Studies of nitrogen immobilization and mineralization in calcareous soils—II: Mineralization of immobilized nitrogen from soil fractions of different particle size and density

Abstract 15NO−3 was immobilized in a calcareous sandy soil and a calcareous clay soil each incubated with glucose and wheat straw. Net mineralization of organic-15N was more rapid in the sandy soil, irrespective of C amendment, and in soils amended with glucose. Intermittent drying and wetting of soils during incubation stimulated mineralization of 15N-labelled and native soil organic-N in all treatments. The availability (percentage mineralization) of recently-immobilized 15N consistently exceeded that of the native soil N. Ratios of the availability of labelled and unlabelled N were similar in the sandy and clay soils but varied according to C amendment, drying and wetting cycle and incubation period. Changes in the distribution of immobilized N amongst soil extracts and soil fractions of different particle size and density were determined during periods of net N mineralization. In straw-amended soils, the organic-15N of a light fraction, sp.gr. Drying and rewetting of soils hastened or magnified changes occurring in the organic-15N of soil fractions, but qualitatively, the pattern of change was similar to that observed with soils incubated under uniformly-moist conditions. The percentage distribution of labelled and unlabelled N suggested that in the long term, the silt fraction will accumulate an increasing proportion of the more stable nitrogenous residues.

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