Does shade tree diversity increase soil fertility in cocoa plantations?

Complex agroforests have been promoted as a potential solutions to address trade-offs between environmental conservation efforts and the need for increased agricultural productivity for smallholder farmers in the tropics. However, the effects of tree diversification on soil fertility in tropical agroforests remain unclear. In this study, we examine whether tree diversification in cocoa plantations is associated with soil fertility benefits and can contribute to soil restoration after deforestation. We tested for positive associations between increasing tree species diversity and increased soil aggregation, soil nutrients and microbial communities across a diversity gradient ranging from cocoa monocultures to complex cocoa agroforests. Secondary forests and primary forests were used as reference ecosystems. Increase in tree diversity within cocoa plantations did not increase soil fertility parameters in topsoil layers or cocoa yields. Mean soil C contents were 8% lower, mean weight diameter of aggregates 48% lower and total bacterial biomass 35% lower in cocoa plantations than in primary and secondary forest systems, whereas soil P content was 22% higher. Across all land-use systems, microbial biomass was greater in sites with higher soil carbon contents and soil aggregation. This suggests soil function restoration in terms of microbial communities, soil C and aggregate stabilization in secondary forests. However, in cocoa plantations tree diversification alone may not be an effective solution to mitigate soil degradation after deforestation. Rather, preserving remaining forests or promoting farming approaches that allow for secondary forest regeneration (e.g. implementing forest strips and regular fallow rotations) might have a more substantial impact on soil health.