| Aiming at the problem of poor soil nutrients caused by nitrogen and phosphorus limitation in Robinia pseudoacacia plantation on the Loess Plateau,this paper explored the dynamic characteristics of nitrogen and phosphorus in R.pseudoacacia at different ages and the synergistic relationship between nitrogen and phosphorus absorption and utilization capacity.R.pseudoacacia of 15 a,27 a,35 a and 45 a forest age were selected.Through the combination of field sampling and laboratory experiments,R.pseudoacacia at different growth and development stages in one year were sampled and analyzed.The contents of carbon(C),nitrogen(N)and phosphorus(P),nutrient use efficiency(NUE,PUE),nutrient reabsorption efficiency(NRE,PRE),fine root biomass,mycorrhizal infection rate,nodule number,rhizobium community and alkaline phosphatase activity of soil,leaves,roots and fine roots in different periods were measured to explore the dynamic characteristics of N and P in different development periods of R.pseudoacacia plantation in the Loess Plateau.The mature period(August)and litter period(November)were selected to explore the relationship between soil C,N,P content and leaf C,N,P content and leaf nitrogen and phosphorus reabsorption efficiency.The nitrogen absorption capacity(fine root biomass,root nodule number,soil nitrogen content and soil nitrogen-phosphorus ratio),phosphorus absorption capacity(mycorrhizal colonization rate,fine root biomass alkaline phosphatase activity,soil phosphorus content and soil nitrogen-phosphorus ratio),nitrogen utilization capacity(leaf nitrogen content,leaf nitrogen utilization efficiency,fine root nitrogen content,fine root nitrogen utilization efficiency and leaf nitrogen reabsorption efficiency),phosphorus utilization capacity(leaf phosphorus content,leaf phosphorus utilization efficiency,fine root phosphorus content,fine root phosphorus utilization efficiency and leaf phosphorus reabsorption efficiency)were selected.Mantel test was used to analyze the synergistic relationship between different abilities and their variation characteristics with forest age.The main conclusions are as follows:(1)The contents of nitrogen and phosphorus in the leaves of R.pseudoacacia at different ages(15a,27 a,35a,45a)showed a downward trend throughout the growing season.The content of nitrogen and phosphorus in branches decreased first and then increased.The highest content of nitrogen in branches appeared in the litter stage,but the period of the highest content of phosphorus in branches was not consistent.The root nitrogen content showed an upward trend,and the change trend of root phosphorus content was not consistent.The root phosphorus content of R.pseudoacacia at 15 a,27a and 45 a forest ages decreased first and then increased,and the 35 a forest age showed an increasing trend.According to the proportion of nitrogen and phosphorus content in each organ,the proportion of nitrogen and phosphorus content in the leaves at the germination stage was the highest in each organ.The proportion of nitrogen content in the leaves of R.pseudoacacia at different ages was 58 %,48 %,54 %,55 %,and the proportion of phosphorus content in the leaves was 62 %,54 %,63 %,69 %,respectively.The leaves were the nutrient supply organs in this period;the proportion of nitrogen and phosphorus in branches and roots increased gradually and became the storage organ of nitrogen and phosphorus during the littering period.The proportion of nitrogen content in branches was26 %,22 %,21 % and 27 % respectively,and the proportion of phosphorus content in branches was 40 %,34 %,33 % and 47 % respectively.The proportion of root nitrogen content was 44 %,48 %,46 % and 45 % respectively,and the proportion of root phosphorus content was 35 %,39 %,35 % and 25 % respectively.The proportion of nitrogen content in the roots of the litter stage was the highest,so the plant roots were the main nitrogen storage organs of different forest ages.The proportion of phosphorus content in the branches of 15 a and 45 a forest age was the highest,so the branches were the main storage organs of phosphorus in the forest age.The proportion of phosphorus content in roots of 27 a and 35 a forest age was the highest at the litter stage,so plant roots were the main storage organs of phosphorus at the litter stage.(2)The effects of soil C,N,P content on C,N,P content and nitrogen and phosphorus reabsorption efficiency of R.pseudoacacia leaves at different forest ages were mainly reflected in 27 a,35a and 45 a forest ages.There was a significant negative correlation between soil C/P and PUE of mature leaves in 27 a forest(P < 0.05),which was consistent with the results of higher total phosphorus content and lower PUE in actual leaves.There was a significant positive correlation between NRE/PRE and soil total phosphorus content(P < 0.05).The lack of soil total phosphorus would stimulate R.pseudoacacia to increase the reabsorption of phosphorus,resulting in the decrease of NRE/PRE at the age of 35 a,and PREwas always higher than NRE,indicating that R.pseudoacacia was always more inclined to reabsorb phosphorus.There was a significant positive correlation between soil total nitrogen content and N/P and total nitrogen content of mature leaves at 45 a forest age(P < 0.05).At the same time,there was a significant positive correlation between NRE/PRE and soil total nitrogen content(P < 0.05),indicating that R.pseudoacacia not only needed to absorb nitrogen from soil,but also needed to increase NRE to alleviate nitrogen deficiency.Compared with mature leaves,the nitrogen and phosphorus contents of 15 a,27a,35 a and 45 a aged R.pseudoacacia leaves decreased,and NUE and PUE increased.The nitrogen and phosphorus contents of senescent leaves were significantly negatively correlated with NRE and PRE(P < 0.05).The PRE of R.pseudoacacia leaves was 1.36(27a)-1.8(35a)times that of NRE,indicating that growth was limited by phosphorus.(3)There was a significant correlation between nitrogen absorption,nitrogen utilization,phosphorus absorption and phosphorus utilization(P < 0.01).At low forest age(15a,27a),leaves had higher leaf total nitrogen content,leaf total phosphorus content,leaf nitrogen and phosphorus reabsorption efficiency,fine roots had higher nitrogen and phosphorus utilization efficiency,and the forest age section had higher AMF and nodule number.At high forest age(35a,45a),phosphorus limitation was stronger,leaves had higher nitrogen and phosphorus utilization efficiency,while fine root total nitrogen content and fine root total phosphorus content were higher,and fine root biomass increased significantly.The results showed that the utilization of nitrogen and phosphorus was preferentially supplied to the growth and development of leaves and other organs at low forest age(15a,27a).Phosphorus absorption depended on higher AMF,and nitrogen absorption depended on the biological nitrogen fixation ability of nodules.At high forest ages(35a,45a),the higher N/P in soil and the lower NRE/PRE in leaves indicated that the forest age had strong phosphorus limitation.The use of nitrogen and phosphorus will give priority to the growth of fine roots,and the absorption of nitrogen and phosphorus is achieved by increasing FRB and AMF and increasing PUE and PRE.With the increase of forest age,the absorption of nitrogen and phosphorus by R.pseudoacacia changed from special mycorrhizal strategy to fine root strategy,and the utilization of nitrogen and phosphorus changed from the growth of aboveground part to the growth turnover of underground fine roots.The results of this study will help us to further understand how R.pseudoacacia forests adapt to nutrient limitation conditions on the Loess Plateau by regulating their own growth.It provides theoretical support for the management,stand optimization and sustainable management of R.pseudoacacia forest,and promotes the ecological benefits of R.pseudoacacia forest. |
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