Soil phosphorus availability after slash-and-burn fires of different intensities in rubber agroforests in Sumatra, Indonesia

Land clearing fires in Sumatra, Indonesia, caused enormous environmental problems for southeast Asia in 1997, but rubber farmers as well as large rubber and oil palm plantation owners continued to slash-and-burn due to the lack of an alternative that is equally quick, cheap, and capable of improving soil fertility. A partial alternative may be found in the reduction of the fuel load before the burn by harvesting and selling a larger fraction of the wood and, thus, changing the intensity of the fire and reducing particulate and greenhouse gas emission. An adequate phosphorus (P) supply is critical to crop production in Sumatra. Thus, it is important to understand the effects of such a reduction in fire intensity on crop P availability. Field and laboratory experiments were conducted to: (1) determine P sorption capacities and affinity constants for a forest soil exposed to different fire intensities in a controlled oven experiment (heat effects only); (2) compare the sorption characteristics of oven-burned soil with field-burned soil (heat and ash addition effects); and (3) determine the effects of fire-induced changes in soil properties on indices of P availability. Sorption experiments using oven-heated forest soil showed an increase in both the maximum amount of P it can hold (P sorption capacity, X(m)) and the strength with which this P is retained to soil particles (affinity constant, K) with heating to 450 degrees C. Field burning resulted in similar increases in X(m) but reduced the affinity constants in the surface 15 cm soil. The increase in X(m) was related to a fire-induced increase in specific surface area of the mineral fraction. The addition of ash in field burns appeared to be responsible for the observed decrease in K. The increase in X(m) is expected to enlarge the need for P fertilizers as soon as solution P declines to pre-burn levels. These mineralogy-based changes in P sorption characteristics of surface soil are expected to be long-term and could reduce the time period for sustained annual crop production, establishment of new rubber plantations from seeds, as well as affect soil loss and sustained rubber production. In managing the intensity of slash-and-burn fires, the farmer, thus, has to balance between short-term gains in P availability and long-term costs in increased P sorption.