Shifting cultivation on the tidal floodplains of Amazonia: impacts on soilnutrient status

Studies of the impact of traditional agroforestry practices on soil nutrien t status in the Amazon Basin have largely been limited to upland sites, man y of which are oligotrophic. However, rural population density in Amazonia is highest on the floodplains of the major rivers and shifting cultivation has been practiced on the floodplains for millennia. We established a slash -and-burn cultivation plot and an untreated forest plot in tidal floodplain successional forest in the Islands of Abaetetuba, Para, Brazil, and sample d soils (0-10 cm) during four phases of the agroforest cycle (n = 9 in each plot for each sampling period): (1) prior to clearing, (2) following burni ng, (3) harvest one (rice at four months), and (4) harvest two (sugar cane at 15 months). During the course of this pilot experiment, background tempo ral variability in SOC, total N and available mineral nutrient pools (P, K, Ca, Mg) exceeded treatment effects, suggesting that soil nutrient pools in this environment are well-buffered against the effects of shifting cultiva tion. These results contrast markedly with those reported for Amazonian upl and sites where nutrient pulses and declines associated with the agroforest cycle have been demonstrated and may restrict the potential for low-input, continuous cultivation. Physical limitations imposed by flooding and fluvi al erosion, rather than nutrient constraints, restrict the potential of agr icultural intensification in the tidal floodplains.