Fertilizer effects on the sustainability and profitability of agroforestry in the presence of carbon payments

Agroforests, where trees are planted with crops, are promoted as an appropriate and sustainable alternative to traditional agriculture as they meet landholder demands for food, income and wood products. This is essential in countries such as Indonesia where much of the landmass is not suitable for intensive cropping (either too steep or the soils are acidic and nutrient poor), yet population pressure and increasing food demand force many farmers to implement such practices on marginal land. These practices, combined with the lack of credit, quality seed and fertilizer have contributed to decreased soil fertility and increased deforestation and soil erosion. Previous research on agroforests suggests, however, that sustainability and profitability are not guaranteed and that the addition of fertilizer to the production system may be required. This is investigated for a hedgerow-intercropping system in the presence of carbon-sequestration payments using a bioeconomic modelling framework that incorporates the SCUAF simulation model. For the assumptions used in this study it was found that fertilizer improves the productivity and sustainability of the system; it is either optimal to grow crops (Zea mays) alone provided 225 kg ha 1 yr 1 of fertilizer is applied, or to completely convert to growing trees (Gliricidia sepium) if fertilizer is not used. This decision depends on the relative prices for crops and carbon. It was also found that there is an incentive for landholders to participate in a carbon-sink project when the previous land use has a continuously decreasing carbon stock, which is the baseline used to determine eligible carbon. Finally, compared with a more complex process model such as WaNuLCAS and data reported in the literature, the process model SCUAF consistently and reliably predicted biophysical interactions for various management regimes of tree/crop systems for use in complex economic analyses.