| Nitrogen(N)and phosphorus(P)are two mineral elements that are most demanded by plants in the terrestrial ecosystem,and they participate in many physiological processes,such as plant growth and metabolism.Most of the nutrients obtained by plants are absorbed by roots,and root nutrition is the premise and guarantee of normal plant growth.Meanwhile,leaf nutrient resorption refers to the process of transferring part of the available nutrients to other organs before aging and shedding of leaf,which is one of the most important ecological strategies for plants to improve their own nutrient utilization efficiency and environmental adaptability.However,there is still a lack of unified understanding on how to regulate the relationship between root nutrient acquisition and leaf nutrient resorption during plant growth to achieve the best growth state when the condition of soil nutrient supply changes.Metasequoia glyptostroboides is the main tree species in the area of the sea-land ecotone and the eastern argillaceous coast and its existing acreage is about 50% of the total acreage of backbone forest area.Metasequoia glyptostroboides have made outstanding contribution to consolidate the ecological security and economic development in the coastal area.In this study,taking the young and middle-aged Metasequoia glyptostroboides plantation in the Dongtai forest farm in Dongtai,Jiangsu province as the research project,through the method of randomized block experiment carried out five N and P addition test.In 2018 and 2020,the author collected the samples of plant organs,litter and soil,analyzed the effects of N and P addition on the characteristics of the plant stoichiometric,leaf nutrient resorption and root nutrient acquisition and revealed the relations between leaf nutrient resorption and root nutrient acquisition.The main research results are as follows:(1)Under the condition of N and P addition,some changes have taken place in organs,litter,soil nutrient and stoichiometric characteristics of plants.N and P addition in each year had no significant effect on the plant C(carbon)content of each forest age,and the structural element stayed relatively stable.Under the condition of N addition,the P content of organs and litter decreased significantly of each forest age in 2018 and 2020.The carbon: phosphorus(C: P)of organs and litter of young plantations in each year increased significantly with the increase of N addition,and the nitrogen:phosphorus(N: P)of organs and litter of young plantations in each year increased significantly with the increase of N addition.Only the carbon: nitrogen(C: N)in the soil of middle-aged plantations decreased significantly in 2020.Under the P addition,the N content of organs and litter of each forest age and each year increased significantly with the increase of P addition,while the P content increased significantly in young plantations and decreased significantly in middle-aged plantations.The C: N and C: P of organs and litter of each forest age in each year decreased with the increase of P addition.(2)Under N addition,the root-soil accumulation factor for nitrogen(RSAF-N)of young forest in each year significantly decreased with the increase of N addition.The year of N addition had direct positive impact on RSAF-N of young forest.Through the change of fine root N content,it had indirect positive impact on RSAF-N.The year of N addition had direct negative impact on RSAFN of middle-aged forest.With the increase of N addition year,RSAF-N decreased.Under P addition,RSAF-N of young forest significantly increased with the increase of P addition.The root-soil accumulation factor for phosphorus(RSAF-P)of young forest in each year significantly decreased with the increase of P addition and in 2020,RSAF-P of middle-aged forest decrease,too.The year of P addition had direct positive impact on RSAF-N.It had indirect positive impact on RSAF-N through the change of fine root N content.With the increase of P addition year,RSAF-N increased.The year of P addition had direct positive impact on RSAF-P of middle-aged.It had indirect positive impact on RSAF-N through the change of fine root N content.With the increase of P addition year and P addition level,RSAF-N and RSAF-P increased and decreased respectively.(3)Under N addition,the nitrogen resorption efficiency(NRE)of young forest in 2018 significantly increased with the increase of N addition.In the year of N addition,the change of the litter P content had indirect positive impact on NRE.NRE increased with the increase of N addition year.The year of N addition had direct negative impact on the NRE of middle-aged forest.With the increase of N addition year,NRE decreased.Under P addition,NRE of young and middle-aged forest in 2018 significantly increased.The phosphorus resorption efficiency(PRE)of the middleaged forest in 2018 significantly increased.The year of P addition had direct negative impact on PRE of young forest.Through the change of P content in leaf,it had indirect negative impact on NRE.With the increase of P addition year,the NRE and PRE of young forest decreased,however the NRE and PRE of middle-aged forest increased.(4)Under N and P addition,the result of root nutrient accumulation factor and leaf nutrient resorption efficiency shows that under N addition,positive linear relation existed between the RSAF-P and PRE of young and middle-aged forest in 2018,negative quadratic function relation existed between the RSAF-N and NRE of young forest in 2020.Under P addition,positive linear relation existed between the RSAF-N and NRE of young forest in 2018.To sum up,with the increase of N,P addition year,nutrient limit of plant growth will ease gradually,the way of plant nutrient acquisition changes from leaf nutrient resorption to leaf nutrient acquisition.The relationship between them change from coordinated variation to trade-off,which embodies the ecological adaptation strategies of plants to nutrient condition changes.Future research will combine soil nutrient availability and nutrient limitation,rationally add fertilizer to achieve the positive accumulation of plant biomass so as to provide theoretical research and technical reference for the realization of the “double carbon” goal of plantation cultivation. |
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