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At temperate forest sites like Harvard Forest in Petersham, Massachusetts, scientists measure soil respiration and other carbon fluxes. (Photo: David Foster)

Nitrogen deposition from atmosphere reduces amount of CO2 emitted from temperate forest soils

Jim Tang

Scientists have watched with concern the increasing amount of nitrogen released into the atmosphere from fossil fuels and fertilizer. While this element is essential to all life, too much nitrogen increases soil acidity and leaches into groundwater. Recently, however, a team of European, American and Chinese researchers found that at low levels, atmospheric deposition of nitrogen has a fertilizer effect, and, in fact, enhances the uptake of carbon dioxide from the atmosphere by stimulating plant growth and reducing organic matter composition. As a result, carbon dioxide emissions from soil are substantially reduced.

Ecosystems Center scientist Jim Tang is one of the scientists whose research was published this week in Nature Geoscience. He and lead author Ivan Janssens and their colleagues analyzed numerous published and unpublished experiments that added nitrogen to forest soils over years in temperate forests where nitrogen deposition ranges from low to high levels.

Tang explained that growing plants absorb CO2 from the atmosphere and convert it into carbon-containing biomass. Dead plant material is subsequently decomposed by soil fungi and bacteria,

which convert the organic matter back to CO2. This study reports that nitrogen not only fertilizes plant growth, but also slows the decomposition of dead organic matter in soils and its associated emission of CO2 into the atmosphere. This negative effect of nitrogen is attributable to reduced production and activity of key enzymes that degrade organic matter in soils. Thus, with increasing nitrogen deposition, soil and plants could offset a certain amount of CO2 emissions from human society.

The scientists noted that the results are drawn from boreal and temperate forests and that the future increase in nitrogen deposition expected in the tropics could have unforeseen consequences for carbon cycling in tropical forest soils.

Janssens, I. A., W. Dieleman, S. Luyssaert, J-A. Subke, M. Reichstein, R. Ceulemans, P. Ciais, A. J. Dolman, J. Grace, G. Matteucci, D. Papale, S. L. Piao, E-D. Schulze, J. Tang and B.E. Law. 2010. Reduction of forest soil respiration in response to nitrogen deposition. Nature Geoscience doi:10.1038/ngeo844 Review