Objective  This paper studies the changes of soil dissolved organic carbon (DOC), microbial biomass carbon (MBC) under four typical forest types in cold temperate forest region with different freezing and thawing frequencies, integrates the measured physical and chemical factors, and reveals the correlation between freezing and thawing alternation and main active organic carbon and other potential driving factors in soil to provide a theoretical basis for regional carbon pool balance in cold temperate forest region.

  Method  The soil litter layer (O layer) and surface mineral soil layer (A layer) under four forest types (Pinus pumila forest, Rhododendron simsii-Betula platyphylla forest, Rhododendron simsii-Larix gmelinii forest and Ledum palustre-Larix gmelinii forest) in the Great Hinggan Mountains, northeastern China of cold temperate zone were studied. Soil samples were cyclically treated with different freezing-thawing frequency accumulation by indoor simulated freezing-thawing alternating method, and the content levels of DOC, MBC, aggregates, pH and water content in the soil were determined. Finally, principal component analysis method was used to explore the correlation between the above multivariate variables.

  Result  On the whole, the contents of DOC and MBC in Ledum palustre-Larix gmelinii forest and Rhododendron simsii-Betula platyphylla forest were higher, while the carbon contents in Rhododendron simsii-Larix gmelinii forest and Pinus pumila forest were lower, and the latter was significantly lower than the former. In the same soil layer and different forest types, the contents of DOC and MBC increased first and then decreased with the accumulation of freezing and thawing frequency, and freezing and thawing significantly increased the content of DOC and decreased the content of MBC. In the soil layer, the contents of DOC and MBC in O layer were significantly higher than those in A layer. In addition, temperature and freeze-thaw frequency also affected the potential driving factors such as active organic carbon, aggregate, pH and water content in soil. With the accumulation of freezing-thawing frequency, the freezing-thawing effect significantly increased the content of DOC and decreased the content of MBC. In soil layer, the contents of DOC and MBC in O layer were significantly higher than those in A layer. In addition, temperature, frequency of freeze-thaw and other factors also affect the potential drivers of soil active organic carbon, aggregates, pH and water content.

  Conclusion   Due to the change of freeze-thaw frequency, the contents of DOC and MBC in soil of different forest types in cold temperate zone change dynamically. Compared with the non freeze-thaw treatment, the DOC content increases and the MBC content decreases after the freeze-thaw cycle, and the broadleaved forest (Rhododendron simsii-Betula platyphylla forest) in the middle altitude area and the Ledum palustre-Larix gmelinii forest in the low altitude area are more conducive to the accumulation of carbon content.