Nitrification and Denitrification of Nitrogen Fertilizers in a Soil Column

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Abstract
Nitrogen transformations occuring within reconstituted Wellwood soil profile samples packed in plastic columns (4.4 cm i.d.) and amended with urea or NO‐3‐N were investigated under isothermal (20°C) laboratory conditions. The surface of the soil column was open to the atmosphere and the bottom was closed. Samples of soil atmosphere were taken regularly through sampling ports located at several different depths and analyzed for N2, O2, and N2O with a mass spectrometer. A steady‐state O2 distribution within the column was established soon after packing, making the surface layer aerobic and the lower layer anaerobic. The depth of the aerobic layer was governed by the gas‐filled pore space of the column. Urea applied uniformly throughout the column was oxidized to NO‐3 within the aerobic layer. The concentration of NO‐3 was highest at the surface and decreased to almost zero near the aerobic‐anaerobic interface. Nitrate transported downward from the aerobic layer was denitrified near the interface. Ammonium in the anaerobic zone was stable in a column at 10.5% gas‐filled pore but gradually declined as incubation at 16.5% gas‐filled pore space progressed. Nitrate N applied to the aerobic layer was transported downward to the anaerobic layer and denitrified. The magnitude of denitrification was greater than that observed with urea possibly due to a higher flux of NO‐3 into the anaerobic layer. Nitrous oxide was observed near the aerobicanaerobic interface with both the NO‐3 treatment and urea (10.5% air porosity treatments). Dinitrogen was the only gaseous dinitrification product ohserved in the column at 16.5% air porosity amended with urea.