TRANSPORT AND TRANSFORMATION OF UREA AND ITS DERIVATIVES THROUGH A MINERAL SUBSOIL

The transport and transformation potential of N compounds derived from urea, applied as a deicing agent during he winter period, have hem in vestigated in a coarse, heterogeneous glacial-contact deposit in south east Norway. The hydrolysis of urea to ammonium-N (NH4+-N) and subsequ ent oxidation to nitrate N (NO3--N) was monitored bg extraction of soi l wafer from five depths (from 70 to 220 em) in a lysimeter trench. Ur ea transport through a soil profile is normally negligible, although u rea is a hydrophilic, uncharged molecule. ride, In this study, urea wa s rapidly transported to at least 220 cm. The measured urea-N concentr ations varied from 2 to 50 mg L-1 throughout the profile. This transpo rt was explained by minimal hydrolytic activity because of soil temper atures close to 0 degrees C, and by different preferential flatu mecha nisms being important during the snowmelt period, Urea hydrolysis sis occurred al all depths, The NH4+-N formed in this in situ hydrolysis w as oxidized to NO3--N. The nitrification started immediately and was m ost rapid at the 70- and 105-cm depth, but was detectable at all depth s down to 220 cm 120 d after solute application. Nitrification also oc curred during the winter following the urea application. One year afte r the urea application, about 40% of the applied urea-N was recovered as NO3-N in file unsaturated zone (0-450 cm), and it is estimated that at least 50% of the applied N will reach the groundwater as NO3--N.