Long-term comparison of the agronomic efficiency and residual benefits of organic and inorganic nitrogen sources for tropical lowland rice

Nitrogen efficiency from Azolla microphylla or Sesbania rostrata green manure, rice straw, and inorganic fertilizer-N was compared in two long-term experiments with irrigated lowland rice (Oryza sativa L.). Treatments included a control and each nitrogen source alone or in combinations that provided 50% of the total applied nitrogen from an organic and inorganic nitrogen source. All nitrogen sources were applied at equivalent nitrogen rates to 19-22 consecutive rice crops. Residual effects were assessed in two subsequent cropping seasons at one site. Lower grain yield, agronomic efficiency (Delta grain per kg total applied nitrogen), and apparent nitrogen uptake were obtained from green manure and rice straw nitrogen as sole or dual nitrogen sources rather than from a standard split application of prilled urea. Compared to prilled urea. residual effects from green manure or rice straw included a significant increase in soil organic carbon and total nitrogen, and greater extractable soil nitrogen in the vegetative growth period. After panicle initiation there was no residual effect on the rate of crop nitrogen accumulation, similar regardless of previous nitrogen source. Recycling of rice straw appeared to have greater potential for reducing fertilizer-N requirements than use of green manure because rice straw is often a wasted resource in irrigated rice systems of the humid tropics, the efficiency of ride straw nitrogen in combination with prilled urea is comparable to green manure nitrogen, and tie increase in soil nitrogen from rice straw was 50-150% greater than from green manure.