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MEPS prepress abstract   -  DOI: https://doi.org/10.3354/meps14117

Baja California Sur mangrove deep peat microbial communities cycle nitrogen but do not affect old carbon pool

M. T. Costa*, E. Ezcurra, O. Aburto-Oropeza, M. Maltz, K. Arogyaswamy, J. Botthoff, E. Aronson

*Corresponding author:

ABSTRACT: Mangroves provide important ecosystem services, including storing carbon belowground for millennia. Mangrove carbon storage relies in part on high primary productivity, but essential to the long-lived nature of this storage is the slow rate of microbial decomposition of peat. In this study, we ask how carbon and nitrogen densities and microbial community composition vary with peat age, and describe the formation of peat deposits over time. At four mangrove sites near La Paz, Baja California Sur, Mexico, we cored the sediments till rejection and obtained 5-cm samples at 20 cm intervals. In these samples we measured organic carbon (Corg), total nitrogen, δ13C, δ15N, and radiocarbon (14C) age. We observed peat carbon densities of 3.4 × 10-2±0.2 × 10-2 g cm-3 and Corg:N ratios of 42 ± 3 and inter-site variation in Corg:N that reflects differing preservation conditions. Recalcitrant organic matter sources and anaerobic conditions leave a strong imprint on peat microbial communities. Microbial community composition and diversity were driven by depth gradients and variation in sediment characteristics, including Corg:N ratio and 14C age. Carbon dating allowed us to reconstruct the accumulation of organic matter over the last 5,029 ± 85 years. Even over this long time scale, though microbes evidently have continuously cycled the peat nitrogen pool, peat carbon density remains effectively unchanged.