As expected, maintenance of anaerobic conditions over most of
the cropping season by continuous
flooding resulted in relatively
low NO3
concentrations throughout the soil profile. In contrast,
highest NO3
concentrations were observed in DFL and DIR,
particularly at the beginning of the cropping season when
maximum concentrations were largely above the limit defined
by the European Nitrate Directive (91/676/EEC) referred to
groundwater. In these plots, oxic soil conditions present before
the onset of
flooding in DFL and throughout the cropping season in
DIR, favored the microbial nitrification of available NH4
+. Nitrate
produced by nitrification in the topsoil moved easily along the soil
profile into the subsoil with percolating waters (Ghosh and Bhat,
1998) leading to the observed increase in concentrations at all soil
depths as well as in the groundwater.