Forest degradation succession often leads to changes in forest ecosystem functioning. Exactly how the decomposition of leaf litter is affected in a disturbed forest remains unknown. Therefore, in our study, we selected a primary Korean pine forest (PK) and a secondary broad-leaved forest (SF) affected by clear-cutting degradation, both in Northeast China. The aim was to explore the response to changes in the leaf litter decomposition converting PK to SF. The mixed litters of PK and SF were decomposed in situ (one year). The proportion of remaining litter mass, main chemistry, and soil biotic and abiotic factors were assessed during decomposition and then we made an in-depth analysis of the changes in the leaf litter decomposition. According to our results, leaf litter decomposition rate was significantly higher in the PK than that in the SF. Overall, the remaining percent mass of leaf litter’s main chemical quality in SF was higher than in PK, indicating that leaf litter chemical turnover in PK was relatively faster. PK had a significantly higher amount of total phospholipid fatty acids (PLFAs) than SF during decomposition. Based on multivariate regression trees, the forest type influenced the soil habitat factors related to leaf litter decomposition more than decomposition time. Structural equation modeling revealed that litter N was strongly and positively affecting litter decomposition, and the changes in actinomycetes PLFA biomass played a more important role among all the functional groups. Selected soil abiotic factors were indirectly driving litter decomposition through coupling with actinomycetes. This study provides evidence for the complex interactions between leaf litter substrate and soil physical-chemical properties in affecting litter decomposition via soil microorganisms.

We collected the leaf litter between late September 2016 and late October 2016 in the primary Korean pine forest (PK) and secondary broad-leaved forest (SF). At the end of the one-month collection period, the stored litter in each forest was mixed and the materials were dried at 65 °C to constant weight before being placed into litter bags. In early November 2016, geographically and micro-topographically similar quadrats were selected to circumvent spatial heterogeneity. Three quadrats were established in PK and SF, each with an area of 30 × 30 m, being placed every 20 m in the forest. Nylon bags (20.0 × 15.0 cm with 0.5 mm mesh) were filled with 10 g of the materials mixed beforehand. A total of 200 litter bags were randomly placed in the mineral soil at a depth of 0–10 cm (at about 45° angle relative to the soil surface) in early November 2016 (corresponding to 0 days) in each quadrat. Leaf litter bags were collected (taking 40 bags at a time in each quadrat) in 30-Apr-2017, 15-Jun-2017, 31-Jul-2017, 16-Sep-2017, and 1-Nov-2017, corresponding to 185, 230, 275, 320, and 365 days from the starting date. Forty litterbags were harvested and homogeneously mixed to create one sample per quadrat at each sample time in PK and SF, respectively (n = 3).