ABSTRACT
The concentration of carbon dioxide (CO2) and other greenhouse gases (GHGs) in the atmosphere has increased considerably over the last four decades. This increase primarily results from the burning of fossil fuels and the conversion of tropical forests to agriculture, with concomitant negative impacts upon the global climate. Agricultural activities account for about 13.5 per cent of total anthropogenic GHG emissions (Rogner et al, 2007) and release mainly nitrous oxide (N2O) and methane (CH4) (about 45 per cent of agricultural GHG emissions each), with CO2 accounting for the remaining share (Baumert et al, 2005). Agricultural N2O and CH4 emissions are expected to increase by 35 to 60 per cent in 2030 due to increased nitrogen fertilizer use, animal manure production and livestock numbers. In contrast, CO2 emissions are likely to remain at the same level due to stable or declining deforestation rates, and increased adoption of conservation tillage practices (Smith et al, 2007). Mitigating agricultural GHGs can be achieved by reducing emissions through more efficient management of carbon (C) and nitrogen (N) flows and by enhancing C storage in soil and vegetation (Smith et al, 2007). Agroforestry systems (AFS) are one means by which the impacts of climate change can be mitigated. The role that AFS play can be increased through payment for ecosystem services (PES) systems that reduce agricultural emissions and increase the quantity of carbon stored. Carbon sequestration (or atmospheric CO2 removal) as an ecosystem service of AFS is generally quantified as the amount of C stored in trees. Nevertheless, increasing tree density in agroforests may also modify soil fluxes of N2O or CH4, which have a global warming potential (GWP) 298 and 25 times higher than CO2, respectively (Forster et al, 2007). Nitrogen-fixing species used as shade trees (e.g. in coffee plantations) may increase soil emissions of N2O (Hergoualc'h et al, 2008) and reduce the soil CH4 sink (Palm et al, 2002).
