Fluxos e estoques de nutrientes, colonização por micorrizas arbusculares e influência das raízes na decomposição da liteira em sistemas agroflorestais e em vegetação secundária na Amazônia central

Abstract

All natural ecosystems are dependent on efficient internal nutrients cycling to maintain functionality. Deforestation occurring in the Brazilian Amazonian disturbs the nutrient cycle and diminishes natural soil fertility. When soil fertility diminishes, the land is abandoned and new areas of primary forest are cut. Abandoned sites generally develop a secondary forest vegetation. However, depending on the intensity of use prior to abandonment, soil erosion can occur and possibly desertification. To prevent the loss of the soil fertility of disturbed and/or abandoned landscapes, some management techniques have been developed, including the implantation of agroforestry systems (AFs). The objective of this study was to examine the recovery of soil fertility after intensive use followed by either AFs or secondary forest. Variables examined for these two land cover types include: the quantity and quality of nutrients in the litter (Chapter 3), the influence of fine roots of cupuaçu (Theobroma grandiflorum) on litter decomposition rate (Chapter 4), and nutrient flux in the soil and colonization of roots by mycorrhizal fungi (Chapter 5). The AFs studied were implanted in 1991 at the experimental field station of EMBRAPA-CPAA (Empresa Brasileira de Pesquisa Agropecuária - Centro de Pesquisa Agroflorestal da Amazônia Ocidental), situated in the SUFRAMA Farming and Ranching District (DAS), north of Manaus, at km 51 of the BR-174 highway. Three types of AFs were included in the study: (1) dominated by palms, (2) dominated by species for wood and fruit production and (3) mixed trees and pasture grasses. The secondary forest areas used in this study were located alongside the AFs. The litter mass stock over the soil was found to be larger under secondary forest (5.9 t ha-1) than in the AFs. However, stocks of calcium (47,8 kg ha-1), total phosphorus (4.4 kg ha-1), magnesium (7.1 kg ha-1) and manganese (0,69 kg ha-1) were larger in the litter of the AFs of than in the secondary forests. The percent of polyphenols in the litter was highest in the AFs dominated by fruit trees and in the secondary forest litter (1.6% in both). Soil pH was less acid (4.5) and the soil concentration of phosphorus was higher (68.8 mg kg-1) in the AFs than in the secondary forests, and was restricted mainly to the uppermost layer of the soil. Roots colonized by mycorrhizal fungi were most abundant (35.8%) in the AF with pasture grasses, again mainly in the upper layer of the soil. Fine roots of cupuaçu had a positive effect on the absorptive removal of calcium, magnesium and manganese from the litter, compared with litter from which roots were excluded. This study concluded that AFs are an effective management technique, aiming at the recovery of natural soil fertility of abandoned and/or degraded Amazonian landscapes. Further study is required to determine whether AFs of higher diversity can intensify this recovery.