Denitrification Losses from Humid Tropical Soils of Puerto Rico
1974
Dubey, H. D. | Fox, R. H.
Profile samples from 5 depths (0–25, 25–50, 50–75, 75–100, and 100–125 cm) from 3 soil types (two Oxisols—Piña sandy loam and Catalina clay, and an Ultisol—Humatas clay) were incubated at 23±1C under field capacity and waterlogged conditions in an oxygen-helium atmosphere. Evolved N₂ and N₂O were determined after 1 and 2 weeks by gas chromatography. Denitrification was related to moisture level, organic matter content, pH, and denitrifying population. Gaseous-N losses occurred almost exclusively from the surface soil (0–25 cm) under waterlogged conditions. Losses ranged from 8 to 31% of the applied NO₃⁻-N. Some loss (7%) also occurred at field capacity from the surface horizon of the Oxisol having the highest organic matter content, Catalina clay. Losses were directly related to organic matter content. Raising the pH of the subsoil only enhanced denitrification slightly. Addition of a denitrifying population and mineral nutrients in the form of a soil inoculant to a subsoil with a raised pH and organic matter content increased denitrification equal to that of surface soil. The lack of denitrification at lower depths is believed to be due to an inadequate supply of rapidly available energy material, low pH and mineral nutrients, and consequent lack of a denitrifying population. Since the surface soil rarely gets waterlogged, it was concluded that little possibility of N losses by denitrification in these humid tropical soils exists.
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