Chemical dynamics in slash-and-burn agriculture

Abstract

The stability of the slash-and-burn system is more a function of the total nutrient stock of the entire ecosystem than the net gain of the soil after slash-and-burn. During the first cycle of land exploitation, slash-and-burn releases a large portion of nutrients stored in the above-ground biomass into the soil. Therefore, the gain of nutrient stock in the soil is at the expense of the plant biomass. Published data on slash-and-burn systems are mostly concerned with the changes in soil nutrient status during cropping. Studies dealing with the dynamics of total nutrient stock in the primary forest ecosystem and the subsequent cropping and fallow cycles are scarce. Conservation of the total mineral nutrient stock of the slash-and-burn system, starting from the clearing of a primary forest followed by burning, cropping and secondary forest fallow, is possible only in an ideal case of shifting cultivation — the equilibrium model. A significant portion of mineral nutrients released from burning may be lost either through erosion and runoff or through leaching (i.e. K, Mg, Ca, nitrate, and sulfate). A portion of the plant nutrients will be removed in harvested crops. Thus, the total mineral nutrient stock in the whole ecosystem gradually declines during the subsequent cycles of fallow and cropping — the depletion model. For various social and economic reasons, slash-and-burn agriculture is being rapidly replaced by new or modified crop production systems throughout the tropics. In strongly weathered soils, external inputs of mineral nutrients are required, particularly, those aiming at sustaining high levels of crop or pasture productivity. Although nitrogen can be replenished through biological N-fixation, other nutrients, especially P and K, must be supplied by external sources. Therefore, a sustainable alternative to the slash-and-burn system should be judged not only by the stability of crop yield but also by the efficiency of the agroecosystem to recycle mineral nutrients as a function of time.