ORGANIC-MATTER AND MICROBIAL BIOMASS IN A VERTISOL AFTER 20-YR OF ZERO-TILLAGE

The effects of 20 yr of tillage practice, crop residue management and fertiliser (urea) N application on organic C, total N, microbial biomass, anaerobic mineralisable N and pH at 0-25, 25-50 and 50-100 mm depths of a fine-textured (65% clay) vertisol were studied. The treatments, in a factorial combination, comprised of tillage (conventional tillage, CT vs zero-tillage, ZT), residue (retained, RR or burned, RB) and urea (0, 23 and 69 kg N ha-1 yr-1) applied at 40-50 mm depth. Wheat and barley were grown for 15 and 3 yr, respectively. All soil properties showed a strong stratification with depth under ZT, RR treatments. Organic C, total N and microbial biomass N were highest and pH lowest in the 0-25 mm layer under ZT, RR and 69 kg N ha-1 yr-1. In the 0-100 mm layer, similar trends were observed where residue was retained or fertiliser was applied but tillage had no effect on organic C and total N although higher microbial biomass was measured in soil under CT than ZT. Therefore, zero tillage, residue retention and fertiliser application results in stratification of soil properties, even in a vertisol.