Characteristics Of Soil Organic Carbon Decomposition Of Forests Under Different Degradation Degrees In Mid-subtropical Zone

Soil organic carbon(SOC)pool plays an important role in the global carbon cycle,and SOC mineralization directly reflects the decomposition process of SOC pool,and significantly affects the atmospheric CO2 concentration.According to the law of forest succession,four kinds of secondary forests in different succession stages(Loropetalum chinensis-Vaccinium bracteatum Thunb-Rhodoendron mariesii Hemsl.et Wils shrubs(LVR).Loropetalum chinensis-Cunninghamia lanceolata-Quercus fabri Hance shrubbery(LCQ)?Pinus massoniana Lamb-Lithocarpus glaber-Loropetalum chinensis coniferous and broad-leaved mixed forest(PLL)?Lithocarpus glaber-Adinandra millettii-Cyclobalanopsis glauca(Thunberg)Oersted evergreen broad-leaved forest(LAG))were selected in hilly area of eastern Hunan.Then soil samples stratified at different depths(0-10 cm,10-20 cm,20-30cm,above 30cm)were collected in April,June,October 2016 in these four kinds of secondary forests.A 49-day laboratory incubation was conducted to measure CO2 evolution,and the data from the incubation experiment were fitted to a three-pool first-order model that separate mineralizable SOC in active,slow and resistant carbon pools.We compared the decomposition characteristics of SOC,distribution characteristics and turnaround time of soil organic fractions in different secondary forests of in subtropical zone,to reveal the formation mechanism and stability of SOC pool and to provide scientific basis for the restoration and reconstruction of secondary forest and management for forest SOC pool in mid-subtropical area.The results showed that:(1)The degradation of secondary forest had a significant effect on the SOC content and organic carbon density.SOC content and density decreased with the degradation level and the trend of SOC in the same soil layer and season showed that:LAG>PLL>LCQ>LVR,and there were significant differences in the same soil layer(P<0.05).The arithmetic mean of the three seasons of the four soil layers was between 2.35-41.23 g·kg-1,which decreases with the increase of soil depth,and the 0-10 cm soil layer was significantly higher than that of other three soil layers(P<0.05).The variation of soil organic carbon density was consistent with the change of organic carbon content.(2)Litter standing crop and soil physical and chemical properties had significant effect on SOC content.There was a significant positive correlation between SOC and litter standing crop,soil water content,total N,total P,hydrolytic N,effective P,C/N ratio,C/P ratio and N/P ratio(P<0.01),and available K(P<0.05).Among them,the correlation coefficient between organic C and total N was the largest,reached 0.946.There was a significant negative correlation between SOC,soil bulk density(P<0.05),and pH value(P<0.01),indicating that SOC,soil physical and chemical properties were closely related.(3)During the 49-day incubation,the average daily mineralization(mineralization rate)of SOC in each soil layer showed obvious characteristics:in the early stage of cultivate(0-13d),the amount of mineralization was large but unstable,and there was a significant decrease.With the prolongation of incubation days,the average daily mineralization was reduced to a certain extent,but the decrease was small.After 17 days,the daily mineralization was relatively low and relatively stable,with only a small decrease.The type of forest had a significant effect on the mineralization rate of organic carbon,in the same sampling season,the order of daily mineralization of organic carbon in the same incubation period was LAG>PLL>LCQ>LVR;At the same time,the seasonal changes of SOC mineralization rate in the same stand was autumn>spring>summer.The vertical change of SOC mineralization rate was significantly higher than that of the other 3 soil layers(P>0.05),and the differences between the adjacent soil layers of the 4 kinds of forest were significantly decreased.(4)The cumulative release amount of soil organic carbon mineralization of the 4 degradation stage of secondary forest showed the same changing trend over time.During the whole incubation period,the early stage of accumulation increase largely,then the growth became slow,and finally reached a stable growth.Degradation of secondary forest had significant effect on soil organic carbon mineralization.The cumulative mineralization of soil organic carbon of 4 stands in the 3 seasons showed:LAG>PLL>LCQ>LVR.The cumulative mineralization of soil organic carbon in summer was significantly lower than that in spring and autumn in the same cultivation time.In the vertical direction,the cumulative mineralization of soil organic carbon of the 4 stage of secondary forest degradation all decreased with the increase of the soil depth,and the cumulative mineralization of soil organic carbon of the 0-10 cm soil layer was significantly different from other 3 soil layers in 3 seasons(P<0.05).(5)Soil organic carbon fractions in the 4 degradation stage of secondary forest were inconsistent.Among them,the active organic carbon content was between 0.02-0.24 g-kg-1,which accounted for 0.33%-0.88%of SOC,and the mean residence time was between 3.75-11.94d.The content of soil active organic carbon decreased gradually from the evergreen broad-leaved forest to the shrub grassland,but the percentage of active organic carbon in SOC was almost unchanged(P>0.05).Slow organic carbon content was between 1.20-22.14 g-kg-1,accounting for 38.16%-72.81%of SOC,and the mean residence time was between 1.90-6.37a.In addition to PLL,the slow organic carbon content comprised the highest percentage of SOC.The content of resistant organic carbon was between 0.61-18.63 g·kg-1 which accounted for 26.58%-61.35%of SOC,and the mean residence time was 574a.The vertical distribution of soil organic carbon fractions in the 4 stands had no difference,and declined with the increase of soil depth.(6)The fractions of SOC were significantly correlated with soil physical and chemical properties.Soil water content,total N,total P,hydrolytic N,available P,C/P ratio and N/P ratio were positively correlated with soil active organic carbon,slowing organic carbon and resistant organic carbon(P<0.01);Soil bulk density and pH value showed significantly negative correlation with three organic carbon fractions(P<0.01).Soil C/N ratio was positively correlated with resistant organic carbon(P<0.05).Total K and available showed no correlation with three organic carbon fractions(P>0.05).