This section is written for faculty who can modify the Overview as appropriate for their students. “Student Instructions” also include introductory material.
Inputs to the terrestrial nitrogen (N) cycle have more than doubled in the past century due to anthropogenic activities, particularly fertilizer use and fossil fuel combustion (Vitousek, 1997; Vitousek et al., 1997). Global anthropogenic N inputs now exceed non-anthropogenic sources (e.g., biological nitrogen fixation). One consequence of this human-driven change in the N cycle is the significant increase in N that is being deposited as wet or dry deposition in forest ecosystems. At the local and regional scale, N deposition near industrialized and agricultural areas can greatly exceed those in unpolluted areas of the world (Vitousek et al., 1997). Rates of N deposition are particularly high in the Midwestern and Northeastern U.S. and in Europe. Since most temperate forest ecosystems evolved under low N conditions, chronic N enrichment would be expected to alter ecosystem properties and processes in fundamental ways.
Aber et al. (1989) suggested that the response of forest ecosystems to N enrichment would vary depending on the duration of the additions. They hypothesized that positive responses (e.g., increases in net primary production—NPP) would be observed in the short-term, while negative responses (e.g., increased nitrate leaching, reduced NPP) would be observed over the long-term. They established the Chronic Nitrogen Addition Experiment at Harvard Forest in 1988 to test these hypotheses. Results from the first 15 years of this experiment were recently detailed in a special issue of Forest Ecology and Management (Aber and Magill, 2004). The purpose of this activity is to illustrate how chronic N additions at Harvard Forest have impacted a range of ecosystem properties and processes, including both plant and soil responses.
Responses to added N at the Harvard Forest have been dramatic (Magill et al., 2004). Long-term fertilization has lead to increased N concentrations in leaf litter and roots. Tree mortality has been high in the high N pine stand (56% by 2002), and biomass accumulation has stopped altogether. The high N hardwood stand shows increased aboveground net primary productivity relative to the control plot, but excess N availability coupled with a severe drought in 1995 lead to high mortality of red maple trees in this plot (72% by 2002). Soil respiration was stimulated by N additions during the first year of fertilization in the hardwood stand; however, in the second year, soil respiration was not significantly different between control and fertilized hardwood plots and was suppressed by up to 25% in the pine fertilized plots (Bowden et al., 2004). After 13 years of continuous N fertilization, soil respiration in the high N plots was suppressed by 40% in both stands. Reductions in soil respiration are concomitant with reductions in microbial biomass, due primarily to a significant reduction in the fungal component of the microbial community (Frey et al., 2004). Further details of the results from the first 15 years of this experiment can be found in Aber and Magill (2004).