Seasonal Dynamics of Greenhouse Gas Emissions from Island-like Forest Soils in the Sanjiang Plain: Impacts of Soil Characteristics and Climatic Factors

Using the static chamber–gas chromatography method, this study investigates the flux characteristics of CO2, CH4, and N2O in the soils of three typical island-like forests in the Sanjiang Plain during the growing season (May to September), as well as their relationships with environmental factors. The results indicate that the soils of the Broadleaf mixed forest, Quercus mongolica forest, and Betula platyphylla forest act as emission sources for CO2 and N2O, with average fluxes of 433.92, 452.41, and 358.17 μg·m−2·h−1 for CO2 and 12.48, 13.02, and 10.51 μg·m−2·h−1 for N2O, respectively. The differences among forest types are not significant. All three forest types serve as sinks for CH4, with average fluxes of −22.52, −23.29, and −0.76 μg·m−2·h−1. The Betula platyphylla forest has a significantly weaker absorption intensity compared to the other types (p < 0.01). The measured environmental factors collectively explain 66.58% of the variability in greenhouse gas fluxes in the island-like forests, with soil temperature, soil moisture, and total nitrogen content being the main influencing factors in the region. Rising temperatures favor the emission of CO2 and N2O and the absorption of CH4 in all three forest types. Increased soil moisture inhibits the absorption of CH4 in the Broadleaf mixed forest and Quercus mongolica forest, while higher levels of alkali-hydrolyzed nitrogen enhance the N2O flux in the Quercus mongolica forest. Soil organic carbon and soil pH significantly influence only the greenhouse gas fluxes of the Betula platyphylla forest.