4.4.3.4.2. Irrigation
Where water deficiencies limit plant growth, irrigation can increase growth
and carbon accumulation in biomass and in the soil-although it can also increase
decomposition rates (Table 4-6). Water is usually
a scarce commodity in dry regions, so it is more often used for higher value
purposes. The arguments presented previously about tradeoffs from fossil fuel
emissions from irrigation apply here as well. In some areas where pasture irrigation
has been implemented, there have been adverse impacts on production through
increased waterlogging and irrigated salinization processes (e.g., Gupta and
Abrol, 1990).
4.4.3.4.3. Species introductions (including legumes
and deep-rooted species)
Introduction of nitrogen-fixing legumes and high-productivity grasses can increase
biomass production and soil carbon stocks (Table
4-6). Some of these species have significant potential to become weeds.
Replacement of native grass pastures with high-productivity perennial grasses
or grass-legume combinations can result in substantial increases in dry matter
production and aboveground biomass (e.g., Montes and Masco, 1996) and even greater
increases in root biomass and soil carbon (Fisher et al., 1994; Rao et
al., 1994, 1998; Guggenberger et al., 1995). The relative increase
is sometimes modified by soil type (high in sandy soils, low in clay soils)
(Guggenberger et al., 1995). Increases in below-ground carbon pools may
result in part from high proportions of resistant root material in these grasses
(Urquiaga et al., 1998), which in some systems may cause pasture decline
in the longer term from nitrogen immobilization (e.g., Robbins et al.,
1989; Robertson et al., 1997). Pasture renovation (light plowing) and
re-sowing with high-productivity grasses can temporarily increase productivity
but are often associated with short-term carbon losses (e.g., Robbins et
al., 1989).
Net gains in ecosystem carbon stocks from replacement of native grass species-such
as those observed in Colombia by Fisher et al. (1994)-depend heavily
on the absence of a need to clear forests to establish the grasses because of
the large carbon stores in the forest. Many of these benefits may also depend
on concomitant intensification of management, including application of fertilizers-placing
constraints in expanding this activity to large areas (Fearnside and Barbosa,
1998).
The effects of these practices (Sections 4.4.3.1 through
4.4.3.4) can be monitored and verified through
repeated field sampling of soil and biomass carbon pools over large areas (see
Section 2.3). Models can provide an indication of the
magnitude of the effect, given verified information on climate, animal types
and densities, grassland species composition, irrigation, and fertilizer levels.
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