Characteristics Of Water And Nutrient Use Of Robinia Pseudoacacia Plantations In Different Forest Ages In Loess Hilly Region

The ecological environment in the loess hilly region is extremely fragile and has become a key area for vegetation restoration and soil and water conservation.At present,the focus of its ecological reconstruction should be mainly on the efficient use and management of existing plantations.In this study,the typical representative area of the Loess Hilly Region is the Zhifanggou Watershed,and the Robinia pseudoacacia plantations with different growth years are the research objects.The research method of combining field investigation and indoor analysis,through measuring leaf-litter-soil nutrient concentration and leaf carbon-nitrogen isotope ratio of Robinia pseudoacacia plantations at different forest ages,to explore the difference between leaf-litter-soil nutrient content and stoichiometric characteristics,combining the differences in water use efficiency reveal the characteristics of nutrients and water use of Robinia pseudoacacia plantations at different forest ages,and provide a theoretical basis for the rational management and efficient management of plantation forests in the region.The main conclusions are as follows:?1?With the increase of forest age,the nutrient content of different organs of R.eudoacacia plantation in the study area was leaf>root>branch,the leaf C and P content increased first and then decreased,reached the maximum in 21 years,and the N content decreased first It increases after a small period and reaches the minimum value in 33 years,but the changing rules of C:N,C:P and N:P with the increase of forest age are not consistent.Forest age has a significant effect on leaf N:P.The leaves of Robinia pseudoacacia showed high C and N in the study area,and the forest age had no significant effect on plant nutrient limitation,but the growth of Robinia pseudoacacia at different forest ages was restricted by P element.?2?With the increase of forest age,the content of litter C in the study area was 17a>13a>33a>21a>43a,and the difference was not significant;the contents of N and P showed an increasing trend and then decreased and the difference was significant?P<0.05?,and all reached the maximum value in 21 years,C:N,C:P,N:P showed a decrease after 13a with the increase of forest age,increased after reaching the minimum value in 21a,and reached the maximum in 33a The law of change after the value decreases again.The litter as a whole showed an elemental pattern with high C and N and low P.Litter C:N is less than 40,but C:P is significantly higher than 480,which is not conducive to nutrient release.The difference of N:P in litter of Robinia pseudoacacia plantation with the age of the forest was not significant,indicating that the rate of litter decomposition and nutrient return in the Robinia pseudoacacia plantation was relatively balanced throughout the growth stage.?3?With the increase of forest age,the contents of soil C and N in the study area are 21a>43a>17a>33a>13a,and the change rule is basically the same with the increase of forest age.The C:N,C:P and N:P values are not consistent with the increasing age.The average status of soil nutrients in the study area is lower than the national soil level,indicating that the soil nutrient storage capacity of the area is poor.The content of soil N and P gradually increased with the increase of forest age,indicating that the plantation community structure gradually became more complex,the diversity of understory preparation increased slowly,and the soil surface nutrients accumulated year by year with the increase of forest age.The availability of element P in the top soil is high;the decomposition of organic N is slow,which affects the absorption efficiency of nutrients by plants.?4?The N and P element nutrient resorption efficiency decreases first and then increaseswith the increase of forest age,reaching a minimum of 41.15%in 21 years,and the 21year reabsorption efficiency is significantly related to 13 years,33 years,and 43 years?P<0.05?.The element P nutrient resorption efficiency decreased first and then increased with the increase of forest age,and then gradually decreased after reaching the maximum value of79.46%in 33 years.Difference?P<0.05?.?5?Different elements have different stoichiometric internal stability.The average value of the internal stability index(H_N,H_P,H_N:P)of the ratio of N,P and N:P in this ecosystem is 8.16,0.40 and 3.77,respectively,and HN is significantly higher than HP?P<0.05?.According to the standard of internal stability,H_P is sensitive,H_N:P is weak steady state,and H_Nis steady state.With the change of forest age,the difference between different organs of the same growth element was significant.Element N showed H_{N branch}>H_{N leaf}>shoot.?6?The ?¹³C of leaves of R.seudoacacia plantations with different forest ages increases with the age of the forest,showing a trend of increasing first and then decreasing.At 21years,?¹³C reaches a maximum value of-27.47‰.The organic quantity fixed per unit water volume reaches the maximum value.Correlation analysis of long-term water and nutrient use efficiency of R.seudoacacia plantations at different forest ages showed that R.seudoacacia had lower water use efficiency and nitrogen use efficiency?C:N?in 13 and 17years,and its significant negative correlation showed that there was a certain The compensation mechanism of mutual restraint gradually disappears as the forest age increases.?7?With the increase of forest age,the trend of water and nutrient use efficiency of Robinia pseudoacacia is different.20 to 27 years have higher water and nutrient use efficiency,and then gradually decrease.