Carbon Stocks and Potential Greenhouse Gas Production of Permafrost‐Affected Active Floodplains in the Lena River Delta.

Arctic warming increases the degradation of permafrost soils but little is known about floodplain soils in the permafrost region. This study quantifies soil organic carbon (SOC) and soil nitrogen stocks, and the potential CH4 and CO2 production from seven cores in the active floodplains in the Lena River Delta, Russia. The soils were sandy but highly heterogeneous, containing deep, organic rich deposits with >60% SOC stored below 30 cm. The mean SOC stocks in the top 1 m were 12.9 ± 6.0 kg C m−2. Grain size analysis and radiocarbon ages indicated highly dynamic environments with sediment re‐working. Potential CH4 and CO2 production from active floodplains was assessed using a 1‐year incubation at 20°C under aerobic and anaerobic conditions. Cumulative aerobic CO2 production mineralized a mean 4.6 ± 2.8% of initial SOC. The mean cumulative aerobic:anaerobic C production ratio was 2.3 ± 0.9. Anaerobic CH4 production comprised 50 ± 9% of anaerobic C mineralization; rates were comparable or exceeded those for permafrost region organic soils. Potential C production from the incubations was correlated with total organic carbon and varied strongly over space (among cores) and depth (active layer vs. permafrost). This study provides valuable information on the carbon cycle dynamics from active floodplains in the Lena River Delta and highlights the key spatial variability, both among sites and with depth, and the need to include these dynamic permafrost environments in future estimates of the permafrost carbon‐climate feedback. Plain Language Summary: Floodplain soil development results from both geological processes, such as sediment erosion and deposition, and biological processes such as vegetation growth. In the Arctic, these processes interact with permafrost to form deep soils, but the carbon stocks and potential decomposition and greenhouse gas emissions from Arctic floodplain soils are relatively unknown. In this study, we investigate carbon stocks and potential decomposition from Arctic floodplain soils to depths of 1 m from a large river delta in Siberia. We show that it is difficult to predict what soil types, carbon stocks, and potential decomposition and emissions are found beneath the surface because the sites vary strongly despite having similar vegetation at the surface owing to the depositional processes that occur in floodplains. Key Points: Active floodplains in the Lena River Delta contain sandy permafrost soils with strongly varying C stocks despite similar surface vegetationCarbon stocks in active floodplains were relatively low but only 40% was found in surface soils indicating the importance of deep soilsPotential carbon loss from incubations showed average aerobic carbon production but very active anaerobic carbon and methane production [ABSTRACT FROM AUTHOR]