Foliar and Soil Chemistry at Harvard Forest Chronic Nitrogen Amendment Experiment 1995-2009

The aim of the chronic N study at Harvard Forest is to increase our understanding of ecosystem nitrogen dynamics in response to elevated nitrogen inputs. In recent, nitrogen deposition in the Northeastern United States has been 10 to 20 times above historic background levels which could possibly saturate the retention capacity of a forest ecosystem. Long-term elevated N deposition typically leads to an increase in the concentration of total foliar N, with or without similar changes in the important base elements such as Ca, Mg and K. This increase in leaf N content also leads to significant shifts in the internal partitioning of N within the leaf. For example, in conifers, N deposition has been shown to significantly increase leaf N present in the form of free amino acids such as arginine. Little is known about N partitioning for hardwoods under these conditions. These changes in N partitioning are possibly connected to leaf function. The present study was conducted to experimentally test whether the alterations in N partitioning do occur due to long-term N deposition and if so do they have a positive or a negative effect on photosynthetic capacity and biomass production. A possible decoupling of the relationship between foliar N and photosynthetic rate may occurs under these conditions. The treatment plots used in this study are part of the Chronic Nitrogen Amendment Study at the Harvard Forest LTER site (42.5°N, 72°W). The site has a temperate climate with monthly temperatures ranging from -7°C in January to 20°C in July. Average annual precipitation is 110 cm (http://harvardforest.fas.harvard.edu). The site averages approximately 8 kg ha-1 year-1 of total N deposition. As reported earlier, the land-use history of the pine and hardwood stands used in this study is very different. Two adjacent stands were chosen for the study: an even-aged red pine (Pinus resinosa Ait.) stand and a 50-year-old mixed hardwood stand that had regenerated naturally after clearcutting in approximately 1945. The hardwood stand is dominated by black and red oak (Quercus velutina Lam.; Q. rubra L.) with significant amounts of black birch (Betula lenta L.), red maple (Acer rubrum L.) and American beech (Fagus grandifolia Ehrh.). The pine site was pastured until 1920, and a red pine (Pinus resinosa Ait.) plantation was established on this site in 1926. Fire destroyed some of the trees in 1957. The hardwood stand has no agricultural land-use history but was substantially damaged by a hurricane in 1938, and was harvested for firewood during 1942-44. In 1988, 30 m x 30 m plots were established in each of the two adjacent stands, red pine and a mixed deciduous each. Starting in 1989, while one plot served as control, six equal doses of NH4NO3 were applied to Low N (50 kg N ha-1 year-1) and High N (150 kg N ha-1 year-1) plots of each stand from May through September each year.