Fire-free alternatives to slash-and-burn for shifting cultivation in the eastern Amazon region: the role of fertilizers

Forest fires in slash-and-burn agriculture rank high in the list of environmental concerns. The objective of this study in Pará State, Brazil, was to evaluate fire-free alternatives to burning of the slashed fallow vegetation for crop production in the humid tropics. Burning, mulching and incorporation of fallow biomass were compared in two successive cropping periods with rice, cowpea and cassava, with and without NPK fertilizer. In the traditional system, with slash-and-burn technology and without fertilizer, the shortening of the fallow period from 10 to 4 years caused a reduction of 30% only in the rice yields of the first cropping period. In the second cropping period, rice yields were lower in the area with 10 years of fallowing and similar with the shorter fallowed area when compared to the first season. Cowpea did not show any differences, whereas cassava yields decreased from the first to the second cropping period in both fallow situations. When fertilizers were applied, fallow periods beyond 4 years and doubled cropping periods appeared to have little effect on yields. Transforming the fallow vegetation into wood chips instead of slash burning reduced rice yields in the first cropping period 47 and 80% after 4 and 10 years of fallowing, respectively, and made cowpea production impossible after 10 years of fallowing. Cassava yields were not affected by fire-free land preparation. Application of fertilizers raised yields from around 0.7 t ha −1 to over 2 t ha −1 for rice, and from around 0.2 t ha −1 or less to around 1.5 t ha −1 for cowpea. Fertilizer use was essential to obtain acceptable yields for the two crops under fire-free land preparation. Residual fertilizer doubled cassava yields. Comparing the two successive cropping periods, rice and cowpea yields increased, on average, 47 and 27%, respectively, in the second cropping period, irrespective of fallow age, but with fertilizer application. Over the entire cropping period, use of fertilizer led to a greater exploitation of soil N and K but not of P. The risk of soil degradation due to intensified cropping by shortened fallow or prolonged cropping periods is considered low, provided that sufficient plant material and nutrients are supplied to the soil by fire-free land preparation and fertilization.