Biomassa, volume e nutrientes de raízes em florestas secundárias na Amazônia Central

Abstract

Forests play a double role in relation to the dynamics of Carbon (C) in the biosphere and can act as a sink, storing C in biomass or source of CO2, through the decomposition or burning of biomass. Deforestation, especially in the Amazon is responsible for about 70 % of Brazilian emissions of greenhouse gases. In this respect, know the stocks and dynamics of carbon in forests are crucial for appropriate environmental and economic decisions. Currently we have allometric equations that estimate, with 94 % confidence, the total biomass (aboveground biomass and thick roots) for the primary forests. However, there is still high scarcity of data and research on the belowground biomass, mainly in secondary forests. The purpose of this study was maily precisely provide data and information of the belowground biomass in secondary forests. We studied the stock of nutrients in the soil and belowground biomass of secondary forests on pastures abandoned in chronosequence (14, 16, 18 and 20 years) which had root biomass of 6.3, 7.0, 11.0 and 11.1 Mg ha-1, respectively. Pastures abandoned after 20 years, had a mean of 11.1 ± 0.88 Mg ha-1; and the capoeiras of 25 years old had mean of 15.0 ± 1.17 Mg ha-1. The general average density of roots was equal to 0.32 ± 0.009 g cm-³, and in the surface layer (S) it was recorded a significant decrease in density values. The average biomass of roots in layers (S) and (A) (the first 10 cm depth) corresponds to 75.6 % of the total biomass, and the layer (A) accounted for 54 % of the total biomass. About 20 % of root biomass, until 30 cm deep, is aboveground. We recorded significant differences in the biomass between the treatments (ANOVA: r ² = 0.165, F = 1.973, p = 0.048) and also according depth (ANOVA: r ² = 0.314, F = 20.875, p <0.05). The soils of pastures abandoned with 14, 16 and 18 years old have a total stock of organic carbon until 30 cm deep equal to 66.5, 70.3 and 59.2 Mg C ha-1, respectively; the pastures abandoned of 20 years old have in averaged 62.7 Mg C ha-1 and the capoeiras of 25 year old have average of 52, 1 Mg C ha-1. These secondary forests have an average of stock of organic carbon in soil of 60.0 ± 6.75 Mg C ha-1. The sandy soils presented higher biomass than clay soils, and these ones showed higher nutrient content. The fine and very fine roots were located mainly in the layer (S). Values Inventory Carbon (IC) in the soil, combined with the IC in roots, provide good estimates of the IC belowground in these ecosystems, which can be added to information related to necromass and aboveground biomass, completing thus the scientific knowledge of stock and carbon dynamics in secondary forests in Central Amazon.