Ammonia Volatilization from Nitrogen Sources Applied to Rice Fields: II. Floodwater Properties and Submerged Photosynthetic Biomass

The effects of (NH₄)₂SO₄, urea, and urea amended with the urease inhibitor, phenyl phosphorodiamidate (PPD), on floodwater properties were studied concurrently as part of a field NH₃ volatilization study. In the (NH₄)₂SO₄ treatment the maximum concentration of ammoniacal-N in the floodwater (≃ 50 g N m⁻³) occurred immediately after its application to the floodwater. Thereafter, floodwater ammoniacal-N concentrations declined rapidly and were negligible 6 d after the application of (NH₄)₂SO₄. Ammoniacal-N concentrations in the urea treatment reached maxima of ≃ 12 g N m⁻³, 3 to 5 d after urea was applied and then declined steadily to negligible concentrations in 7 d. Application of PPD (1% wt/wt) along with urea delayed the buildup of ammoniacal-N in floodwater until 5 to 7 d after N was applied although the maximum ammoniacal-N concentration in the floodwater was comparable to that obtained in the urea treatment. Floodwater pH displayed a marked diurnal pattern throughout the experiment in the urea-amended and background fields. In contrast, pH in the floodwater in the (NH₄)₂SO₄-amended field was buffered at ≃ 8.00 for the first 2 d, probably because high concentrations of NH₄HCO₃ formed in floodwater. Diurnal fluctuations in pH prevailed after 3 d when ammoniacal-N concentrations had declined substantially. The partial pressures of NH₃ (pNH₃) in the floodwater in all treatments were synchronized with diurnal temperature and pH changes in the floodwater. The maximum pNH₃ in floodwater was similar in both the (NH₄)₂SO₄- and urea-amended fields even though ammoniacal-N concentrations were initially significantly higher in the former. Total titratable alkalinity in floodwater increased after urea and urea/PPD were applied but declined following the application of (NH₄)₂SO₄. The initial levels of alkalinity in floodwater were double the content of alkalinity in irrigation water, and it is surmised that evaporation and/or respiration contributed significantly to alkalinity in the floodwater. Enumerations of algae present in the flooded soil showed the biomass to be small and dominated by non-N₂-fixing blue-green algae (Syanophyceae). This biomass, however, was associated with marked diurnal fluctuations in floodwater pH, which coupled with the accumulated alkalinity, were the major factors contributing to the rapid NH₃ loss following the application of (NH₄)₂SO₄ and urea to the floodwater.