Different variations in soil CO, CH, and NO fluxes and their responses to edaphic factors along a boreal secondary forest successional trajectory.

Forest succession is an important process regulating the carbon and nitrogen budgets in forest ecosystems. However, little is known about how and extent by which vegetation succession predictably affects soil CO, CH, and NO fluxes, especially in boreal forest. Here, a field study was conducted along a secondary forest succession trajectory from Betula platyphylla forest (early stage), then Betula platyphylla-Larix gmelinii forest (intermediate stage), to Larix gmelinii forest (late stage) to explore the effects of forest succession on soil greenhouse gas fluxes and related soil environmental factors in Northeast China. The results showed significant differences in soil greenhouse gas fluxes during the forest succession. During the study period, the average soil CO flux was greatest at mid-successional stage (444.72 mg m h), followed by the late (341.81 mg m h) and the early-successional (347.12 mg m h) stages. The average soil CH flux increased significantly during succession, ranging from -0.062 to -0.036 mg m h. The average soil NO flux was measured as 17.95 μg m h at intermediate successional stage, significantly lower than that at late (20.71 μg m h) and early-successional (20.85 μg m h) stages. During forest succession, soil greenhouse gas fluxes showed significant correlations with soil and environmental factors at both seasonal and successional time scales. The seasonal variations of soil GHG fluxes were mainly influenced by soil temperature and water content. Meanwhile, soil MBN and soil NO-N content were also important factors for soil NO fluxes. Structural equation modelling showed that forest succession affected soil CO fluxes by changing soil temperature and microbial biomass carbon, affected soil CH fluxes mainly by changing soil water content and soil pH value, and affected soil NO fluxes mainly by changing soil temperature, microbial biomass nitrogen, and soil NO-N content. Our study suggests that forest succession mainly alters soil nutrient and soil environment/chemical properties affecting soil CO and NO fluxes and soil CH fluxes, respectively, in the secondary forest succession process.