Indicating soil quality in cacao-based agroforestry systems and old-growth forests: The potential of soil macrofauna assemblage
Soil quality or health is a fuzzy concept that has been vigorously criticized due to the extreme variability of soil and the difficulty of linking soil indicators to soil functions and sustainability. In most soil quality studies some obvious factors or typologies are used as a basis to select the "best indicators" of soil quality, i.e. those that best explain the differences among the plots under study. This is not the case for a variety of natural or agro-ecosystems including the Talamanca cacao-based agroforestry systems (AFS), which present neither a pre-established typology nor a clear framework to evaluate their soil quality. This situation required a selection of indicators based on the literature that was oriented by the non-equilibrium thermodynamic theory. A framework was elaborated through full and minimum indicator sets of baseline soil physical and chemical indicators, along with macrofauna groups. A minimum set of four well-accepted abiotic soil quality indicators (bulk density, sum of bases, pH and carbon) was able to separate cacao AFS plots and forests into five distinct clusters along a low-to-high "soil quality" gradient. The AFS rated as "good" soil quality did not differ from the forest. Abundances of selected macrofauna groups were well correlated with these indicators and helped elucidate the soil quality clusters identified. In particular, high predator abundance indicated proper energy flow and confirmed the high abiotic soil quality, thus confirming the potential of macrofauna groups as apt soil quality indicators. However, these indicators need to be tailored to local conditions. Consequentially, cacao-based AFS in Talamanca are able to conserve soil and provide a high level of soil-related ecological services. Considering the soil an open system where the non-equilibrium thermodynamic theory applies successfully guided indicator selection and could help to reformulate the soil quality definition.