Effect of Freezing–Thawing Cycle on Soil Active Organic Carbon Fractions and Enzyme Activities in the Wetland of Sanjiang Plain, Northeast China

To determine the response of soil active organic carbon fractions and enzyme activities to freezing-thawing cycles (FTCs), an FTC simulation experiment of Deyeuxia angustifolia wetland soil samples collected from Sanjiang Plain, Northeast China, were examined. The results showed that the water-extracted organic carbon (WEOC), microbial biomass carbon (MBC) concentration and three enzyme activities were affected by FTCs. FTCs increased WEOC concentration, and the small amplitude (0 °C ± 5 °C) and the low water content interaction had the maximum impact, which was nearly 9.0–70.4%, while the large amplitude and the low water content interaction had the minimum impact, which was about 3.6–50.7%. FTCs significantly decreased MBC concentration, amylase, invertase and catalase activities. During the large amplitude (0 °C ± 10 °C) and high water content interaction impacted heavily on MBC, which was approximately 4.2–41.45%. As the number of FTC increased, WEOC concentration increased followed by a gradually decline in contrast that MBC concentration and three enzyme activities showed an opposite tendency. There was a significant correlation between soil active organic carbon fractions and enzyme activities. In addition, negative correlation was observed between WEOC and MBC. This may suggested that increased WEOC by FTCs plays an important role in soil microbes.