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

Abstract

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.

Nitrogen acquisition strategies of mature Douglas‐fir: a case study in the northern Rocky Mountains

“…However, N taken up from soil following bud break dwarfed the magnitude of N remobilization; by the time bud break occurred (late May to mid‐June in 2016, mid‐June to late June in 2017), root uptake of N from the soils was likely the dominant source. During this period, soil moisture levels were high (Yano et al 2019), soil N supply and availability rates were high (Figs. 9, 10), and transpiration rates were high (Looker et al 2018), enhancing rates of N mass flow to the roots (Lambers et al 1998, Matson et al 2002).…”