Root biomass variation of cocoa and implications for carbon stocks in agroforestry systems

Cocoa (Theobroma cacao L.) root systems are typically assumed to contribute a small portion of carbon (C) to total C stocks in cocoa agroecosystems. Yet there are almost no direct measurements of cocoa coarse root biomass to support this assumption, presumably due to the difficulty of measuring coarse roots in situ and the risk to farmers' livelihoods. Instead, root biomass is commonly estimated using allometry based on forest data, which might not be accurate for perennial crops given their range of management conditions. In this study, we bridge conventional methods of quantifying coarse root biomass with non-destructive application of ground penetrating radar to estimate cocoa belowground biomass (BGB) and C stocks in an agroforestry system in Ghana. BGB was measured for cocoa grown with shade trees (Entandrophragma angolense or Terminalia ivorensis) and in monoculture. BGB estimates showed good accuracy, with a relative root mean square error of 7% from excavated plants. It was estimated that 15-year-old cocoa hold approximately 6.0kgCplant(-1) in coarse root biomass and have a root to shoot ratio of approximately 0.23. However, the results indicate that proportionally more biomass was allocated to roots for cocoa grown in mixture with shade trees. Plot scale estimates show that cocoa roots contributed 5.4-6.4MgCha(-1), representing 8-16% of C stocks in all live tree biomass (cocoa+shade trees), depending on shade tree management. Our findings illustrate a promising approach for non-destructive BGB inventories of perennial crops. It is highlighted that although commonly used pan-tropical allometric equations may broadly function in estimating BGB for cocoa, this approach assumes proportional allocation between aboveground biomass and BGB, which may translate into inaccuracies in C stock inventories across diverse cocoa agroecosystems.