Chemical estimation of nitrogen mineralization in paddy rice soils: II. Comparison to greenhouse availability indices

A simple nitrogen (N) availability index is needed to improve fertilizer recommendations and, thus, reduce the opportunity for over‐ or under‐fertilization of rice (Oryza sativa L.). Consequently, a study was conducted to develop a chemical extraction procedure that will estimate the potentially mineralizable N made available to rice during the growing season. The ammonium‐nitrogen (NH4+‐N) extracted with 0.05M KMnO4 (potassium permanganate) + 0.5M H2SO4 (sulfuric acid) (acidified KMnO4), 0.05M K2Cr2O7 (potassium dichromate) + 0.5M H2SO4 (acidified K2Cr2O7), 1.0M HC1 (hydrochloric acid), and 0.5M C2H2O4 (oxalic acid) was compared to total N uptake (TNU) by ‘Newbonnet’ rice grown under greenhouse conditions. Comparisons were also made between distillation of the entire soil‐extractant suspension (TOT) and distillation of the supernatant after centrifugation (SUP). Anaerobic incubation for 7, 14, and 21 d was also included for comparison. The indices evaluated decreased in ability to predict TNU in the order: anaerobic incubation > acidified KMnO4 > acidified K2Cr2O7 = HC1 > oxalic acid. Anaerobic incubation 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 availability was oxidation with acidified KMnO4 (r2 = 0.75**). Extraction with HC1 (TOT) and acidified K2Cr2O7 (TOT) were similar and extraction with oxalic acid was the least effective method. Pre‐incubation of each soil for 30 d prior to anaerobic incubation did not improve the relationship with TNU by rice in the greenhouse.