Nitrogen enrichment reduces parasitism in an annual hemiparasite

Anthropogenic activities, such as fossil fuel combustion and synthetic fertilizer synthesis, have resulted in 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 co-occuring Stipa (host) species. We found elevated N decreased haustoria formation and host seeking behavior by roots but did not stimulate growth, suggesting N enrichment may reduce parasitism without associated loss of parasite biomass. We confirmed parasitism decreased host performance and delayed onset of parasite flowering, and that the strength of parasitism was positively related to host biomass. We also found S. lepida may be preferred over S pulchra as a host. Collectively, these results highlight the far-reaching ecological effects of N pollution on host-parasite interactions within plant communities.化石燃料燃烧、合成肥料生产等人为活动已导致生态系统发生氮沉降（N deposition），并提升了生态系统内的氮素可利用水平。许多寄生植物适应低氮环境，演化出从宿主体内摄取水分、氮素及其他限制性营养物质的生理机制。人为氮沉降可能干扰这类宿主-寄生植物的互作关系，进而改变生态系统养分循环，最终导致生物多样性下降。 为探明氮富集（N enrichment）对植物寄生作用发生率与强度的影响，本研究在模拟受氮沉降影响的城市区域氮添加水平下，开展了宿主-寄生植物性能测定试验与体外根系生长试验。本研究选取一年生半寄生植物（hemiparasite）火焰草（Castilleja exserta），以及与其同域分布的针茅属（Stipa）宿主植物作为研究对象。 研究结果显示，高氮水平会抑制根系的吸器（haustoria）形成与宿主搜寻行为，但并未促进寄生植物的生长，这表明氮富集可能在不造成寄生植物生物量损失的前提下，削弱植物寄生作用的强度。本研究证实，寄生作用会降低宿主的生长性能，并延迟寄生植物的开花起始时间；同时寄生作用强度与宿主生物量呈显著正相关关系。此外，本研究还发现该寄生植物相较于美丽针茅（Stipa pulchra），更偏好以赖特针茅（Stipa lepida）作为宿主。 综上，本研究结果凸显了氮污染对植物群落内宿主-寄生植物互作关系的深远生态影响。