Impacts of repeated forest fires and agriculture on soil organic matter and health in southern Amazonia

Abstract

The Arc of Deforestation, a Cerrado-Amazon transition region, faces large-scale agricultural expansion and land degradation, fostering fire spread and recurrence, particularly during extreme droughts driven by climate change. While previous research has focused on aboveground changes, the impacts of fires on soil functions, such as carbon (C) storage and nutrient cycling, remain understudied. This study investigated how forest-to-agriculture conversion and fire frequency in standing forests affect soil C and nitrogen (N) in the Arc of Deforestation. We also examined the effects of these disturbances on stable isotopes (δ¹³C and δ¹⁵N) and how the changes in Soil Organic Matter (SOM) influence other soil health indicators. Four treatments were considered: (i) annual forest burns; (ii) forest burning every 3 years; (iii) undisturbed forest; and (iv) long-term agriculture. Annual burns depleted soil C stocks (0-30 cm) by 16 %, triennial burns by 19 %, and long-term agriculture by 38 %, compared to the undisturbed forest. Annual and triennial burns reduced aboveground C stocks by 40 % and 60 %, respectively, showing that fire impacts on soil and vegetation differed. In burned forests, δ¹³C indicated no grass invasion, remaining below −27 ‰ and rising slightly (∼ 2 ‰) with depth. In agricultural areas, the δ¹³C was uniform (−24 ‰) due to C₄ plants’ influence and soil mixing from tillage. The relationships between soil C and health indicators suggested that SOM depletion from fire and agriculture disrupted soil structure and negatively affected nutrient cycling. Our findings revealed long-lasting fire legacies in tropical forest soils, with SOM loss and harmful effects on soil health persisting nearly a decade after the fire, but no evident effect of fire frequency was found. Soil degradation was less severe when the forest was repeatedly burned than when converted to agriculture. Conservation and management practices prioritizing forest protection, wildfire prevention, and halting agricultural expansion are needed to preserve soil health in the Amazon.