Towards net-zero chocolate: carbon and nutrient budgets for compost use on cocoa farms

Chocolate has one of the highest carbon footprints of all food types because cocoa is associated with deforestation. Cocoa established on recently cleared rainforest is initially productive, but then productivity declines as soils become depleted of nutrients. Further expansion of cultivated land by deforesting tropical rainforests is environmentally costly, socially unacceptable, and inherently unsustainable.

Therefore, strategies are urgently required to maintain and restore the productivity of existing farms.

Ghana is the second largest producer of cocoa globally. Most cocoa is grown on 1-5 hectare smallholdings. Inorganic fertilisers are expensive and unobtainable for smallholder cocoa farmers.

Average yields are low thereby impacting livelihoods and there is a large gap between potential and realised yield. When cocoa is harvested the beans are extracted from the husks on-farm and the husks are typically piled up and left to rot. Our preliminary research has demonstrated that making compost from cocoa pod husks can recycle nutrients and improve soil fertility.

Farmers in Ghana have started adopting this practice. However, considerable knowledge gaps remain that prevents farmers from adopting compost more widely or cocoa buyers from subsidising compost production.

The compost nutrient release characteristics are unknown and therefore the extent to which composts can maintain or improve cocoa yields and replace inorganic fertilisers is uncertain. Because compost addition increases soil organic matter content, it is likely that nutrient use efficiency improves over time if compost is applied annually, but this has not yet been investigated.

It is also unclear what the fate of nutrients (e.g. N, P, and K) applied in composts is in terms of losses via leaching and gaseous emissions and immobilisation in the soil. We hypothesise that the nutrient use efficiency will be affected by the nitrogen source that is used to compost the cocoa pod husks (e.g. poultry manure or leguminous tree prunings), and the ratios of the compost feedstock ingredients, because these influence C/N ratios, which in turn influences mineralisation and immobilisation.

Filling these knowledge gaps would enable us to quantify the role that cocoa pod husk composts could play in increasing cocoa yield and quality and calculating the carbon and nutrient budgets required to determine its potential to contribute towards net zero chocolate production.

The aim of this PhD project is to calculate carbon and nutrient budgets for cocoa pod husk compost use on cocoa farms and identify the optimum compost use strategy to reduce the carbon footprint of cocoa farms while maintaining yields. The project will leverage on 15 pre-existing field compost and biochar trials established in 2021 across different cocoa growing regions of Ghana during a recent GCRF/BBSRC-funded research project, currently supported with funding from a Licensed Cocoa Buyer.

Cocoa pod husk composts are applied at 5 rates annually and yields monitored. Objective 1 is to collect soil and bean samples from plots across the field experiments and quantify the relationship between soil amendments, soil chemical/physical properties, and cocoa yield/quality to establish the link between soil fertility and bean quality. Objective 2 is to establish a pot-scale experiment to quantify leaching and gaseous losses of nutrients under controlled conditions using isotopic probing approaches Soils will be incubated with composts made using different ratios of feedstocks (pod husks, poultry manure, leguminous tree prunings).

Objective 3 is to monitor greenhouse gas emissions and leachate, alongside key explanatory chemical, physical, and biological soil properties over time on selected experiments/plots in the field. These measurements, together, will allow the calculation of nutrient and carbon budgets. Objective 4 is to model soil C and N dynamics using established biogeochemical models