Fine-root morphology in cacao-based systems responds to soil penetration resistance rather than macroporosity

Fine root morphology plays a key role in helping perennial crops like cacao (Theobroma cacao) cope with compacted soil conditions. However, how soil physical constraints such as penetration resistance and macroporosity influence fine root traits in perennial agroforestry systems remains poorly understood, especially under tropical field conditions. This study investigates how soil penetration resistance (SPR) and macroporosity influence fine root morphological and biomass traits under three cacao land-use systems in Southeast Sulawesi, Indonesia. We combined measurements of SPR and soil water content collected in 2015 with previously obtained data on root traits and macroporosity from 2014 field activities to explore their associations. Results show that SPR differed significantly across land-use systems, with the highest values found in cacao monocultures (∼2.5 MPa) and lower values in agroforestry systems (<2.0 MPa). The root length-to-weight ratio was positively correlated with SPR (p < 0.05), suggesting that fine-root assemblages within cacao systems may respond to compacted conditions by developing finer roots. In contrast, no clear association was found between root development and macroporosity. These observations point to SPR as a more influential factor than macropore abundance in determining root morphology and underline the potential benefits of agroforestry in mitigating compaction-related constraints in cacao-based systems.