Characteristics Of Tron-aluminum Oxides Fixation And Mineralization Of Soil Organic Carbon Under Mixed Plantation Of Pinus Massoniana And Erythrophleum Fordii And Monoculture Plantations

Forest soil organic carbon is an important part of the global carbon cycle,and its accumulation and decomposition will directly affect the terrestrial ecosystem carbon storage and global carbon balance.One of China’s important strategies to deal with climate change is to vigorously develop artificial forests.China has the largest number of artificial forests and the fastest growing area in the world.Our country’s subtropical region is the priority area for building artificial forests.It is the development trend of plantation management in this area to change coniferous pure forest into high value local mixed coniferous and broad-leaved pure forest.Soil iron and alumina oxides are important active minerals in soil,which affect and control the dynamic process of soil organic carbon sequestration and decomposition.However,there is insufficient research on the effects of different iron and alumina oxides on soil organic carbon content in forest soil.At the same time,there are few studies on soil organic carbon mineralization characteristics of artificial mixed forest and pure forest of native conifer tree species in subtropical region,and the internal mechanism leading to the difference of mineralization characteristics is not clear.The study was conducted in three native coniferous and broadleaf planted forests including a pure Pinus massoniana coniferous plantation,a pure Erythrophleum fordii broadleaf plantation,and a mixed plantation of these species located in our country’s subtropical region.We collectted soil samples（0～20,20～40,40～60cm）from different soil layers of each stand,and analyzed the relationship between different forms of iron and alumina oxides and soil organic carbon,and studied the mineralization characteristics of soil organic carbon by mineralization culture experiment.The soil organic carbon mineralization process was fitted by double exponential equation,and we explore the effects of stand type and soil depth on soil organic carbon mineralization.The results of this study can provide theoretical basis for in-depth understanding of soil organic carbon sequestration and emission mechanism of native tree species plantation in this region,and provide scientific basis for the selection of tree species and planting pattern in the construction of artificial forest based on soil organic carbon sequestration and stability.The main research conclusions are as follows:（1）Soil organic carbon（SOC）and its active organic carbon components were significantly affected by stand type and soil depth.In each soil layer,SOC content of mixed plantations of Pinus massoniana and Erythrophleum fordii was the highest,followed by Erythrophleum fordii plantation,and Pinus massoniana plantation was the least.Soil microbial biomass carbon（MBC）,easily oxidized carbon（EOC）and particulate organic carbon（POC）were the most abundant in Erythrophleum fordii plantation.The content of soil organic carbon and its active organic carbon in different stands decreased with the deepening of soil layer.（2）The content of free iron oxide（Fe_d）in soil of the three stands was the highest among all forms of iron and alumina oxides.The content of different forms of iron and alumina oxides is in order as follows:Free iron oxide（Fe_d）>Free alumina oxide（Al_d）>Amorphous iron oxide（Fe_o）>Amorphous alumina（Al_o）>Complexed iron oxide（Fe_p）>Complexed alumina oxide（Alp）.There was a significant negative correlation between Fe_dand soil organic carbon content,while there was a significant positive correlation between Fe_o,Fe_p,Al_pand soil organic carbon content.From the perspective of soil iron and alumina oxides,the soil of the three stands had a strong promotion effect on soil organic carbon mineralization due to the high content of Fe_d,while the content of Fe_o,Fe_pand Al_pwas low,but it was conducive to the accumulation of soil organic carbon.The soil mineralization capacity of Pinus massoniana plantation was affected by the Fe_dcontent,which was significantly higher than that of the other two stands,resulting in poor soil organic carbon sequestration.The contents of Fe_o,Fe_pand Al_pin the Erythrophleum fordii plantation and mixed plantations of Pinus massoniana and Erythrophleum fordii were higher than those in Pinus massoniana plantation,which resulted in the soil organic carbon retention effect of the two forests was stronger than that of Pinus massoniana plantation.（3）The results of mineralization culture for 40 days showed that the mineralization rate of soil organic carbon in the three stands slowed down with the extension of time and then stabilized.Among them,the period of rapid decline is 1～4 days,slow decline is 4～28 days,28 days after the leveling off state.The rate of soil organic carbon mineralization in each stand decreased with the deepening of soil layer.In terms of the mineralization rate of soil organic carbon in the whole section,Erythrophleum fordii plantation>Pinus massoniana plantation>mixed plantations of Pinus massoniana and Erythrophleum fordii.The accumulative mineralization amount and mineralization rate of organic carbon have similar trend.The accumulative mineralization of organic carbon was significantly affected by stand,soil layer and their interaction effects.The soil organic carbon mineralization intensity of Pinus massoniana plantation was the highest,indicating that the soil organic carbon stability was poor,while the soil organic carbon mineralization intensity of mixed plantation was significantly lower than that of the two pure plantations,indicating that the soil organic carbon was more stable and the soil organic carbon retention effect was better.In general,the mineralization intensity decreased with the deepening of soil layer,indicating that the stability of soil organic carbon in the bottom soil was better than that in the top soil layer.（4）The determination coefficients（R²）of soil organic carbon mineralization were 0.969～0.999.The fitting results showed that the content of easily decomposed organic carbon（C_o）was the highest in the surface layer（0～20 cm）and the middle layer（20～40 cm）.In the bottom layer（40～60 cm）,C_ocontent of Pinus massoniana plantation was the highest.In terms of the C_o/SOC value of the whole soil profile,Pinus massoniana plantation>Erythrophleum fordii plantation>mixed plantations of Pinus massoniana and Erythrophleum fordii,indicating that the mixed plantations had the strongest soil carbon sequestration ability.The mineralization rate（k）of decomposed organic carbon in Erythrophleum fordii plantation was the highest in all soil layers,indicating that activity of the soil microbial in Erythrophleum fordii plantation was higher,which could effectively improve the mineralization rate of C_o.From the whole profile,the forests with the highest mineralization rate（h）of refractory organic carbon were divided into Pinus massoniana plantation,indicating that the soil microorganisms in Pinus massoniana plantation had strong ablilty to decompose and utilize refractory organic carbon.（5）Correlation analysis showed that soil physicochemical properties and active organic carbon components significantly affected soil organic carbon mineralization.The cumulative mineralization of organic carbon was positively correlated with SOC,MBC,DOC,EOC and POC,and negatively correlated with TP.C_owas positively correlated with SOC,active organic carbon components and NO₃^--N.k was positively correlated with POC,and negatively correlated with TP and p H.h was positively correlated with SOC,TP and NH₄⁺-N.C_o/SOC values were positively correlated with EOC and POC.Redundancy analysis（RDA）showed that EOC were the most important contributor to the variation of soil organic carbon mineralization characteristics,and were also the main soil factor that caused the significant difference of soil organic carbon mineralization characteristics between 0～20 cm soil layer and 20～40 cm and40～60 cm soil layer.In conclusion,in terms of soil organic carbon sequestration and stability,it is better to construct mixed plantations of Pinus massoniana and Erythrophleum fordii and Erythrophleum fordii plantation than Pinus massoniana plantation in this area.