Slash-and-burn agriculture disrupts the carbon storage potential and ecosystem multifunctionality of Amazon's secondary forests

Author(s)

Bieluczyk, Wanderlei ; Duarte, Marina Pires ; Martins, Guilherme Lucio ; Camargo, Plinio Barbosa de ; Noronha, Norberto Cornejo ; Piccolo, Marisa de Cassia ; Tsai, Siu Mui

• Document type: Journal article
• Source: AGRICULTURE ECOSYSTEMS & ENVIRONMENT; v. 381, p. 15-pg., 2024-12-07.

Abstract

Urgent action is needed in the Amazon to halt deforestation, repair agricultural damage, and restore forests to revive ecosystemic functions such as carbon (C) storage and soil health. A critical and demanding challenge, especially in sandy soils, is ceasing the slash-and-burn in smallholder farming livelihoods to preserve ecosystem services of primary and secondary forests. Here, we examined (i) the recovery of secondary forests in structure, litter layer, and soil health, as well as C storage post-agricultural abandonment of extremely sandy Amazonian soils (> 89 % sand), and (ii) the extent of loss of these gains when a secondary forest is burned for agricultural reconversion. We tracked secondary forests at 2, 5, 10, and 20 years, including slash-and-burning the 20-year-old forest. Our methods included analyzing C stocks in soil, litter, and plants, forest vegetation ecological indexes, litter quality assessed through nitrogen (N), C, and lignocellulose contents, delta C-13 to indicate organic matter origin, and seven additional soil health indicators. Soil delta C-13 ranged from-27.1 to-28.8 parts per thousand across the sites, indicating a negligible influence of tropical grasses on the soil's organic matter and suggesting that pastures were not previously cultivated in these areas. Secondary forest growth accumulated 0.24 and 2.97 Mg C ha(- 1 )y(- 1 ) in litter and trees, respectively. Yet, soil C stocks did not show significant changes during 20 years of forest regeneration. Over 18 years, the forest increased the vegetation diversity fourfold and litter N by 41 %, improving forest structure and litter quality. This progress in organic matter aboveground contributed to improved soil biological activity and nutrient storage, facilitating soil health and multifunctionality regeneration as the forest aged. However, slash-and-burn resulted in a 67.6 Mg C ha(- 1 ) loss, reverting levels below those of the 2-year-old forest. Returning to agriculture also depleted soil cation exchange capacity, bulk density, and fauna activity, degrading soil's chemical, physical, and biological functions to levels comparable to or worse than those in the youngest forest. We conclude that Amazonian lands abandoned after long-term agriculture still offer potential for ecological restoration, with secondary forests capable of regenerating multiple ecosystem functions, even in sandy soils. However, a single slash-and-burn reverses 20 years of progress and degrades soil health further. Recognizing smallholder farmers' poverty and reliance on slash-and-burn, we advocate for educational and socioeconomic support to stop fires and encourage sustainable agriculture, including bioeconomy incentives and environmental compensation to sustain the perpetuation and benefits of secondary forests in the Amazon. (AU)

• FAPESP's process: 21/00976-4 - Amazon PyroCarbon: quantifying soil carbon responses to fire
• Grantee: Plínio Barbosa de Camargo
• Support Opportunities: Research Projects - Thematic Grants
• FAPESP's process: 23/18333-8 - Quantifying Soil Organic Carbon Responses to Fire at the Landscape Scale in the Amazonia
• Grantee: Wanderlei Bieluczyk
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 22/05561-0 - SOIL BIODIVERSITY AS THE CENTRAL ROLE OF PLANT-SOIL FEEDBACKS IN AMAZON AGROFORESTRY SYSTEMS
• Grantee: Guilherme Lucio Martins
• Support Opportunities: Scholarships in Brazil - Doctorate
• FAPESP's process: 23/02576-9 - Carbon cycling in degraded soils and its contribution to the regeneration of forests in the Eastern Amazon
• Grantee: Jéssica Adriele Mandro
• Support Opportunities: Scholarships in Brazil - Doctorate