Large differences in shade tree species effect on soil fertility and cocoa agroforest multifunctionality explained by litter traits

1. Manipulating plant functional diversity to improve agroecosystem multifunctionality is a central challenge of agricultural systems worldwide. In cocoa agroforestry systems (cAFS), shade trees are used to supply many services to farmers, yet their impact on soil functioning, particularly soil fertility, and cocoa yields is likely to vary substantially among tree species. 2. Here, we compared the impact of five shade tree species (Canarium schweinfurthii, Dacryoides edulis, Milicia excelsa, Ceiba pentandra, Albizia adianthifolia) and unshaded conditions on the functioning of poor sandy savannah soils within eight cocoa farms in Central Cameroon. We assessed the effects of multiple plant functional traits, as well as leaf litterfall and fine root biomass, on a range of soil functions and on cocoa yield. 3. Shade trees generally improved several soil functions, including soil pH, NH4+, NO3- and Olsen P content, biomass production of a bioassay, and soil total C and N content, while leaving cocoa yields unchanged. However, as expected, these effects varied largely among species. Improvements of soil functions were low under the two fruit trees, Canarium and Dacryodes, medium under the legume tree Albizia, and high under the two timber trees, Milicia and Ceiba. Low litter recalcitrance was most strongly associated to increases in soil variables representing soil fertility such as N and P availability, whereas soil C and N content increased with litter Ca restitution. 4. We demonstrated here that agroecosystem multifunctionality is substantially influenced by the functional traits of shade trees, and increased with the level of dissimilarity between traits of shade and cocoa trees (cocoa showing the lowest leaf litter quality). Selection of shade trees based on their functional traits appears as a promising practice to adequately manage soil functioning. In this context, future research will need to extent this approach to other belowground traits and other aspects of multifunctionality, including long-term cocoa health and yield, in order to fully assess the beneficial role of shade trees on these agroecosystems.