Priming Effect of 15N‐Labeled Fertilizers on Soil Nitrogen in Field Experiments

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Abstract
Evidence of the “priming effect” on the uptake of soil N by additions of rather conservative amounts of fertilizer N was examined in data from two recently reported field experiments. In these experiments, urea and oxamide each labeled with 15N were compared on adjacent locations in successive years with ‘Sudax SX11’ Sorghum‐sudan hybrid (Sorghum sudanense) as the test crop. The priming effect in the first experiment was calculated from data from four cuttings during the first year and from a residual cutting during the second year. For the second experiment, data were used from the three cuttings harvested during the year the fertilizers were applied.Additions of N fertilizer increased the uptake of soil N by 17 to 45% in the first experiment in 1966 and by 8 to 27% in the second experiment in 1967. In the residual cutting of the first experiment, increases in uptake of soil N ranged from less than 0 to 29%. The increase in uptake of soil N by the crops was speculated to be due to stimulation of microbial activity by N fertilizers which increased mineralization of soil N, thus making more soil N available for use by plants.

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