Iron’s role in soil organic carbon (de)stabilization in mangroves under land use change

Abstract Mangroves are coastal hotspots for carbon storage and yet face multiple threats from anthropogenic activities. Here we explore the role of Fe-mediated organomineral interactions (FeOMI) in soil organic carbon (SOC) stabilization and its sensitivity to land use change (LUC) in Amazonian mangroves. First, we show that the poorly crystalline Fe oxides represent the most effective Fe phase for retaining SOC in pristine mangroves. Contrary to prevailing assumptions about high stability induced by FeOMI, thermal analysis revealed an increase in organic matter stability after Fe removal from samples. The results thus indicate that Fe protects more labile SOC fractions, otherwise vulnerable to biological degradation. Despite the fragile equilibrium of FeOMI under dynamic redox conditions in mangroves, this balance sustains a substantial pool of labile SOC (approximately 8% of total SOC). The studied LUC scenario led to massive loss of Fe-bound OC as less crystalline phases were either degraded or transformed into more crystalline ones, losing the efficiency in retaining SOC. We show that the conversion of mangroves to pastures and shrimp ponds, which are pervasive globally, trigger important biogeochemical changes driving SOC persistence, and therefore have major implications for the intended restoration and carbon capture in mangroves areas formerly disturbed.