Landscape Change Affects Soil Organic Carbon Mineralization and Greenhouse Gas Production in Coastal Wetlands

Plant invasion and aquaculture activities have drastically modified the landscape of coastal wetlands in many countries, but their impacts on soil organic carbon (SOC) mineralization and greenhouse gas production remain poorly understood. We measured SOC mineralization rate and soil CO2 and CH4 production rates in three habitat types from 21 coastal sites across the tropical and subtropical zones in China: native mudflats (MFs), Spartina alterniflora marshes (SAs), and aquaculture ponds (APs). Landscape change from MFs to SAs or APs increased total and labile fraction of SOC, as well as carbon mineralization rate and greenhouse gas production, but there were no discernible differences in SOC source‐sink dynamics between SAs and APs. SOC mineralization rate was highest in SAs (20.4 μg g−1 d−1), followed by APs (16.9 μg g−1 d−1) and MFs (11.9 μg g−1 d−1), with CO2 as the dominant by‐product. Bioavailable SOC was less than 2% and was turned over within 60 days in all three habitat types. Proliferation of S. alterniflora marshes and expansion of AP construction had resulted in a net increase in soil CO2‐eq production of 0.4–4.3 Tg yr−1 in the last three decades. Future studies will benefit from better census and monitoring of coastal habitats in China, complementary in situ measurements of greenhouse gas emissions, and more sampling in the southern provinces to improve spatial resolution. Plain Language Summary Wetlands are one of the largest reservoirs of soil carbon and play an importance role in the global terrestrial biogenic carbon cycle. Coastal wetlands are major sinks for carbon due to high sedimentation rate and burial of organic matter. However, landscape modifications due to invasive plants and aquaculture activities have profoundly impacted the carbon source‐sink dynamics in coastal wetlands. We compared the soil organic carbon turnover and greenhouse gas (CO2 and CH4) production between native mudflat, Spartina marshes, and aquaculture ponds in five coastal provinces across the tropical‐subtropical gradient in China. Landscape modification of native mudflats increased soil carbon mineralization rate and greenhouse gas production, predominantly as CO2, and the effect was consistent across the large geographical and climate gradients. Our results provide a better insight into the carbon dynamics in impacted wetlands across a large geographical range. Key Points Landscape change in coastal mudflats increased soil carbon turnover and emission Landscap