It’s Not Just About Snowmelt: Ecosystem Fates of N Deposition During Experimental Rainfall in the Colorado Front Range

Abstract:

Climate is changing in the Colorado Front Range, leading to declines in snowpack below 2500 m and a greater proportion of total annual precipitation falling as rain. Several decades of research in alpine ecosystems have demonstrated the links among climate, hydrologic response, and the fate of atmospheric nitrogen (N) deposition – another driver of ecosystem change. A greater proportion of precipitation that falls as rain will likely affect the patterns and persistence of soil moisture, as well as the fate, availability, and transport of N. To explore these interactions, we conducted tracer experiments in July using ¹⁵N-nitrate (NO₃^-) and lithium bromide (LiBr) during experimental rainfall events (intensity and storm duration with a recurrence interval of 2-5 years) in a high montane catchment within the Boulder Creek Critical Zone Observatory. For four weeks following the application of rainfall and tracers, we measured soil water concentrations of Br^-, ¹⁵NO₃^-, and N species daily, followed by recoveries of ¹⁵N species in bulk soils and microbial biomass (collections in August and October). We found that hillslope aspect (north- versus south-facing) and position were significant determinants of soil ¹⁵N-NO₃^- recoveries, while soil depth and time since rain event were significant determinants of ¹⁵N recovery in the microbial pool. Dry conditions during the growing season led to little movement of tracers in soil solution. Overall, microbial uptake and hydrologic transport of N was greater on the north- than the south-facing slopes. Our results point to the importance of soil properties (e.g., presence/absence of organic horizon, degree of macroporosity), which differ greatly by hillslope aspect, in controlling ecosystem N retention. These findings present an important piece in the story of atmospheric N’s fate along the elevation gradient from the plains to the alpine, and highlight that soil organic matter is a significant component of ecosystem N storage in high montane forests.