Monday, November 15, 2010, 12:00PM - 1:00PM
Eve-Lyn S. Hinckley
INSTAAR postdoctoral scholar
Full title: "Investigation of hydrological flows and the fate of nitrogen deposition using 15N tracer studies in the Boulder Creek Critical Zone Observatory."
Increased deposition of reactive nitrogen (Nr) to the Colorado Front Range has changed ecosystem stoichiometry, microbial transformation rates, and aquatic community structure over the last decade. While several studies have examined N loading to the alpine zone, little attention has focused on lower elevations, including subalpine and montane zones. To understand Nr impacts in these areas, we instrumented plots along a north-south cross-section in a forested montane catchment that is part of the Boulder Creek Critical Zone Observatory. We applied 15N-nitrate and lithium bromide tracers during spring snowmelt and a simulated summer rain event, the two major hydrologic events mobilizing N in these systems. Following each application, we measured tracer constituents in solution waters, as well as N isotopic composition and concentrations in soils, vegetation, and microbial pools in order to 1) determine the residence time, export pathways, and biological uptake of deposited N, and 2) develop an understanding of the seasonal hydrologic forcing and soil profile characteristics that determine the annual N budget. Our data show that source dynamics (e.g., variation in snowpack melting) control N movement through the soil profile during snowmelt; preliminary data suggest that the majority of all tracers moved out of snow-covered north-facing slope plots over the course of 40 days, versus during a two-day melt event in the south-facing slope plots. In contrast, local soil properties (e.g., texture and structure) exert control during rainfall events; N retention on the south-facing slope was greater than on the north-facing slope by a factor of six, leading to more N uptake by vegetation and microbial pools. 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.