Effects of permafrost thaw on N-cycle processes in a thermokarst system

Northern peatlands store large amounts of carbon (C) as well as nitrogen (N) which amounts to ∼80 % of global C and N peatland stocks, making them important C and N reservoirs. With Arctic amplification warming the Arctic nearly four times faster than the global average, an increased permafrost thaw was observed even in very cold polar regions such as the Canadian High Arctic, where thaw depths already exceeded scenarios projected to occur by 2090, altering hydrology, geomorphology as well as nutrient cycling in the landscape, caused by but not limited to increased thermokarst formation. Thermokarst describes a landscape occurring when ice-rich permafrost, which is highly vulnerable to climate change due to lack of sufficient thermal buffering, thaws altering microbial decomposition of soil organic matter (SOM), including N pathways. Considering the effects of global warming on permafrost-affected peatlands in the Arctic, it is likely that the active layer will continue to deepen and thaw more and more permafrost and therefore, expose more formally frozen SOM to microbial decomposition, priming the N-cycling and increasing the N availability. Our work explores the changes in N-cycling in thermokarst landscapes, by incubation of soils with 15N stable isotope tracing to assess organic N depolymerization, N-mineralization and nitrification rates over time. Permafrost soils from the continuous permafrost zone on the uplands east of the Mackenzie Delta (Northwest Territories, Canada) from 3 different depths in the active layer and the upper permafrost, in two phases of thermokarst development were investigated. We performed a 15N tracing experiment, by incubating soils with a 15N-protein for 9 days and estimated 15N in dissolved organic N, microbial N and nitrate as well as ammonium. Our results show changing N-cycle processes with depth, as well as with progress of thermokarst stages. Generally microbial N uptake in active layers was favoured over N mineralization, while the contrary was the case in permafrost layers. This pattern might be connected to a microbial N-limitation in the upper soil layers leading to increased microbial N demand. In permafrost layers microbes show higher rates of N mineralization (ammonification), i.e., they excrete inorganic N, most likely because of a carbon limitation. With progressing thermokarst development a shift form microbial uptake focused processes to mineralization pathways was observed in the active layer. This trend might be due to increased N availability as ground collapses as a result of thawing and mixes the soil layers, leading to decomposition of previously frozen SOM. Permafrost layers favoured ammonification, however, samples from secondary thermokarst sites showed signs of N limitation at the end of incubation, most likely because of the long-term exposure of microbes to available SOM leading to depletion of the N stocks. With this work we contribute to unravelling the changes in N-cycle pathways in the thawing Arctic, shining a light on the consequences of climate change on these remote ecosystems. This study was funded by the Marie Skłodowska-Curie Actions H2020-MSCA-IF-2020 within “NITROKARST” project (Grant agreement 101024321)