Wood, nematodes, and the nematode-trapping fungus Arthrobotrys oligospora

Researchers have proposed that Arthrobotrys oligospora and related fungi trap soil nematodes to obtain nitrogen and thereby compete saprophytically for carbon and energy in nitrogen-poor environments, including litter and wood. The current study tested two hypotheses concerning this model. The first was that wood decomposition would be enhanced if both large numbers of nematodes (a potential nitrogen supply) and A. oligospora (a cellulolytic organism that can use that N supply) were present. The second was that A. oligospora trapping activity would increase if large numbers of nematodes were added to soil containing abundant carbon (a wood dowel or chip). Although the first hypothesis was supported by an in vitro experiment on agar (A. oligospora degraded much more wood when nematodes were present), neither hypothesis was supported by an experiment in vials containing field soil. In soil, wood decomposition was unaffected by the addition of A. oligospora or large numbers of nematodes. Whereas A. oligospora trapped virtually all nematodes added to agar cultures, it trapped few or no nematodes added to soil. Given that the fungal isolate was obtained from the same soil and that the fungus increased to large numbers (>1×10³ propagules g⁻¹ soil), the failure of A. oligospora to trap nematodes in soil is difficult to explain. Soil nitrate levels, however, were high (71 mg kg⁻¹), and it is possible that with lower nitrate levels, trapping in soil might be stimulated by wood and nematodes.