| Sumario: | This study presents some aspects of nitrogen fractionation in the volcanic ash soils of the Andean Highland of Pasto, Nariño, southwest of Colombia . In this area the average annual rainfall and temperature are 781 mm and 13°C respectively. The rock types and pyroclastic materials are of andesitic-dacitic composition.
The volcanic ash soils studied have adequate amounts of total-N (about 4,000 ppm) and organic-N (95 per cent of total-N). Native fixed-NH4 was found to range from 32.4 to272.6 ppm and, on average, varied from 1.90 to 4.13 per cent of total-N, whilst exchangeable-NH4 values were between 28.0 to 78.0 ppm varying, on average, as a fraction of total-N from 1.40 to 2.43 per cent. Both, the average concentration of native fixed-NH4 and exchangeable-NH3-N ranged from 0.19 to 29.5 ppm. (on average, 0.16 to 0.32 per cent of total-N).
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