The fate of reactive nitrogen differs by hillslope aspect in montane forests of the Colorado Front Range, U.S

Background/Question/Methods

Over a decade of research in the alpine zone of the Colorado Front Range has shown that nitrogen (N) deposition originating from low elevation, developed areas has changed ecosystem stoichiometry, microbial transformation rates, and aquatic community structure.  Markedly less research has occurred in the montane zone, which sits at the current snow line and may be vulnerable both to climate change impacts on precipitation and temperature, as well as increased N loading.  We conducted ¹⁵N-nitrate and lithium bromide (LiBr) tracer studies during spring snowmelt – the major hydrologic event in this system – to determine the fate and transport of N in a forested montane catchment that is part of the Boulder Creek Critical Zone Observatory.  At the onset of snow melt, we applied the tracers to instrumented plots along a hillslope cross-section, including crest, midslope, and toeslope positions, as well as north- and south-facing aspects.  Integrating hydrological theory with measurements of N species and tracers in soil, soil water, microbial biomass, and vegetation provided a means of determining residence time, export pathways, and biological uptake of deposited N. 

Results/Conclusions

We found that tracers broke through immediately to 0.3 m depth on the north-facing slope, where the seasonal snowpack was melting, and transported completely out of soil waters within 40 days.  Additionally, 5% of the applied ¹⁵N was recovered in vegetation.  On the south-facing slope, which does not develop a snowpack, tracers were transported to depth only during spring storms, and approximately 22% of the ¹⁵N was recovered in vegetation.  Tracer persisted in soil waters through the 54-day observation period.  These results suggest that N residence time is longer on the south- than the north-facing slope, due to sporadic melt water transport, and biota that are adapted to respond rapidly to N availability in the spring.  Our results lend insights that are important for understanding catchment-scale timing of N transport to streams and the role of mid-elevation forests in metabolizing deposited N within the larger landscape context, from alpine to plains.