Nitrogen enrichment reduces parasitism in an annual hemiparasite

Anthropogenic activities, such as fossil fuel combustion and synthetic fertilizer synthesis, have resulted in nitrogen (N) deposition and elevated N availability in ecosystems. Many parasitic plants are adapted to low N environments and have evolved mechanisms to sequester water, N, and other limiting nutrients from hosts. Anthropogenic N deposition may perturb these host-parasite interactions, thereby altering nutrient cycling and ultimately reducing biodiversity. To test how N enrichment affects the incidence and strength of plant parasitism, we performed parasite-host performance assays and in-vitro root growth assays under supplemental N levels representative of urban areas experiencing N-deposition. We focused on the annual hemiparasite Castilleja exserta and two co-occurring Stipa host species (S. lepida and S. pulchra). Elevated N decreased haustoria formation and host-seeking behavior by roots but did not affect growth, suggesting N enrichment may reduce parasitism without associated loss of parasite biomass. We confirmed parasitism decreased host performance, and that the strength of parasitism was positively related to host biomass and earlier flowering. We also found S. lepida may be preferred over S. pulchra as a host. Collectively, these results indicate that N pollution altered parasitism in C. exserta and highlight the far-reaching ecological effects of N pollution on host-parasite interactions within plant communities.