Soil nitrogen dynamics in riparian zones of two Arctic rivers

Riparian zones act as protective buffers, filtering land-to-river nutrient inputs through retention and nitrogen (N) gas losses. While rapid Arctic warming and permafrost thaw may enhance terrestrial nutrient inputs to aquatic systems, comprehensive assessments of N cycling in Arctic riparian soils are lacking. Conventionally, these zones are considered inactive due to N limitation of Arctic soils and low temperatures, leaving their N cycling unexplored. To address this, we investigated N pools and turnover processes in riparian zones of two Arctic rivers in Finland and Greenland. We combined measurements of the extractable N pool and in-situ nitrous oxide (N2O) flux with 15N-labeling techniques to quantify gross mineralization, nitrification, potential denitrification, and potential dissimilatory nitrate reduction to ammonium (DNRA). Our findings reveal that mineral N cycling in high-latitude riparian zones is more active than previously assumed. Despite low temperatures and low to intermediate soil organic carbon content, we found high gross N mineralization rates, alongside detectable rates of gross nitrification and potential DNRA. We attribute the unexpectedly high N activity in C-poor soils to low C/N ratios, which create a stoichiometric imbalance, thereby enhancing N mineralization. The presence or absence of vegetation largely determined the dominance and magnitude of these pathways. While dense vascular plant cover was associated with higher extractable N pools and gross mineralization rate, low nitrification rates and elevated DNRA potential indicated a tight N cycle. Conversely, sparsely vegetated patches with cryptogamic biocrusts acted as N2O hotspots. In bare sandy soils adjacent to the river, we found high gross nitrification rate and detectable N2O losses, possibly driven by riverine organic N inputs. Taken together, this research shows that Arctic riparian zones may play an important role in reducing terrestrial N inputs to the rivers, which should be considered when studying the fate of N in the rapidly changing Arctic.