Increasing soil organic carbon and nitrogen stocks along with secondary forest succession in permafrost region of the Daxing'an mountains, northeast China

Secondary forest succession plays a vital role in the dynamics of soil organic carbon and nitrogen and is recognized as an important driving force for the recovery of soil properties. In order to explore the effect of forest succession on the stocks of soil organic carbon and nitrogen in permafrost region of the Daxing'an Mountains, northeast China, the Betula platyphylla forest at the early succession stage; the Betula platyphylla-Larix gmelinii mixed forest at the middle succession stage and the pure Larix gmelinii forest at the climax forest stage were selected to compare soil organic carbon and soil nitrogen stocks along forest succession stage. The results showed that the concentration of soil organic carbon (SOC) and total soil nitrogen (SN) all increased during forest succession, particularly in 0-40 cm depth. However, the C:N ratio changed slightly. The concentration of SOC and SN and the ratio of C/N significantly decreased with increasing of soil depth along forest succession. The stocks of total soil organic carbon (TSOC) and total soil nitrogen (TSN) increased from 77.69 Mg ha(-1) to 130.50 Mg ha(-1) and from 7.46 Mg ha(-1) to 13.90 Mg ha(-1), respectively, during forest succession, but the accumulation were mainly in later stage of succession from Betula platyphylla-Larix gmelinii forest to Larix gmelinii forest. TSOC and TSN showed obvious surface aggregation, with more than 58% of TSOC in the soil of 0-20 cm depth and more than 50% of TSN in the soil of 0-40 cm depth. TSOC and TSN decreased with increasing soil depth. To sum up, our study highlighted that secondary forest succession greatly increased the sequestration capacity of SOC and SN of forest soil in the future Daxing'an Mountains, northeast China. The result is of great significance for understanding the simultaneous effects of forest succession and climate change, and improving forest resource management measures and strategies in permafrost region. (C) 2020 The Authors. Published by Elsevier B.V.