Science 320, 1181 (2008)

Credit: J. BRANDENBURG/MINDEN PICTURES/FLPA

Careful measurement of the fixed nitrogen in water that drips through the foliage of boreal forests (pictured above) at a dozen sites in northern Sweden has helped researchers pin down how that biome manages its nitrogen budget.

Shortly after forest fires, canopy dripwater delivers sufficient nitrogen to provide for cyanobacteria on the forest floor. The cyanobacteria thus reduce the rate at which they fix nitrogen from the air, Thomas DeLuca of the Wilderness Society in Bozeman, Montana, and his colleagues have found.

Because the 12 sites had undergone varying periods of post-fire recovery, the authors were able to show that cyanobacteria increase their nitrogen fixation rates as the feather mosses they live on grow back, and as the nitrogen content of rainwater dips. This feedback mechanism may provide some buffer to nitrogen pollution from human sources.