Change in carbon and nitrogen stocks in soil under 13 years of conventional or zero tillage in southern Brazil

The objective of this study was to determine in a long-term experiment (13 years) the effect of three different crop rotations (R1: wheat (Triticum aestivum)-soybean (Glycine max), R2: wheat-soybean-vetch (Vicia villosa)-maize (Zea mays), and R3: wheat-soybean-oat (Avena sativa,a)-soybean-vetch-maize) under zero tillage (ZT) and conventional tillage (CT) on the stocks of soil organic matter (SOM) in a clayey Oxisol soil of Passo Fundo, Rio Grande do Sul. At the end of 13 years, soil samples were taken to a depth of 100 cm, and analysed for bulk density, chemical composition and C-13 natural abundance. Under a continuous sequence of wheat (winter) and soybean (summer) the stock of soil organic C to 100 cm depth under ZT (168 Mg ha(-1)) was not significantly different (LSD at P = 0.05 of 11 Mg ha(-1)) to that under CT (168 Mg ha(-1)). However, in the rotations with vetch planted as a winter green-manure crop (R2 and R3), soil C stocks were approximately 17 Mg ha(-1) higher under ZT than under CT. Between 46 and 68% of this difference occurred at 30-85 cm depth. The C-13 abundance data indicated that under ZT the decomposition of the original native SOM was not affected by the different composition of crops in the different rotations, but under CT the rotations R2 and R3, which included vetch and maize, stimulated the decay of the original native SOM compared to the continuous wheat/soybean sequence (RI). It appears that the contribution of N-2 fixation by the leguminous green manure (vetch) in the cropping system was the principal factor responsible for the observed C accumulation in the soil under ZT, and that most accumulated C was derived from crop roots. (C) 2003 Elsevier B.V. All rights reserved.