Effect of green manure and floodwater algae on diurnal fluctuations of floodwater pH and depth of aerobic soil layer under lowland rice conditions

Abstract

The present investigation was undertaken to elucidate the effect of floodwater algae and green manure on floodwater pH and depth of aerobic soil layer which are mainly responsible for the nitrogen losses in lowland rice production systems. The study was conducted in an environmental chamber using a sandy loam soil. Cylindrical plastic bottles 7 cm in diameter were used and 5 cm soil layer covered with 5 cm deep floodwater was created in each bottle to simulate submerged rice conditions. The presence of algae in the floodwater increased the floodwater pH owing to removal of CO2 during photosynthesis during the day, the value decreasing again during the night. Addition of green manure, without algal growth, depressed the pH for the first week and after 4 weeks of incubation. With algae, the application of green manure in vitro resulted in lower diurnal pH increases especially for the first 2 weeks of incubation and a consistently higher morning pH throughout the study. The addition of green manure eliminated the aerobic soil layer below the soil-water interface, presumably because green manure cause a high O2 demand owing to increased microbial activity which results in a lower Eh value.