Effects Of Phosphorus Addition On Plant Root Characteristics And Leaf Traits In An Alpine Meadow Of The Northwestern Sichuan,China

Phosphorus is an important element required for plant growth.Nitrogen deposition has exacerbated the potential phosphorus limitation on the Qinghai-Tibet Plateau.Alpine meadows,as the representative grassland type of the Qinghai-Tibet Plateau,are very sensitive to environmental changes.Many studies have focused on the vegetation growth and community dynamics of alpine meadows,but there are few reports on the response of underground ecological processes,especially the dynamic changes of plant roots to phosphorus addition.In this paper,four fertilization gradients(0,10,20,30 g/m~2)were set up in the alpine meadows of northwestern Sichuan.The micro root window technology and a combination of field investigation and indoor analysis were used to monitor the difference of plants for two consecutive years changes in root-level dynamics,species leaf properties,plant community characteristics,and soil microenvironment changes in order to clarify the interconnections between above-ground ecological processes in alpine meadows under phosphorus addition.The results showed that:1.Phosphorus addition generally increased the total biomass and coverage of the community,of which P20 increased the biomass and coverage of grass and forbs,P10increased legume biomass,and P30 increased coverage of sedge.Phosphorus addition increased the Shannon-Wiener index and Pielou index,reduced the Simpson diversity index of plants,and the species richness tended to decrease.2.Overall,P10 and P30 increased the specific leaf area of legume plants,and P20 and P30 increased the specific leaf area of grass plants,but phosphorus addition had no significant effect on the specific leaf area of forbs and sedge;The content of N in the leaves of grass and legume plants was increased;P30 increased the content of P in the leaves of the plants but decreased the C:P and N:P in the leaves.3.Phosphorus addition increased root turnover,root production,mortality,and standing crop.The peak value of standing crop appeared in August-September;P20increased root N content,and phosphorus addition reduced root C content but reduced P content.However,the impact of phosphorus addition was weak.4.P30 increased soil(10-20 cm)β-1,4-N-acetylglucosidase(NAG)activity and decreased soil acid phosphatase(AP)activity,but P10 and P20 weakened AP enzyme activity Not significant;overall phosphorus addition reduced soil(10-20 cm)N:P enzyme activity ratios and all were greater than the global average(0.44),but had no significant effect on soil C:N enzyme activity ratio and soil C:P enzyme activity ratio influences.5.Phosphorus application increased soil compactness,reduced soil p H and soil moisture;P20 and P30 as a whole increased soil total phosphorus content,and reduced C:P and N:P;phosphorus application also improved soil quick-effect Nitrogen and available phosphorus content.6.Structural Equation Modeling(SEM)analysis revealed that phosphorus addition indirectly affected plant roots,leaf properties,and community characteristics by affecting soil available nutrient.In summary,the phosphate fertilizer directly changed the physical and chemical properties of the soil,which in turn affected plant roots,leaf attributes,and vegetation community characteristics,and significantly increased underground and aboveground biomass.Phosphorus supplementation improved the availability of nutrient in the"food chain"of the alpine meadow ecosystem,making soil available nutrient a link between the above-ground and underground ecological processes.According to the"optimal allocation"model theory of ecological economics,plants were more inclined to distribute nutrient to the above-ground parts through soil,root systems,etc.to compete for light resources to promote growth,resulting in an underground part having a greater feedback effect than the above-ground part.In addition,phosphorus application made the soil more available phosphorus.More phosphorus was transported to the leaves by the root system,which resulted in incomplete synergy between the root system and leaf nutrient storage.The soil N:P,leaf N:P and root N:P were all less than 13,and the ratio of soil N:P enzyme activity was greater than 0.44,indicating that phosphorus addition may trigger potential nitrogen limitation in alpine meadows,which in turn affected the stability of ecology system structure and function in alpine meadow.The optimal nutrient addition scheme for alpine meadow should be combined with nitrogen and phosphorus.