4.2.1. Land Use, Management, and Ecosystem Carbon Balances (continued)

The main factors that determine the rate and duration of carbon gain in a given ecosystem after a management change follow:

• The form the newly stored carbon takes. Carbon is stored in terrestrial ecosystems in diverse organic forms with a wide range of mean residence times (Balesdent and Mariotti, 1996; Harrison, 1996; Skjemstad et al., 1996; Trumbore and Zheng, 1996). The mean residence time (MRT) is the average time a carbon atom spends in a given pool. Some carbon-for example, much of the carbon in fresh litter or non-woody biomass-is ephemeral and returns quickly to the atmosphere (Stott et al. 1983). Other carbon, such as that in woody materials or "active" soil organic matter fractions, may persist for decades (Table 4-3). The most persistent stock of carbon is stable soil carbon (including charcoal) that may have an MRT as high as 1000 years or more because of chemical inertness or interaction with soil minerals (Balesdent and Mariotti, 1996; Paul et al., 1997a). The amount and duration of carbon gain within an ecosystem depend on the temporal dynamics of these different pools: Transient pools may increase rapidly but quickly level off, whereas carbon that is incorporated into more stable pools can produce slow but long-term increases. Consequently, the initial impact of land-use or management change occurs disproportionately in pools with shorter residence times (Cambardella and Elliott, 1992; Huggins et al., 1998a), whereas increases in stable soil pools occur slowly over a much longer time period (see Figure 4-3).
• The degree of land-use or management change. The potential carbon gain depends on the carbon stock potential of the new practice relative to that of the previous practice (Greenland, 1995). Consequently, a wholesale change from one land use to another (e.g., cropland to forestland) often elicits the largest response in carbon stock change. Within a given land use, potential carbon sequestration depends on how far the current carbon status is below the eventual maximum storage of the newly adopted practice. All else being equal, an ecosystem that has been under effective carbon-conserving management for several years has less potential for further storage than one that has been severely depleted of carbon. In ecosystems in which carbon stocks are already near maximum, the primary focus may be preserving existing stocks.
• The persistence of current management and climatic regimes. Continued accumulation of carbon in response to new management or land use depends on continued application of that new practice; a reversal can lead to partial or complete loss of previous gains (Dick et al., 1998; Stockfisch et al., 1999). The long-term pattern of carbon accumulation also is responsive to changes in climatic conditions. For example, accumulations of stored carbon may be susceptible to loss from accelerated decomposition under higher temperatures in future decades (Jenkinson, 1991; Trumbore et al., 1996), although losses may be at least partially offset by the effects of CO2 fertilization (see Chapter 1). As carbon stocks increase, so does the potential for future losses from regressive management or unfavorable climatic conditions.

The effect of a new management or land use on atmospheric CO2 cannot be judged solely on the basis of net carbon storage within the ecosystem (see Figure 4-4). In many "managed" ecosystems, there is significant removal of carbon in harvested product. Some of this harvested carbon may accumulate in long-term repositories (e.g., wood products), and some is quickly returned to the atmosphere via respiration (e.g., agricultural products) (see also Figure 2-2). Thus, the full impact of a new management practice on atmospheric CO2 can be assessed only by including net changes in off-site carbon stocks.

Energy use-notably that from fossil fuel used to establish and maintain a given land use-also affects the net exchange of carbon between the ecosystem and the atmosphere (Figure 4-4). Consequently, if a management or land-use change affects energy use, the corresponding CO2 emission affects the net carbon balance. For example, if an effort to increase soil carbon in cropland requires higher fertilization, the CO2 from energy involved in manufacturing that fertilizer may partially offset soil carbon gains (Flach et al., 1997; Janzen et al., 1998; Schlesinger, 1999). By replacing fossil fuel, biofuels can reduce the net emission of CO2 (Cole et al., 1997).

Figure 4-3: Predicted response of different pools of soil organic matter for an agricultural soil converted to forest in northeastern United States of America (Gaudinski et al., 2000). Early response reflects changes in the relatively small pools with mean residence time (MRT) <10 years (leaf and root residues). Pools with intermediate MRT (10-100 years; including humified organics in litter layers) dominate the overall response because this pool contains most organic matter in this soil. Persistent carbon pools (MRT >100 years) do not change appreciably over a 100-year period. MRT = average time spent by a carbon atom in a given reservoir.

Some land management practices may also affect emission of GHGs other than CO2, thereby augmenting or offsetting CO2 sources and sinks. For example, wetland restoration may increase methane emissions (see Fact Sheet 4.18); greater use of nitrogen fertilizers to enhance crop productivity (Fact Sheet 4.1) may enhance N2O emissions; biomass burning emits CH4 and N2O; and conversion of arable land to grassland may reduce N2O emissions (Fact Sheet 4.7). Emissions from these activities are already estimated and reported in a country's GHG inventory under the Revised Guidelines for National Greenhouse Gas Inventories, hereafter referred to as the IPCC Guidelines (IPCC, 1997); nevertheless, these emissions must be considered when a Party is contemplating the adoption of any new carbon-conserving practices.

Figure 4-4: Simplified view of carbon cycle in an ecosystem and associated off-site carbon. Designations are as follows: P = photosynthesis; R = plant respiration [includes respiration from herbivory and abiotic respiration (e.g., fire)]; H = harvest; L = litter fall; D = decomposition; E = CO2-C emission from energy use in the ecosystem; O = oxidation of harvested carbon (e.g., consumption/respiration of food products, burning); A = amendment of ecosystem with off-site organic carbon (e.g., biosolids, wood chips). Net change in stored carbon = P - (R + D + H) - O. Net effect of ecosystem on atmospheric CO2 = (E + O + D + R) - P. Some practices can reduce net emissions by substitution; for example, biofuels reduce E by substitution with O.

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