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

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 (1) examined how carbon and nitrogen densities and microbial community composition vary with peat age and (2) describe the formation of peat deposits over time. At 4 mangrove sites near La Paz, Baja California Sur, Mexico, we cored the sediments until rejection and obtained 5 cm samples at 20 cm intervals. In these samples, we measured organic carbon (C org ), total nitrogen, δ 13 C, δ 15 N, and radiocarbon ( 14 C) age. We observed peat carbon densities of 3.4 × 10 -2 ± 0.2 × 10 -2 g cm -3 , C org :N ratios of 42 ± 3, and inter-site variation in C org :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 and sediment characteristics, including C org :N ratio and 14 C age. Carbon dating allowed us to reconstruct the accumulation of organic matter over the last 5029 ± 85 yr. Even over this long time scale, though microbes have evidently continuously cycled the peat nitrogen pool, peat carbon density remains effectively unchanged.