Vertical Spatial Heterogeneity Of Priming Effect Of Soil Organic Matter Decomposition In Subtropical Forests At Different Elevations

Through the decomposition of soil organic matter,the soil carbon（C）pool can transport about 60 Pg C carbon dioxide to the atmospheric carbon pool every year.After exogenous C is introduced into soil,it can strongly affect the decomposition rate of soil original organic matter in a short time,this phenomenon is defined as priming effect（PE）,which is an important process in the C cycle of terrestrial ecosystem and can regulate the transformation of soil C sink-source relationship.At present,priming effect has been proved to be a common phenomenon in different types of ecosystems,and it has great spatial heterogeneity.However the previous research is often focused on the horizontal spatial heterogeneity of priming effect,but the research on vertical spatial heterogeneity of priming effect is much less,leading to a unclear vertical spatial variation law of priming effect.Moreover,global climate change can affect the priming effect intensity by changing the amount of vegetation C input into the soil.However,there are still great uncertainties about the change law of priming effect of forest soil organic matter decomposition at different elevations,and whether there are differences in the response of priming effect to the change of external C input among elevation gradients,as well as the underlying regulation mechanism.To clarify above scientific problems,this research takes the soil of Cunninghamia lanceolata plantation at 750,950 and 1150 above elevation in Mazongling Forestry Station of Anhui Province as the research object,and carries on the indoor culture experiment for 97 days.¹³C labeled Chinese fir leaf litter represents C input.The effects of various C input treatments(CK treatment:no litter,L1,L2 and L3 treatments showed the effects of adding ¹³C labeled Chinese fir leaf litter with 2.5%,5%and 10%soil organic carbon)on the priming effect of soil organic matter decomposition are tested at different elevations.The purpose of this research is to clarify the vertical spatial heterogeneity of the priming effect of soil organic matter decomposition in Chinese fir plantation and the responses of the priming effect of various amounts of Chinese fir leaf litter at different elevations,and to reveal its potential regulation mechanism.Seeking litter management that can scientifically promote soil carbon storage,the main results are as follows:（1）There are differences in soil physical and chemical properties（such as soil carbon content and clay content）and microbial biomass among elevation gradients.The C input increased the different elevations mineralization rate of soil organic matter and the cumulative amount of soil carbon mineralization.According to the dynamics of priming effect,the culture period was divided into two stages:pre-culture（0-37 days）and middle and late culture（38-97 days）.The cumulative mineralization in the early stage of culture was higher than that in the later stage of culture.The input of C input and elevation gradient had significant effects on the mineralization rate of soil organic matter.The cumulative mineralization of C input between elevation gradients increased with the increase of C input,and increased with the increase of elevation.The unstable components in soil carbon pool and C input were decomposed preferentially,and the relatively difficult decompose components of C input（lignin and polyphenols）in the middle and later stage of culture limited the utilization of C input.（2）The priming effect induced by C input is generally positive.The priming effects induced by the same amount of C input in different elevations are not consistent,and there is obvious vertical spatial heterogeneity.The priming effect intensity at low elevation of 750 was higher under all C input treatments,while that at 950 above elevation was generally lower.A variety of statistical analysis results show that the differences of soil physical and chemical properties and microbial community and function between elevation gradients regulate the vertical spatial heterogeneity of priming effect,which can explain69%of the spatial variation of priming effect.In terms of soil physical and chemical properties,the content of soil dissolved organic carbon is the primary factor of soil cumulative priming effect,and soil C/N also regulate the priming effect intensity through mediating microbial stoichiometry.In terms of microbial community structure,the utilization of C input by r-strategists such as bacteria can induce the priming effect to increase rapidly in the early stage.Predominantly fungi K strategy using the components of difficult decomposition,to promote the cultivation of middle of soil organic matter decomposition.The competition for nutrients between r-strategists and K strategists can affect the community structure of microorganisms.Exogenous C input leads to the increase of microbial secretion of extracellular enzymes related to carbon,nitrogen and phosphorus,and the activity of microbial extracellular enzymes has an important effect on the priming effect.The microbial biomass and extracellular enzyme activity of soil at 750 above elevations are higher than those at 950 above elevations,which is also an important cause for the spatial variation of priming effect in soils along elevation gradient.（3）Due to the different properties of soil at different elevations,the dynamic of priming effect intensity is different with the increase of C input,and the priming effect is the strongest at high amount of C input.In the early stage of culture,due to the mechanism of preferential utilization of substrate,the priming effect induced by medium and high amount of exogenous C input was weakened.The priming effect increased with the increase of C input in 950 soil and 1150 soil.At a high amount of C input,it is easy to cause the imbalance of microbial nutrient stoichiometry,and enhance the long-term nutrient demand for nitrogen,obtaining nitrogen through the mineralization of soil organic matter by microorganisms,which enhances the intensity of priming effect.In this study,we found that there was vertical spatial heterogeneity in the priming effect induced by C input along elevation gradient,and the priming effect varied with C input in soils at different elevations.These results show that the differences of soil physical and chemical properties,microbial community,extracellular enzyme activity and their interaction affect the intensity of priming effect at different elevations.The stimulation effect during culture is regulated by microbial substrate priority utilization mechanism,stoichiometric decomposition mechanism,and microbial nitrogen mining mechanism.This study revealed the vertical spatial variation of the priming effect of forest soil at different elevations in the subtropics and the response of the priming effect to the increase of exogenous input at different elevations.The results can not only provide data support for the study of vertical spatial heterogeneity of priming effect,but also provide scientific litter management and forest management technology for improving forest soil fertility.