Comparative Effects of an Organic and Inorganic Nitrogen Source in Flooded Soils

The availability of N from organic and inorganic sources in soils is controlled by contrasting microbiological processes. Nitrogen from an added organic source is made gradually available through mineralization whereas immobilization and N loss mechanisms act to decrease inorganic-N availability with time. To define these initial processes, it is necessary to trace the availability of N from both added and native sources and to measure biological activity while varying a controlling factor such as temperature. A laboratory incubation study was designed to compare (NH₄)₂SO₄ and vetch as added N sources in a flooded soil (Clear Lake clay, Typic Pelloxererts, fine, montmorillonitic, thermic). Nitrogen-15 labeled materials were added at rates of 0, 50, or 100 µg N/g in factorial combination to 50 g samples of soil and placed in vessels forming columns 10- to 12-cm deep. Temperature regimes of 20, 25, and 30°C were utilized. Available (soluble and exchangeable) NH⁺₄-N and CO₂ production were periodically measured over a 30-d incubation time. Vetch-N underwent a gradual mineralization paralleling that of soil-N. Levels of total NH⁺₄-N in the vetch treatments were positively affected by temperature and showed a highly significant correlation with CO₂ production. Levels of total (labeled + unlabeled) NH⁺₄-N were consistently higher throughout the incubation period in the (NH₄)₂SO₄ treatments as compared to vetch treatments at equal rates of addition. Labeled (NH₄)₂SO₄-N rapidly disappeared from the available form, however, and a simultaneous “priming effect” on soil—N mineralization occurred. These two events were quantitatively correlated. The priming effect was not correlated with CO₂ production. These findings support the concept that the priming effect of labeled fertilizer-N on soil-N mineralization is an apparent effect due to the interchange of labeled available-N with unlabeled organic-N with no net effect on N mineralization.