CHEMICAL ESTIMATION OF NITROGEN MINERALIZATION IN PADDY RICE SOILS .1. COMPARISON TO LABORATORY INDEXES

A reliable, rapid procedure for estimating native soil nitrogen (N) mi neralization potential in paddy rice (Orysa sativa L.) has eluded rese archers. While several have been proposed, no technique has been suffi ciently reproducible to be implemented in any soil testing program. Th erefore, the objective was to develop a chemical extraction procedure as an index to estimate N mineralization in silt loam rice soils of th e Southern United States Rice Belt. Samples of silt loam soils were co llected from strategic locations throughout the rice-growing region of Arkansas during 1990 and 1991. Anaerobic incubations were performed a s an index of potentially mineralisable N. Extraction with acidified p otassium permanganate (KMnO4), acidified potassium dichromate (K2Cr2O7 ), hydrochloric acid (HCI), 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 so il-solution suspension (TOT). The acidified KMnO4 and acidified K2Cr2O 7 extractions gave the best overall indices of N availability. The aci dified KNnO4 and acidified K2Cr2O7 sucessfully predicted N mineralizat ion 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 ev aluated. The oxalic acid TOT extractable ammonium-nitrogen (NH4+-N) wa s significantly related to anaerobic incubation. More N was extracted with the TOT methods than SUP methods, probably due to alkaline hydrol ysis during distillation of TOT methods. Oxidation with KMnO4 released the most N, while HCl and oxalic acid extracted the least.