Response of soil constituents to freeze-thaw cycles in wetland soil solution

Soil freeze-thaw cycles in the winter-cold zone can substantially affect soil carbon, nitrogen and phosphorus cycling, and deserve special consideration in wetlands of cold climates. Semi-disturbed soil columns from three natural wetlands (Carex marsh, Carex marshy meadow and Calamagrostis wet grassland) and a soybean field that has been reclaimed from a wetland were exposed to seven freeze-thaw cycles. The freeze-thaw treatments were performed by incubating the soil columns at −10 °C for 1 d and at 5 °C for 7 d. The control columns were incubated at 5 °C for 8 d. After each freeze-thaw cycle, the soil solution was extracted by a solution extractor installed in each soil layer of the soil column, and was analyzed for dissolved organic carbon (DOC), NH₄⁺-N, NO₃⁻-N and total dissolved phosphorus (TDP). The results showed that freeze-thaw cycles could increase DOC, NH₄⁺-N and NO₃⁻-N concentrations in soil solutions, and decrease TDP concentrations. Moreover, the changes of DOC, NH₄⁺-N, NO₃⁻-N and TDP concentrations in soil solutions caused by freeze-thaw cycles were different in various sampling sites and soil layers. The increments of DOC concentrations caused by freeze-thaw cycles were greater in the wetland soil columns than in the soybean field soil columns. The increments of NH₄⁺-N concentrations caused by freeze-thaw cycles decreased with the increase of soil depth. The depth variation in the increments of NO₃⁻-N concentrations caused by freeze-thaw cycles in the wetland soil columns was different from that in the soybean field soil columns. The decrements of TDP concentrations caused by freeze-thaw cycles were greater in columns of Carex marsh and Carex marshy meadow than in columns of Calamagrostis wet grassland and the soybean field. The study results provide information on the timing of nutrient release related to freezing and thawing in natural versus agronomic soils, and have implications for the timing of nutrient application in farm fields in relation to water quality protection.