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Harvard Forest REU Symposium Abstract 2007
- Title: How can stream exports of N be used to understand forest nutrient retention?
- Author: Michele E Rolph (Boston University)
- Abstract:
Headwater streams integrate nutrient, organic matter, and sediment fluxes from forested watersheds. I examined inorganic and organic N dynamics in Bigelow Brook, a headwater stream draining a mature, healthy hemlock forest in the Prospect Hill tract of Harvard Forest (Petersham, MA). Bigelow Brook’s geomorphology, hydrology, and watershed ecology are well characterized and efforts are underway to estimate annual carbon and nutrient fluxes to downstream ecosystems. Preliminary research suggested that Bigelow Brook has surprisingly low concentrations of nitrate, despite the fact that New England receives high atmospheric N inputs. My project had three objectives: 1) to better document inorganic (NH4+ and NO3-) and organic N (DON) concentrations in Bigelow Brook, 2) to evaluate the bioavailability of organic matter fluxes, and 3) to evaluate inorganic and organic N retention across the forest soil and stream interface. The relative concentrations of NH4+, NO3- and DON varied with season. During the growing season, DON (0.056±0.003 mg/L) was surprisingly greater than inorganic N concentrations (NO3--N = 0.011±0.002 mg/L and NH4+-N = 0.024±0.001). Downstream fluxes of DON were closely linked to summer storm events suggesting a rapid flushing of organic matter from forest soils. These results suggest that Bigelow Brook’s watershed routinely retains inorganic forms of N, yet is susceptible to large DON losses during storm events. Throughout the summer, % bioavailability of organic matter never exceeded 20% suggesting that the majority of DON exported from the watershed is refractory and is transported to downstream ecosystems. Cation and anion resin bags were incubated in riparian soil to estimate NO3- and NH4+ accumulation, and soil cores were used to determine net N mineralization. These pending data will help to identify potential hotspots of biogeochemical activity and determine relative amounts of nutrients being stored in the riparian zone. Bigelow Brook needs to be the focus of future research that continues to quantify stream and riparian zone biogeochemistry to more fully understand controls on nutrient retention and loss from forested ecosystems.
- Research Category: Soil Carbon and Nitrogen Dynamics; Watershed Ecology
