Protection of previously intensively grazed land and reversion of cultivated lands to perennial grasslands (e.g., in "set-aside" programs) often increases mean aboveground and below-ground biomass and soil carbon (Tables 4-5 and 4-6). The rate of sequestration will decline with time over a period of about 50 years (McConnell and Quinn, 1988) or longer (Burke et al., 1995). The rate of carbon storage and total carbon storage could be increased with fertilization and in some cases may be nearly doubled (Huggins et al., 1998b); additional sinks may be available if afforestation is promoted in such lands (Barker et al., 1995). If these lands are returned to crop production, there will again be a rapid loss of carbon from the soils (Lindstrom et al., 1999), with the size of losses dependent on the management used (Section 4.4.2). Thus, a continuity of set-aside policies and management purposes is required to maintain or increase soil carbon reliably.
Where plant growth is limited by soil nutrient availability as well as by water, fertilization can result in large growth responses, as well as increased biomass and soil carbon pools (e.g., Schwab et al., 1990; Haynes and Williams, 1992; Schnabel et al., 2000; Table 4-6). This effect will increase if legumes are introduced in conjunction with fertilizers (Barrow, 1969). The response to increased fertility is generally greater where grazing is either reduced or removed (Cameron and Ross, 1996; McIntosh et al., 1997b). Normal management practice, however, is to increase the harvesting intensity of the additional biomass produced (e.g., Winter et al., 1989), potentially leading to little change in carbon stocks. Fertilizer responses vary significantly between systems and species and can result in undesirable changes such as increased dominance of weedy species. Emissions of oxides of nitrogen, decreases in methane oxidation, and impacts from leaching of nutrients also must be included in the assessment of tradeoffs involved in adopting these practices.
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