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Responses of Northern U.S. Forests to Environmental Change
ISBN 0-387-98900-5

Chapter 10: Effects of Soil Warming on Carbon and Nitrogen Cycling

Lindsey E. Rustad, Jerry M. Melillo, Myron J. Mitchell, Ivan J. Fernandez, Paul A. Steudler, and Patrick J. McHale

Experimental evidence supports the hypothesis that an increase in soil temperature of 1.0-3.5oC can have significant effects on below ground C and N cycling in northern U.S. forests. Soil C and N cycling are important because of potential feedbacks to the atmosphere which could affect climate change, because of the relationship of these cycles to forest productivity and health, and because of the potential for nutrient export from watersheds to sensitive downstream wetlands and coastal water bodies.

The responses of soils processes to experimental warming are mixed. Soil respiration and N mineralization showed significant and consistent increases regardless of site or treatment. Oxidation of CH4, N2O flux, and litter decomposition showed variable responses that depended on litter quality, N availability, and soil moisture. Because of this complexity, it is not possible to definitively state whether Northern U.S. forest soils will be a net source or sink as a consequence of atmospheric warming. However, the balance of experimental evidence and observations suggests that increased soil respiration and litter decomposition, together with decreasing soil organic matter with increasing temperature, will result in a net efflux of C from the soil to the atmosphere. Other possible responses that could counteract this effect include increased N availability and therefore increased NPP in N limited ecosystems, which could increase the rate of C uptake by plants. However, in N-saturated systems, which are rarer (but increasing from N deposition) than N-limited systems, C uptake could decrease as a consequence of deteriorating forest health.

If temperature changes are small, regional effects on ecosystems are likely to be insignificant relative to more pronounced effects from harvesting, insects and diseases, and other disturbances. Nonetheless, because of the ubiquitous nature of prospective warming, even a small effect spread over a large area could be significant.

Below: Location of soil warming and gradient studies in the Northeastern U.S.
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