How much soil C stocks will change as a result of a climatic change will depend on how much a particular factor (such as litter input) will change, and on how sensitive soil C storage is to that particular factor. If we disregard the possibility of a soil being C-saturated, an x% increase in litter input can be expected to increase soil C store by x% once the system has returned to steady state. Similarly, if the specific decomposition rate is changed by X/o, the steady-state soil C store can be expected to change by about x% as long as x is not too large. These are the simple consequences of simple first-order kinetics. During the transition from one steady state to another, litter inputs will differ from respiration. However, at steady state the two must be equal, whatever the level of the C store.

Matters become less evident when the problem is considered in more detail. The simple first-order decomposition rate is an aggregate of conditions (such as temperature) that directly affect rates, and decomposer properties that determine the transformation of C compounds. It seems that many of the decomposer-related properties can have a much larger effect on the C store than the change in the property itself (Ágren et al., 1996). The positive and negative feedbacks that are not fully understood or cannot be quantified further complicate estimation of the effects of climatic changes on SOM. An example of such a feedback is that decreased soil tilling leads to increased stability of aggregates. This leads to decreased decomposition, but the greater SOM leads to increased water-holding capacity, which in turn leads to increased decomposition rates. However, it seems that the first-order assumption works well over a wide range of conditions. Katterer and Andrén (1999) showed that it was adequate to explain the influence of management in 99 long-term agricultural experiments in northern Europe.

Finally, land use and management have a large impact on whether soils function as sources or as sinks for C (Paustian et al., 1997b). Rasmussen et al.

(1998) concluded, in a review of results of long-term agroecosystem experiments, that returning residues to soil over long periods has transformed many temperate soils from sources to sinks. Soil erosion leads to increased emission of CO2 by exposing C locked within aggregates to decomposition. In general, soil management methods, such as no-till or reduced tilling, cover crops, mulching, etc., that are used to reduce soil erosion result in sequestration of C in the soil (see Chapter 3, this volume). All these methods aim at increased aggregate stability.

Another way to reduce the risk of C losses from arable land is to avoid cultivation of marginal lands, such as organic soils. The C content in the subsoil may be increased by cultivating deep-rooted crop species or varieties. Lal et al. (1998) have described in detail the potential of US cropland to sequester C and mitigate the greenhouse effect.

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Guide to Alternative Fuels

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