Subsoil accumulation of mineral nitrogen under polyculture and monoculture plantations, fallow and primary forest in a ferralitic Amazonian upland soil

Schroth, G.; Silva, L.F. da; Seixas, R.; Teixeira, W.G.; Macedo, J.L.V.; Zech, W.

Subsoil accumulation of mineral nitrogen under polyculture and monoculture plantations, fallow and primary forest in a ferralitic Amazonian upland soil

1999

Central Amazonia is characterized by high and intensive rainfall and permeable soils. When rainforests are cleared for agricultural use, the efficient nutrient recycling mechanisms of the forests are disrupted and the nutrient availability in the topsoil is increased by fertilization, thereby increasing the potential for nutrient leaching. In this study, the distribution of mineral N in the upper two meters of a ferralitic upland soil was evaluated as an indicator for nutrient leaching and for the potential contribution of the subsoil to crop nutrition. A perennial polyculture system with four tree crops and a leguminous cover crop at two fertilization levels was compared with a monoculture plantation of peach palm (Bactris gasipaes), spontaneous fallow and primary rainforest. Mineral N accumulated principally as nitrate in the subsoil under all agricultural crops and also under the primary forest, although to a lesser extent. Within the polyculture system, there were significant differences in N accumulation between the tree crop species, and for one of the species (Theobroma grandiflorum) also between fertilization levels. The principal sources of subsoil N were mineral fertilizer and presumably N from the mineralization of leguminous biomass and soil organic matter. The N losses from the agricultural systems and the absence of yield responses of the tree crops to N fertilization indicated that agricultural production was not limited by N at this site, or that N was too rapidly leached to be taken up efficiently by the crops. None of the tree crop species seemed to be efficient in capturing leached N. Strategies are discussed for reducing N losses from agricultural systems with perennial crops, including the development of site- and species-specific fertilizer recommendations, closer tree spacing, and the encouragement of lateral and vertical tree root development.

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