Effects Of Clear-cutting And Stand Conversion Of Chinese Fir Forest On Soil Respiration And Stand Carbon Stocks

As the main plantation species in China,Chinese fir?Cunninghamia lanceolata?has the characteristics of fast growth and high yield,and chinese fir plantation is a carbon storage with large storage potential and fast growth rate.However,the quality of Chinese fir stand is declining and the carbon sequestration capacity has been weakened due to the management of multiple generations of continuous cropping.Therefore,large-scale broad-leaved transformation of low-efficiency pure Chinese fir plantation in the southern of China has been conducted,aiming to develop into an uneven-aged mixed coniferous and broadleaved forest with complex structure.Clear-cutting and replanting broad-leaved trees not only changed stand structure,but also had effects on soil physical and chemical properties as well as soil respiration.In this study,an experiment was carried on a low-efficiency pure Chinese fir plantation with three treatments using the static chamber-gas chromatography to determine the soil respiration rate.This plantation was converted to a Chinese fir-broadleaved platation by clear-cuting and replanting broadleaved trees with low density?CL?and Clear-cutting and replanting broadleaved trees with high density?CL?,taking uncut?UC?as control.Monthly soil respiration in the 4th and 6th years?2016,2017,2018?after clear cutting were measured.The seasonal soil physical and chemical properties were measured in the 6th year after clear-cutting.The main soil factors of affecting soil respiration was analyzed.The changes in stand structure and biodiversity,and the effect of clear cutting on the carbon sequestration and soil carbon storage of forest stands in the 6th year?2018?after the clear cutting were studied.The mainly results of the studyare as follows:?1?In the 4th,5th,and 6th year after clear-cutting and replanting,the soil respiration of UC,CL,and CH treatments were 199.79,286.73,and 306.06 mg·m^-2·h^-1,174.06,241.86,and 257.13Mg·m^-2·h^-1;198.93,273.17 and 286.59 mg·m^-2·h^-1,respectively.Compared with UC,CL and CH increased soil respiration by 43.52%and 53.15%,38.95%and 47.72%,37.32%and 44.07%in the 4th,5th and 6th year,respectively.Month and treatment have significant effects on soil respiration and the interaction between them were detected.?2?During the 4th to 6th year after clear-cutting and replanting,there was a marked increase of soil temperature.In the 4th year after the clear cutting,the soil moisture of CH was significantly higher than that of UC and CL,and there was no significant difference after that.In the 6th year after clear-cutting and replanting,compared with UC and CL,CH significantly increased soil NH₄⁺-N content?4.94%?and soil MBC and MBN content?47.39%,66.35%and 4.29%,18.46%?.,and decreased soil NO₃^--N content?22.73%?..The soil WSOC content significantly decreased to 17.90%in CL,and increased to 19.70%in CH.?3?Random forest method showed that,soil temperature,WSOC,NO₃^--N and MBN were the main factors affecting soil respiration,among which soil temperature was the most important.?4?After 6 years of clear-cutting,tree species composition in CL and CH treatment changed,which developed from pure Chinese fir forest to mixed forest of Chinese fir and broad-leaved.However,the control was still pure Chinese fir plantation.The clear-cutting and replanting significantly increased the increament of DBH and height growth and stand volume growth rate as well as carbon storage of broad-leaved trees;and decreased carbon storage of Chinese fir,shrub,herb,litter layer and total biomass storage.