Effects Of Drought On Root Exudation Rates And Their Associated Priming Effects On Soil Organic Carbon Decomposition In Evergreen Broad-leaved Forests

Evergreen broad-leaved forest is a typical vegetation type in the subtropical region of China,with a series of ecological functions such as carbon sequestration,climate regulation,and soil water conservation.It is also recognized as an important ecological barrier in the subtropical region.In recent years,the frequency and intensity of extreme drought events have continued to increase in subtropical areas,which seriously threaten ecosystem stability and carbon sequestration in subtropical forests.Root exudates play a key role in plant-soil-microbe interaction and the decomposition of soil organic carbon in forest ecosystems.However,little is known about how drought stress affects the quantity and quality of root exudates and associated soil priming effects,which limits the understanding of soil organic carbon accumulation in forest ecosystems in the context of global climate change.In this study,four coexisting evergreen broad-leaved forests（Schima superba,Lithocarpus glaber,Castanopsis carlesii,Castanopsis sclerophylla）in the drought-simulation platform（control and 70%rain fall reduction）at Zhejiang Tiantong National Forests Ecosystem Observation and Research Station were selected as target tree.The root exudates were collected using an in situ collection device,and then root exudates of total C,N,sugar,organic acid,and amino acid were measured by laboratory instruments.We also quantified relative growth rate,root morphological traits,mycorrhizal infection rates,and soil properties to examine the driving factors underlying variations in root exudation under drought conditions.Based on the data from field measurements,the ¹³C isotope-labeled（sugar,organic acid,and amino acid）gradient addition experiments were set up to elucidate the soil priming effects mediated by root exudation rates of total C and its components.Our main findings are as follows:（1）Effects of extreme drought on root exudation rates of total carbon and its components in evergreen broadleaved forests.Root exudates rates of total carbon and its components were measure by in situ collection.Our results showed that drought stress significantly decreased root exudation rates of total C,sugar,and amino acid by17.8%,30.8%,and 35.0%,respectively,but increased root exudation rate of organic acid by 38.6%.These drought-induced changes in root exudation rates were largely associated with drought-induced changes in tree growth rates,root morphological traits,and mycorrhizal infection rates.Specifically,trees with relatively high growth rates were more responsive to drought for root exudation rates compared to those with relatively low growth rates,which were closely related to root morphological traits and mycorrhizal infection rates.These findings highlight the importance of plant growth strategy in mediating drought-induced changes in root exudation rates of total C and its components in evergreen broadleaved forests.（2）Effect of root exudate inputs on soil priming effects under drought stress.Based on data from field experiments,we determined the ratio of sugar,organic acid,amino acid addition（60:35:2）,and the gradient of ¹³C isotope-labeled organic solvents(0,5,7.5,10μg C g^-1 soil d^-1)to quantify the soil priming effects induced by root exudate inputs under drought stress.We also quantified soil microbial and physicochemical properties to explore the drivers of soil priming effects.Results showed that drought significantly suppressed the soil priming effects by 36.0%.The soil priming effects increased with root exudate inputs in both control and drought treatments,but the magnitude was significantly different（Slope=2.4 vs.1.7,P<0.05）.Effect of root exudate inputs on soil priming effect under drought stress was closely related to changes in soil microbial carbon use efficiency,microbial copy number,and microbial community structure.Among these,microbial carbon use efficiency was the most important driver of soil priming effects.These results highlight that drought significantly suppressed the soil priming effect induced by root exudate inputs,and microbial carbon use efficiency can explain the variation in priming effects induced by root exudate inputs under drought.（3）Effects of different root exudate components on the soil priming effects under drought stress.¹³C isotope-labeled sugar,organic acid,and amino acid were added separately with the same carbon addition gradient as in the above section.We also quantified soil microbial properties and mineral protection structures to examine the driving factors underlying variations in soil priming effects.Results showed that organic acid addition induced a larger positive priming effect（+22.0%）,while glucose addition induced a smaller positive priming effect（+14.1%）.In contrast,amino acid addition induced a negative soil priming effect（–16.2%）.Drought significantly reduced the priming effect of three root-exudate components by 40.2%,24.9%,and 25.4%,respectively.The inhibitory effect of drought on soil priming effect increased with the dose of root-exudate inputs.The soil priming effects induced by different root exudate components under drought stress were closely related to soil microbial properties and mineral protection structures.Our results suggest that the changes in root-exudate components under drought can also profoundly affect soil organic carbon decomposition processes,which could help elucidate carbon sink and stability in subtropical forest ecosystems under drought.In summary,this study systematically examined the root exudate dynamics and associated soil priming effects under drought stress in evergreen broadleaved forests.Results showed that drought stress could significantly alter the root exudation rates of total C and its components in evergreen broadleaved forests,and the magnitude and direction of drought-induced changes in root exudates are affected by the plant growth strategy.Drought significantly reduced the soil priming effects induced by root exudate inputs,and microbial carbon use efficiency can explain the variation of soil priming effect under drought;Diverse root exudate components were able to drive soil priming effects through differential soil microbial properties and mineral protection structures,and drought could significantly reduce the soil priming effects induced by the three root exudate components.This study could enhance the understanding of soil carbon decomposition processes in evergreen broadleaved forests in the context of climate change,and provides support for terrestrial ecosystems’carbon process models and the management of subtropical forests in China.