Effects Of Understory Removal And Selective Cutting On Spatial Heterogeneity Of Soil Organic Carbon And Soil Respiration In Pinus Massoniana Forest

Conducting some reasonable forest managements to improve stand productivity and to reduce the rate of CO₂ emission from forest soil to the atmosphere has become an important way to enhance forest carbon sequestration function in forestry.The complex effect of forest management on above-ground and underground ecological processes results that it is import but hard to detect forest carbon pool and carbon emission process under different forest managements in detail.In order to estimate accurately the role of forest management practices on forest carbon pool,understanding the spatial variability of soil organic carbon and soil respiration in a small scale is of great significance for scientific understanding of regulation mechanism of soil carbon pool.Pinus massoniana forest is the largest and most important forest type in the Three Gorges Reservoir area.However,traditional extensive management and strong human disturbance result that most of P.massoniana stands are low quality and efficiency,which need more sustainable managements to improve their ecological service functions.In order to clarify the impact of conventional forest management practices on soil carbon reservoir and carbon emission process,aerially seeded P.massoniana stands with similar site condition in Jiulingtou treefarm in Zigui county,Hubei province was chosen to study.Four 50 m×50 m fixed monitoring sample plot in this area were conducted using the following managements respectively:（1）control measure（CK:maintaining the original stand status）,（2）understory removalmeasure（UR:regularly clearing all shrub layers,including shrubs and regeneration of broad-leaved trees,but retaining the younger trees of P.massoniana）,（3）non-P.massoniana cutting（NPMC:selective cutting DBH≥4cm of individuals except of P.massoniana with a cutting intensity of 15%）,and（4）dominant P.massoniana cutting（DPMC:selective cutting P.massoniana individuals which above the average DBH in the sample plot with a cutting intensity of 70%）.After three years,we measured thespatial heterogeneity of stand composition and structure,soil physical and chemical properties,soil labile organic carbon,soil enzyme activity,soil respiration under these different forest managementtreatments,and detect the main factors influenced on soil carbon emission and its related processes in order to provide scientific basis for sustainable forest managements in this region.The main results showed that:1)Species component and diversity were changed after understory removal or selective cutting measures.Compared to that in CK plot,the biodiversity of arborouslayer was decreased but that of shrub layer and herb layer wasincreased in UR,NPMC and DPMC plots.Furthermore,both two selective cutting measures NPMC and DPMC increased the aggregation degree of stand spatial structure and improved stand mingling intensity.Overall,DPMC treatment led to a retrogressive succession of P.massoniana forest,while UR and NPMC led to a decline of forest stability.2)Both understory removal and selective cutting measures had little effect on soil physical characters,whileled to a change of soil nutrient content and a higher spatial heterogeneity of soil active organic carbon.Compared with the CK plot,total organic carbon content decreased by 12.91%,2.41%and 3.00%and microbial biomass carbon content decreased by 19.83%,33.99%and 27.66%in UR,NPMC and DPMC plot,respectively.Soil light fraction organic carbon in UR,NPMC and DPMC plot were less but not significant than that in the control plot.The dissolved organic carbon content increased by 21.68%under NPMC and 19.90%under DPMC treatment,while it decreased by 7.06%under UR treatment.Furthermore,three artificial managements also increased the spatial heterogeneity of soil labile organic carbon.Soil total organic carbon,total nitrogen,carbon nitrogen ratio,DBH,leaf area and soil moisture are the main factors affected the spatial variation of soil labile organic carbon.By changing stand structure,soil physical and chemical properties and microenvironment,understory removal and selective cutting measures can affect soil organic carbon and increase the instability of soil organic carbon.3)Understory removal and selective cutting measures significantly decreased the activities ofβ-glucosidase by a range of 32.62%-57.28%,acid phosphatase by 35.74%-44.09%and peroxidase in soil by 8.24%-33.20%,while increased the activities of leucine amino peptidase by 42.71%-53.68%and polyphenol oxidase by 8.59%-71.74%.Understory