The relatively high concentrations of NH4+ observed in the early
stages of the cropping season could be attributed to the combined
effects of fertilizer N application, limited plant uptake, net N mineralization of labile organic matter (e.g. incorporated crop
residues), and, in the flooded plots, inhibition of nitrification due to anoxic soil conditions (Cucu et al., 2014). During this period and particularly in the two dry seeded treatments, N supply was
probably greater than the crop N requirements resulting in
measurable peak soil solution NH4
+ concentrations in the rooted
soil layer. Later on in the cropping season, rapid N uptake by the
crop was probably responsible for the relatively low NH4
+
concentrations even after urea application