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

Abstract

Mangrove soils with high organic carbon (C_org) content are likely to contain C_org that is vulnerable to remineralization during land use changes. Mangrove conversion to different land uses might deplete soil C_org stocks causing variable carbon dioxide emissions, but the extent of these emissions and the fraction of soil C_org (i.e., labile or stable/recalcitrant) that is mostly lost is poorly understood. Here, we study mangrove soil C_org degradability and its susceptibility to mineralization after mangrove disturbance. We measured changes in soil properties, organic matter (OM) stability and C_org 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_org characteristics. Shrimp pond soils contained the lowest OM-C_org pools, consisted mostly of stable OM (i.e., recalcitrant and refractory; 36.0 ± 5.7% of the total OM) and enriched δ¹³C_org (-22.6 ± 2.7‰). Conversely, control mangrove soils had the largest OM-C_org pools consisting of a large unstable OM fraction (i.e., labile; 46.4 ± 4.2%) and lighter δ¹³C_org (-26.8 ± 0.4‰) being characteristic of C_org 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_org stocks in the upper meter, representing potential emissions of ~1200, 800, and 400 Mg CO₂ ha^-1, 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_org stability among sites can be used to predict potential carbon dioxide produced during mineralization, hence aid at prioritizing areas for conservation, restoration or management.

Keywords: Blue carbon ecosystems; Effluent discharge; Land use change; Organic matter stability; Shrimp ponds.