Effects of <i>Pinus sylvestris</i> var. <i>mongolica</i> reforestation on soil properties after 30 years of postfire recovery—comparison with old-growth natural forests

Abstract Boreal forests play crucial roles in sustaining carbon sequestration but are increasingly threatened by severe wildfires. Wildfires not only reduce available timber and release carbon but may also negatively affect soil properties with potential effects on the reestablishment of planted and natural forests. In the Greater Khingan Range region of Northeast China (52°N ~ 53.5°N), large-scale reforestation efforts were conducted in 1988–90 in which Pinus sylvestris var. mongolica (Mongolian pine) was used to restore ecological functions after severe wildfires. However, the status of soil restoration in the Mongolian pine reforestation plantations remains unclear. In this study, we assessed and compared stand characteristics and soil properties from 0 to 40 cm soil depth in both Mongolian pine plantations and old-growth natural Mongolian pine forests (hereafter referred to as natural forests). While the vegetation biomass of the reforested stands was found to be quite similar to that of the natural forests, the results indicate that the litter stock in Mongolian pine plantations was 49.8% lower than that in natural forests. Soil nutrient contents in plantations were 31.9%–36.9% lower than those in natural forests at the 0–10 cm soil depth. There were no significant differences in soil nutrient contents between plantations and natural forests at the 10–20 and 20–40 cm soil depths except for the nitrogen content, which was significantly lower by 31.3% in plantations compared to that of natural forests at the 10–20 cm soil depth. Furthermore, the enzyme activities, particularly β-glucosidase and exoglucanase, were 30.7% and 29.3% lower in plantations compared with those of natural forests at the 0–10 cm soil depth, respectively. At 10–20 and 20–40 cm soil depths, β-glucosidase and exoglucanase activities were, on average, 17.9% and 2.6% lower, respectively, but these differences were not statistically significant. The positive correlations between litter stock, enzyme activities, and soil carbon and nitrogen contents highlight the critical role of litter input and microbial processes in maintaining soil fertility. Notably, the soil carbon and nitrogen contents in plantations reached ~60% of those in natural forests at the 0–10 cm soil depth. Our findings suggest that while vegetation biomass was recovered comparably well 30 years postwildfire, soil functioning did not yet reach the same level as the old-growth forest. We note that wildfire management to prevent additional wildfires may be important to give soils more time to recover in the study area.