Plastic Responses Of Fine Root And Mycorrhizal Functional Traits Of 23 Tree Species To Phosphorus-rich Patches In A Subtropical Evergreen Broad-leaved Forest

Phosphorus（P）is one of the key limiting factors for plant growth in terrestrial ecosystems,and fine root nutrient acquisition plasticity is an important way for plants to improve the absorption of soil P.At present,there are few in situ studies on the plastic responses of fine roots to soil P availability（such as P-rich patches）of mature trees in subtropical natural forests,and most of the existing studies are based on the soft traits of fine roots（such as morphological and architectural traits）,ignoring the hard traits（such as root physiological and mycorrhizal fungi traits）,which are crucial for P acquisition.Furthermore,there are many related studies on root economic spectrum,which reveal the interspecific variations of root traits but can not be used to predict the plant nutrient acquisition plasticity.How plants coordinate the plastic responses of root nutrient acquisition traits remains unclear.So,the following scientific questions are raised:Are there any differences in the plastic responses of fine root and mycorrhizal nutrient acquisition traits to P-rich patches among different tree species?What are the relationships among the plastic responses of these traits?Can fine root traits predict the plastic responses of these traits?How do soil P availability affect the plastic responses of these traits?In this study,23 tree species（including evergreen broadleaved（EB）and deciduous broadleaved（DB）tree species,arbuscular mycorrhizal（AM）and ectomycorrhizas（ECM）tree species）in a subtropical evergreen broad-leaved forest were selected.The control and P-rich patches were established by root bag method.The fine root and mycorrhizal nutrient acquisition traits of different tree species were measured,including the fine root morphological traits（root average diameter（AD）,specific root length（SRL）and root tissue density（RTD））,architectural traits（specific root tip density（SRT）and specific root fork density（SRF））,proliferation traits（root length growth rate（LGR）and root mass growth rate（MGR））,physiological traits（root acid phosphatase（RAP）and root exudation rate（RE））and mycorrhizal traits（mycorrhizal colonization rate（MC）and extramatrical hyphal length（EHL））.The fine root chemical traits（root nitrogen（RN）and P concentration（RP））and soil P availability indicators(p H,soil available P(PO₄^3--P),ratio of soil microbial biomass N concentration to microbial biomass P concentration（MBN/MBP）and ratio of soilβ-1,4-N-acetylglucosaminidase to soil acid phosphatase（SNAG/SAP）)were also measured.Based on these above,the plastic responses of fine root and mycorrhizal nutrient acquisition traits to soil P-rich patches were explored,so as to further understand the strategies of belowground P nutrient acquisition of subtropical tree species and their adaptation to soil environmental changes.The main results are as follows:（1）Response of soil P availability and fine root nutrients of different tree species to P-rich patches:Tree species had significant effects on the R^3-_（PO4-P）,and P addition significantly increased soil available P concentration in the root bags of EB and AM tree species（P<0.05）.Tree species also had significant effects on the R_RN and R_RNP（P<0.01）,and P addition significantly increased the RN in the root bags of EB tree species（P<0.05）.（2）The interspecific differences in the plastic responses of fine root and mycorrhizal nutrient acquisition traits to P-rich patches:Tree species had significant effects on the R_AD,R_RTD,R_LGR,R_MGR,R_REand R_EHL（P<0.05）.Tree species showed diversity in their plastic responses of fine root and mycorrhizal traits:with tree species exhibiting plastic responses of only single trait,multiple traits,or even showing no significant plastic response.In addition,leaf habits had a significant effect on the R_RE（P<0.05）,while mycorrhizal types had no significant effect on the response ratio of any trait（P>0.05）.P addition significantly increased the SRT and significantly reduced the RE of DB tree species,reflecting the P acquisition strategy of DB tree species.（3）Synergies or tradeoffs among plastic responses of fine root and mycorrhizal nutrient acquisition traits:The R_SRL and R_SRF were positively correlated with the R_SRT,but were negatively correlated with the R_RTD.The plastic response of the absorptive root proliferation plasticity was independent from that of hyphal proliferation in the acquisition of soil P.Some relationships were affected by leaf habits:the negative relationship between R_SRL and R_RE existed in the DB species,but not in the EB species.Some relationships were affected by mycorrhizal types:the positive relationship between R_SRL and R_SRF,and the negative relationship between R_RAP and R_EHL were detected in the AM species,but not in the ECM species.The plastic responses of fine root and mycorrhizal fungi traits were multidimensional,as indicated by the principal component analysis（PCA）.（4）Predictions of common fine root traits to plastic responses of fine root and mycorrhizal nutrient acquisition traits:The thinner-root tree species enhanced exploitation of the P-rich patches through the mycorrhizal plasticity,while the thicker-root tree species through the fine root physiological plasticity.EB tree species with lower SRL had higher R_LGR,while DB tree species with higher SRL increased the R_MGR.ECM tree species with lower RP increased the R_SRL and decreased the R_RTD,while AM tree species had no trait response related with RP.（5）Predictions of soil P availability to plastic responses of fine root and mycorrhizal nutrient acquisition traits:In the soil with lower available P concentration,ECM tree species could enhance the acquisition of soil P by increasing the R_SRL and decreasing the R_RTD,while AM tree species had no trait plastic response related with the soil available P.In the soil with higher MBN/MBP,DB tree species could increase the R_MGR,while EB tree species had no trait plastic response correlated with the soil MBN/MBP.In the soil with lower SNAG/SAP,ECM tree species could enhance soil P acquisition through increasing the R_SRL and R_SRT,while AM tree species had no trait plastic response linked with the SNAG/SAP.In summary,the plastic responses of fine root and mycorrhizal nutrient acquisition traits to P-rich patches differed among tree species,but leaf habit and mycorrhizal type had no significant effect on the plastic responses of most traits.There were a series of synergies or trade-offs in the plastic responses among fine root traits and between fine root and mycorrhizal traits,but the interspecific variations of these trait plastic responses were multidimensional,which enable tree species to better adapt to a variety of nutrient niches.The fine root diameter,SRL,RP and soil P availability can predict some plastic responses of these traits,which depended on leaf habits or mycorrhizal types.Tree species can acquire P from soil through only one or a combination of these trait plasticity,which reflects the diversity in belowground nutrient acquisition strategies.