Mangrove encroachment into salt marshes may enhance carbon retention but reduce surface accretion in coastal wetlands

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Coastal Wetlands are increasingly recognized as carbon (C) sinks capable of storing more C per area than all other ecosystems, particularly in their soils.  As warmer temperatures facilitate poleward expansion of mangroves into saltmarshes worldwide, it is essential to understand how this regime shift may alter wetland resilience to sea-level rise and C sequestration. We maintained patch-scale (3 ´ 3 m) vegetation (marsh or mangrove) along a gradient of ten plot-scale (24 ´ 42 m) mangrove densities in a coastal wetland (Port Aransas, Texas, USA). We isolated the effects of vegetation from abiotic conditions to test how vegetation regime shifts alter factors affecting carbon storage and soil accretion and how effects scale spatially. Plant canopy in mangrove patches intercepted more PAR (87.66 ± 3.46 %) than marsh patches (7.62 ± 3.46 %; P<0.001), and average soil temperature was lower under mangrove canopy (P< 0.01) and displayed a hump-shaped relationship with plot-scale mangrove density (P = 0.01; r²=0.63). Breakdown rates (k d^-1) of cellulose were lower in mangrove (0.005 ± 0.0007) than marsh (0.006 ± 0.0007) patches (P = 0.047), and wood k decreased with increasing plot mangrove density (P = 0.013; r²=0.39). Breakdown of marsh litter (Batis maritime) was much faster than mangrove litter (P<0.01).  Root biomass (marsh: 1051.86 ± 222.32 g m^-2, mangrove: 2131.97 ± 257.47 g m^-2; P < 0.001) and percent SOM (marsh: 8.1 ± 0.46%, mangrove: 10.62 ± 0.77%; P = 0.001) were higher in mangrove than marsh patches. Sediment accretion rates were similar in marsh and mangrove patches (0.02  ± 0.003 mm d^-1 ;P > 0.05), but overall plot accretion decreased with increasing mangrove density (P= 0.0007; r² =0.566).  Wood breakdown was positively related to surface accretion (P=0.008; r²=0.16).  Our results show that mangroves alter microclimate, organic matter quality, organic matter breakdown and soil carbon stocks, indicating increased C retention at multiple spatial scales, but dense mangrove stands may reduce surface accretion rates in the marsh interior.