Effects Of Experimental Warming On Root Traits In An Alpine Meadow Ecosystem

Roots are an important organ for plants to absorb soil water and nutrients.Fine root,as the most active portion in an integrated root system,participate in the material exchange and energy flow of the ecosystem through continuous growth,death,turnover and decomposition,and drive the carbon and nitrogen cycle of the ecosystem,which is the mainly source of soil carbon pool.Soil is the largest carbon pool in terrestrial ecosystems.Therefore,small changes in soil carbon pools may significantly alter the global carbon cycle.Alpine meadow in Qinghai-Tibetan Plateau is a sensitive area of climate change,the speed of climate warming in the Tibetan Plateau region is 2-3 times higher than the global average over.Exploring the root seasonal dynamics,root functional traits and root decomposition patterns in response to experimental warming in alpine meadow is a great significance to reveal the underground carbon cycling processes of future climate change on alpine meadows and even on global terrestrial ecosystems.This study used meta-analysis to synthesis the responded of plant root biomass(total root biomass,fine root biomass,coarse root biomass,and root: shoot ratio)to experimental warming,and the responses of root biomass to warming magnitude,warming duration,and warming method and the warming responses of root biomass in relation to background environmental conditions(ecosystem type,mean annual air temperature,mean annual precipitation,aridity index)were also investigated.An experimental warming(control,high warming > 2℃,low warming < 2℃)in Sichuan Zoige Alpine Wetland Ecosystem National Observation and Research Station on eastern Qinghai-Tibet Plateau was be selected to clarify mechanisms of belowground fine root response to warming.The fine root seasonal dynamics(root production,root mortality,standing root length and root turnover rate)were monitored by minirhizotron;the fine root functional traits were studied by the soil core;and fine decomposition of four functional groups was explored by the litter bags.The results showed that:(1)Experimental warming increased BGB,BNPP,root turnover rate due to soil drought,and decreased community-level root average diameter,root tissue density and increased specific root length,specific root area,as well as root length density,indicating a shift in strategy from resource conservation to acquisition in response to warming in the alpine meadow.In addition,with increasing temperature magnitude,the association among root traits became closer,indicating plant fine-root can accommodate environmental conditions via phenotypic plasticity.(2)The results of fine root functional traits responded to three soil depths(0-10 cm,10-20 cm and 20-40cm)showed that soil depth was an important spatial factor influencing fine root functional traits.Along with soil depth,fine-root traits exhibited consistent variation with experimental warming,the fine root functional traits gradually shifted towards resource acquisition traits.However,the controlling factor was inconsistent in different soil depths.Soil moisture and nutrients play a more and more important role from top to subsoil.(3)The meta-analysis collected 611 paired observations from 151 published peerreviewed articles.The results showed that Simulated warming significantly increased fine root biomass by 8.87%,but had no significant effects on total root biomass,coarse root biomass,and root: shoot ratio;Medium magnitude of warming(1-2 ℃)significantly increased fine root biomass and root: shoot ratio by 14.57% and 23.63%,respectively.The short-to medium-term(<5 years)warming enhanced fine root biomass,while long-term warming(<5 years)had a trend to decrease it.Both open-top chamber and infrared radiators significantly increased fine root biomass by 17.50% and 12.16%,respectively,while heating cables significantly decreased fine root biomass by 23.44% and coarse root biomass by43.23%;The warming responses of root biomass were inconsistent in different ecosystem types.Specifically,warming significantly increased fine root biomass by 21.03% in tundra ecosystems.The response of fine root biomass to simulated warming had significant and negative correlations with the background climate,such as mean annual air temperature,mean annual precipitation,and aridity index.(4)Analyzed the seasonal dynamics of fine root in alpine meadow from 2018 to 2020.Experimental warming not only significantly increased fine root standing stock,production and mortality in different soil depths in 2019(Undisturbed year),but also maintained a continuous increase in fine root production and standing stocks from May to July,2019,which was related to the input of above-ground litter and soil nutrient content.In addition,warming significantly increased fine root production at the beginning of the growing season,indicating that experimental warming advanced fine root phenology in alpine meadows.mortality dynamics.Moreover,the influence of warming on fine root dynamics strengthened in top soil was related to the decrease in soil resource effectiveness with increasing soil depth.(5)Extreme flooding during the 2018 and 2020 growing seasons showed that flooding events had a direct impact on the alpine meadow ecosystem,significantly reduced alpine meadow fine root production and standing stocks.Extreme flooding had changed the plant composition of alpine meadow,increasing the proportion of grass and legume,which had an indirectly affected on fine root production,mortality,standing stocks and root turnover in the year following the flooding event(2019).By comparing the fine root production and standing stocks of the control and warming treatment from 2017 to 2020 revealed that experimental warming significantly increased fine root production and standing stocks from 2019,which suggested that the extreme flooding event amplify the response of fine root seasonal dynamics to the experimental warming.(6)Based on the study of fine roots decomposition(<0.05 mm)of four plant functional groups in alpine meadows showed that the decomposition days and species had a significant effect on the decomposition of fine roots.Although the experimental warming had no significantly affect the decomposition of fine roots,the results of the decomposition experiment for one year revealed that the experimental warming had promoted the decomposition of forb and legume,which may be related to the species fine root characteristics.The decomposition of different plant functional groups in the alpine meadow had obviously stages,specifically the decomposition rate of the first stage was higher than that of the second stage,which was revealed to the initial chemical composition content of the four species.