The effect of irrigation on soil C stocks is a result of the balance between the effect of this practice on C input through crop residues and of C losses through microbial decomposition of soil organic matter (SOM). This study assessed the influence of sprinkler irrigation on SOM dynamics and on the total C stocks in a subtropical Acrisol under no-tillage (NT) and conventional tillage (CT) in a long-term experiment (8 years) in Universidade Federal do Rio Grande do Sul, RS, Brazil. Irrigation increased the C addition (about 8 kg C ha -1 yr -1 per mm precipitation) in both tillage systems, but this was not enough to increase the soil C stocks (0-20.0 cm) because irrigation also increased the decomposition rate of SOM by 19% in the CT soil and by 15% in NT soil. In the top layers (0-2.5 and 2.5-5.0 cm), the C concentration was higher in the NT than in CT soil, while the opposite trend was observed in the deepest layer (10.0-20.0 cm). Thus, the total C stocks in the whole 0-20.0 cm layer did not differ between tillage systems. The oat residue decomposition rate in NT soil increased with irrigation, which corroborates the higher SOM decomposition rates estimated for the irrigated soil. Based on these results, it becomes clear that the establishment of a high input cropping system able to counterbalance the higher SOM decomposition rates is a crucial point in the maintenance or improvement of soil C stocks in irrigated tropical and subtropical croplands.
