EFFECTS OF STUBBLE AND N-FERTILIZATION MANAGEMENT ON N-AVAILABILITY AND UPTAKE UNDER SUCCESSIVE RICE (ORYZA-SATIVA-L) CROPS

Experiments were conducted in fields which had a history of nil to four rice (Oryza sativa L.) crops during the previous four summers. Incorporating stubble after each harvest reduced soil nitrate-N content between crops, but increased soil N mineralization potential. During the fourth successive crop, plots where stubble had been incorporated after the previous three harvests had an average 21% more soil NH4N and 22% more N uptake than plots where stubble had been burnt. Soil fertility fell rapidly with increasing numbers of crops, and the unfertilized fifth crop accumulated approximately half the N (60 kg N ha-1) found in the unfertilized first crop (116 kg). Fertilizer N alleviated the effects of annual cropping; the application of 210 kg N ha-1 to the fifth crop (uptake of 156 kg N ha-1) resulted in similar N uptake to the first crop fertilized with 50 kg N ha-1 (154 kg N ha-1). Applying N at sowing had no significant effect on soil NH4-N concentration after permanent flood (PF), while N application at PF resulted in increased NH4-N concentration and N uptake until panicle initiation (PI). N applied at PI increased soil NH4-N concentration at least until the microspore stage. Management factors such as stubble incorporation and increasing N application rate, maintained N supply and enabled successive rice crops to accumulate similar quantities of N at maturity. © 1990 Kluwer Academic Publishers.