Soil phosphorus dynamics in a humid tropical silvo-pastoral system
The Costa Rican silvopastoral system of Erythrina berteroana and native grass pasture has undergone notable changes below ground over the 40-month study (baseline measurements taken in August 1987 and final measurements in December 1990). The significant declines in soil pH and exchangeable monovalent cations with concurrent increases in extractable Al and Fe suggest that the soil system is still undergoing an equilibrium shift from forest to pasture ecosystems, even 15-20 years after rain forest clearing. Although the most dramatic changes in soil chemical properties occur within the first five years following forest clearing, more gradual changes associated with nutrient leaching are possible over the period described (Bushbacher et al., 1988; Werner, 1984). Alternatively. it is also possible that the observed changes in soil chemical properties, when viewed in a longer time-scale context, merely represent normal fluctuations associated with the pasture ecosystem's own dynamic equilibrium. The significant increase in soil C relative to N in only the non-grazed tree treatment in both surface (0-15 cm) and subsurface (15-30 cm) horizons implies that organic C is accumulating to a greater extent than N in this system. In additon, N may not have accumulated as much as C because N mineralized from decomposing Erythrina leaves may have stimulated mineralization of soil organic N reserves, resulting in a temporary decrease in TKN (Boernemisza, 1966). The presence of leguminous trees and tree pruning increases the C:N ratio in the surface soil to a greater extent than recycling from pasture grass clippings alone (control treatment). In summary, the increase in the C:N ratio in the non-grazed tree tratment relative to the grazed treatments supports the hypothesis that the lack of grazing is analogous to fallow conditions and that leguminous trees enhance the fallow status of the system. Since there is no nutrient or organic matter export from the non-grazed system, one should expect organic C accumulation over time. Although there was a treatment-induced (non-grazed treatment) increase in soil organic C, organic P reserves decreased among all farms regardless of treatment. Simultaneously, available P (NAHCO3-EDTA extractable) increased. With treatment means pooled by farm, there was an increase in P mineralization from the experiment's initiation to its end. This increased mineralization was either another manifestation of gradual soil chemical changes associated with post-deforestation equilibrium shifts or an indication of enhanced nutrient turnover associated with changes in pasture management (from continuous to rotational or no grazing).
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| Format: | biblioteca |
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Columbus, Ohio (EUA)
1992
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| Subjects: | PROPIEDADES FISICO-QUIMICAS SUELO, FOSFORO, PASPALUM CONJUGATUM, HOMOLEPSIS ATURENSIS, ERYTHRINA, MICORRIZAS ARBUSCULARES VESICULARES, ESTIERCOL, BIOMASA, SISTEMAS AGROSILVOPASCICOLAS, TROPICOS HUMEDOS, |
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