Indicating soil quality in cacao-based agroforestry systems and old-growth forests: The potential of soil macrofauna assemblage

Abstract

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.

Introduction

Soil quality or health is a fuzzy concept officially defined as “the continued capacity of the soil to function within ecological and land-use boundaries, to sustain productivity, promote the quality of air and water, and maintain plant, animal and human health” (Doran and Safley, 1997). This concept was vigorously criticized (Lancaster, 2000, Letey et al., 2003, Rossi et al., 2009) and considered non-objective (Lancaster, 2000), mainly because no absolute reference to an optimal soil quality could be identified (Letey et al., 2003, Rossi et al., 2009). This is a consequence of extreme soil variability and the difficulty of linking soil indicators to sustained productivity, air or water quality or health. Determining soil quality as officially defined requires long-term experiments with indicators from the soil, the crops, the environment and the society that lives from this soil. To the best of our knowledge, no such experiment has been done. Instead, scientists work with soil indicators more or less linked to soil functions (Andrews et al., 2002, Arshad and Martin, 2002, Bastida et al., 2008) and compare soils under different practices or uses without monitoring air and water quality or plant, animal or human health. For these reasons Letey et al. (2003) claimed that soil quality could not be measured per se and that only management practices should be compared to determine which one enhanced some particular soil indicator at stake. Indeed, most of the prior studies on soil quality aimed at comparing land-uses (Saviozzi et al., 2001, Rousseau et al., 2010), cropping systems (Glover et al., 2000, Andrews et al., 2002), cultural practices (Koutika et al., 2005, Rousseau et al., 2006), forest management practices (Burguer and Kelting, 1999, Pang et al., 2006) or soil recovery levels (Wang and Gong, 1998, Dawson et al., 2007). In all these cases some obvious factors or typologies could be 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. In the case of Talamanca, the cacao-based agroforestry systems (AFS) have no pre-established typology, as they represent a population of highly diverse agrosystems, all of which are unique. This situation applies to a variety of natural or agro-ecosystems that require a clear framework to evaluate and monitor their soil quality. In this context, soil quality must rely only on indicators previously established in the literature. Among the large number of indicators that efficiently detect differences between soils and management practices, the physical and chemical indicators are the most used (see Appendix). Soil biological indicators are also required to indicate soil quality as officially defined (Bastida et al., 2008, Ritz et al., 2009), but no consensus exists on “universal” indicators. Soil macrofauna assemblage efficiently compared soil quality among land uses (Lavelle et al., 2006, Rossi and Blanchart, 2005, Rousseau et al., 2010), agricultural intensification levels (Decaëns and Jiménez, 2002) and AFS (Barros et al., 2003), including cacao AFS (Moço et al., 2010). However, only earthworms are scarcely used as soil quality indicators while other macrofauna were never mentioned in the “soil quality” literature (see Appendix). Given the essential role of macro-invertebrates in soil formation and nutrient cycling and their potential as a soil quality indicator, the conditions in which they may be used as indicators need to be clarified with particular respect to their relation to soil abiotic indicators.

Agroforestry systems have been increasingly promoted as sustainable productive systems for their continuous vegetation cover, diversification of production and effective nutrient cycling (Schroth et al., 2001, Hartemink, 2005). Their use is particularly relevant when soils have low agricultural potential and farmers have poor access to inputs. Therefore, simple and adapted methods of soil quality indication that include biologically mediated soil processes are required to enhance the management and promote the use of these systems. In Talamanca, the region is suffering forest fragmentation due to the cultivation of monocropped bananas and plantains. In this context, indigenous cacao-based AFS are good ecological connectors (Harvey and González-Villalobos, 2007, Harvey et al., 2006) and provide many of the forest ecological functions and associated services (Schroth and Harvey, 2007). Additionally, the forest matrix degradation severely erodes soil biodiversity (Mathieu et al., 2005, Rossi et al., 2010); thus, cacao AFS could act as sources or connectors to conserve soil biodiversity in the landscape.

We elaborated a framework to evaluate and compare soil quality among different cacao AFS and old-growth forest patches in Talamanca using a full indicator set (FIS) and a minimum indicator set (MIS) of physical and chemical soil indicators along with macrofauna groups (Lavelle et al., 2003). The specific objectives of this study were: (1) to define a minimum indicator set of soil quality based on the literature; (2) to classify cacao AFS and forest patches according to the minimum indicator set; (3) to select soil macrofauna groups that best explain abiotic soil quality indicators and evaluate their potential to indicate soil quality.