Laboratory studies on the influence of the earthworm Eukerria saltensis (Beddard) (Oligochaeta: Ocnerodrilidae) on overlying water quality and rice plant establishment

Abstract

Dense infestations of a peregrine oligochaete worm, Eukerria saltensis (Beddard), have been linked to rice crop failures in southern New South Wales, Australia. The influence of E. saltensis on water quality and rice plant establishment was investigated in a series of laboratory experiments using a flooded Riverina clay soil. Worm densities of 20 and 40 per container (2548 and 5096 worms/m2, respectively) significantly increased water turbidity after 7 days incubation. Longer incubation periods led to turbidity levels of over 500 NTU being achieved (40 worms per container, 21 days incubation). Water pH was significantly reduced by densities of 10, 20, and 40 worms per container after 7 days under cyclical illumination, however in continuous darkness significant changes in pH related to worm density were only found after 21 days incubation. Nitrogen as NH4+ and total phosphorus increased significantly in the overlying water in response to increasing worm densities after 21 days incubation, however nitrogen as nitrate/nitrite and soluble phosphorus did not. Algal production (measured as extracted chlorophyll a concentration) was unaffected by the worms, reflecting the low concentrations of available phosphorus in all treatments. Rice plants grown in containers with worms produced significantly longer and heavier shoots than control plants. Root systems were unaffected, and there was no evidence of root abrasion. Evaluation of rice seed stratification in the soil profile indicates that rice seeds can be passively transported below the soil surface by the feeding and tunnelling activity of E. saltensis. Our results suggest that E. saltensis impedes the establishment of aerially-sown rice crops primarily by increasing water turbidity. Plants respond to high turbidity by partitioning more of their growth into shoot production, and consequently become vulnerable touprooting through wave action, particularly as the soil loses compaction because of worm activity. Maintaining the lowest possible water levels during rice crop establishment has helped farmers to minimize these effects.