Closing the nutrient loops in (peri-)urban farming systems through composting

Abstract

Organic amendments are used to improve soil fertility and maintain agricultural fields in a productive state. Despite these benefits, the use of organic amendments is limited in many developing countries. The overall objective of this thesis is therefore to provide a better understanding of current waste management practices in developing countries and ensure sustainable crop production via the biotransformation of urban waste into a high-quality soil amendment. First, I aimed at determining the causes for the limited use of organic amendments in small-scale urban farming systems. I interviewed 220 urban farmers in Ethiopia and found that competition for agricultural waste between fuel, feed and soil amendment is a major cause for the limited use of organic amendments. I demonstrated that allocation of agricultural waste for soil amendment is linked with farmers’ livelihood strategies. I also studied variation in compost demand among different farmer groups, and the socio-economic variables which explained these variations. Gaseous losses of ammonia and greenhouse gas (GHG) emissions occur during composting of nitrogen-rich urban waste. Several technologies could reduce these losses. However, these technologies are inadequate to fit within the broader farming systems because they are expensive. The second aim of this thesis was to develop low-cost methods to mitigate N losses and GHG emissions from composting, while retaining its fertilising value. Composting by earthworms (vermicomposting) is proposed as a low-cost strategy for minimising N losses and GHG emissions. Using a wide range of substrate qualities (C:N ratio, labile C sources) and other factors (earthworm density, amount of input, and moisture), I showed that vermicomposting reduced N losses and GHG emissions compared with traditional thermophilic composting, but the magnitude of the earthworm effect varied between substrates. Earthworms also change the quantity and composition of dissolved organic carbon during composting. Another low-cost strategy is to delay the addition of N-rich substrates during composting. I demonstrated that addition of nitrogen-rich substrate after the thermophilic phase reduced N losses. Delayed addition of N-rich substrates increased N2O emissions, but reduced CH4 emissions. Delayed addition resulted in compost that was as stable and effective at completely eradicating weed seeds as traditional composting. In conclusion, urban waste compost should be considered as alternative source for soil amendment, particularly in developing countries with competition for agricultural waste. Technologies such as vermicomposting and delayed addition of N-rich substrate are recommended to increase or maintain the nitrogen content of compost, reduce N losses and mitigate GHG emissions.