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

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AbstractThe 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.