Effect Of Nitrogen Loading On Seasonal Nitrogen Internal Cycling And Seedling Quality Of Quercus Variabilis

Nitrogen(N)is important for plant growth,reproduction and stress tolerance,and it is often a limited resource in many forestry ecosystems.Plants have developed a variety of adaptations to optimize the use of N.One of these adaptations is storage and remobilization of N for supporting main plant functions,which is called internal cycling of N.The remobilization of N contributes 50-90%of the total N in the early spring and reduces the dependence on soil N during periods of high N demand.N remobilization is affected by storage N level,and it could be also influenced by environmental conditions.Forest plantations in temperate climates are usually established during the winter and spring.However,in many parts of the China,trees can be subjected to drought during the spring.The effect of spring drought on N remobilization and N uptake is still not clear.The remobilized N in the next spring comes from the storage reserves and N storage is contributed by N uptake during the hardening period and N resorption.The effect of N uptake during the hardening period on N resorption and storage is still not clear.To study all these questions could provide theoretical basis for fertilization.The objective of this study is Quercus variabilis Blume seedlings.This study analyzes(1)how the size of N storage in seedlings interacts with drought stress to affect N remobilization,uptake and new growth.(2)and analyzes how seedling sizes interacts with N-loading during the hardening period to affect N resorption,uptake and N,NSC storage using 15N-enriched fertilizer.To understand how seedlings use storage N under spring drought and how seedling use N uptake during the hardening period to promote the reserves of NSC and N could provide practical guidance of fertilization.(3)The study is also investigating the independent and interacting effects of exponential N fertilization during the rapid growth period and N loading during the hardening period on growth,nutrient acquisition and accumulation during different growth phases 1 year after seedlings transplanting in a nursery.The main results and conclusions are as below:(1)During bud burst and initial shoot elongation(T1),restricted watering,which induced a moderate water stress,did not affect the amount of N remobilized from roots,the major source of stored N source at this growth stage.This suggests that high N storage can partially counteract the negative effect of moderate water stress on early growth.At T1,water stress did not affect N uptake,and high N content seedlings absorbed significantly less soil N than low N content seedlings did.At T3,in contrast,water stress was the main determinant for N uptake,with drought-stressed plants showing lower uptake than well-watered plants.We conclude that moderate drought does not inhibit N remobilization from the major storage organ at early growth stages in spring,and that increasing N storage of planted seedlings through N-loading during the hardening period can mitigate the negative effect of moderate spring drought during the next growing season on growth.(2)After the buds set during the hardening period,part of the N uptake allocated in the leaves of seedlings and then retranslocated to the storage organs before the defoliation.The resorption efficiency of small and big seedlings were 39%and 62%,respectively.N resorption efficiency was negative correlated with green leaf N concentration,big seedlings with lower N concentrations had higher resorption efficiency compared to small seedlings.And these differences lead to the different amount of uptake N during the hardening period storaged in the perennial organs of small and big seedlings.The contribution of uptake N during the hardening period to the total N storage was 11-39%and it was affected by the N applied rate during the hardening period and the seedling size.N availability in leaves increased by N applied during the hardening period led to improved photosynthesis and supported accelerate the capture and assimilation of carbon.N uptake during the hardening period significantly increased the non-structure carbon of both sizes.(3)High exponential N fertilization during the rapid growth period benefited nutrient reserves and subsequent transplanted seedling growth and tissue nutrient storage at the end of the rapid growth and hardening phases.N loading during the hardening period with 36 mg N increased stem dry mass and tissue nutrient content at the end of the hardening phase.At the conclusion of establishment,exponential N fertilization during the rapid growth period and N loading during the hardening period showed a significant interaction for N and K uptake from soil.At the end of the rapid growth and hardening phases,high exponential N fertilization during the rapid growth period consistently increased nutrient uptake.Enhanced N and K uptake occurred following application of 36 mg N of N loading during the hardening period at the end of the hardening phase.Distinct roles for exponential N fertilization during the rapid growth period and N loading during the hardening period on 3 phases of transplanted seedlings demonstrated the necessity to evaluate fertilization in terms of nutrient reserves and subsequent transplanting performance in consecutive phases.Combining 50 mg N seedling-1 during the rapid growth period with 36 mg N seedling-1 during the hardening period yielded ideal transplanting performance for Q.variabilis seedlings.Overall,moderate drought does not inhibit N remobilization from the major storage organ at early growth stages in spring,and that increasing N storage of planted seedlings through N-loading during the hardening period can mitigate the negative effect of moderate spring drought during the next growing season on growth.High N storage can be accomplished by N-loading during the hardening period and N uptake during the hardening period does not stimulate growth but increase NSC concentration in seedlings and different sizes of Q.variabilis seedlings show different effects.Combining 50 mg N seedling-1 during the rapid growth period with 36 mg N seedling-1 during the hardening period yields ideal transplanting performance for Q.variabilis seedlings.