Belowground Carbon Dynamics Of Secondary Forest Ecosystems In Northeastern China

Forest ecosystems in northeastern China play an important role in both local and national carbon budgets because of the large area and huge amount of carbon storage. The spatial and temporal changes in soil surface CO₂ flux (R_S), the major CO₂ source to the atmosphere from terrestrial ecosystems, directly influence the local and regional carbon budgets. Soil carbon, a key component of ecosystem carbon budgets, is the largest carbon pool of forest ecosystems in high latitude regions. However, few data on R_S and soil carbon storage were available in this region. In this study, we used infrared gas exchange analyzer (Li-Cor 6400) to measure the R_S and related biophysical factors, used trenching-plot approaches to determine heterotrophic (R_H) and autotrophic respiration (R_A), and examined effects of soil thermal and moisture on soil respiration and heterotrophic respiration for six temperate forest types: Mongolian oak (dominated by Quercus mongolica), poplar-birch (dominated by Populous davidiana and Betula platyphylla), mixed-wood (no dominant tree species), hard-wood forests (dominated by Fraxinus mandshurica, Juglans mandshurica, and Phellodendron amurense), Korean pine (Pinus koraiensis) and Dahurian larch (Larix gmelinii) plantations. They represented secondary forest ecosystems in northeastern China.Soil temperature, soil moisture and their interactions significantly (p< 0.01) influenced the R_s and R_h, but their effects depended on forest ecosystem and soil depth. In the growing season, the soil temperatures at 10 cm depth for the forest ecosystems did not differ significantly but soil moisture did. The R_s for the oak, pine, larch, hardwood, mixed-wood, and poplar-birch stands varied from 1.89 5.23, 1.09 -4.66, 0.95- 3.52, 1.13 - 5.97, 1.05 - 6.58, and 1.11 -5.76μmolCO₂·m^-2·s^-1, respectively; the contribution of root respiration to soil respiration (RC) varied from 34.31-37.36%, 33.78-37.02%, 14.39-35.75%, 32.40-51.44%, 17.94-47.74% and 39.72-46.65%, correspondingly. The seasonality of R_S and RC showed similar patterns of bell-shaped curves. However, the mainly driven factors might be different. The former were driven by soil temperature and moisture, while the latter was by soil temperature and root phenology for RC. The temperature sensitivity coefficient of soil respiration (Q₁₀) for the forest ecosystems ranged from 2.32 to 3.55. The Q₁₀ showed a convex curve along with soil moisture gradient, increasing when soil moisture increased from 30.19% to 40.7%, and then decreasing when extremely high soil moisture in the hardwood forest probably impeded activities of soil microbes and plant roots which in turn decreased decomposition rates and soil CO₂ emission.The comprehensive statistical models of Rs including soil temperature, soil moisture and their interactions as independent variables explained 67.5 - 90.6% variations in R_S and 46.5-71.5% in R_h. These models were used to estimate the annual fluxes of soil respiration, root respiration and heterotrophic respiration for the forest ecosystems. There were significantly differences in annual fluxes of R_S, R_H and R_A for the six ecosystems. The annual fluxes of R_S for mixed-wood, hard-wood, poplar-birch, Mongolian oak forests, Korean pine, and Dahurianlarch plantations were 951, 892, 812, 678, 596, and 451 gC-mV1, respectively. The annual RH were significantly greater than the Ra, ranging from 337 540 gCm'V^nd 88-331 gOm" 2-a"x for Rh and Ra, respectively.The soil orgainic carbon (SOC) contents for broadleaved and coniferous forests varied from 2.63- 66.29 gkg1 and 42.15- 49.15 gkg"1, respectively. The mean SOC densities were 15.57 kg-m"2 and 17.16 kgm'2for broadleaved and coniferous forests, respectively; and the mean SOC turnover periods were 32 and 48 years, respectively. The Rs and its components (Ra and Rh) were highly positively correlated to SOC content. The degree of correlation depended on soil profile horizons (R2 = 0.747-0.933). The SOC content decreased with soil depth in a specific forest ecosystem, whereas the SOC density and turnover period increased with soil depth.This study provided a great deal of measured data and reseach framework for belowground carbon pool and flux in representative forest ecosystems in northeastern China, and illustrated that soil CO2 efflux in these ecosystems was mainly driven by soil thermal and hydrological regimes, combined with soil organic carbon contents. Therefore, the carbon sink/source capacity of those ecosyterns may change under global climate change conditions, which emphasized the significance and necessity of further studies on this subject.