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.
Highlights
Once the litter layer and protective canopy cover of natural vegetation is removed by clearing land for agriculture, tropical soils are exposed to sunshine and harsher micro climatic conditions such as higher temperatures, lower relative humidity and lower soil moisture due to greater insolation (Hoffmann 2003)
We investigated the vertical distribution of fine root (\ 2 mm), soil organic carbon, soil aggregate stability and soil macroporosity as well as soil infiltration on the five different land use systems
At a depth of 10–20 cm, degraded forests had the highest Lrv, followed by complex agroforestry, annual crop field, simple agroforestry, and the lowest is in cocoa monoculture
Summary
Once the litter layer and protective canopy cover of natural vegetation is removed by clearing land for agriculture, tropical soils are exposed to sunshine and harsher micro climatic conditions such as higher temperatures, lower relative humidity and lower soil moisture due to greater insolation (Hoffmann 2003). These unfavourable conditions combined with changes in litter quality and quantity allow the rapid disappearance of any new surface litter. Soil organic matter is considered to be a key characteristic in judging soil
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