Estoques e mecanismos de estabilização de C e N em solos da região central da Amazônia

Abstract

Carbon (C) and nitrogen (N) preservation in Amazonian forest soils is fundamental to the maintenance of its sustainability. The soils from this region being responsible for a C sequestration potential of 126 to 141 Tg year-1. Forest maintenance depends on nutrient cycling of litter that is deposited above soil and because of that, studying this region is important to understand the dynamic and stabilization of C and N in this environment. Stabilization of C in soils is governed by the following mechanisms: recalcitrance of organic material, physical and chemical protection. There are five factors responsible for C accumulation on soils: vegetation, climate, relief, origin material and time. The objective of this work was to evaluate the influence of the relief factor on carbon and nitrogen stocks in soils from Urucu, Central Amazonia. Soil samples (disturbed and undisturbed) were collected, identified and prepared in air dried soil. The soils physical and chemical attributes were determined. With results of C and N levels their stocks were calculated until 100 cm depth. To evaluate the mechanisms responsible for C stabilization on soils a physical fractioning by organic matter density to obtain the mass of the free light fraction (FLF) and intra-aggregate light fraction (IAFL) in ten soil profiles under different drainage and vegetation conditions. Carbon Stocks vary from27,4 to 230,3 Mg ha-1 and N stocks vary from 8,5 e 42,0 Mg ha-1. Distribution of C and N levels follows as represented in most soils with the exception of Espodossolos that represented a light increase in depth. The FLF contents found in well drained soils that are under the influence of Upland Dense Tropical Rainforest, varied from 58,5 g kg-1 of soil to 291,4 g kg-1 of soil and IALF varied from 58,5 g kg-1 of soil to 291,4 g kg-1 of soil. In soil profiles found in poorly drained situation under Upland Open Tropical Rainforest influence, in river plains it was noticed that contents from 40,8 g kg-1 of soil to 57,6 g kg-1 of soil and FLF and from 6,45 g kg-1 of soil to 16,8 g kg-1 of soil of IALF. In soils found in terraces under Upland Open Tropical Rainforest influence, the quantities found in FLF were from 113,2 g kg-1 of soil to 842,4 g kg-1 of soil and from IALF from 32,8 g kg-1 of soil to 72,2 g kg-1 of soil. The obtained correlation among clay variables, C, N, N stock, C stock, FLF and IALF demonstrates the role of clay in stabilization of C and N in soils. The organic matter light fractions (FLF and IALF) are also preserved in soils with higher clay levels. These fractions are considered the first C and N input in soils and preserving them is associated with higher C and N stocks in soils. The formation of organo-minerals complexes between clay fraction and organic matter, through chemical connection cation bridge, by the presence of aluminum ion, it is an important chemical protection mechanism in Urucu soils. The relief factor influences C and N levels and stocks by providing different drainage conditions that influences not only the kind of vegetation but also the soil organic matter decomposition.