High nitrogen enrichment buffers plant photosynthesis against herbivory damage

Abstract Herbivorous insects and nitrogen deposition are key drivers of ecosystem productivity under global change. However, their combined effects on plant photosynthesis, particularly during insect outbreaks, remain poorly understood in natural ecosystems. We conducted a decade‐long, multi‐level nitrogen‐addition experiment in an alpine meadow on the Qinghai–Tibet Plateau, with insect‐herbivory outbreak treatments nested within each nitrogen level. Our results showed that insect outbreaks consistently suppressed leaf photosynthesis in dominant species, but this effect was strongly modulated by nitrogen level. Under low nitrogen supply (2–8 g N m −2 year −1 ), herbivory significantly reduced net photosynthetic rates by 45.4%–60.6%, whereas this reduction was roughly halved at higher nitrogen levels. Across the nitrogen gradient, the decline in photosynthesis scaled linearly with increasing nitrogen availability, suggesting a nitrogen‐dependent reduction of herbivore impact. This pattern likely resulted from a trade‐off of nitrogen resource allocation between reduced investment in alkaloid synthesis and increased photosynthesis in facing insect herbivory and nitrogen enrichment, reflecting a shift from resistance‐based to tolerance‐based defence strategies. Overall, these findings reveal the non‐linear nature of the insect herbivory effect in natural ecosystems when experiencing variable nitrogen levels, providing new insights into understanding and predicting grassland productivity responses to the interactions of nitrogen enrichment and insect outbreaks. Read the free Plain Language Summary for this article on the Journal blog.