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 lo ng-term experiments with irrigated lowland rice (Oryza sativa L.). Tre atments included a control and each nitrogen source alone or in combin ations that provided 50% of tile total applied nitrogen from an organi c and inorganic nitrogen source. All nitrogen sources were applied at equivalent nitrogen rates to 19-22 consecutive rice crops. Residual ef fects were assessed in two subsequent cropping seasons at one site. Lo wer grain yield, agronomic efficiency (Delta grain per kg total applie d nitrogen), and apparent nitrogen uptake were obtained from green man ure and rice straw nitrogen as sole or dual nitrogen sources rather th an from a standard split application of prilled urea. Compared to pril led urea. residual effects from green manure or rice straw included a significant increase in soil organic carbon and total nitrogen, and gr eater extractable soil nitrogen in the vegetative growth period. After panicle initiation there was no residual effect on tile rate of crop nitrogen accumulation, similar regardless of previous nitrogen source. Recycling of rice straw appeared to have greater potential for reduci ng fertilizer-N requirements than use of green manure because rice str aw is often a wasted resource in irrigated rice systems of the humid t ropics, the efficiency of ride straw nitrogen in combination with pril led urea is comparable to green manure nitrogen, and tie increase in s oil nitrogen from rice straw was 50-150% greater than from green manur e.