Chronic nitrogen additions at the Harvard Forest (USA): the first 15 years of a nitrogen saturation experiment

The delivery of reactive forms of nitrogen to the environment through the sum of agricultural and industrial activities now exceeds that from natural processes. Potential negative effects on forests were first proposed in 1985, and in the ensuing two decades, the process of N saturation has become a well-established and generally understood phenomenon, with a few remaining, significant unknowns. One goal of this special section in Forest Ecology and Management is to report in detail on results from the first 15 years of chronic nitrogen additions to two contrasting forest types at the Harvard Forest in Petersham, MA, USA, with special reference to these two central questions. As similar projects elsewhere come to an end, the Harvard Forest experiment remains as one of the few on-going, long-term N saturation experiments. Longevity has enhanced the value of the chronic N experiment, and lead to a series of collaborative studies on plant, soil and microbial responses. Another goal of this special issue is to bring together and present the findings resulting from a diverse set of measurements enabled by the presence of this long-term experiment. A total of 11 papers are presented, in addition to this brief introduction. (C) 2004 Elsevier B.V. All rights reserved.