Vienna, Austria – Chocolate is the food of the gods. The name of the tree from which the confection originates, Theobroma cacao, combines the Greek words for god (theós) and food (brôma). This small evergreen tree grows in tropical forests rich in both biodiversity and carbon. Over the past 40 years, demand for chocolate, the product of processing cacao pods, has surged, says Isabella Steeley, a doctoral student at the University of Sheffield in England. With that demand, she says: "more cocoa needs to be produced."
Meeting it often means clearing tropical forests, replacing diverse, carbon-dense ecosystems with cacao plantations. Another path is to increase yield on existing farms. Average cacao yields are about 480 kilograms per hectare, but potential yields may be ten times greater, Steeley says.
In a study to be presented at the European Geosciences Union General Assembly this week, Steeley examines whether enhanced rock weathering can improve soil fertility and yields in two cacao systems in Brazil's degraded, fragmented Atlantic Rainforest. One system involves reforesting degraded pastures with rows of cacao and shade trees in a commercial farm. The other, the traditional cabruca system, intersperses cacao trees within native forest. It preserves more forest than plantations, but is less productive, a compromise between yield and biodiversity, Steeley says.
In intact forests, plants recycle nutrients: roots suck them from the soil while organic matter replenishes them. Clearing trees breaks that cycle. Because of the heat and rainfall, tropical soil isn't very sticky, which results in low nutrient retention. Moreover, soil become acidic, dropping uptake of already limited nutrition. Once cacao trees become established, their yields typically decline after a couple decades, while acidity facilitates the uptake of toxic elements like aluminum or cadmium. Enhanced rock weathering involves adding finely crushed rock: here, an andesitic basalt dust produced in Brazil. As the rock dust weathers, it neutralizes acidity, improving nutrient availability and potentially supplying essential elements to crops. And, it removes atmospheric CO2.
Steeley will share results from the first two years of a three-year study. Soil improvements were strongest in the commercial cacao farm, suggesting such farms could help reconnect fragmented rainforest.
Her team also introduces a new way to quantify how much rock dust has weathered, enabling calculation of how much carbon has been sequestered. Early results indicate that cabruca soils may capture more CO₂ than commercial farms through enhanced rock weathering, raising the possibility that smallholders could fund soil amendment costs by selling carbon credits.
Because most cacao is produced on farms smaller than 50 hectares, improving yields could directly benefit local communities. "This work is a collective effort, with local farmers and agronomists supporting the research," Steeley says. Farmers are "really excited about any kind of innovation that can help sustain their livelihoods."
Text written by Alka Tripathy-Lang.
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