Forest fires and land use change compromise soil biological health in the Amazon: insights from soil organic carbon and enzymatic activities

Forest fires and land use change represent major threats to the Amazon biome. While aboveground effects have been extensively studied, soil impacts remain poorly understood. We analyzed how forest burning and forest-to-pasture conversion affect soil organic carbon (SOC) and enzymatic activities (arylsulfatase and beta-glucosidase) as ecosystem disturbance indicators in two key Amazon regions: Manaus (AM) and Santarém (PA), Brazil. We evaluated topsoil (0-10 cm) of primary forests (controls), burned forests and pastures using a four-quadrant framework integrating short-term (enzymes) and long-term (SOC) indicators. This approach classifies treatments into four health states relative to reference conditions: healthy (high SOC, high average specific enzyme activity - ASEA), recovering (low SOC, high ASEA), degrading (high SOC, low ASEA) and unhealthy (low both), adopting 75% of primary forest values as thresholds. In Manaus, two burned forests and one pasture were classified as degrading, while one burned forest and one pasture reached unhealthy status. In Santarém, one pasture and one burned forest were classified as degrading, while three burned forests and one pasture were categorized as recovering. After grouping, while SOC showed no significant differences among treatments in both regions, enzymatic activities were reduced by disturbances. Burned forests and pastures exhibited 60-70% lower arylsulfatase activity in Manaus and 48-65% lower beta-glucosidase activity in both regions compared to primary forests. These results demonstrate that forest fires and forest-to-pasture conversion compromise soil biological health and multifunctionality in the Amazon, with SOC and enzymes together serving as indicators of ecosystem degradation.