Parasitic plants indirectly regulate decomposition of soil organic matter

Parasitic plants have been shown to affect soil-organic-matter (SOM) decomposition, but the mechanism is unknown. As arbuscular mycorrhizal fungi (AMF) can affect decomposition and compete with parasitic plants for carbon, we hypothesized that parasitic plants can indirectly regulate decomposition by suppressing AMF. We conducted two experiments in which Bidens pilosa was inoculated with AMF or not, and parasitized with Cuscuta australis or not. In hyphae-in-growth bags, we provided SOM in one experiment as 13C-/15N-labelled maize leaves and in the other as phytate P. In the presence of AMF, Parasitization increased the 13C but decreased the organic P remaining in the bags. In the absence of the parasite, AMF decreased the 13C and increased the organic P remaining in the bags. Our results demonstrate that parasitic plants can indirectly regulate the decomposition of organic materials by suppressing the effect of the extraradical AMF hyphae on decomposition.