Environmental and economic impacts of agri-environmental schemes designed in French West Indies to enhance soil C sequestration and reduce pollution risks. A modelling approach

摘要

Agri-environmental schemes (AESs) are the main public policy instrument used in Europe to encourage farmers to adopt environmentally-friendly farming practises. Some AESs designed in French West Indies to replace N fertilizers with composts to reduce nitrate pollution and enhance C sequestration have been unsuccessful because few farmers adopted them despite the subsidies offered for the provision of environmental benefits. To explain this low adoption rate, we assessed the agri-environmental and economic impacts of two AESs and compare them with the most widely-applied strategy based on inorganic N fertilizer (NFER), and with an organic strategy based on sewage sludge (SLUD), a free organic amendment. The first AES was proposed in 2007 (AESold) and only concerned with the use of composts. The second was proposed in 2014 (AESnew) and combines the use of composts and inorganic N fertilizer at a rate 25% lower than NFER. The study was applied to water yam using a crop model to obtain agri-environmental indicators over a period of ten years, which were then used to calculate economic outputs for small and large farms. Although AESold increased C sequestration by 300% and reduced nitrate leaching by 80% compared to NFER, it also reduced yields (13%) and net income for farmers (30%). The subsidy offered by AESold did not compensate the loss of productivity, which explains its low rate of adoption. AESnew and SLUD increased C sequestration (350% and 400%) and reduced nitrate leaching (45% and 34%), and maintained yields and net income after five years of implementation. Yields and net income during the first five years were 5–10% lower than under NFER. Although the land area concerned by SLUD is limited because of regulatory constraints, AESnew could be a satisfactory policy instrument in French West Indies because it promotes environmental benefits and maintains economic income in the medium term for smallholder using family labour. The economic performance of AESs was lower for large farms; the adoption rate could be improved for these farmers through the implementation of mechanization to reduce labour costs. For both farm types, it may be necessary to increase subsidies during the first five years to offset yield losses during this period and the fixed and transition costs attached to adoption.