Human disturbance drives loss of soil organic matter and changes its stability and sources in mangroves.

Mangrove soils with high organic carbon (C) content are likely to contain C that is vulnerable to remineralization during land use changes. Mangrove conversion to different land uses might deplete soil C stocks causing variable carbon dioxide emissions, but the extent of these emissions and the fraction of soil C (i.e., labile or stable/recalcitrant) that is mostly lost is poorly understood. Here, we study mangrove soil C degradability and its susceptibility to mineralization after mangrove disturbance. We measured changes in soil properties, organic matter (OM) stability and C pools and sources across a mangrove disturbance gradient (i.e., pristine forests, degraded mangroves receiving domestic sewage and shrimp farm effluents, and shrimp ponds). Results showed that the conversion of mangroves to shrimp ponds caused the most severe changes in soil properties, OM and C characteristics. Shrimp pond soils contained the lowest OM-C pools, consisted mostly of stable OM (i.e., recalcitrant and refractory; 36.0 ± 5.7% of the total OM) and enriched δC (-22.6 ± 2.7‰). Conversely, control mangrove soils had the largest OM-C pools consisting of a large unstable OM fraction (i.e., labile; 46.4 ± 4.2%) and lighter δC (-26.8 ± 0.4‰) being characteristic of C from a mangrove origin. Conversion of mangroves to shrimp ponds and its degradation by shrimp farm and domestic sewage effluents caused a loss of 97%, 61%, and 35% of soil C stocks in the upper meter, representing potential emissions of ~1200, 800, and 400 Mg CO ha, respectively. These losses were explained by enhanced OM mineralization of unstable fractions driven by the loss of the physico-chemical protection provided by fine-grained soils and vegetation cover. The differences in C stability among sites can be used to predict potential carbon dioxide produced during mineralization, hence aid at prioritizing areas for conservation, restoration or management.