Permafrost thaw and release of inorganic nitrogen from polygonal tundra soils in eastern Siberia

Abstract

Abstract. The currently observed climate warming will lead to substantial permafrost degradation and mobilization of formerly freeze-locked matter. Based on recent findings, we assume that there are substantial stocks of inorganic nitrogen (N) within the perennially frozen ground of arctic ecosystems. We studied eleven soil profiles down to one meter depth below surface at three different sites in arctic eastern Siberia, covering polygonal tundra and river floodplains, to assess the 15 amount of inorganic N stores in arctic permafrost-affected soils. Furthermore, we modeled the potential thickening of the seasonally unfrozen uppermost soil (active) layer for these sites, using the CryoGrid2 permafrost model and representation concentration pathway (RCP) 4.5 and 8.5 scenarios. The first scenario, RCP4.5, is a stabilization pathway that reaches plateau atmospheric carbon concentrations early in the 21 st century; the second, RCP8.5, is a business as usual emission scenario with increasing carbon emissions. The modeled increases in active layer thickness (ALT) were used to estimate 20 potential annual N mobilization from permafrost-affected soils in the course of climate-induced active-layer deepening. We observed significant stores of inorganic ammonium in the perennially frozen ground of all investigated soils, up to 40-fold higher than in the active layer. The modeled ALT increase until 2100 under the RCP8.5 scenario was between 19 ± 3 cm and 35 ± 6 cm, depending on the location. Under the RCP4.5 scenario, the ALT remained stable in all investigated soils. Our estimated mean annual N release under the RCP8.5 scenario is between 8 ± 3 mg m -2 and 81 ± 14 mg m -2 for the different 25 locations, which reaches values up to the order of magnitude of annual fixation of atmospheric N in arctic soils. However, the thawing induced release of N represents only a small flux in comparison with the overall ecosystem N cycling.

Experimentally increased nutrient availability at the permafrost thaw front selectively enhances biomass production of deep‐rooting subarctic peatland species

“…Thawing and subsequent mineralization of permafrost may release considerable amounts of plant‐available nitrogen (N) into subarctic peatlands (up to 0.3 g N m −2 year −1 in the coming decade) (Keuper et al., ). This is consistent with observed increases in N‐availability with soil depth in other tundra types (Beermann et al., ; Harms & Jones, ; Sistla et al., ; Wild et al., ) and enhanced inorganic N export from permafrost‐thaw influenced soils to streams (Frey & McClelland, ; Jones, Petrone, Finlay, Hinzman, & Bolton, ). This “new” global change‐induced N‐source at the permafrost thaw front at depth is additional to the predicted and observed increase in N‐availability in shallower soil layers due to atmospheric deposition or increased N‐mineralization rates by higher air temperatures (Rustad et al., ; Weedon et al., ).…”

“…Thawing and subsequent mineralization of permafrost may release considerable amounts of plant-available nitrogen (N) into subarctic peatlands (up to 0.3 g N m À2 year À1 in the coming decade) (Keuper et al, 2012). This is consistent with observed increases in N-availability with soil depth in other tundra types (Beermann et al, 2016;Harms & Jones, 2012;Sistla et al, 2013;Wild et al, 2015) and enhanced inorganic N export from permafrost-thaw influenced soils to streams (Frey & McClelland, 2009;Jones, Petrone, Finlay, Hinzman, & Bolton, 2005).…”

Depth‐based differentiation in nitrogen uptake between graminoids and shrubs in an Arctic tundra plant community

“…Warmer temperatures stimulate microbial activity and increase mineralization rates of organic matter (Davidson & Janssens, ; Rustad et al., ), particularly in the upper soil, where fresh plant litter decays and temperature is more influenced by air temperature than deeper in the soil. In the deeper soil layers, frozen ancient soils contain a large amount of organic matter, including nutrients (Beermann et al., ; Kokelj & Burn, ; Zimov, Schuur, & Chapin, ). Climate warming deepens the thawing depth of permafrost in the growing season and thus extra nutrients are mobilized in deeper soil (Frey & McClelland, ; Keuper et al., ).…”

Projecting Permafrost Thaw of Sub‐Arctic Tundra With a Thermodynamic Model Calibrated to Site Measurements

“…Given the increases in deep soil temperature, active layer depth, permafrost thaw depth, and active layer depth simulated here, we expect that the contribution of soil respiration to the net ecosystem carbon balance to outpace concurrent increases in plant uptake. For well‐drained tundra, permafrost thaw can increase plant access to nutrients, such as nitrogen, leading to an increase in carbon uptake and shifts in plant community composition (Aerts et al., 2006; Beermann et al., 2016; Mack et al., 2004; Salmon et al., 2016; Shaver et al., 2007). However, warmed soil and permafrost thaw also stimulate soil respiration (Bracho et al., 2016; Cheng et al., 2017; Natali et al., 2011), which can offset increases growing season carbon uptake through increases in winter soil respiration (Commane et al., 2017; Grogan & Chapin, 1999; Natali et al., 2019; Webb et al., 2016).…”