Introduction

Global climate change is a result of high atmospheric concentration of greenhouse gases (mainly CO2, N2O, CH4, and flu-orochlorocarbons). Experts predict an increment of the average global temperature of 3°C to 5°C by the end of the present century if the accumulation of these greenhouse gases

(GHGs) continues (Intergovernmental Panel on Climate Change [IPCC], 2001).

Agricultural activities take place on approximately 35% of the world's land. Land use and changes in land use constitute an important source of GHGs, being responsible for 25% of CO2, 50% of CH4, and 70% of N2O emitted by all human activities (Agriculture and Agri-Food Canada, 1998; IPCC, 2001). The adoption of new management practices by farmers may help lower global GHG emissions associated with food production. It has been demonstrated that beneficial management practices (BMPs) help to restore air quality by converting atmospheric carbon (C) and nitrogen (N) into soil organic matter (SOM) (Agriculture and Agri-Food Canada, 1998). Carbon constitutes nearly 58% of the SOM, and improves the physical, chemical, and biological conditions of soil, and favors higher yields in crops (Herrick and Wander, 1998).

Little information exists on soil carbon stored under different climate conditions and land use systems, and on changes in soil organic carbon (SOC) content induced by management practices on sloping land or on hillside agriculture. Also, a standard methodology to monitor SOC changes in hillside farming systems does not exist. Studies conducted around the world indicate it is possible to capture atmospheric C in soils through BMPs that increase the aboveground dry matter, and reduce both soil erosion and the microbial oxidation of SOM. These practices include minimum tillage, crop rotation with legumes, cover crops, afforestation, tree barriers, pastures, and the addition of compost and animal and vegetal waste (Lal, 1998). Mexico has great potential to capture C in original forestland and hillside landscapes that are now being cultivated. Simulation models have been developed to estimate changes in C stocks; however, these models have not been validated for hillside agriculture conditions.

Concern expressed by national and international agencies about global climate change and certain agricultural activities practiced by rural communities on hillsides in Mexico, led to the establishment in 1999 of the Sustainable Management on Hillside Agriculture Project (PMSL) in three socially marginal regions in Oaxaca State (Mazateca,

Cuicateca, and Mixe). This project was sponsored by the Mexican Ministry of Agriculture, Livestock, Rural Development, Fisheries, and Foodstuffs (SAGARPA), Oaxaca State, and the Global Environment Facility with the World Bank. Several indigenous groups reside in the selected target areas under conditions of severe poverty. The objectives of this project were (1) increase understanding of C stocks associated with secondary-growth forest and agricultural hillside systems managed under traditional and modern agricultural practices, (2) determine actual soil fertility conditions, and (3) characterize the spatial variability of SOC and of soil properties associated with soil fertility.

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