Transformations of fertilizer nitrogen in soil

Organo-mineral sedimentation fractions of an N¹⁵-labelled Chester silt loam surface soil were analyzed for total carbon, total nitrogen, and nitrogen extractable at 100°C by 0.5N sodium pyrophosphate. The nitrogen of the pyrophosphate extract was fractionated into: I, NaOH-distillable N; II, 6N H₂SO₄-hydrolyzable NaOH-distillable N following I; III, ninhydrin-reactive NaOH-distillable N following II; and IV, total N on the extract residue following III. Nitrogen not extractable by pyrophosphate treatment was designated from V. Subsamples of each soil fraction were incubated under waterlogged conditions, and MgO-distillable nitrogen was determined on the extracted incubate as an estimate of nitrogen mineralization capacity. The distillate from each nitrogen determination was analyzed for N¹⁵ by mass spectrometry.

Total nitrogen and per cent nitrogen mineralization increased, and C/N ratios decreased, as fraction particle size decreased. In contrast, total nitrogen and nitrogen mineralization generally decreased, and C/N ratios increased, for a given particle size, with an increase in intensity of ultrasonic dispersion treatment. Atom per cent excess N¹⁵ data revealed that a general labelling of all soil nitrogen forms examined had occurred, and that the degree of labelling decreased in those materials which required greater intensities of ultrasonic vibration for dispersion. Apparently, fertilizer N¹⁵ had been incorporated in a wide range of soil nitrogen components, from the most readily extractable to those which remained in extracted residues, at each level of microaggregate stability. Tagging was highest in the nitrogen of the > 53 µ organic separate, in the water-solubilized NaOH-distillable nitrogen, and in the nitrogen mineralized during incubation. Allocation of the various total and N¹⁵-labelled nitrogen forms among soil fractions for the maximum dispersion treatment, t₁₈₀ min, showed that the <2 µ plus water-solubilized materials contained 85 per cent of the NaOH-distillable soil nitrogen, the form concluded to be most active biologically in light of the high correlations obtained between it and mineralized nitrogen (r ranged from 0.90 to 0.98). It was also shown that the sum of the amounts of nitrogen mineralized by soil fractions separated at a given dispersion intensity became equivalent to the amount mineralized by the whole soil only when the quantity of NaOH-distillable nitrogen extracted by water during the fractionation step was added. This finding further supported the hypothesis that the two forms of nitrogen probably have a common source, or sources, among soil nitrogen components.

Authors and Affiliations

1. U.S. Soils Laboratory, Soil and Water Conservation Research Division Agricultural Research Service, USDA, Beltsville, Maryland

   F. W. Chichester

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