EVALUATION OF AMMONIUM RELEASE BY ALKALINE PERMANGANATE EXTRACTION AS AN INDEX OF SOIL NITROGEN AVAILABILITY

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Abstract

The relation of potentially mineralizable soil nitrogen, No to hydrolytic and oxidative release of NH4-N from soil organic matter by extraction with alkaline permanganate was investigated using 62 soils representing several major agricultural regions in the United States. The NH4-N was recovered by steam-distilling 1-g samples of soil during extraction of 4 min with several concentrations of NaOH and KMnO4 in different combinations. The same concentrations of NaOH and KMnO4, were used to determine amounts of NH4-N released by hydrolysis (B) during steam distillation. Oxidative release was estimated as the difference between total NH4-N produced during alkaline permanganate extraction (BOx) and that derived by NaOH distillation. These NH4-N fractions were determined using inital NaOH concentrations ranging from 0.25 to 0.625 N, alone or with amounts of KMnO4 ranging from 0.032 to 0.5 g/10 ml of extractant. For all combinations of NaOH and KMnO4, the correlation coefficients relating B and BOx consistently were high (r = > 0.9). Correlations between the independent variables B and (BOx - B), ranged from r = 0.17 to 0.90. Multiple regressions of No on B and BOx were associated with coefficients of multiple correlation, R, that were relatively constant (generally, 0.8 to 0.9), despite the diversity of reagent concentrations used for extraction. THese and other considerations indicate that the NH4-N released by NaOH hydrolysis (B) and by KMnO4 oxidation (BOx - B) was derived, in large part, from the same soil organic sources (e.g., amides, amines, amino acids, and amino sugars). With few exceptions, correlations of No with BOx were as good as, or better than, simple correlations of No with B or (BOx - B). Thus, the extra effort required for separate NaOH and NaOH-KMnO4 extractions would not appear to be justified. The alkaline permanganate methods thus far reported by various investigators, as well as modifications evaluated in this study, offer a less precise and reliable basis for predicting No than does acid permanganate extraction for measuring oxidative release of NH4-N.

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