Changes to soil organic matter decomposition rate and its temperature sensitivity along water table gradients in cold-temperate forest swamps

High-latitude forest swamp soil stores a large amount of carbon (C), because of its anaerobic condition and lower temperature. The combination of decreasing water table by artificial drainage and climate warming exert a considerable influence on soil C storage. In this study, we explored how the soil organic matter decomposition rate (Rs) and its temperature sensitivity (Q10) are altered with water-table gradients in high-latitude swamp soil. We sampled soil from cold-temperate forest swamps at three water-table gradients (permanently flooded, seasonally flooded, and non-flooded) in the high-latitude areas of Genhe and Mohe in China. We then conducted an incubation experiment using a new mode of varying incubation temperature, with rapid and continuous measurements of Rs. Our results showed that Rs at 20 °C (R20) was significantly higher in seasonally flooded (SF) swamp soil, compared to permanently flooded (PF) or non-flooded (NF) soil. Soil redox condition mainly controlled R20 during the early drying stage of forest swamps, while substrate quantity and quality dominated the latter stage. The highest Q10 was observed in SF soils in Genhe, but this was not significant compared to the Q10 in soils from Mohe. There were clear regional differences in Q10 driven by changes to soil substrate (dissolved organic C and nitrogen) and physicochemical properties. Our findings indicate that high-latitude forest swamp soils may emit more CO2 and act as a C source under the scenarios of artificial draining and climate warming.