Estoque e labilidade da matéria orgânica em um argissolo sob sistemas de produção de eucalipto

Abstract

The adoption of conservation management systems contributes to the maintenance or increment of the soil organic matter content (SOM), with positive reflexes on the soil, air and water quality. Scarce are the studies that evaluate alterations on the stock and lability of the SOM in forest production and pasture systems, especially in the south of Brazil. The monitoring of these variables along the time is an indicator of the quality of the management system used. It was aimed at to quantify the stock of total organic carbon (TOC) and total nitrogen (TN) in the whole soil and in the free light fraction (FLL) and to infer on the lability of the SOM in production system of eucalyptus (Eucalyptus saligna) with 13 years (E1) and 20 years (E2), comparatively the natural forest (MATA) and to an area of the braquiaria pasture (Brachiaria brizantha) associated with the pensacola (Paspalum lourai) and clover (Trifolium sp.) (PAST), considered as references. Soil samples deformed and no-deformed it were collected in the layers 0,00-0,05; 0,05-0,10; 0,10-0,20; 0,20-0,40; 0,40-0,60 and 0,601,00m in a Alfisol of Butia-RS. In the fraction <2mm (whole soil) and in FLL obtained by flotation in NaI 1,8 g cm-3, it was quantified the TOC and TN pools by use of the dry combustion analyzer. The commercial production of eucalyptus and pasture systems maintained of the TOC and TN stocks of in the whole soil in most of the soil layers evaluated. The increase of the cultivation time of the areas of eucalyptus from 13 (E1) to 20 years (E2) it promoted significant increases in the TOC stock in larger depth (0,60-1,00m). In the whole soil, the TOC stocks increased in the order E1<MATA<E2<PAST, values from 130,33 to 151,74 Mg ha-1. The constant addition and maintenance of the crop residues to the soil and use of the reduced tillage in the commercial eucalyptus and pasture systems contributed to the maintenance of the stocks of TOC and TN in the free light fraction (FLL) of the SOM along the time, comparatively to the forest area. The index of lability of the soil organic matter in the system of eucalyptus production (E1), based on the proportion of TOC in FLL in relation to the whole soil, it was similar to the observed for the forest area in all of the layers evaluated, larger than to the obtained in the superficial layer (0,00-0,05m) and less than in the layer 0,05-0,20m for the pasture system.