Kg. Cassman et al., LONG-TERM COMPARISON OF THE AGRONOMIC EFFICIENCY AND RESIDUAL BENEFITS OF ORGANIC AND INORGANIC NITROGEN-SOURCES FOR TROPICAL LOWLAND RICE, Experimental Agriculture, 32(4), 1996, pp. 427-444
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.