Rapid Permafrost Thaw Removes Nitrogen Limitation and Rises the Potential for N2O Emissions

Abstract

Ice–rich Pleistocene permafrost deposits (Yedoma) store large amounts of nitrogen (N) and are susceptible to rapid thaw. In this study, we assess whether eroding Yedoma deposits are potential sources of N and gaseous carbon (C) losses. Therefore, we determined aerobic net ammonification and nitrification, as well as anaerobic production of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) in laboratory incubations. Samples were collected from non-vegetated and revegetated slump floor (SF) and thaw mound (TM) soils of a retrogressive thaw slump in the Lena River Delta of Eastern Siberia. We found high nitrate concentrations (up to 110 µg N (g DW)−1) within the growing season, a faster transformation of organic N to nitrate, and high N2O production (up to 217 ng N2O-N (g DW)−1 day−1) in revegetated thaw mounds. The slump floor was low in nitrate and did not produce N2O under anaerobic conditions, but produced the most CO2 (up to 7 µg CO2-C (g DW)−1 day−1) and CH4 (up to 65 ng CH4-C (g DW)−1 day−1). Nitrate additions showed that denitrification was substrate limited in the slump floor. Nitrate limitation was rather caused by field conditions (moisture, pH) than by microbial functional limitation since nitrification rates were positive under laboratory conditions. Our results emphasize the relevance of considering landscape processes, geomorphology, and soil origin in order to identify hotspots of high N availability, as well as C and N losses. High N availability is likely to have an impact on carbon cycling, but to what extent needs further investigation.

Microbiogeochemical Traits to Identify Nitrogen Hotspots in Permafrost Regions

“…Based on measured N 2 O emissions, N 2 O loss under optimal conditions may be 54.8 mg N 2 O-N m −2 per year, which is 0.14% of the initial inorganic N content of exposed Yedoma [87]. Marushchak et al [87] reported from bare, disturbed slump floor sites with high water content (WFPS ≥ 67%), but also undisturbed, dry sites (WFPS = 6-15%, mean of 13 ± 6) an N 2 O uptake [87] (Tables 1 and S2, Figure 5B) being in contrast to a study at the same site where even nonvegetated bare slump with intermediate water content (WFPS of 63%) had significant in situ N 2 O emissions (2.03 ± 1.39 mg N 2 O-N m −2 d −1 ) [256]. High N 2 O emissions on slump floor sites were detected only with measurable nitrate content (RTS Lena River [87]; Table S1).…”

“…Lower, similar, or higher rates of C mineralization and thus gaseous C released as CO 2 and CH 4 have been described at hillslope thermokarst landforms compared to undisturbed neighboring sites [88,257,276,277,286,287,297]. In comparison to undisturbed soils, soils in hillslope thermokarst landscapes are generally characterized by low C/N ratios and higher but highly variable inorganic N availability, dominated by ammonium, [29,87,125,213,256]. The increased inorganic N content is most likely due to the following: (1) thermo-erosion exposes mineral soils and thaws permafrost subsoil layers, which typically contain higher con-centrations of inorganic nitrogen (mainly ammonium) than topsoils [28,42,77,103,117,213], (2) physical mixing increases aeration and drainage, which promotes aerobic N mineralization [186,208,213], (3) reduced or absent uptake of plant nutrients due to disturbed vegetation.…”

“…For RTS sites, a distinction was made between two subsites: slump floor (SF) and thaw mound (TM). 1 and 2), undisturbed site: Holocene cover deposits with moss, sedge and dwarf shrub vegetation communities on top of the RTS, disturbed thermokarst site: Yedoma revegetated with grasses (Arctagrostis arundinacea) subdivided into slump floor (SF, SF3V in [256]; SF3 in [257], Yedoma mixed with younger Holocene) and thaw mounds (TM, TM1V in [256] dominated by late Pleistocene Yedoma) as described in Marushchak et al [87]. Data are mean values ± standard division calculated from the mean values of the cited publications, therefore they are mainly variations between sampling years.…”

“…When differentiating between subsides, soils of thaw mounds in particular had increased nitrate content with a higher DIN/DON ratio (means of all sites 27.7 ± 36.3 µg NO 3 − -N g dw −1 , 0.4 ± 0.6 µg NH 4 + -N g dw −1 , 4.9 ± 3.0 data of one site see at Table S1)). In contrast, mixed Holocene and Pleistocene sediments at the slump floors were mainly enriched in ammonium and had a lower DIN/DON ratio (means of all sites 8.3 ± 15.3 µg NO 3 − -N g dw −1 , 9.4 ± 7.7 µg NH 4 + -N g dw −1 , 0.8 ± 0.8, data of one site see at Table S1 [256]).…”

“…The net mineralization and net nitrification rates of slump floor soils measured in laboratory incubations increased with the thaw stage of RTS in Siberia. The lowest rates were observed on undisturbed, vegetated control sites (C/N 38), followed by young, non-vegetated disturbed sites (C/N ~15, both mainly with negative rates), and the highest rates were observed on revegetated, stabilized sites (gross N mineralization 15.1 ± 10.1 µg N g dw −1 d −1 , net N mineralization 4.0 ± 4.9 µg N g dw −1 d −1 , net nitrification 1.6 ± 1.5 µg N g dw −1 d −1 , in aerobic incubations 4-5 • C, Tables 1 and S1 [87,256]).…”