removal and selective cutting measures also increased the variation of soil enzyme activity.The variation of hydrolase activity was consistently higher than that of oxidoreductase.The main factors influenced on the spatial variation pattern varied in different forest managements,such as mean DBH within the range of 4 m diameter in CK plot,mean DBH within the range of 4m diameter and litter amount in UR plot,the total organic carbon,total nitrogen,microbial biomass nitrogen,microbial biomass carbon,water content,carbon nitrogen ratio,and mean DBH within the range of 3 m and 9 m diameter in NPMC plot,while total organic carbon,total nitrogen,light fraction organic carbon,total phosphorus,microbial biomass nitrogen,microbial biomass carbon,and mean DBH within the range of from 2 m to 10 m diameter in DPMC plot.Overall,stand structure parameters were the main factors affect the spatial variation of soil enzyme activity in CK and UR plots,while stand structure parameters,soil physical and chemical properties,as well as microbial carbon and nitrogen together regulated the spatial distribution of soil enzyme activity in NPMC and DPMC.4)Compared with the CK plot,UR treatment reduced soil respiration rate by 26.29%,while NPMC and DPMC improve soil respiration rate by 5.14%and 60%,respectively.Similarly,the soil respiration temperature sensitivity in P.massoniana forest decreased by9.32%in UR treatment,while the temperature sensitivity increased by 3.11%in NPMC and30.05%DPMC.The annual soil respiration fluxes were 1.82、1.48、2.10、2.90 kg·CO₂·m^-2 in CK,UR,NPMC and DPMC plot,respectively.The only 5 cm soil temperature could explain27.9%-51.7%of seasonal variation of soil respiration,while the combination of soil temperature and humidity could explain 28.7%-60.0%,and the combination of soil temperature and litter productivity could explain 64.8%-80.3%of seasonal variation of soil respiration under different forest management practices.These results indicated that understory removal treatment could reduce soil respiration rate and annual soil respiration fluxes while selective cutting treatments could improve them.Soil temperature and the dynamic variation of litter productivity were the main factors to explain the seasonal variation of soil respiration in P.massoniana forest.5)The spatial heterogeneity of soil respiration was caused by structural factors in CK and UR plot,while that of NPMC treatment was controlled by structural factors and random factors together.The spatial correlation of soil spatial heterogeneity was weak in DPMC plot.Dissolved organic carbon and mean DBH in the range of 6 m could explain 27.3%of the spatial variation of soil respiration in NC plot.The combination of basal area in the range of 3m diameter and total nitrogen jointly explained 30.1%of the variation of soil respiration in UR plot.Total organic carbon could account for 53.0%and 14.9%of the spatial variation of soil respiration in NPMC and DPMC plot,respectively.The required sampling numbers of soil respiration for 5 m sampling scale at the 95%confidence levelscales in CK,UR,NPMC and DPMC treatments were 384,339,256,484 within 5%error level and 96,85,64 and 121 with10%error level,respectively.The results showed that the dominant factors on spatial heterogeneity of soil respiration are different under different forest management practices.Stand structure parameters and soil physical and chemical properties affect the spatial variability of soil respiration in CK and UR treatment,while soil total organic carbon is the decisive factor for the spatial variability of soil respiration in both selective cutting treatments NPMC and DPMC.In conclusion,understory removal and selective cutting measures reduced stand quality,soil physical and chemical properties,but improved the instability of soil organic carbon,resulting in the degradation of soil quality.The dynamic changes of soil temperature and litters mass are the main influencing factors on seasonal variation in soil respiration under different forest management practices.Futhermore,understory removal and selective cutting measures also significantly increase spatial heterogeneity of soil organic carbon,soil enzyme activity,soil respiration at small scales.Stand structure and soil physical and chemical properties together contributed to the spatial variation of soil active organic carbon and soil respiration.The different forest management practices based on selective cutting levels and cutting targets will resulted tothe differences in key regulatory factors related to the heterogeneous ecological processes of soil organic carbon and soil respiration.