Effects of freeze-thaw cycles and the soil water content on carbon and nitrogen changes in different soil types of Heilongjiang Province, China

To reveal the influence of freeze-thaw cycles (FTCs) on soil carbon and nitrogen changes, six typical soils in Northeast China were selected as the research objects to conduct a FTC simulation test in an artificial climate chamber. Three soil volumetric water contents (10%, 20%, 30%) and eight FTCs (0, 2, 4, 6, 8, 10, 15, 20) were set. The results showed that the soil organic carbon (SOC) and microbial biomass carbon (MBC) contents of different soil types under the FTCs initially exhibited a downward and then an upward trend, while the dissolved organic carbon (DOC) content exhibited an upward and then a downward trend. Otherwise, the fourth and sixth FTCs were the key points of change. The SOC, MBC and DOC contents in paddy fields were higher than those in dry fields, showing upward and then downward trends spatially from northeast to southwest. The SOC and MBC contents in each soil type were the highest at the 20% water content, and the DOC content gradually increased with increasing water content. The ammonium nitrogen (NH4+-N) content in different soil types at different water contents under the FTCs showed an upward trend first, then a downward trend and finally an upward trend. The NH4+-N content in paddy fields was higher than that in dry fields. The nitrate nitrogen (NO3--N) content showed a downward trend first, then an upward trend and finally a downward trend. The NO3--N content in dry fields was higher than that in paddy fields. The NH4+-N contents in the three soil types on the Sanjiang Plain were significantly higher than those on the Songnen Plain. The NH4+-N and NO3--N contents showed upward trends with increasing water content, but the differences were not significant. The results have implications for the study of different types of soils and provide references for research on the mechanism of soil carbon and nitrogen transformation in typical farming areas in Northeast China.