Effects of freezing and thawing on soil active organic carbon and enzyme activity in the Sanjiang Plain wetlands

The aim of this study was to analyze the influence of freezing-thawing cycles(FTCs) on soil active organic carbon fractions and soil enzyme activity in different types of soils. To this end, a laboratory freezing-thawing simulation test was carried out using soil samples from undisturbed Deyeuxia angustifolia wetlands(UDAW), disturbed Deyeuxia angustifolia wetlands(DDAW), and rice paddy fields(RP) in Sanjiang Plain. Soil samples were taken from different layers of each soil type(0-10 cm, 10-20 cm, and 20-30 cm). Two FTC amplitudes, namely,(-10-10℃) and(-5-5℃), were tested. Soil samples were then processed during the 3 rd, 6 th, 10 th, and 15 th FTCs, and dissolved organic carbon(DOC), microbial biomass carbon(MBC) and enzyme activity was compared in relation to FTCs(0). The results demonstrated that, in the three wetland soils, the dissolved DOC concentration increased after freezing-thawing cycles. The DOC concentration in each type of soil decreased with soil depth. DOC concentration increased the most in UDAW, followed by DDAW, and then RP. In contrast, MBC concentration and cellulase, sucrose and amylase activity in the soils decreased after freezing-thawing cycles. The effects of FTCs on MBC and the aforementioned activity of the three enzymes gradually weakened with increases in soil depth. MBC concentration and the three enzymes activities presented the greatest reduction in UDAW, followed by DDAW, and then RP. The effects of FTCs on DOC concentration in soils under a FTC amplitude of-5-5℃ were greater than that under a FTC amplitude of-10-10℃, but the opposite phenomenon was observed with respect to the effects of FTCs on MBC concentration and the three enzymes. The number of FTCs had a significant influence on the active organic carbon fractions(DOC and MBC) and the three enzymes activities(P