into the soil [68]. Due to limited vertical drainage, melting water was detained in surface soil, and absorbed or degraded by organic matter in soil, leading to a decrease in DOC concentration [39,60,[68][69][70]. In comparison, gentle slopes could facilitate the near surface flow path and augment the contact area and time between water and soil that is rich in organic matter, which would accelerate the humification of macromolecular organic matter and increase the DOC concentration [38,39,46,71,72].…”

Arctic concentration–discharge relationships for dissolved organic carbon and nitrate vary with landscape and season

“…Our finding of NO 3 − dilution behavior during most flow events in the Tundra catchment emphasizes the relatively low N‐availability and high N‐demand typical of these Alaskan tundra landscapes (Fig. 2a), despite increasing NO 3 − concentrations later in the thaw season (Treat et al 2016; Khosh et al 2017). In contrast, in the Alpine catchment specifically, the consistent pattern of negative DOC β across all sampling years (Fig.…”

“…6a) can likely be attributed to lower ecosystem productivity generating organic carbon and, subsequently, storing organic carbon in the subsurface of this landscape (Keuper et al 2012;Harms et al 2016). The absence of a seasonal trend for Alpine NO 3 − β may have been the result of the dominance of snowmelt driving flow or high rates of nitrification across the season, resulting in a consistently enriched NO 3 − signal and depleted landscape NO 3 − sources (Harms and Jones 2012;Khosh et al 2017). Generally, our observations across the different landscapes suggest that precipitation events that occur anytime during the thaw season can disproportionately contribute DOC or NO 3 − to downstream exports and potentially trigger nutrient limitation events in terrestrial environments.…”