Too Much of a Good Thing

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Science  15 Apr 2005:
Vol. 308, Issue 5720, pp. 326
DOI: 10.1126/science.308.5720.326d

The widespread agricultural use of nitrogenous fertilizers in recent decades has doubled the amount of available nitrogen in the global ecosystem. Although higher levels of N generally cause an increase in primary productivity (the rate at which new plant growth is produced via photosynthesis), they also cause a loss of diversity.

To understand the mechanisms linking N supply to diversity, Suding et al. conducted a series of N fertilization experiments across a range of North American ecosystems and assessed the functional and ecological correlates of declining diversity in nearly 1000 plant species. One-third of species losses from the experimental plots were attributable to the initial rarity of these plant species. In most other cases, losses could be attributed to physiological or morphological traits of species. In particular, perennials and species with N-fixing symbioses (such as legumes) were more prone to local extinction after N fertilization, and native species tended to fare worse than non-natives. The relative importance of the trait- specific effects (versus initial abundance) varied across ecosystems; for example, there was a disproportionate loss of legumes from tallgrass prairie. Thus, these experiments generate predictions of how patterns of plant diversity will decline as N loading continues to increase. — AMS

Proc. Natl. Acad. Sci. U.S.A. 102, 4387 (2005).

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