Volume 8, Number 33: 17 August 2005
In a recent review of the pertinent scientific literature, Mutuo et al. (2005) discuss the potential for agroforestry to sequester carbon and mitigate greenhouse gas emissions from soils in the tropics, where they define agroforestry as "land-use systems in which woody perennials are grown in association with herbaceous plants (crops, pastures) or livestock, in a spatial arrangement, a rotation, or both." The rationale for the study resides in the fact, as they describe it, that "intensified agricultural practices lead to a reduction in ecosystem carbon stocks, mainly due to removal of aboveground biomass as harvest and loss of carbon as CO2 through burning and/or decomposition," such as occurs in response to tillage operations. In addition, they report that upland forests typically consume methane, and that "conversion of tropical forest soils to agriculture in general has been shown to reduce the sink strength for methane (Keller et al., 1990; Keller and Reiners, 1994; Steudler et al., 1996; Verchot et al., 2000)."
How significant are these phenomena?
Mutuo et al. report that "conversion of forest and pastures to continuous cropping systems has consistently led to declines of 50-60% in soil C [carbon] stocks," citing the work of van Noordwijk et al. (1997) and Guo and Gifford (2002). In addition, they say that carbon losses that result from converting natural forests to logged forests in Brazil, Indonesia and Cameroon range from 100-150 Mg C ha-1, citing Palm et al. (2000) and Hairiah et al. (2001), and that further conversion to continuous cropping or pasture systems leads to the loss of almost all aboveground C stocks and about 25 Mg C ha-1 from the soil organic matter pool in the top 20 cm of soil.
Reversing the process, Mutuo et al. write that "if croplands and pastures were rehabilitated through conversion to tree-based systems, this would certainly result in net aboveground C sequestration and also in belowground C in the case of conversions from cropland," suggesting that resulting carbon sequestration would range from 10-70 mg C ha-1 in the vegetation and 5-15 Mg C ha-1 in the topsoil over a period of 25 years, citing the work of Murdiyarso et al. (2000), Palm et al. (2000) and Hairiah et al. (2001). They also report "there is evidence that roots in agroforestry systems can have a time-averaged C stock ranging from about 6 Mg C ha-1 for shifting cultivation to about 20 mg C ha-1 for tree fallows in the top 0-50 cm soil depth."
In the case of the major greenhouse gas methane (CH4), Mutuo et al. say that data from different countries confirm that upland primary and secondary forests are CH4 sinks, with a mean CH4 consumption rate of 30 µg C m-2 h-1, citing the work of Tomich et al. (1998), Palm et al. (2002) and Tsuruta et al. (2000). They also note that agricultural systems can decrease the sink strength by 50% or more ... or even produce a net flux of CH4 to the atmosphere.
How might these phenomena be impacted by the ongoing rise in the air's CO2 content? Interestingly, woody plants of the type that are grown in agroforestry operations are some of the most positively-responding plants to atmospheric CO2 enrichment. Consequently, agroforestry makes more sense than ever in a CO2-accretting atmosphere, as it capitalizes on the growth-promoting prowess that trees experience when exposed to this productivity promoting and water- and nutrient-use efficiency enhancing trace gas of the atmosphere, which benefits its practitioners economically while enabling the operation's woody plants to slow the rate-of-rise of the air's CO2 and CH4 concentrations and thereby reduce the impetus for further greenhouse gas-induced global warming.
Sherwood, Keith and Craig Idso
References
Guo, L.B. and Gifford, R.M. 2002. Soil carbon stocks and land use change: a meta analysis. Global Change Biology 8: 345-360.
Hairiah, K., Sitompul, S.M., van Noordwijk, M. and Palm, C.A. 2001. Carbon stocks of tropical land use systems as part of the global carbon balance: effects of forest conversion and options for clean development activities. Alternatives to slash-and-burn (ASB) Lecture Note 4. ICRAF, Bogor, Indonesia.
Keller, M., Mitre, M.E. and Stallard, R.F. 1990. Consumption of atmospheric methane in soils of Central Panama: Effects of agricultural development. Global Biogeochemical Cycles 4: 21-27.
Keller, M. and Reiners, W.A. 1994. Soil-atmosphere exchange of nitrous oxide, nitric oxide, and methane under secondary succession of pasture to forest in the Atlantic lowlands of Costa Rica. Global Biogeochemical Cycles 8: 399-409.
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Mutuo, P.K., Cadisch, G., Albrecht, A., Palm, C.A. and Verchot, L. 2005. Potential of agroforestry for carbon sequestration and mitigation of greenhouse gas emissions from soils in the tropics. Nutrient Cycling in Agroecosystems 71: 45-54.
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