Short-term stability and function of charcoal in soil and its relevance to Ghanaian subsistence agriculture

Abstract

Maintaining an adequate level of soil organic matter and nutrients cycling is crucial to the success of any soil management in the humid tropics. Cover crops, compost, or manure additions have been used successfully to supply nutrients to crops but the benefits of such amendments are often short-lived in the tropics, since decomposition rates are high. This study focused on charcoal which, when utilised as a soil amendment is termed ‘biochar’. Throughout this thesis, the terms ‘charcoal’ and ‘biochar’ are used interchangeably depending on the context. Charcoal exhibits physiochemical properties potentially suitable for soil improvement as well as for the safe and long-term storage of carbon in the environment. As a way of investigating its recalcitrance as a carbon store, O:C ratios have been shown to reflect the extent of oxidation and therefore decomposition of charcoal. This study aimed to fill a gap in the research by describing the effects of biochar on the water retention capacity of soil under laboratory conditions. It also provides a detailed snapshot of the first ten years of recalcitrance under natural conditions, using X-Ray Photoelectron Spectroscopy (XPS) to determine elemental oxygen and carbon ratios of whole charcoal fragments and how these are affected by the surrounding environment over time. Mechanisms of oxidation are described, showing how both biotic and abiotic factors influence the degradation of charcoal in the soil. It also investigated how these properties affect the nutrient and water retention capability of charcoals of different ages in the laboratory. Results of charcoal/soil mixtures showed significant reduction in nitrate leachate losses with no reduction in performance over a ten year period of residing in the soil. It was also shown that charcoal addition to a sandy soil resulted in a significantly increased available water content. Both these results were argued to support the idea that charcoal is of potential beneficial amendment to sandy, degraded soils.