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During the past three decades, agroforestry has become recognized the world over as an integrated approach to sustainable land use because of its production and environmental benefits. Its recent recognition as a greenhouse gas-mitigation strategy under the Kyoto Protocol has earned it added attention as a strategy for biological carbon (C) sequestration. The perceived potential is based on the premise that the greater efficiency of integrated systems in resource (nutrients, light, and water) capture and utilization than single-species systems will result in greater net C sequestration. Available estimates of C-sequestration potential of agroforestry systems are derived by combining information on the aboveground, time-averaged C stocks and the soil C values; but they are generally not rigorous. Methodological difficulties in estimating C stock of biomass and the extent of soil C storage under varying conditions are compounded by the lack of reliable estimates of area under agroforestry. We estimate that the area currently under agroforestry worldwide is 1,023 million ha. Additionally, substantial extent of areas of unproductive crop, grass, and forest lands as well as degraded lands could be brought under agroforestry. The extent of C sequestered in any agroforestry system will depend on a number of site-specific biological, climatic, soil, and management factors. Furthermore, the profitability of C-sequestration projects will depend on the price of C in the international market, additional income from the sale of products such as timber, and the cost related to C monitoring. Our knowledge on these issues is unfortunately rudimentary. Until such difficulties are surmounted, the low-cost environmental benefit of agroforestry will continue to be underappreciated and underexploited.

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Litter plays a vital role in the nutrient cycling of plantations and agroforests. Silvicultural interventions can alter litter production and decay rates, thereby varying nutrient fluxes. We evaluated the effect of various thinning densities on litter dynamics of 9-year-old Acacia mangium Willd. stands. To quantify litterfall, we placed traps at four random grid points in 24 plots in which none, one-third, one-half, or two-thirds of stems had been removed. In each plot, 48 litterbags were also placed to evaluate litter decay. Annual litterfall ranged from 5.73 (two-thirds thinning) to 11.18 Mg·ha−1 (unthinned) and showed a significant linear relationship to basal area (p < 0.0001). Nitrogen (N), phosphorus (P), and potassium (K) concentrations were highest during the wet season, when litterfall production was low, implying an inverse relationship between litterfall quality and quantity. The highest annual N, P, and K additions (82.9, 3.3, and 71.9 kg·ha−1, respectively) occurred in the unthinned stands. High thinning intensities resulted in accelerated decay rates, which we attribute to changes in microenvironment. Soil N concentrations were highest in the one-half thinning treatment, followed by the two-thirds treatment, signifying a trade-off between litterfall production and decay. The highest soil organic C concentrations were in the unthinned stands, reflecting the potential of high stand densities for promoting C sequestration.

Conservation of biodiversity and mitigation of global warming are two major environmental challenges today. In this context, the relationship between biodiversity (especially plant diversity) and soil carbon (C) sequestration (as a means of mitigating global warming) has become a subject of considerable scientific interest. This relationship was tested for homegardens (HG), a popular and sustainable agroforestry system in the tropics, in Thrissur district, Kerala, India. The major objectives were to examine how tree density and plant-stand characteristics of homegardens affect soil C sequestration. Soil samples were collected at four depths (0-20, 20-50, 50-80, 80-100 cm) from HG of varying sizes and age classes, and their total C content determined. Tree density and plant-stand characteristics such as species richness (Margalef Index) and diversity (Shannon Index) of the HG were also determined. Results indicated that the soil C stock was directly related to plant diversity of HG. Homegardens with higher, compared to those with lower, number of plant species, as well as higher species richness and tree density had higher soil carbon, especially in the top 50 cm of soil. Overall, within 1 m profile, soil C content ranged from 101.5 to 127.4 Mg ha -1 . Smaller-sized HG (\0.4 ha) that had higher tree density and plant-species density had more soil C per unit area (119.3 Mg ha -1 ) of land than larger-sized ones ([0.4 ha) (108.2 Mg ha -1 ). Soil C content, especially below 50 cm, was higher in older gardens. The enhanced soil-C storage in species-rich homegardens could have relevance and applications in broader ecological contexts.