Responses of Northern U.S. Forests
to Environmental Change
Chapter 10: Effects of Soil Warming on Carbon and
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.
Location of soil
warming and gradient studies in the Northeastern U.S.
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