Understanding how changing soil nitrogen affects plant–pollinator interactions

Many pollinating insects, across taxa and regions, have declined during the twentieth century. Amongst the drivers of these trends, soil eutrophication and acidification caused by nitrogen (N) have not been broadly researched. Anthropogenic influences have greatly increased the global deposition of N to soils during the past century; this is increasingly recognised as a threat to global biodiversity. The fundamental role of soil in plant growth and health means that alterations to soil conditions will likely have consequences for plant–pollinator interactions. Soil N can be a substantial driver of the species structure of botanical communities, often reducing species richness due to quick growth of competitive grasses. Floral traits, relevant to pollinators, such as phenology, morphology, and nectar and pollen production and quality can also be affected by soil N. We currently lack sufficient research to determine if and how pollinators will be impacted by these changes. This review collates the research and evidence of how soil N affects botanical species composition and relevant floral traits, and discusses how pollinating insects and plant–pollinator interactions might be impacted. We conclude by identifying the key knowledge gaps in this subject; the lack of research that includes the pollinators into studies of how N additions affect botanical traits, poor understanding of inter-specific variation in botanical responses to N, synthesis of botanical traits to form a comprehensive understanding, and the inclusion of other abiotic and biotic drivers into studies.