Chemical estimation of nitrogen mineralization in paddy rice soils: I. Comparison to laboratory indices

Abstract

A reliable, rapid procedure for estimating native soil nitrogen (N) mineralization potential in paddy rice (Orysa sativa L.) has eluded researchers. While several have been proposed, no technique has been sufficiently reproducible to be implemented in any soil testing program. Therefore, the objective was to develop a chemical extraction procedure as an index to estimate N mineralization in silt.loam rice soils of the Southern United States Rice Belt. Samples of silt loam soils were collected from strategic locations throughout the rice‐growing region of Arkansas during 1990 and 1991. Anaerobic incubations were performed as an index of potentially mineralisable N. Extraction with acidified potassium permanganate (KMnO₄), acidified potassium dichromate (K₂Cr₂O₇), hydrochloric acid (HCl), and oxalic acid [(COOH)₂], were evaluated at extractant:soil ratios ranging from 5:1 to 1:1. Also, distillation of centrifuged extracts (SUP) was compared to distilling the entire soil‐solution suspension (TOT). The acidified KMnO₄ and acidified K₂CnO₇ extractions gave the best overall indices of N availability. The acidifed KMnO₄ and acidified K₂Cr₂O₇ sucessfully predicted N mineralization at all ratios evaluated. A significant relationship with anaerobic incubation was observed for all HCl TOT ratios evaluated, except the 3:1 in 1991. The HCl SUP methods were not significant for all soils evaluated. The oxalic acid TOT extractable ammonium‐nitrogen (NH₄+‐N) was significantly related to anaerobic incubation. More N was extracted with the TOT methods than SUP methods, probably due to alkaline hydrolysis during distillation of TOT methods. Oxidation with KMnO₄ released the most N, while HCl and oxalic acid extracted the least.