Carbon and nitrogen dynamics in the East China Sea (ECS) shelf sediments were examined at 13 sites, combining geochemistry, metagenome, and reaction-transport modeling. The model yielded a decrease in total particulate organic carbon (POC) degradation rates across the shelf from 3 to 0.6 mmol C m^-2 d^-1 caused by decreases in the sedimentation rates and POC accumulation rates. Total POC degradation rates in muddy area cores were dominated by organoclastic sulfate reduction (OSR) and aerobic respiration, followed by organotrophic iron oxide reduction, canonical (organotrophic) denitrification, and organotrophic manganese oxide reduction. For sandy area cores, OSR and aerobic respiration together account for 88-94% of total POC degradation. The elevated denitrification and DNRA rates in the muddy area compared to the sandy area is primarily due to the greater liability and higher degradation rates of POC. In addition to canonical denitrification, chemolithotrophic denitrification, likely sulfide, iron and manganese-dependent, contributed approximately 20% to total nitrate removal in the muddy area, but it was negligible in the sandy area. This was consistent with microbiological data showing more abundant Fe, Mn, and sulfur oxidation genes and their positive correlation with denitrification genes in the muddy sediments. The relative contribution of denitrification in nitrate reduction decreased while anammox gained more importance with increasing water depths. In contrast, the relative fraction of DNRA did not display a clear depth-dependent trend. Synthesizing data from additional 139 sites, the regional POC degradation and burial rates were estimated to be and 1.7 Mt C yr⁻¹ and 8.3 Mt C yr⁻¹, respectively. Despite covering less than 30% of the ECS shelf, the muddy area accounts for 63% of total buried organic carbon. The general high organic carbon burial efficiency (CBE), particularly in the muddy area, on the ECS shelf is attributed to high sediment accumulation rates but relatively low POC degradation rates. Overall, this study reveals marked differences in carbon and nitrogen dynamics between the muddy and sandy areas on the ECS shelf, which are likely linked to variations in sediment accumulation rates and organic carbon lability.

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