The Natural Fix?

As land under tropical agriculture occupies a wide range of soil types and climates, the capacity for carbon sequestration can differ considerably. In hot and dry areas where soil has been de- graded, implementation can restore carbon and prevent further losses. In humid climates the potential for carbon sequestra- tion can reach one tonne per ha. According to some estimates, degraded soils represent half of the world’s carbon sequestra- tion potential (Lal 2004a). One management practice with a high potential for carbon sequestration in tropical areas is agroforestry. In agroforest- ry systems, food production is combined with tree planting. Because of the trees, agroforestry systems store more carbon as plant biomass and have a higher potential for soil carbon sequestration than conventional agricultural systems (Nair et al. 2009). Biodiversity benefits may also be realised. Aver- age carbon storage by agroforestry practices is estimated at around 10 tonnes per ha in semi-arid regions, 20 tonnes per ha in sub-humid and 50 tonnes per ha in humid regions, with sequestration rates of smallholder agroforestry systems in the tropics being around 1.5–3.5 tonnes of carbon per ha per year (Montagnini and Nair 2004). In addition, agroforestry systems can reduce the pressure on natural forests thereby having indirectly a positive effect on carbon storage in the lat- ter (Montagnini and Nair 2004). However, as with conventional agricultural systems, sustain- able management practices also need to be adopted in agrofor- estry systems to ensure carbon sequestration and sustainable water use. In some systems, interference interactions between crop species and trees planted as part of agroforestry measures may have a negative impact on crop yields (Garcia-Barrios 2003). In these circumstances, compromise solutions may be best, aiming to store reasonable rather than maximum amounts of carbon while still ensuring profitability from crops (Verchot et al. 2005).

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