Abandoned pasture

Mobilization of water from the soil is closely related to root depth and root density in each layer of soil. Fine roots of active B. brizantha pastures, established in deeply weathered clayey soils in eastern Amazonia, reach depths of 8 m or more (Nepstad et al. 1994). In abandoned pastures (50% B. humidicola and P. maximum cover and 50% invading shrubs and small trees), fine roots ( < 1 mm in diameter) were found at depths of 12 m (Nepstad et al. 1994). Fine-root biomass in the superficial soil layers of an active pasture in Paragominas, eastern Amazonia, was 3 times higher than that found in an adjacent primary forest area. Fine root biomass in the active pasture decreased by a factor of 100 between the surface and 6 m depth. In an abandoned pasture area, the distribution pattern of fine-root biomass was similar to that observed in the deeper soil layers of the forest ecosystem. This pattern is associated with the fine roots of the existing dicotyledonous invading species. 

Under normal conditions of water availability, values for LAI in pastures of B. brizantha have been measured above 4.0. However with the establishment of a water deficit in the soil, these values decrease to below two or even lower in pastures of P. maximum (Roberts et al. 1996). A similar situation is found in abandoned pastures in eastern Amazonia, where a reduction of approximately 68% of green tissue has been observed in the dry season, while in an adjacent area of primary forest this reduction was only 16% (Nepstad et al. 1994). Primary forests, which have deep root systems and little seasonal variation in LAI, maintain stable subcanopy microclimatic conditions and transpirational flux, even during the dry season. Because of an evergreen forest canopy, the return of the rainy season has less impact on the microclimate near the soil in the forest than in the pastures, and the deep soil water stores are also more efficiently recharged in the forest. 

As abandoned pasture areas become increasingly invaded by shrubs and small trees, which in time lead to secondary forest formation, the biogeochemical cycles of plant nutrients and the hydrological cycle are expected, eventually, to resemble the cycles originally found in the primary forest. The rate of recuperation of biogeochemical cycles in secondary forests and the factors that influence those rates, however, deserve further attention. 

Buschbacher, R., C. Uhl, and E. A. S. Serrao. 1988. "Abandoned pastures in eastern Amazonia K. Nutrient stocks in the soil and vegetation." Journal of Ecology 76 682-699. 

Nepstad, D. C. 1989. Forest regrowth in abandoned pastures of eastern Amazonia limitations to tree seedlings survival and growth. Ph.D. Thesis, Yale University, New Haven. 

Nepstad, D. C., C. Uhl, C. A. Pereira, and J. M. C. da Silva. 1996a. A comparative study of tree establishment in abandoned pasture and mature forest of eastern Amazonia. Oikos 76 25-39. 

Vieira, 1. C. G., C. Uhl, and D. C. Nepstad. 1994. The role of the shrub Cordia multispicata Cham, as a succession facilitator in an abandoned pasture, Paragominas, Amaz6nia. Vegetatio 115 91-99. 

Nutrients removed in grain, leaf, and wood biomass should be replaced via organic and inorganic fertilizers otherwise the system ceases to be sustainable. On recently deforested sites, additional nutrients may not be required for several years before productivity declines set in. Abandoned pastures, however, show a combination of soil chemical and physical constraints to crop production and will require inputs to alleviate soil compaction and nutrient deficiencies (principally phosphorus and calcium). 

On degraded and abandoned pastures, we hypothesize that a modest input of chemical fertilizers (25 kg N, 50 kg P, 50 kg 25 kg Ca, and 25 kg of Mg per hectare) will be essential to prime the nutrient cycling pump of agroforestiy systems. Ideally, these nutrients should be added to the first crop established. 

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