Frações orgânicas e agregação do solo em diferentes sistemas de produção orgânico

Abstract

The adequate soil usage and management affect directly its quality, and they are fundamental aspects in the development of a sustainable agriculture. The objective of this study was to evaluate the influence of agricultural organic systems in the different organic matter fractions and soil aggregation, and to correlate them with changes in physical and chemical properties of an Udult soil. The study was developed in an area of the Integrated System of Agroecological Production (SIPA), located in Seropédica, Rio de Janeiro State, Brazil. The plots selected presented the following cultivation and crops treatments: conventional tillage (corn/beans), no tillage (eggplant/corn), passion fruit and Desmodium sp consortium, fig cultivation, and agroforest system (AFS). The soil samples were taken in two depths (0-5 and 5-10 cm) and two periods of the year, the first in November 2005 (summer) and the second in June 2006 (winter). The physical properties analyzed were: bulk density (Ds), particle density (Dp), granulometry, total volume of pores (VTP%), aggregate stability by the considered mean weight diameter (DMP) and mean geometric diameter (GMD) indexes; and granulometric fractioning of soil organic matter, obtaining the particulate organic carbon organic (COp) and minerals associated organic carbon (COam). The chemical analyses consisted of: total organic carbon (TOC), and chemical fractioning of soil organic matter, with determination of organic carbon in humin (C-HUM), humic acids (C-FAH) and fulvic acids (C-FAF) fractions. They were calculated the carbon stocks in the chemical and physical fractions of the soil organic matter. In relation to the physical properties, the soil texture was sandy loam, in all areas and sampled depths, and the Ds values did not indicate restriction to root development of the crops. The area with conventional tillage and corn/beans presented the smallest DMP and GMD values. In the summer season, the largest DMP and GMD values were verified in the 0-5 cm superficial layer. While in the 5-10 cm depth, the largest values were found in the winter season. The COp was more adequate than TOC to evidence differences among the crops and agroecological systems, when analyzed the 0-5 cm depth and in the summer season. The COam presented differences among the areas in the two evaluated depths. The management systems adopted in the SIPA, in each soil sampling season, influenced TOC levels, mainly in the 5-10 cm depth. The C-HUM was the fraction that contributed to most of the TOC, standing out in the area under fig coverage, in both depths and in summer. By using the average values of C-FAH it was possible to identify alterations in the soil related to the agroecological management, in the surface layer and for the summer season sampling, for all the crops and systems. Standing out that in the no tillage system, where the contents of C-FAH were largest, in the two depths, when compared to the area with conventional tillage. The C-FAF presented this same behavior, only that for the depth of 5-10 cm and in the winter, especially for the area with conventional tillage, with largest content of that fraction when compared to the no tillage system.