Changes in soil organic matter content and quality in Amazonian mangrove forests converted to shrimp farms

Francisco Ruiz^1*Tiago Osório Ferreira¹Matheus Sampaio²Cornelia Rumpel^3,4Marie-France Dignac^3,4François Baudin⁵Xose Luis Otero^6,7Angelo Fraga Bernardino⁸

• ¹Universidade São Paulo, Escola Superior de Agricultura Luiz de Queiroz, Departamento de Ciência do Solo, Piracicaba, São Paulo, Brazil
• ²Katholieke Universiteit Leuven, Leuven, Belgium
• ³Institut d'Ecologie et des Sciences de l'Environnement de Paris, Paris, France
• ⁴Sorbonne Universite, Paris, France
• ⁵Institut des Sciences de la Terre de Paris, Paris, France
• ⁶Universidade de Santiago de Compostela Centro de Investigacion Interdisciplinar en Tecnoloxias Ambientais, Santiago de Compostela, Spain
• ⁷Universidade de Santiago de Compostela, Santiago de Compostela, Spain
• ⁸Universidade Federal do Espirito Santo, Vitoria, Brazil

Mangroves are highly efficient carbon sinks, yet they are increasingly threatened by aquaculture expansion. Moreover, the long-term impacts of shrimp farming on soil organic matter (SOM) quality and carbon sequestration in Amazonian mangroves remain poorly understood, particularly regarding changes in SOM composition and their consequences for ecosystem resilience. This study compares soil organic matter (SOM) in pristine mangroves, mangroves converted to shrimp ponds, and mangroves adjacent to shrimp ponds in the Brazilian Amazon. We showed that pristine mangroves soils had higher soil organic carbon (SOC) ranging from 25 to 32 g.kg⁻¹, higher C/N ratio (15.6– 20.9), and stable δ¹³C (-27.5 to -27.0 ‰) and δ15N (3.6 to 3.3 ‰), with SOM rich in lignin, carbohydrates, and polyaromatic compounds. In contrast, mangroves converted to shrimp ponds had lower SOC (<10 g.kg⁻¹), lower C/N (~12), and enriched δ¹³C and δ¹⁵N, indicating shrimp feed inputs and nitrogen enrichment. The SOM was also lipid-rich and thermally less stable. Soils in mangroves adjacent to shrimp ponds exhibited only minor alterations, suggesting high resilience; however, effluent-driven SOM mineralization and reduced thermal stability are latent risks. Overall, shrimp farming decreases SOM content and alters its quality, undermining long-term carbon storage in both converted and nearby mangrove areas. Our findings highlight the importance of regulating effluent discharge and promoting more sustainable aquaculture practices to mitigate SOM degradation and safeguard the carbon sequestration capacity of adjacent mangroves.