Temperature is one of the fundamental environmental variables governing microbially mediated denitrification and anaerobic ammonium oxidation (anammox) in sediments. The GHG nitrous oxide (N 2 O) is produced during denitrification, but not by anammox, and knowledge of how these pathways respond to global warming remains limited. Here, we show that warming directly stimulates denitrification-derived N 2 O production and that the warming response for N 2 O production is slightly higher than the response for denitrification in subtropical sediments. Moreover, denitrification had a higher optimal temperature than anammox. Integrating our data into a global compilation indicates that denitrifiers are more thermotolerant, whereas anammox bacteria are relatively psychrotolerant. Crucially, recent summer temperatures in low-latitude sediments have exceeded the optimal temperature of anammox, implying that further warming may suppress anammox and direct more of the nitrogen flow towards denitrification and associated N 2 O production, leading to a positive climate feedback at low latitudes.

To evaluate biogeochemical impacts of shelf‐scale submarine groundwater discharge (SGD), the northern South China Sea (NSCS) and the East China Sea (ECS) shelf were chosen for this study. Based on a three end‐member mixing model and a Ra box model, SGD fluxes on NSCS (west and east of the Pearl River Estuary, WPRE and EPRE, respectively) and ECS shelf in winter were estimated to be 3.8–9.5 × 109, 1.4–2.2 × 109 and 0.7–2.2 ×1010 m3 d−1, respectively. Our results were equivalent to the SGD flux to the entire Mediterranean Sea, and were an order of magnitude greater than fluxes to the South Atlantic Bight. SGD associated nutrient fluxes into WPRE, EPRE and ECS were almost in the same order of magnitude, 0.2–2.4 × 103 mol m−1 d−1 for dissolved inorganic nitrogen, 1.2–9.8 mol m−1 d−1 for soluble reactive phosphorus, and 0.4–3.4 × 103 mol m−1 d−1 for dissolved silicate. Moreover, SGD delivered 1.1–2.8 × 104, 0.6–0.9 × 104, 1.7–5.1 × 104 mol m−1 d−1 dissolved inorganic carbon and 1.1–2.7 × 104, 0.5–0.8 × 104, 1.6–4.8 × 104 mol m−1 d−1 total alkalinity to WPRE, EPRE and ECS, respectively. Shelf‐scale SGD is a significant source of nutrients and carbon, and may pose great impacts on regional marine ecosystems.

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