| Introduction | Introduction
The productivity of most temperate forest ecosystems is limited by the
availability of nitrogen. Some regions have seen dramatic increases
in rates of nitrogen deposition as a result of human activities.
As nitrogen deposition increases, it may act as a fertilizer to increase
the productivity of forests. However, if the supply of nitrogen continues
to increase, a complex series of alterations to soil and plant biogeochemistry
may affect productivity, competition, and microbial community structure.
As water and other nutrients become more limiting relative to nitrogen,
these ecosystems may approach "nitrogen saturation" (Ågren
and Bosatta, 1988; Aber et al., 1989; Aber, 1992; Aber et al., 1998).
The nitrogen saturation model hypothesizes that a complex suite of changes
occurs as nitrogen deposition increases, ultimately leading to forest decline
and soil acidification (figure 1).
Figure 1. Hypothesized response of temperate forest
ecosystems to long-term nitrogen additions. In Stage 1, N-mineralization
increases, which results in increased NPP. In stage 2, NPP and N-mineralization
decline due to decreasing Ca:Al and Mg:N ratios, and soil acidification.
Nitrification also increases as excess ammonium is available. Finally,
in Stage 3, nitrate leaching increases dramatically (modified from Aber
et al 1998).
As plant-microbial competition for nitrogen decreases, a greater proportion of labile ammonium will be nitrified to nitrate (figure 2-the nitrogen cycle). Greater rates of nitrification may result in an increased flux of nitrogen gas to the atmosphere and in increased leaching of nitrate to ground- and stream-water, which may lead to contaminated drinking water and eutrophication of aquatic ecosystems. The effects of nitrogen saturation have been studied using natural gradients
in nitrogen deposition (Friedland et al., 1991; McNulty
et al., 1991; McNulty et al., 1996) and controlled nitrogen-fertilization
experiments (Gundersen, 1991; Magill et al., 1997;
Boxman et al., 1998). This overview of nitrogen saturation is divided
into the following sections:
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