Effects on Greenhouse Gas (CH4, CO2, N2O) Emissions of Conversion from Over-Mature Forest to Secondary Forest and Korean Pine Plantation in Northeast China

Abstract

This study aimed to evaluate the seasonal variations of Greenhouse Gas fluxes (CH4, CO2, and N2O), Greenhouse Gas (GHG) emissions, and Global Warming Potential (GWP) over the extent of the regions and understand the controlling factors. CH4, CO2, and N2O fluxes were measured along with their environmental variables from the over-mature forest, Korean pine plantation, and five 60-year-old natural secondary forests in mountainous regions in Northeast China from May 2015 to April 2016. The results revealed that secondary forests, except for Betula platyphylla forest, significantly increased CH4 absorption by 19.6% to 51.0% and 32.6% to 67.0% compared with over-mature forest (OMF) and Korean pine plantation (KPP). Five secondary forests significantly increased CO2 flux by 32.9% to 78.6% and 14.1% to 53.4% compared with OMF and KPP, respectively. According to the annual statistics, the N2O fluxes had significant differences among seven forest types and decreased in the following order: mixed deciduous forest (MDF) > OMF > KPP > Populous davidiana forest (PDF) > hardwood forest (HWF) > Mongolian oak forest (MOF) > Betula platyphylla forest (BPF). The CH4 absorption and CO2 emission peaks occurred in summer, while the peak N2O fluxes occurred in spring. Stepwise multiple linear regression showed that CH4 and CO2 fluxes from soils were strongly influenced by air and soil temperature, soil volumetric water content (SVWC), nitrate nitrogen (NO3−-N), ammonium nitrogen (NH4+-N), and soil organic carbon (SOC) across the whole year. Air temperature, SVWC, pH, NO3−-N, and NH4+-N were the dominant factors controlling N2O fluxes from OMF and five secondary forests (except for BPF). No significant relationships were observed between these environmental factors and N2O fluxes from KPP and BPF. Additionally, the total cumulative CH4, CO2, and N2O fluxes were –13.37 t CH4 year−1, 41,608.96 t CO2 year−1, and 3.24 t N2O year−1, and the total cumulative GWP were 42,151.87 t CO2 eq year−1 through the whole year in seven forest types at the Maoershan Ecosystem Research Station in Northeast China. For the annual GWP per hectare, secondary forests and KPP averaged a higher GWP by 33.7%–80.1% and 17.9% compared with OMF. This indicates that the effects of early human activities have not been completely eliminated in the middle stage of KPP and secondary forests.