Herbaceous Retrogression and Soil Carbon Losses

Simulations with the CENTURY biogeochemistry model (Parton et al, 1987, 1988, 1993) parameterized for assumed presettlement conditions ("light" grazing, fire at 10-year intervals), soil texture, and climate of the La Copita projected that soil organic carbon (SOC) would have been on the order of 2500 g m-2 to a depth of 0-20 cm (Hibbard, 1995). The SOC values from this assessment were then used as a baseline against which historic effects of heavy, continuous livestock grazing were evaluated. In a subsequent model run, intensification of grazing and removal of fire were initiated in 1850, a date approximating the advent of widespread, unregulated livestock grazing in southern Texas (Lehman,

Onset of Heavy Grazing

Lowland

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(Clay Loam)

___Upland

1800

1950

2000

1850 1900 Year

FIGURE 3 Simulation model reconstruction of changes in soil organic carbon (0-20 cm) on an upland, sandy loam soil and a lowland, clay loam soil at the La Copita Research Area in southern Texas, USA (from Hibbard, 1995). Solid lines depict steady-state SOG values expected for the climate of the site under light grazing and fire every 10 years (preset -tlement conditions). Dashed lines depict changes in SOC predicted to occur on two major soil types after the onset of heavy, continuous livestock grazing and cessation of fire. Steady-state values for heavy, continuous grazing and cessation of fire are within 5% of field measurements on present-day grasslands.

1750

1800

1950

2000

1850 1900 Year

FIGURE 3 Simulation model reconstruction of changes in soil organic carbon (0-20 cm) on an upland, sandy loam soil and a lowland, clay loam soil at the La Copita Research Area in southern Texas, USA (from Hibbard, 1995). Solid lines depict steady-state SOG values expected for the climate of the site under light grazing and fire every 10 years (preset -tlement conditions). Dashed lines depict changes in SOC predicted to occur on two major soil types after the onset of heavy, continuous livestock grazing and cessation of fire. Steady-state values for heavy, continuous grazing and cessation of fire are within 5% of field measurements on present-day grasslands.

1969). This simulation produced a 16-29% reduction in SOC of sandy loam upland and clay loam lowland soils, respectively (Fig. 3). These results appear reasonable in that the model-generaled steady-state SOC levels for sandy loam uplands (2062 g m-2) approximated the average pool sizes measured in present-day grassland communities on these soils (2087 g m-2). Further, the grazing-induced reductions in SOC predicted by the simulation were comparable to field measurements reported for other grazed grasslands (Bauer et al, 1987; Frank et al., 1995; but see also Milchunas and Lauenroth 1993). CENTURY simulations did not explicitly include potential erosion losses. The fact that simulated historic changes in SOC approximated those currently observed at the site therefore suggests that such losses may have been minimal. This inference seems reasonable, since the La Copita landscapes have relatively little topographic relief (1-3% slopes) and show no obvious physical signs of erosion (pedestals, rills, gulleys). Furthermore, soil profile structure in low-lying portions of the landscape shows no pedogenic evidence of significant translocation of soils from uplands. Elevated C and N pools in soils of developing woody communities (summarized later) thus appear to be the result of in situ accumulations induced by trees and shrubs rather than losses from grazed grasslands.

The present-day herbaceous vegetation is dominated by a low cover of ephemeral dicots and short-statured, weakly perennial grasses. In contrast, herbaceous vegetation on relict, protected grasslands in the region is characterized by mid- to tall-statured perennial grasses whose potential productivity (500-600 g m-2; SCS, 1979) is two to three times that which has been recorded at La Copita (<270 g m-2; Vega, 1991; Hibbard, 1995). Thus, it is reasonable to conclude that soil C and N storage has declined in herbaceous communities over the past century as a result of changes in species composition, microclimate, and biomass production attributable to heavy, continuous livestock grazing on this site.

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