Abstract
In Arctic ecosystems, increasing temperatures are driving the expansion of nitrogen (N) fixing shrubs across tundra landscapes. The implications of this expansion to the biogeochemistry of Arctic ecosystems is of critical importance, yet many details about the contributions of N-fixation to the nutrient economy, especially N in the form of either nitrate or ammonium, remain unknown. To address this knowledge gap, the spatiotemporal variability of NO3- and its environmental and edaphic controls were investigated at an alder dominated permafrost tundra landscape in the Seward Peninsula, Alaska, USA. Soil pore water was collected in and around patches of alder shrubland growing along a well-drained hillslope. δ15N¬¬ and δ18O of soil pore water confirmed that NO3- was produced through microbial degradation of N-rich alder shrub organic matter. Soil pore water collected within alder shrublands had NO3- concentrations of up to 58 mg L-1 (15.50 ± 24.42 mg L-1) and differed significantly from both upslope (0.11 ± 0.25 mg L-1; p < 0.05) and downslope locations (0.10 ± 0.06 mg L-1). Temporal variation in NO¬¬3- within and downslope of alder shrubland patches corresponded to precipitation events, where NO3- accumulated in the soil was flushed downslope during rainfall. Finally, enrichment of both δ15N¬¬ and δ18O at wetter downslope locations indicate that denitrification buffered the mobility and spatial extent of NO3-. These findings have important implications for shrub expansion in N-limited permafrost systems, and the mobility of nutrients as tundra landscapes respond to a warming Arctic.
