Quantifying how changing mangrove cover affects ecosystem carbon storage in coastal wetlands

Abstract Despite overall global declines, mangroves are expanding into and within many subtropical wetlands, leading to heterogeneous cover of marsh–mangrove coastal vegetation communities near the poleward edge of mangroves’ ranges. Coastal wetlands are globally important carbon sinks, yet the effects of shifts in mangrove cover on organic‐carbon ( OC ) storage remains uncertain. We experimentally maintained black mangrove ( Avicennia germinans ) or marsh vegetation in patches ( n = 1,120, 3 × 3 m) along a gradient in mangrove cover (0–100%) within coastal wetland plots ( n = 10 , 24 × 42 m) and measured changes in OC stocks and fluxes. Within patches, above and belowground biomass ( OC ) was 1,630% and 61% greater for mangroves than for recolonized marshes, and soil OC was 30% greater beneath mangrove than marsh vegetation. At the plot scale, above and belowground biomass increased linearly with mangrove cover but soil OC was highly variable and unrelated to mangrove cover. Root ingrowth was not different in mangrove or marsh patches, nor did it change with mangrove cover. After 11 months, surface OC accretion was negatively related to plot‐scale mangrove cover following a high‐wrack deposition period. However, after 22 months, accretion was 54% higher in mangrove patches, and there was no relationship to plot‐scale mangrove cover. Marsh ( Batis maritima ) leaf and root litter had 1,000% and 35% faster breakdown rates ( k ) than mangrove ( A. germinans ) leaf and root litter. Soil temperatures beneath mangroves were 1.4°C lower, decreasing aboveground k of fast‐ (cellulose) and slow‐decomposing (wood) standard substrates. Wood k in shallow soil (0–15 cm) was higher in mangrove than marsh patches, but vegetation identity did not impact k in deeper soil (15–30 cm). We found that mangrove cover enhanced OC storage by increasing biomass, creating more recalcitrant organic matter and reducing k on the soil surface by altering microclimate, despite increasing wood k belowground and decreasing allochthonous OC subsidies. Our results illustrate the importance of mangroves in maintaining coastal OC storage, but also indicate that the impacts of vegetation change on OC storage may vary based on ecosystem conditions, organic‐matter sources, and the relative spatiotemporal scales of mangrove vegetation change.