Effects of Tubificid Worms on Denitrification and Nitrification in Stream Sediment

Abstract

Labeled nitrate-N in the form of K¹⁵NO₃ was used to determine the effects of the tubificid oligochaetes, Tubifex tubifex and Limnodrilus hoffmeisteri, on the N transformation processes in stream sediment contained in laboratory columns. Denitrification and nitrification occurred simultaneously and were enhanced significantly in the presence of the worms. The denitrification rate over 48 d at 15 °C in the sediment that contained the worms was 90 mg N/m²∙d⁻¹; the simultaneous rate of nitrification was 69 mg N/m²∙d⁻¹. The corresponding rates for denitrification and nitrification in sediment without worms were 50 mg N/m²∙d⁻¹ and 29 mg N/m²∙d⁻¹. The rate of CO₂ evolution from sediment containing worms and overlain with nitrate-N solution was about 21% greater than that of sediment with worms overlain with distilled water. It was concluded from experiments using chloride as tracer that denitrification could be enhanced by the worms at least in part as a result of accelerated movement of nitrate-N into the sediment. However, when a column of glass beads was used as substrate for worms, and with no organic matter present, the concentration of nitrate in an overlying solution declined at an appreciable rate. This suggested that denitrification occurred in or on the worms, a possibility supported by the observation that denitrifying bacteria could be isolated from both the exterior and the gut contents of the worms.Key words: tubificid worms, stream nutrient cycling, denitrification, nitrogen cycling