Abstract: A sandy loam was incubated under floodwater in the laboratory either in the dark or in the light (7 h day, 20 °C; 17 h night, 15 °C) and with four N sources [control, ammonium carbonate [(NH4)2CO3], ammonium chloride (NH4Cl), potassium nitrate (KNO3)]. In the dark, floodwater pH rose steadily from 6.4 to about 7.5 over 60 days in the control, KNO3 and (NH4)2CO3 treatments, but with NH4Cl pH decreased to 5.8. In the light, algal growth began to affect the floodwater pH after 9 days. At the end of the night, pH values were similar for all treatments to those kept in the dark. During the day, pH changes depended on the morning pH value: the daily increase was zero at pH 5.6 rising to a maximum of about 2 units at pH 6.3 and falling again at higher pH values. Changes in the carbonate equilibria in response to CO2 removal by algal photosynthesis partly explain the results, but increasing inputs of acid are also implicated below pH 6.3 possibly due to reduced volatilization and increased nitrification. Redox potential (Eh) in the floodwater was little affected by N treatment until algal growth began. Eh then decreased each day as pH rose and recovered during the night. The daily decrease in Eh per unit increase in pH rose from about 10 to 90 mV pH–1 over the incubation period. Initially, therefore, O2 concentration must have been increased during the day by algal photosynthesis (values <59 mV pH–1), but later O2 concentration must have fallen, due possibly to the decomposition of algal cells. The presence of algae initially increased the depth of the aerobic soil layer, but eventually an algal mat settled on the soil surface acting as a zone of O2 demand as the algae decomposed. more...