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

Abstract

AbstractThe 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.