Effective soil incubations for studying nitrogen cycling in permafrost soils — practical and ecological considerations

Soil nitrogen (N) cycling and nitrous oxide (N₂O) production and consumption dynamics have received relatively little attention in Arctic biogeochemistry, largely because of the classical understanding of general N limitation and negligible N gas losses. Now, this understanding is becoming outdated by recent studies, which have revealed active N cycling in permafrost-affected soils by looking into processes and habitats previously ignored and possibly also because of actual intensification of soil N cycling with warming and permafrost thaw. Since enhanced N availability versus N limitation is of crucial importance for future carbon balance and ecosystem-climate feedback in the rapidly warming Arctic, there is a dire need for more information on N cycling in permafrost soils. Soil incubations can be used to study N turnover rates in controlled conditions and to tease apart various processes, but they need to be well designed to realistically represent the soil N cycle in its full complexity.

Here, we summarize our experience from N cycling studies by soil incubations over the years, with an emphasis on the ongoing Thaw-N project which investigates the fate of permafrost N following thaw. We give our suggestions for the balancing act between simple experiments that can be easily conducted over large numbers of samples to improve spatial representativeness, and detailed experiments with advanced methods to uncover the actual rates of individual processes and link them with microbial activities. . Our experience highlights the role of microbial versus substrate limitation in shaping the soil N cycle as time passes following disturbances, such as permafrost thaw. We also discuss the special challenges of studying N₂O production in comparison to the other, more commonly studied greenhouse gases carbon dioxide and methane.