Effects of pH and Oxygen on Phosphorus Release from Agricultural Drainage Ditch Sediment in Reclaimed Land, Kasaoka Bay, Japan

Abstract

Agricultural farms are considered to be non-point sources of phosphorus (P) loads to rivers and lakes. Sediment can act as a source or sink of P under different environmental conditions. Inorganic P is the major form, and is a very useful indicator to evaluate the potential release of P in sediment. This study aimed to evaluate the effects of pH and aerobic/anaerobic conditions on P release from sediment by the changes of P fractions before and after incubation. Sediment samples were collected from a drainage ditch of a livestock farm on reclaimed land in Kasaoka Bay, Japan. Experiments were conducted in a 200-mL glass serum bottle with 8 – 10 g fresh sediment and 100 mL of 0.02 M KCl solution, adjusted to pH 4, 7, or 10, and incubated under aerobic or anaerobic conditions. Phosphorus was released under both aerobic and anaerobic conditions until day 5. The amount of P released under anaerobic conditions increased continuously towards day 10 while that under aerobic conditions decreased after day 5 to the end of experiment (p < 0.05). Thus, sediment acted as a sink of P under aerobic conditions and as a source of P under anaerobic conditions. Sediment fractionations indicated that loosely sorbed P (loosely-P) and iron-bound P (Fe-P) fractions were the main sources of P released to the overlying water. The amount of P released from sediment at acidic pH was higher than that at neutral or alkaline pH under anaerobic conditions (p < 0.05). However, under aerobic conditions, the amount of P release was higher at alkaline pH than that at acidic or neutral pH (p < 0.05).