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 the winter period, have been investigated in a coarse, heterogeneous glacial-contact deposit in southeast Norway. The hydrolysis of urea to ammonium-N (NH4+-N) and subsequent oxidation to nitrate-N (NO3-N) was monitored by extraction of soil water from five depths (from 70 to 220 cm) in a lysimeter trench. Urea transport through a soil profile is normally negligible, although urea is a hydrophilic, uncharged molecule. In this study, urea was rapidly transported to at least 220 cm. The measured urea-N concentrations varied from 2 to 50 mg L-1 throughout the profile. This transport was explained by minimal hydrolytic activity because of soil temperatures close to 0 degrees C, and by different preferential flow mechanisms being important during the snowmelt period. Urea hydrolysis occurred at all depths. The NH4+-N formed in this in situ hydrolysis was oxidized to NO3-N. The nitrification started immediately and was most rapid at the 70- and 105-cm depth, but was detectable at all depths down to 220 cm 120 d after solute application. Nitrification also occurred during the winter following the urea application. One year after the urea application, about 40% of the applied urea-N was recovered as NO3-N in the unsaturated zone (0-450 cm), and it is estimated that at least 50% of the applied N will reach the groundwater as NO3-N