Restoration of soil health by Amazonian secondary forests is severely eroded by slash-and-burn recurrence

Secondary forest regeneration is a promising path to restore soil health in the Amazon, but slash-and-burn for agricultural reconversion may reverse decades of the soil's multifunctional progress. This study examined (i) how secondary forests rehabilitate soil multifunctionality after long-term agriculture in the eastern Amazon and (ii) the extent of soil functional loss if the forest is re-burned after two decades of ecological restoration. We investigated contiguous secondary forests at 2, 5, and 20 years, including post-slash-and-burn of the 20-year-old forest, all on soils with over 85 % sand on a small farm practicing shifting agriculture. We structured a soil health assessment using thirteen physical, chemical, and biological soil indicators, and calculated index scores based on soil functions. Secondary forests improved soil aggregation, cation exchange capacity, and nitrogen and carbon levels, enhancing soil's physical, chemical, and biological functions and increasing soil multifunctionality by 13 % over 18 years. While slash-and-burn increased available P and exchangeable Ca, Mg, and K, nutrient levels remained poor for cassava cultivation on the farm. Furthermore, nitrogen, cation exchange capacity, carbon, macrofauna indexes, and aggregate stability dropped to levels comparable to or lower than the 2-year-old forest, reducing the soil health score by 11 %. We conclude that secondary forests effectively restore soil multifunctionality in the eastern Amazon, but a single slash-and-burn recurrence reverses two decades of soil health recovery. We advocate educational and socioeconomic support for small farmers reliant on slash-and-burn, promoting sustainable agroforestry and compensating those who perpetuate secondary forests, given their soil health benefits demonstrated in this study.