Effects Of Thinning On The Soil Heterotrophic Respirations And CH₄ Fluxes Of Oak-pine Mixed Forests In Qinling Mountains

Oak-pine mixed forest is the main forest type in the Qinling Mountains,which is of great significance to the carbon fixation and carbon cycle of forest ecosystem in the whole region.As a common forest management measure,how thinning affects soil greenhouse gas emissions in pine-oak mixed forests during the growing season is not very clear.In this paper,the mixed forest of pine and oak in Xinkuang Forest Farm of Ankang,Shaanxi Province was taken as the research object.The monthly variation and temperature sensitivities of soil greenhouse gas effluxes in the growing season of oak-pine mixed forests in Qinling Mountains with different thinning years were investigated by static box-gas chromatography.The main results and conclusions of this paper are as follows:1.In this study,the soil samples of 0-20 cm soil layer were measured in three treatments of thinning different years ago（no thinning,thinning 4 years ago and thinning 12 years ago）.It was found that the soil microbial biomass carbon contents in the thinning plots 4 years ago and thinning plots 12 years ago were significantly lower than that in the control plots（no thinning）（P<0.05）.Soil p H increased significantly（P<0.05）after thinning.The content of soil organic carbon decreased,and the thinning plots 4 years ago decreased significantly（P<0.05）.There was no significant difference in the content of dissolved organic carbon,but it increased slightly after thinning.In general,most of the soil physical and chemical indexes of the thinning plots12 years ago were closer to the level of the control plots than the thinning plots 4 years ago.2.To clarify the monthly variations of greenhouse gas emissions/absorption fluxes in the growing season of oak-pine mixed forests in Qinling Mountains,we measured the concentrations of carbon dioxide and methane with time by static chamber-gas chromatography.Among them,soil heterotrophic respiration showed a“bimodal”variation.There was a significant time variation characteristic,which was characterized by high flux in summer and low flux in autumn,with peaks in June and October.There were no significant differences in the cumulative fluxes of CO₂ among the three treatments.However,the thinning plots 4 years ago and 12 years ago increased slightly compared with the control,the thinning plots 12 years ago(1.13±0.02 kg·m^-2)>the thinning plots 4 years ago(1.10±0.05 kg·m^-2)>the control plots(1.06±0.04 kg·m^-2).During the growing season,soil CH₄ showed an absorption sink,which also showed a“bimodal”variation rule with significant time variation characteristics.Meanwhile,CH₄ showed a high absorption in spring and summer and a low absorption in autumn.The peak appeared in May and August.There was no significant difference in the cumulative CH₄ uptake fluxes between the three treatments.The thinning plots 4 years ago(0.28±0.05 g·m^-2)>the control plots(0.23±0.06 g·m^-2)>the thinning plots 12 years ago(0.12±0.02 g·m^-2).3.Through the exponential model and linear regression fitting,we found that soil temperature and soil moisture were significantly positively correlated with CO₂fluxes（except the thinning plots 4 years ago）.By calculating the temperature sensitivity,it was further found that the temperature sensitivity Q₁₀ of soil CO₂ emissions increased after thinning,the control plots（1.19）<the thinning plots 12 years ago（1.46）;However,there was no significant correlation between soil CH₄ uptake and the soil temperature.In summary,thinning increased the amount of direct sunlight in the forest,which increased soil temperature and accelerated the decomposition rates of soil organic carbon in the forests.Soil CO₂ emission rates increased during the growing season,while the effect was not significant.Soil methane is an absorption sink throughout the growing season,which offsets part of the soil carbon loss to some extent.After 12 years of thinning,the absorption of CH₄ in plots decreased,while the absorption of CH₄ in the plots increased after 4 years of thinning.There was no significant relationship between treatments.Soil temperature and moisture are the critical factors affecting CO₂ fluxes,which showed a significant positive correlation.In addition,the temperature sensitivity of soil heterotrophic respiration increased due to the decrease of organic matter in heterotrophic respiration substrate.The emission and absorption mechanisms of CH₄ needs to be further studied.This study provides a theoretical basis for the carbon sink management of oak-pine mixed forests in Qinling Mountains.