The Slow Burn: How Repeated Forest Fires and Agricultural Expansion Are Degrading Southern Amazon Soils
In the shadow of the Amazon’s lush canopy, a silent upheaval is underway. A consortium of researchers from Brazil, the United States, and the United Kingdom has unveiled disruptive findings regarding the long-term degradation of soil health in the southern Brazilian Amazon—a region increasingly afflicted by recurrent forest fires and expanding agricultural frontiers. The study, centered around the Tanguro Research Station located in the fiercely contested transition zone between the Amazon Rainforest and the Brazilian Cerrado, delineates the intricate and severe consequences these anthropogenic pressures exert on fundamental soil properties, including carbon and nitrogen stocks vital for ecosystem function.
The Amazon’s “Arc of Deforestation” is emblematic of the global tension between environmental preservation and agricultural development. Here, human-induced fires—ignited not by natural wildfire cycles but by a combination of expanding cattle ranching, soybean cultivation, and the intensification of agricultural practices—coupled with intensified droughts amplified by climate change, are persistently scarring native forests. According to Mário Lucas Medeiros Naval, lead author and researcher at the Center for Nuclear Energy in Agriculture (CENA-USP), these fires are particularly insidious because the native Amazonian biome is not adapted to frequent burning. Unlike the Cerrado ecosystem, where fire acts as a natural ecological reset, the Amazon suffers irreversible harm when subjected to these repeated conflagrations.
Using a robust experimental design, this research compared four distinct land-use scenarios: undisturbed forest, forest subjected to annual burning, forest burned every three years, and lands converted fully to agriculture with conservation-minded practices such as no-till farming and crop rotation. Over 150 hectares within the private legal reserve in the Tanguro station’s jurisdiction, each treatment spread over 50 hectares, provided ample spatial scope for collecting representative soil samples and metrics. Such extensive sampling enabled a thorough investigation of both physical and chemical indicators of soil health, including organic matter content, nitrogen concentrations, and cation exchange capacity (CEC)—a critical indicator of nutrient retention in soils.
The findings were stark and unambiguous. Soils in areas subjected to annual fires showed an average 17% reduction in carbon stocks, while those burned more sporadically every three years showed a 19% decrease. Conversion to agriculture yielded the most dramatic loss, slashing carbon stocks by as much as 38%. These statistics are disconcerting because carbon stored in soils plays a vital role not only in maintaining soil fertility and nutrient cycling but also in global climate stabilization by sequestering atmospheric CO2. Intriguingly, even no-till agricultural systems, traditionally considered a soil-friendly farming method, failed to maintain soil carbon levels on par with unburned forests.
Perhaps more disturbing than raw carbon losses is the collaterally damaged soil chemistry and physical structure. The study demonstrated significant declines in cation exchange capacity, an essential soil property governing the absorption and retention of positively charged nutrients such as potassium, calcium, and magnesium. Reduction in CEC implies diminished soil fertility and a compromised ability to support productive vegetation, which, in turn, further exacerbates vulnerability to erosion and nutrient leaching. These attributes underscore how cumulative damage from repeated burning alters the soil’s nutrient economy and resilience, potentially stifling forest regeneration and agricultural productivity alike.
Furthermore, these deleterious effects persisted even nine years after the last fire, signaling that soil recovery in these ecosystems is tremendously slow, if not functionally arrested. The slow rebound of both carbon and nitrogen pools indicates that once degraded, these soils become long-term carbon sources rather than sinks, undermining broader efforts to mitigate climate change through forest conservation and sustainable land management. It highlights an urgent need to rethink the intensity and frequency of fires, as well as the methodologies deployed in local agricultural expansion.
The biological and structural characteristics of this transition forest itself also exacerbate its susceptibility. Defined as a “Cerradão” formation—a dense, low-biodiversity forest with trees reaching up to 20 meters in height—this landscape melds traits of both Amazonian rainforest and Cerrado savanna. Not as towering as the deep Amazon upland forests yet more forested than the open savanna, this biome occupies a delicate ecological niche sensitive to disturbances. Unfortunately, this intermediate status has rendered it a hotspot for agricultural incursion, particularly for soybean monocultures, which aggressively encroach on native forest territory.
The broader implications of this study extend far beyond the borders of the Brazilian Amazon. Forest fires and land-use changes in tropical biomes contribute disproportionately to global greenhouse gas emissions, biodiversity loss, and disruptions in hydrological cycles. The Amazon, in particular, acts as a global climate regulator through massive carbon storage and evapotranspiration processes that influence regional and global rainfall patterns. Continued degradation, especially driven by human-induced fires, may trigger tipping points where the region shifts from a carbon sink to a carbon source, exacerbating climate change and forest dieback.
The authors advocate strongly for multifaceted policies that can restrain agricultural frontier expansion and intensify fire prevention strategies. A compelling recommendation is the promotion of diversified agroforestry systems—a land management approach that combines trees with crops or livestock, enhancing biodiversity and fostering carbon storage more effectively than conventional farming. Agroforestry not only mitigates fire risk but also bolsters soil health through organic matter inputs, root system complexity, and improved nutrient cycling.
Moreover, the study underscores the urgent need for international cooperation and research investments directed toward understanding fire effects in Amazonian soils across various locations. Part of a larger collaborative project known as Amazon PyroCarbon, funded by organizations such as Brazil’s FAPESP and the United Kingdom’s NERC and UKRI, this endeavor aims to provide a comprehensive map of fire impacts ranging from the southern Amazon to other regions, enabling better-informed land-use decisions at both national and global scales.
This complex interplay between ecological vulnerability, human activity, and climate stressors confronts the global community with a pivotal question: how to foster food security while preserving one of Earth’s most critical natural treasures? Researchers like Plínio Barbosa de Camargo contend that developing alternatives to current agricultural paradigms is essential, not only for halting environmental decline but also to secure sustainable livelihoods for the local populations dependent on the land.
In sum, this illuminating research paper reveals that the Amazon’s soils are under siege—not just from immediate deforestation but from the cascading, corrosive effects of recurrent fires and land conversion. The long-term degradation of soil organic matter, nutrients, and structural integrity paints a grim picture of ecosystem health and resilience. If current trajectories persist, the consequences will reverberate through global carbon cycles, biodiversity conservation, and climate stability. The time for decisive action, integrated policy frameworks, and innovative land management practices is both urgent and ethically imperative.
Subject of Research:
Impacts of repeated forest fires and agriculture on soil organic matter and health in southern Amazonia
Article Title:
Impacts of repeated forest fires and agriculture on soil organic matter and health in southern Amazonia
News Publication Date:
25-Mar-2025
Web References:
doi:10.1016/j.catena.2025.108924
www.fapesp.br/en
References:
Naval, M.L.M., Bieluczyk, W., Camargo, P.B., Feldpausch, T. et al. (2025). Impacts of repeated forest fires and agriculture on soil organic matter and health in southern Amazonia. Catena. https://doi.org/10.1016/j.catena.2025.108924
Image Credits:
Mário Lucas Medeiros Naval/CENA-USP
Keywords:
Amazon deforestation, forest fires, soil degradation, carbon stocks, nitrogen stocks, cation exchange capacity, Cerradão, agroforestry, no-till agriculture, soil organic matter, climate change, Amazon PyroCarbon project
