Climate Change

If the Earth's temperature rises due to the greenhouse effect, we can expect soils to be warmer, especially at high latitudes. Except in some deserts, the rate of decomposition in soils increases with increasing temperature - as seen both in compilations of literature values (Raich & Schlesinger, 1992) and nearly all studies that have imposed experimental warming (Rustad et al., 2001). The rate of soil respiration7 [(Eq. 2)] doubles with a 10°C rise in temperature - that is, the Q10 of the relationship is about 2.0 (Katterer, Reichstein, Andren, & Lomander, 1998; Kirschbaum, 1995; Palmer-Winkler, Cherry, & Schlesinger, 1996). The greatest response is found in samples of surface plant debris and in soils from cold climates (Lloyd & Taylor, 1994). Nearly all models of global climate change predict a loss of carbon from soils as a result of global warming (McGuire, Melillo, Kicklighter, & Joyce, 1995; Schimel et al., 1994). However, Melillo et al. (2002) suggest that the liberation of N during enhanced decomposition of soil organic matter may also stimulate plant growth and carbon uptake, partially compensating for the carbon losses from soils.

As a result of cold, water-logged conditions, large quantities of organic matter accumulate in boreal forest and tundra soils (Harden, O'Neill, Trumbore, Veldhuis, & Stocks, 1997; Trumbore & Harden, 1997). Radiocarbon measurements indicate limited turnover, but nearly all the organic matter is found in labile fractions that will be easily decomposed should the climate warm (Chapman & Thurlow, 1998; Lindroth, Grelle, & Moren, 1998). In the tundra, melting of permafrost and concomitant lowering of the water table may lead to a large increase in decomposition (Billings, Luken, Mortensen, & Peterson, 1983; Moore & Knowles, 1989). Indeed, Oechel et al. (1993), Oechel, Vourlitis, Hastings, and Bochkarev (1995) found evidence of a large loss of soil organic matter in tundra habitats as a result of recent climatic warming in Alaska, and Goulden et al. (1998) found a significant loss of carbon from soils during several warm years that caused an early spring thaw in a boreal forest of Manitoba. Recent measurements of European forests show greater respiration, and lower net carbon uptake, by forests at high latitudes, perhaps as a result of climatic warming during the past several decades (Valentini et al., 2000). In response to global warming, large losses of CO2 from boreal forest and tundra soils could reinforce the greenhouse warming of Earth's atmosphere (Woodwell, 1995).

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