Effects of long-term nitrogen and phosphorus addition on plant defence compounds in a freshwater wetland

•12-yr N addition and 10-yr P addition experiments were conducted.•Nutrient addition decreased the production of defence compounds in plant leaves.•Leaves are more sensitive to relative resource availability compared with stems.•Nutrient enrichment altered plant carbon allocation to secondary metabolites.•Nutrient enrichment would influence nutrient cycling in freshwater wetlands.