Soil organic matter (SOM) strongly influences soil parameters, such as fertility, structural stability, water infiltration and retention, and biological activity, all of which contribute to the maintenance of crop productivity and environmental sustainability. The objective of this study was to determine in a long-term experiment (17 years) the effect of two different crop rotations (1. wheat-soybean, 2. wheat-soybean-vetch+oat-maize) under two tillage systems (CT and ZT) on the organic C balance in a clayey Oxisol soil to a depth of 100 cm in Cruz Alta, Rio Grande do Sul. Estimates of the proportions of the soil C derived from the original SOM and the crop residues were made using two techniques based on d¹³C natural abundance. All these crops were fertilised according to soil analyses. After 17 years, soil samples were taken to 100 cm depth, for chemical analysis and bulk density. Total N in plant and soil samples was determined using the Kjeldahl technique.  Soil organic C and ¹³C natural abundance were determined using an automated CN analyser (LECO CHN 600) and a continuous-flow isotope-ratio mass spectrometer, respectively. The results showed that : (i).The soil C and N stocks in 0-100cm were significantly lower in rotation 1 (wheat/soybean) in relation to rotation 2 (wheat/soybean - oat/soybean - vetch+oat/maize). (ii) In rotation 1 the soil C and N stocks under ZT did not differ significantly from those under  CT , and (iii) Similar estimates of treatment effects on the original soil organic C were obtained using the two d¹³C methods. Neither the tillage system nor the different rotations had any significant effect on the rate of decomposition of the original SOM. The incorporation of C derived from the residues in the SOM was higher in rotation 2.

Keywords: ¹³C natural abundance, Carbon stock, Crop rotation, Soil organic matter, Tillage system.