Forests' Small Appetite for Nitrogen

BALTIMORE--A glut of nitrogen washing over the land from car and factory exhaust and crop fertilizers is degrading water and air quality and even altering precarious balances in species diversity. But researchers have thought there might be one reason to cheer this surfeit of nitrogen: The nutrient should fertilize tree growth, spurring forests to soak up human-made carbon dioxide (CO2) that would otherwise fuel global warming. New results presented here last week at the Ecology Society of America meeting, however, suggest nitrogen might not be spurring forest growth much at all.

The amount of nitrogen that falls on ecosystems has more than doubled since the 1960s, fed by nitrogen compounds from fossil fuel burning and ammonium in manure and fertilizers (Science, 13 February, p. 988). Ecologist Knute Nadelhoffer of the Marine Biological Laboratory in Woods Hole, Massachusetts, wanted to explore whether nitrogen fertilization might help explain how terrestrial ecosystems store 1800-billion kilograms of carbon each year.

So in 1991 and 1992 his team sprayed plots of red pine and oak in the Harvard Forest with water doped with nitrogen-15, a tracer isotope. At the end of the growing season, they fed wood, root, and soil samples into a mass spectrometer, which measures elemental balances. The results were surprising: Only 5% to 25% of the nitrogen ended up in the wood, and the rest apparently went into soils or leached away. Until now researchers had often assumed that as much as 80% of the nitrogen added to a forest went into its vegetation. Comparable results have come out of an experiment in Maine and in six European forests, Nadelhoffer reported. "Global scenarios where most of the nitrogen goes into trees, we just don't think are supported by this," he said.

University of Colorado, Boulder, ecologist Alan Townsend cautions that there could be some lag time in how long trees take to sop up the nitrogen sprayed on the soil, and the sites didn't cover all forest types. Still, if the finding holds up, "it has some pretty big implications" for trying to find the missing carbon sink, says Townsend. "We're going to have to start looking somewhere else."