Influence Mechanism Of Root Exudates On Activation Of Active And Inert Soil Carbon Pools

Soil is the largest organic carbon(C)pool in terrestrial ecosystems,about three times in the atmospheric C pool and four times in the vegetation C pool.Therefore,even small changes in soil organic carbon(SOC)would have significant effects on atmospheric CO₂concentrations.Terrestrial primary production may continuously increase with global warming and elevated CO₂,which could accelerate soil organic matter(SOM)decomposition through the priming effect(PE)by plant carbon(C)inputs such as litter or root exudates.Priming effect is the phenomenon that the input of fresh organic matter causes the change of microbial decomposition rate of the nature SOM.At the same time,the decomposition of SOC accelerated,thus reducing soil C storage.The different proportions of these two effects determine the feedback relationship between climate change and soil C pool.The“microbial nitrogen mining”,“stoichiometric decomposition”and“abiotic destabilization”hypotheses have been proposed as the mechanisms of the PE on SOM decomposition.However,the relative intensity of PE for different SOM fractions with different turnover rates and the main underlying mechanisms remain elusive.Here,we compared the PE of three soil types under inputs of three types of compounds of root exudates in a 90-day incubation experiment.Soil CO₂-C respired was captured from soils of 15 years fallow,15 years bare and 23 years bare plus additional 815-day laboratory incubation(relatively active,intermediate,and resistant SOM,respectively)with the addition of ¹³C-labeled glucose,glycine,and oxalic acid.The static lye absorption method was used to capture the CO₂ of soil respiration in the whole process,the soil respiration rate and the change of?¹³C value were measured,and the CO₂ from the soil and root exudates was distinguished by the ¹³C isotope tracing method,and the ole-field and bare-fallow treatments were used to distinguish different C pools,explore the priming effects of soil C pool decomposition at different turnover rates.We found that oxalic acid caused the strongest PE among three substrates,enhancing the decomposition rate of active,intermediate and resistant SOM by 20.8%,58.4%and62.8%relative to the control,respectively.This result supports the“abiotic destabilization”hypothesis that oxalic acid may alleviate SOM-mineral interactions and thus cause a strong positive PE.Moreover,partly consistent with the“stoichiometric decomposition”hypothesis,glycine caused a stronger PE of intermediate and resistant SOM(but not active SOM)than glucose.Specifically,glucose stimulated the decomposition of active,intermediate and resistant SOC decomposition by 17.3%,20.8%and 21.5%,respectively,while glycine promoted it by 12.0%,36.9%and 52.6%,respectively.Taken together,we conclude that the PE of the slow-cycling intermediate and resistant SOM is potentially more vulnerable to priming by various root exudates compared to the fast-cycling active SOM,and the“abiotic destabilization”and“stoichiometric decomposition”hypotheses are important in explaining the PE of SOM decomposition under root exudates inputs.