CHEMICAL ESTIMATION OF NITROGEN MINERALIZATION IN PADDY RICE SOILS .2. COMPARISON TO GREENHOUSE AVAILABILITY INDEXES

A simple nitrogen (N) availability index is needed to improve fertiliz er recommendations and, thus, reduce the opportunity for over- or unde r-fertilization of rice (Oryza sativa L.). Consequently, a study was c onducted to develop a chemical extraction procedure that will estimate the potentially mineralizable N made available to rice during the gro wing season. The ammonium-nitrogen (NH4+-N) extracted with 0.05M KMnO4 (potassium permanganate) + 0.5M H2SO4 (sulfuric acid) (acidified KMnO 4), 0.05M K2Cr2O7 (potassium dichromate) + 0.5M H2SO4 (acidified K2Cr2 O7), 1.0M HCI (hydrochloric acid), and 0.5M C2H2O4 (oxalic acid) was c ompared to total N uptake (TNU) by 'Newbonnet' rice grown under greenh ouse conditions. Comparisons were also made between distillation of th e entire soil-extractant Suspension (TOT) and distillation of the supe rnatant after centrifugation (SUP). Anaerobic incubation for 7, 14, an d 21 d was also included for comparison. The indices evaluated decreas ed in ability to predict TNU in the order: anaerobic incubation > acid ified KMnO4 > acidified K2Cr2O7 = HCI > oxalic acid. Anaerobic incubat ion was the best index of N availability for rice. The 14-d incubation predicted TNU (r2 = 0.82*) better than the 7- or 21-d incubation (r2 = 0.71 and 0.65*, respectively). The best chemical index of N availa bility was oxidation with acidified KMnO4 (r2 = 0.75*). Extraction wi th HCI (TOT) and acidified K2Cr2O7 (TOT) were similar and extraction w ith oxalic acid was the least effective method. Pre-incubation of each soil for 30 d prior to anaerobic incubation did not improve the relat ionship with TNU by rice in the greenhouse.