Agroforestry enhances soil C, N and P contents with non-proportional C–N–P responses: A global meta-analysis

Agroforestry is widely promoted for soil restoration, but its effects on the coordinated responses of soil C, N and P remain uncertain. We synthesized 1902 observations from 103 studies worldwide to assess agroforestry effects on soil C, N and P contents, C:N:P stoichiometry, microbial biomass and microbial diversity relative to monoculture controls. Because bulk-density data were not consistently reported, we evaluated soil nutrient contents rather than areal nutrient stocks. Agroforestry increased SOC, TN and TP, but their responses were non-proportional: SOC increased more strongly than TN and TP, and only C:N increased significantly. SOC and TN responses were significantly coupled but sub-proportional, whereas SOC–TP and TN–TP relationships were weak or non-significant. Microbial biomass N, microbial C:N and N:P, and fungal Chao1 also responded to agroforestry. Moderator analyses identified MAP and MAT as the most supported moderators of SOC responses and initial soil N:P as the most supported moderator of TN responses; no robust moderator was identified for TP. Overall, agroforestry increased soil C, N and P contents and altered multiple microbial indicators. However, C, N and P responses were non-proportional and context dependent, and the ecological direction of these stoichiometric shifts could not be determined from the available data. Agroforestry sustainability should therefore be assessed using SOC together with soil nutrient responses, C:N:P stoichiometry and microbial indicators.