Effects of algae and fertilizer-nitrogen on pH, Eh and depth of aerobic soil in laboratory columns of a flooded sandy loam

A sandy loam was incubated under floodwater in the laboratory either in the dark or in the light (7 h day, 20 degrees C; 17 h night, 15 degrees C) and with four N sources [control, ammonium carbonate [(NH4)(2)CO3], ammonium c hloride (NH4Cl), potassium nitrate (KNO3)]. In the dark, floodwater pH rose steadily fl-om 6.4 to about 7.5 over 60 days in the control, KNO3 and (NH4 )(2)CO3 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 n ight, pl-l 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 in crease was zero at pH 5.6 rising to a maximum of about 2 units at pH 6.3 an d falling again at higher DH 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 d ue 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 nigh t. The daily decrease in Eh per unit increase in pH mse from about 10 to 90 mV pH(-1) over the incubation period. Initially, therefore. O-2 concentrat ion must have been increased during the day by algal photosynthesis (values <59 mV pH(-1)), but later O-2 concentration must have fallen, due possibly to the decomposition of algal cells. The presence of algae initially incre ased the depth of the aerobic soil layer, but eventually an algal mat settl ed on the soil surface acting as a zone of O-2 demand as the algae decompos ed.