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.