High Carbon Sequestration Potential of Tropical Agroecosystems

A recent IPCC study on land use, land-use change, and forestry (Watson et al., 2000) documented the large potential for tropical agroecosystems to sequester carbon. The tropics have two major advantages over the temperate regions. Trees grow faster under high year-round temperatures and high solar radiation. In addition, many tropical soils are depleted of carbon because of unsustainable land use practices. Table 1.2 illustrates the potential for carbon sequestration in the tropics.

Land use intensification practices usually start from a high carbon stock base, resulting in annual sequestration

Table 1.2 Carbon Sequestration Rates and Annual Potential of Agricultural Practices by 2010

Carbon Sequestration Rate

Practice (tons C/ha/year)

Land Use Intensification (Global)

Croplands (reduced tillage, 0.36 rotations, cover crops, fertilization, and irrigation)

Forest lands (forest regeneration, 0.31 better species, silviculture)

Grasslands (better herds, woody 0.80 plants, and fire management)

Lowland rice production 0.10

Land Use Change (Tropics)

Agroforestry (conversion from 3.10 unproductive croplands and grasslands at humid tropical forest margins, and by replenishing soil fertility in subhumid tropical Africa)

Improved pastures in subhumid 2.8 without a tropical South America (conversion from native pasture to deep-rooted improved grasses and legumes)

Tropical deforestation legume, 7.C with a legume

Annual Potential by 2C1C (million tons C/year)

-1644

Source: From Watson, R.T., I.R. Noble, B. Bolin, N.H. Ravindranath, D.J. Verardo, and D.J. Dokken. 2000. Land Use, Land-Use Change and Forestry. Cambridge University Press, London; New York. With permission. Data also provided by M. Fisher, personal communication, 2003.

rates of tenths of a metric ton per hectare, in both tropical and temperate regions. Because of the large areas, the total carbon sequestration potential by 2010 ranges from 50 to 168 Mt (million metric tons) per year, except for paddy rice production because of methane emissions associated with it.

Transforming unproductive tropical croplands or grasslands into highly productive agroforestry and improved pasture systems results in annual carbon sequestration rates of a higher order of magnitude. Trees are periodically harvested in agroforestry systems. Thus, these calculations refer to time-averaged carbon, which takes into account carbon removals associated with harvesting (Palm et al., 1999). The high sequestration rates in these land-use change categories are due to a drastic increase in biomass production. Either originally fallow lands have lost much of their system carbon stock in the agroforestry systems (Sanchez and Jama, 2002) or a new sink of carbon has been developed in the subsoil (Fisher et al., 1994). Given the large areas to which these conditions apply, the overall potential for additional carbon sequestration is huge. Conversion to tropical agroforestry has the potential to soak up 390 Mt of carbon per year, equal to about one-fifth of annual carbon emissions of the United States from all sources.

The importance of avoiding further deforestation is evident from data shown in Table 1.2. The magnitude of emitted carbon is enormous, and avoidance of such emissions by preventing deforestation will play a major role in the global carbon cycle.

The magnitude of carbon sequestration in developing countries through systems described in Table 1.2 depends to a major extent on rainfall regimes. The carbon sequestration potential per hectare of such systems is lowest in the semiarid tropics and highest in the humid tropics, with the subhumid tropics in between (Schroeder, 1994). Hotspots could be identified at a similar scale of resolution as the well-established biodiversity hotspots. However, there are tradeoffs between carbon sequestration per hectare, and the number of hectares that can be put into such systems. The carbon sequestration potential in the Sahel is in the range of 0.25 to 0.05 tons of carbon per hectare per year (Sloger, 2003), or one-tenth of what land use change with legume-based pastures or agro-forestry can yield in the subhumid and humid tropics. However, there are large areas of degraded lands in the dry areas stretching from Morocco to Mongolia for which land use change could make a major difference, even if the sequestration rates are low on a per hectare basis.

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