| Casuarina equisetifolia was the most important protective forest species in the southeast coast of China.However,due to the neglect of management and its own nutrient fixation,soil nutrients are no longer sufficient to support the normal growth of trees,resulting in the decline of large-area C.equisetifolia Plantations to varying degrees,which seriously restricts the construction and development of the coastal protective forest system.Fertilization is the most rapid and effective method to stabilize and improve the ecological service value of C.equisetifolia plantation.In this study,The young(5 years),Middle-aged(12 years)and Mature(25years)C.equisetifolia forests in Pingtan Island,Fujian Province were taken as the research objects.The experiment of adding nitrogen,phosphorus and potassium was carried out by using the internal growth method to determine the functional and morphological characteristics,stoichiometric properties and metabolic enzyme activities of different fine roots.The water use efficiency was estimated by using the stable isotope method,and the nutrient capture strategy for the morphological and physiological characteristics of fine roots of different forest ages was analyzed,And determine the important factors that affect the water use efficiency of fine roots under different nutrient additions,in order to provide a theoretical basis for the stable and healthy growth of C.equisetifolia Plantation in coastal sandy land with increasingly frequent seasonal drought.The main results are as follows:(1)Nitrogen addition significantly increased the specific root length,C concentration,N concentration and C/N of C.equisetifolia,and decreased C/K;Phosphorus addition significantly increased the total root length,total root surface area,specific root surface area,C/N and C/K of fine roots,and decreased the concentration of K,C/P,N/P and K/P of fine roots;Potassium addition significantly increased the tissue density,C concentration,P concentration,C/K and N/K of fine roots of C.equisetifolia,and significantly decreased K concentration,C/P,N/P and K/P.(2)Forest age affects the sensitivity of different fine root functional morphological characteristics and stoichiometric traits to nutrient addition.The response of absorbing root morphological characteristics and stoichiometric traits to nutrient addition plasticity is more sensitive than that of transporting root,which reflects that there is a trade-off between different fine root functional morphological characteristics and stoichiometric traits of different forest ages in the strategy of nutrient resource acquisition and allocation.(3)The morphological characteristics and stoichiometric strategies of nutrient uptake and utilization by fine roots of C.equisetifolia at different forest ages were different.Under the condition of nitrogen addition,the young forest absorbed nutrients mainly through the variation of morphological characteristics,increasing the density of absorbing root tissue and prolonging the life of fine roots;Through the variation of fine root morphological characteristics and stoichiometric characters,the middle-aged forest can increase the specific root length and specific surface area of absorbing roots,build a dense root network,and increase the number of root transportation pipelines to obtain nitrogen nutrient resources;The morphological and stoichiometric characteristics of fine roots in mature forests were not significant,and there was no bias to invest in nitrogen absorption.Under phosphorus addition,the absorption strategies of fine roots of C.equisetifolia at different forest ages were opportunistic strategies.The young forest mainly obtained phosphorus resources by increasing the metabolism rate and growth rate of absorption roots through the variation of stoichiometric characteristics of absorption roots;The middle-aged forest can not only increase the physiological and biochemical activities of fine roots,but also expand the absorption range of fine roots to obtain nutrient resources through the two-way variation of morphological and stoichiometric characteristics of absorbing roots;The mature forest mainly obtains phosphorus nutrient resources by absorbing the variation of root morphological characteristics,increasing root life and expanding the absorption range of fine roots.Under potassium addition,the young forest mainly obtains potassium nutrient resources through the variation of stoichiometric characters of absorbing roots,increasing the metabolism rate of absorbing roots and the growth rate of roots;The middle-aged forest not only increases the metabolic rate and root growth rate,but also expands the root absorption range to obtain phosphorus nutrient resources through the absorption of root stoichiometry and morphological characteristics variation;The fine roots of mature forest had no significant plasticity response to the increase of potassium nutrient content.(4)The effects of fine roots of C.equisetifolia at different ages on the activities of metabolic enzymes in nutrient uptake were slightly different.Under nitrogen addition,nitrogen addition increased GS activity,IAAO activity and ACP activity of absorbing roots of young forest,but there was no significant difference in enzyme activity of absorbing roots of other forest ages.Under phosphorus addition,IAAO activity of young forest was significantly increased,and IAAO activity of transportation roots in C.equisetifolia control and phosphorus addition treatment was significantly higher than that of absorption roots;The ACP activity of transporting roots decreased,while that of absorbing roots increased at different ages of C.equisetifolia.Under potassium addition,the control group of middle-aged forest was the highest,which was significantly higher than that of potassium treatment.The GS activity of transport roots of middle-aged forest increased significantly,and that of transport roots of young forest decreased significantly.IAAO activity of absorbing roots in young forest was significantly increased.(5)The water use efficiency(WUE)of fine roots of C.equisetifolia at different forest ages showed young forest age > mature forest > middle forest age.The water use efficiency had different rules in different fine root functions,indicating that different nutrient additions affected the carbon water coupling mechanism of fine root functions.There were significant differences in the contribution of fine root morphological characteristics,stoichiometric plasticity response and metabolic enzyme variation to the change of water use efficiency under different nutrient addition treatments.In the variation of morphological characteristics,the total surface area of fine roots increased with the addition of nitrogen and phosphorus,and the contribution of fine root biomass increased with the addition of potassium to fine root water use efficiency was the largest.In the variation of stoichiometric characters,the concentration of N and K in fine roots decreased with the addition of nitrogen,the concentration of c/n and c/k increased,the concentration of K in fine roots increased with the addition of phosphorus,the c/n decreased,the C/P in fine roots increased with the addition of potassium,and the decrease of the concentration of N and K contributed the most to the water use efficiency.In the variation of metabolic enzyme activity,the increase of Cpr activity,the decrease of GS activity,the increase of NR activity,the decrease of Cpr activity and ACP activity,the increase of GS activity and the decrease of IAAO activity and NR activity with the addition of potassium have a great contribution to the improvement of fine root water use efficiency.To sum up,C.equisetifolia with different nutrient additions can coordinate the internal relationship of fine root morphology and physiology,and implement a diversified nutrient absorption strategy.Due to the difference in photosynthesis level and the cost of building and maintaining fine root carbon,the fine roots of C.equisetifolia with different forest ages make selective investment in acquiring various nutrient resources,so as to achieve the maximum nutrient capture efficiency.In addition,the contribution of changes in fine root morphological and physiological traits to the increase of water use efficiency was different under different nutrient addition treatments.Therefore,in the process of production and management,in order to ensure the healthy growth of C.equisetifolia and cope with the increasingly frequent seasonal drought,corresponding tending and management schemes can be adopted according to the growth and development characteristics of fine roots at different growth stages and the corresponding water use efficiency characteristics of C.equisetifolia. |
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