PARENT SESSION
Oral Session # 98: Biogeochemistry V: Forests; Nitrogen.
Presiding: RL Sanford
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Meeting Room 204.

Effects of nitrogen addition to the canopy at the Howland Ameriflux Site, ME and to the forest floor at the Harvard Forest LTER Site, MA on the foliar carbon and nitrogen metabolism and forest productivity.

Minocha, Rakesh^*,1, Hollinger, David¹, Long, Stephanie¹, Dail, Bryan², Magill, Alison³, Aber, John³, ¹ USDA Forest Service, Durham, NH, USA² University of Maine, Orono, ME, USA³ University of New Hampshire, Durham, NH, USA

ABSTRACT- Atmospheric nitrogen (N) deposition may cause forest decline or enhance its productivity depending on the initial N status of the soil, and the rate and duration of N deposition. Continuous deposition of N can move ecosystems towards nitrogen saturation, a sate that can lead to forest decline. Previous studies from our group show a strong correlation between soil nutrient deficiencies and/or chronic N addition and increased foliar free putrescine (an organic polyamine) levels, indicating that foliar putrescine can potentially be used as a marker of general stress in visually healthy trees. Free amino acids, especially arginine, also increased in response to chronic N input. Arginine could thus be an indicator of excess nitrogen inputs in otherwise nitrogen limited forest soils. The objective of the present study was to determine changes in foliar physiology (cellular polyamines, amino acids, proteins and chlorophyll levels) at the Howland Experimental Forest following a relatively low level of nitrogen application to canopy (18 Kg N ha^-1 y^-1) since 2001 and at the Harvard Forest LTER Site where a similar experiment underway since 1989 is adding 50 and 150 Kg N ha^-1 y^-1of N to the forest floor. We predicted that at the Howland site, the smaller foliar N addition will not exceed the capacity for N uptake by vegetation and may lead to greater C sequestration. This is unlikely to put physiological stress on trees. Our preliminary data for first two years at Howland show no change in the physiology of the needles due to canopy N application. However, significant changes in foliar physiology were observed after 6 years of chronic N addition at Harvard Forest. The long-term goal is to be able to better understand the N deposition levels that elicit a stress response in a forest where N availability is assumed to limit net primary productivity.

Key words: Nitrogen Deposition, Harvard Forest LTER Site, Howland Ameriflux Site, Foliar Physiology