Soil organic matter in coconut-based agroforestry systems in Vanuatu : a key to sustainability?

Traditional Melanesian farming systems include slash-and-bum, short cultivation and fallow periods. The extension of coconut plantations has modified these systems and led to a variety of complex agroforestry systems that combine annual, forage, and perennial crops such as cocoa or mango with coconuts. As there is no chemical input or tillage in any of these systems, soil organic matter (SOM) plays a crucial role in soil fertility conservation. Moreover, in these systems, agroecological functioning is closely linked with soil fertility dynamics to ensure sustainability. In recent years, the reduction in the economic viability of copra production due to the decline in world market copra prices, together with the high population growth in the Vanuatu Archipelago, raises questions about the sustainability of these cropping systems. What are the long-term impacts on soil fertility? A synchronic approach was adopted to analyse temporal SOM patterns in different coconut-based cropping systems. SOM contents were assessed by near infra-red spectrophotometry (NIRS). High SOM contents were measured (mean 13% in the top soil). SOM appeared to be preserved in smallholder coconut-based cropping systems. A slight SOM decrease was observed after 20 years of continuous cultivation, except in agropastoral systems. A comparison with coexisting industrial systems using tillage and fertilizers is currently under study. A comprehensive analysis of SOM patterns in Melanesian traditional and industrial coconut-based farming systems is proposed and the sustainability of the various options is discussed. (Texte intégral