Crop residue management to conserve soil organic matter content in sugarcane-based crop rotations

In rice-plant cane-ratoon cane and cowpea-plant cane-ratoon cane rotations, rice produced a significantly greater root biomass (1•5 t/ha) than cowpea (0•7 t/ha), but, overall, the cowpea-plant cane-ratoon cane rotation added little more residues (7•9 t/ha) to the soil than the rice-plant cane-ratoon cane rotation (7•5 t/ha). Organic matter (OM) and total N in the soil not only increased more after cowpea (OM = 0•78%, N = 0•048%) compared with rice (OM = 0•66%, N = 0•024%) but also remained at a higher level throughout the crop sequence. Time of N application to sugarcane did not influence significantly the OM and N contents of the soil. However, placement of N at 15 cm depth along stubble rows in the ratoon crop improved the OM and N contents of the soil more than the top dressing of N. Sugarcane yields were significantly higher in cowpea-plant cane-ratoon cane rotation than those in the rice-plant cane-ratoon cane rotation. Split applications of N at 45 and 90 days after planting produced a significantly higher plant-cane yield than other split applications, but did not influence ratoon-cane yield significantly.