Soil organic matter decline and associated degradation of soil and environmental conditions under conventional tillage in tropical and subtropical regions underline the need to develop sustainable soil management systems. This study aimed first to evaluate the long-term effect (9 years) of two soil-tillage systems (conventional tillage: CT, and no-tillage: NT) and two cropping systems (oat (Avena strigosa Schreb)/maize (Zea mays L.): O/M; and oat+common vetch (Vicia sativa L.)/ maize+cowpea (Vigna unguiculata (L.) Walp): O+V/M+C without N fertilization on total organic carbon (TOC) and total nitrogen (TN) concentrations in a sandy clay loam Acrisol in southern Brazil. The second objective was to assess soil potential for acting as an atmospheric CO2 sink. Under NT an increase of soil TOC and TN concentrations occurred, in both cropping systems, when compared with CT. However, this increase was restricted to soil surface layers and it was higher for O+V/M+C than for O/M, The O+V/M+C under NT, which probably results in the lowest soil organic matter losses (due to erosion and oxidation) and highest addition of crop residues, had 12 Mg ha(-1) more TOC and 0.9 Mg ha(-1) more TN in the 0-30.0 cm depth soil layer, compared with O/M under CT which exhibits highest soil organic matter losses and lowest crop residue additions to the soil. These increments represent TOC and TN accumulation rates of 1.33 and 0.10 Mg ha(-1) per year, respectively. Compared with CT and O/M, this TOC increase under NT and O+V/M+C means a net carbon dioxide removal of about 44 Mg ha(-1) from the atmosphere in 9 years. NT can therefore be considered, as it is in temperate climates, an important management strategy for increasing soil organic matter. In the tropicals and subtropicals, where climatic conditions cause intense biological activity, in order to maintain or increase soil organic matter, improve soil quality and contribute to mitigation of CO2 emissions, NT should be associated with cropping systems resulting in high annual crop residue additions to soil surface. (C) 2000 Elsevier Science B.V. All rights reserved.
