Sustainability assessment of slash-and-burn and fire-free agriculture in Northeastern Pará, Brazil

Slash and burn agriculture, largely dependent on the duration of the fallow period to restore the productivity of the land, is extensively practiced by small landholders in Pará State, Brazil. Due to a mounting demographic pressure, fallow periods have been shortened, and signs of agronomic and ecological failure such as decreasing crop yields and structural and compositional depletion of the secondary vegetation have been observed. In order to circumvent these problems, fire-free agricultural management practices have been proposed. Among these practices, the most important are the (i) use of a bush-chopper for mulching (instead of burning) the biomass, (ii) enrichment of the secondary vegetation with nitrogen-fixing, fast growing trees to improve nutrient recovery/fixation during the fallow period, and (iii) soil fertilization following mulching, for optimal crop absorption of nutrients. A system overview of these management practices shows that most of the inputs for production correspond to free contributions from the environment. The release of nutrients contributed by the slash-and-burn operation represents a major input to yield, reaching almost one half of all nonrenewable contribution in this system. As this major input is replenished by fallow, with no requirements from the economy, the slash-and-burn system shows a larger emergy yield ratio. On the other hand, the input of fertilizer required in the fire-free management, added to the purchased labor and services needed for mulching, result in a higher environmental loading ratio, and a smaller emergy yield ratio for this system. Hence, due to dependency on purchased inputs, the fire-free management shows a smaller sustainability index. In order to meliorate the adoption perspective of fire-free agriculture, incentives could be paid for soil organic matter buildup, as compensation to carbon sequestration under a policy scenario of global climate change prevention.