Plant N Resorption And Its Impacts On N Retention In Korean Pine And Broad-leaved Mixed Forest In Changbai Mountain

Nitrogen（N）resorption as a key strategy for N conservation in plants,and is often influenced by soil nutrient availability and N content within the plant.N resorption is also regulated by enzymatic remobilization during leaf senescence.Most current studies focused on the impacts of environmental variables and plant nutrients status on N resorption efficiency（NRE）,while plant internal physiological regulation is still unclear.Forests as a primary ecosystem in land,play a vital role in alleviating global climate change and regulating terrestrial biogeochemical cycles.The impacts of deposited N on the forest ecosystems mainly depend on the amount of N retained and its allocation among ecosystem components.N resorption can directly affect N retention in ecosystems by altering leaf N content and litter quality,and indirectly influence the distribution of deposited N in ecosystems via changing plant N uptake at the same time.In addition,N resorption can also provision N for plant growth,reduce plant’s reliance on soil N sources,which to some extent alleviates soil N limitation and increases plant carbon（C）sequestration.Therefore,N resorption plays an important role in regulating N cycling and climate change.Previous studies on the retention and allocation of deposited N mainly were based on short-term experiments（3-18 months）,while studies on the impacts of N resorption on the long-term retention of deposited N in temperate forests remain scarce,and the allocation dynamics of different form of deposited N over a long-time scale remain unclear.Moreover,further studies are needed to explore the ratio of N resorption to N requirement and its influencing factors.Here,we evaluated the relative contribution of altitude-induced alterations in environmental factors and plant enzymes to the variations of NRE and identified the main factors in regulating NRE.Using the stable isotope ¹⁵N tracing technique to trace the turnover of natural N deposition in forests,we clarified the mechanism of long-term N retention in a temperate forest from the perspective of N resorption via quantifying the ratio of N resorption to N requirement and revealed the temporal dynamics of different forms of deposited N allocation in the forest ecosystem.Finally,a meta-analysis was conducted to identify the impact factors of the ratios of N resorption to N requirement,and revealed the implication of this ratio for forest C sequestration.The main findings of this study are as follows:（1）We revealed the internal regulatory mechanism of NRE response to elevation change via measuring plant enzyme activity and soil physical and chemical variables.We found that NRE of Q.mongolica and F.mandshurica increased with altitude,while a decrease in NRE of T.amurensis,A.mono and A.pseudosieboldianum occurred along the altitudinal gradient,but the relationships between F.mandshurica and A.mono and altitude did not reach a significant level.The responses of different tree species in glutamate dehydrogenase（GDH）in senescing leaves to altitude variation trended towards the same pattern with those of NRE.The responses of NRE to increasing altitude were species-specific,which mainly caused by the variation of leaf GDH,an enzyme responsible for N translocation.GDH activity in senescing leaves was not only affected directly by the altitude-induced variations in soil temperature and moisture,but also influenced indirectly by specific leaf area（SLA）and green mature leaf N content that regulated by altitude.GDH activity in senescing leaves explained the largest proportion of the variation in NRE compared to soil and climatic factors,demonstrating that different tree species had different physiological regulation strategies to conserve N under similar environment.Our results emphasized the key role of plant enzymes in regulating NRE,further interpreted the inconsistent phenomenon between soil total N content and NRE.（2）We investigated the long-term retention and allocation of different forms of deposited N in ecosystems via the application of ¹⁵NH₄NO₃ and NH₄¹⁵NO₃ in a Korean pine and broad-leaved mixed forest of Changbai Mountain and evaluated the ratios of N resorption to N requirement.Results showed that under ¹⁵NO₃^-treatment,the total ¹⁵N recovery of the ecosystem declined from 98.6%at 14 days after tracer application to 87.2%at 2600 days after tracer application and that under ¹⁵NH₄⁺treatment declined from 112.6%to 73.6%during the same period.At the community level,the ratios of N resorption to N requirement reached 49.4%and N resorption fluxes increased with increasing altitude.At 14 days after tracer application,litter layer was the largest sink whose ¹⁵N recovery reached 84.2%under ¹⁵NH₄⁺treatment,while litter layer as the second largest sink,with 33.9%¹⁵N tracer recovered under ¹⁵NO₃^-treatment.The ¹⁵N recovery of litter layer gradually decreased with sampling time under both treatments,only retained 4.2%-4.4%¹⁵N tracer at 2600 days after tracer application.Similarly,the ¹⁵N enrichment in lightly decomposed litter（L-layer）and partially decomposed litter（F-layer）gradually decreased with sampling time,while that in highly decomposed litter（H-layer）showed a trend of increasing and then decreasing with sampling time.At 14 days after treatment,the top soil layer（0-5 cm）was another important sink whose ¹⁵N recovery reached 40.3%and19.4%under ¹⁵NO₃^-and ¹⁵NH₄⁺treatment,respectively.The ¹⁵N recovery and enrichment in the whole soil horizon decreased and then increased with sampling time,the ¹⁵N signal was detected in the deeper soil layer at the later stage of the experiment,and the response of ¹⁵N enrichment in deeper soil layer to temporal variation was opposite to that in top soil layer.The recovery of ¹⁵N in plants gradually increased with sampling time.Among them,¹⁵N recovery under ¹⁵NO₃^-and ¹⁵NH₄⁺treatment increased from 24.4%and 9.1%at 14 days after tracer application to 29.3%and 27.9%at 2600 days after tracer application.Our results indicate that most of deposited N are still retained in the ecosystem over a longer time scale.However,the higher ratio of N resorption to N requirement at the community level implies that our studied temperate forest had a conservative N cycling,which promoted N retention in the ecosystem.In addition,retained deposited N could be further transferred among ecosystem components with time.（3）Using a meta-analysis,we revealed the impacts of environmental factors and mycorrhizal types on the ratios of N resorption to N requirement.Results showed that the ratios of N resorption to N requirement in ectomycorrhizal（EM）trees decreased with mean annual precipitation（MAP）,mean annual temperature（MAT）,soil total N content（TN）and atmospheric CO₂ concentration and was significantly lower than that in arbuscular mycorrhizal（AM）trees.An in situ ¹⁵N tracing experiment further confirmed this stronger reliance on N resorption for AM trees,which is mainly attribute to the greater ability for N uptake of EM roots.Furthermore,the N requirement and uptake in EM trees increased with elevated atmospheric CO₂ concentration,while the ratios of N resorption to N requirement showed an opposite trend,indicating that different strategies may exist between AM and EM trees to alleviate progressive N limitation（PNL）.In summary,I found our studied forest is still not N saturated and serves as an important sink for deposited N.N resorption plays an important role in N retention in forest ecosystems,especially for the N-limited temperate forests.N resorption is stimulated because of the enhanced GDH activity in senescing leaves,which heightens the ratio of N resorption to N requirement,resulting in relatively closed N cycle in the ecosystem and reducing the losses of deposited N from the ecosystem.Especially for AM trees,N resorption contributed more to plant N requirement,which could alleviate progressive N limitation to some extent under rising atmospheric CO₂ concentration in the future.This study reveals the important role of N resorption in N retention in the temperate forest from the perspective of internal plant N cycling,which not only helps to better understand and predict N cycling in forest ecosystems under climate change,but also provides a theoretical basis for improving Earth system models.