Can cocoa agroforestry restore degraded soil structure following conversion from forest to agricultural use?

Abstract

Alternating degradation and restoration phases of soil quality, as is common in crop-fallow systems, can be avoided if the restorative elements of trees and forests can be integrated into productive agroforestry systems. However, evidence for the hypothesis of ‘internal restoration’ in agroforestry is patchy and the effectiveness may depend on local context. We investigated to what extent cocoa (Theobroma cacao, L.) agroforestry can recover soil structure and infiltration in comparison to monoculture systems across the Konaweha Watershed, Southeast Sulawesi. We compared soil organic carbon, fine root length and weight, soil aggregate stability, macroporosity and infiltration from three soil layers at five land use systems: i.e. degraded forests, 9–14 years old of complex-cocoa agroforestry, simple-cocoa agroforestry, monoculture cocoa and 1–4 years old annual food crops, all with three replications. In general, roots were concentrated in the upper 40 cm of soil depth, contained of 70% and 86% of total fine root length and weight. Compared to simple agroforestry and cocoa monoculture, complex agroforestry had greater root length and weight in the topsoil, even though it attained only half the values found in degraded forests. Higher root density was positively correlated to soil organic carbon. In upper soil layers, complex agroforestry had slightly higher soil aggregate stability compared to other agricultural systems. However, no significant difference was found in deeper layers. Complex agroforestry had higher soil macroporosity than other agricultural systems, but not sufficient to mimic forests. Degraded forests had two times faster steady-state soil infiltration than agricultural systems tested (13.2 cm h−1 and 6 cm h−1, respectively), relevant during peak rainfall events. Compared to other agricultural systems, complex agroforestry improves soil structure of degraded soil resulting from forest conversion. However, a considerable gap remains with forest soil conditions.

Soil Organic Matter, Mitigation of and Adaptation to Climate Change in Cocoa–Based Agroforestry Systems

“…At the interface of aboveground and belowground effects and contributions to both mitigation and adaptation climate change agendas, cocoa agroforestry can indeed be 'climate-smart', contributing to resilience and policy synergy [74]. As detailed in a recent study in Southeast Sulawesi [75], the improvements of cocoa agroforestry relative to cocoa monoculture are modest steps in the direction of physically restoring forest soil conditions.…”

Climate change adaptation in and through agroforestry: four decades of research initiated by Peter Huxley

“…However, root-mediated mineralisation (for example by phosphatase release) may work in the opposite direction. Whilst most of the literature on AF and C sequestration stays at the level of tree planting (Kumar and Nair 2011;Anderson and Zerriffi 2012;Jose and Bardhan 2012;Chapman et al 2020) or measured soil C stocks (Shi et al 2018;Chatterjee et al 2018;Corbeels et al 2019), some studies have delved deeper into the processes of root-aggregate interactions (Albrecht and Kandji 2003) and simultaneous effects on soil macroporosity (bulk density) and C org concentrations (Hairiah et al 2020a, b;Saputra et al 2020). Other studies have addressed the lack of visibility of trees in agricultural lands in current C accounting systems at the national scale (Rosenstock et al 2019a, b, c), as precondition for economic incentives to reach the farmgate.…”

Balancing litterfall and decomposition in cacao agroforestry systems

“…In annual cropping systems, in contrast, the litter layer is not permanent as a consequence of intensive management, which has been linked to a decline in soil fertility (Asase et al 2008). Belowground carbon (C) stocks in annual cropping systems are around 40% lower than in forests (Hairiah et al 2020), due to low root weight and low turnover in annual cropping systems (Saputra et al 2020), and low C input into the soil through litter decomposition. In a mixed system such as cacao agroforestry, the litter layer consists of a mixture of leaves from cacao and various shade tree species.…”

“…Complex agroforestry systems usually have a closed, multi-strata canopy compared to cacao monocultures. Complex cacao agroforests improve the structure of degraded soils resulting from forest to agriculture conversion (Saputra et al 2020), and increase the soil water buffering capacity, which enhances the drought tolerance of the system (Gusli et al 2020). Shade trees in agroforestry systems can also improve soil fertility (Wartenberg et al 2019).…”