Snowpack influences spatial and temporal soil nitrogen dynamics in a western U.S. montane forested watershed

Declines in winter snowpack have increased the severity of summer droughts in western U.S. forests, with the potential to also impact soil available nitrogen (N). To understand how snowpack controls spatiotemporal N availability, we examined seasonal N dynamics across elevation, aspect, and topographic position (hollow vs. slope) in a forested watershed in the northern Rocky Mountains. As expected, peak snow‐water equivalent (SWE) was generally greater at higher elevations and on north‐facing aspects. However, the effects of topographic position and snowdrift led to variability in snow accumulation at smaller spatial scales. Spatial patterns of the snowpack, in turn, influenced soil moisture and temperature, with greater SWE leading to generally higher soil moisture levels during the summer and smaller temperature fluctuations throughout the year. Wetter conditions in spring or fall generally supported greater inorganic N pools, but at the driest locations (low‐elevation slope), pulses of N mineralization in summer may have played important roles in overall N dynamics. More importantly, soil moisture during the summer appeared to be more influenced by antecedent snowpack from the previous year than by current‐year summer rain. Subsequently, N mineralization under snowpack may be strongly influenced by soil moisture and temperature conditions from the previous fall, before snowpack accumulation. Together, our results indicate that snowpack strongly influences N dynamics beyond the current growing season in western coniferous forests through mediation of soil moisture and temperature, and suggest that further decline in winter snowpack may affect these forests through constraints in both water and N availability. In temperate forests, nitrogen (N) is often considered to be the most common limiting nutrient (Vitousek and Howarth 1991), and many experimental studies demonstrate increases in aboveground productivity with increased N availability (summarized in LeBauer and Treseder 2008). In semi‐arid western U.S. conifer forests, water and N likely co‐limit tree growth because the water and N cycles are tightly coupled through biological N transformation and leaching (Schimel et al. 1997, Wang et al. 2015). Furthermore, because water availability in these ecosystems largely depends on snowmelt water, spatial distribution and inter‐annual variations in snowpack may indirectly control N availability during the growing season. For example, the amount of snowpack and the timing of spring snowmelt collectively affect proximate soil moisture during the snow‐free