Effects Of Drought On Root Respiration And The Mechanisms

In the context of global climate change,the frequency,intensity and duration of extreme drought events will significantly increase over time.Extreme drought and its consequent changes in soil moisture could strongly affect terrestrial carbon cycle,especially for belowground carbon processes.As the second-largest flux in terrestrial ecosystems,soil respiration consists of soil heterotrophic and autotrophic(root)respiration,which are both considerably influenced by soil water content.Previous studies on responses of soil carbon fluxes to drought mostly focused on soil respiration and heterotrophic respiration.It remains unknown how root respiration responds to drought and what mechanisms underpin the biological and abiotic regulations.In this study,we first explored potential mechanisms controlling responses of root respiration to drought in different ecosystems by analysing a global dataset of drought simulation experiments.Secondly,we monitored root respiration in a long-term rainfall exclusion experiment in a subtropical evergreen broad-leaved forest to investigate the roles of soil environment and key biological processes in response to extreme drought.By conducting a greenhouse drought experiment on four dominant tree species with different hydraulic characteristics in subtropical forests,we further revealed the differential regulatory mechanisms influencing responses of fine root respiration to drought among tree species.The main findings were as follows:(1)By analysing the global dataset of simulated drought experiments,we found that drought significantly inhibited root respiration and specific root respiration in natural ecosystems(e.g.,forests and grasslands).The total and specific root respirations decreased by 21% and 10%,respectively.With the increase of drought intensity and the decrease of soil water content,the extent to which root respiration was inhibited by drought increased gradually.Among the biotic factors,photosynthesis,fine root biomass and root/shoot ratio were important factors affecting the responses of root respiration to drought.Under drought,both root respiration and specific root respiration declined with the decreased photosynthesis.In addition,drought caused a reduction in fine root biomass and an increase in root/shoot ratio at a global scale,which both consequently led to decreased root respiration.These results suggest that plants may change carbon allocation strategy under drought(e.g.,allocating a larger portion of carbon to belowground than aboveground parts)to mitigate the adverse effects of drought on plant physiology.(2)Based on the long-term monitoring of rainfall exclusion experiment in the subtropical forest,we found that extreme drought significantly reduced root respiration by 35%.The significant inhibited effects of drought on root respiration in the growing season were different from non-growing season with no significant change.Soil moisture and temperature were the most important environmental factors affecting root respiration.Although drought treatment did not change soil temperature,the relationships between root respiration and soil temperature or moisture altered under drought.The effect of fine root biomass on root respiration showed opposite patterns between the control and the treatment groups.Specifically,as fine root biomass increased,root respiration increased in the control group,but decreased in the drought group.Moreover,drought promoted the growth of fine root biomass,while,the stimulated root biomass did not give rise to the increase in root respiration.These results suggest that plants allocated more carbon for root construction rather than metabolic activities.(3)To further explore the mechanisms of the fine root respiration in response to drought and their differences among species,we conducted a greenhouse experiment with persistent drought.We found that drought significantly reduced fine root respiration of the four subtropical tree species.The responses of fine root respiration to drought exhibited significant differences among root orders.Specifically,the respiration of first-order roots was the most sensitive to drought,sensitivity of fine root respiration to drought decreased with the increase of root order rank.Plant photosynthesis played a key role in regulating fine root respiration in response to drought.Fine root respiration increased with fine root nitrogen content and specific root length,and decreased with mean diameter.Drought had the strongest effects on fine root respiration of Phoebe chekiangensis,and the weakest effects on Schima superba.Under drought,biotic factors dominated the regulations on the responses of fine root respiration to drought,explaining 75% of the total variation in fine root respiration.These results indicate that the different responses of fine root respiration to drought among species were associated with hydraulic traits of plants.In comparison with other species,the substantially rapid decline of photosynthesis in Phoebe chekiangensisd under drought might result from decreased stomatal activities,causing the strongest responses of fine root respiration.In summary,this study explored the pattern and controlling factors of root respiration in response to drought.We firstly examined the general pattern of changes in root responses under drought at a global scale.Secondly,we investigated the roles of soil environment and biological processes in regulating the effects of long-term experimental drought on root respiration in a subtropical evergreen broad-leaved forest.We also revealed the differential mechanisms underlying the different responses of fine root respiration to drought among species.These results will improve our understanding on the carbon dynamics of terrestrial ecosystems in future climate change and provide some insights and a data basis for accurately predicting the carbon cycle by Earth system models.