Phosphorus concentrations in growth rings of trees as indicators of P availability and recycling efficiency in forest ecosystems

长期监测点的叶面磷浓度呈负趋势，表明在过去的20 - 30年里，许多欧洲森林的磷树营养有所下降。造成这一现象的可能原因包括土壤酸化、高氮沉降和树木生长改善导致的养分失衡、土壤石灰化以及干旱和高种子产量年份频率增加。对生长年轮中元素浓度的分析提供了机会来检测磷以及可能与土壤酸化（树木年轮中的Ca、Al）、富营养化(N)和其他环境波动有关的其他营养素的长期营养趋势。此外，树干横截面中P的树木年轮剖面可以评估木质生物质中P的再循环效率，从而有助于解决DFG优先计划生态系统营养的核心问题之一。在优先计划的第一阶段，我们已经建立了激光烧蚀-电感耦合等离子体质谱（LA-ICP-MS）方法适合于测量生长环中的P和其他元素。然而，该方法需要进一步发展，以捕捉生长环内营养物质的空间异质性。对1.3 m高度的冷杉树芯进行的初步研究结果表明，P循环系统的P循环效率并不高于P获取系统，在最贫瘠的地点，硬木中的P含量与持续的土壤酸化无关。目前正在对更多的地点和森林木进行进一步的分析。在这里，我们建议将这一分析扩展到包括树木的年轮和树皮，从而包括具有更高生理活性的茎区。我们假设在树木年轮中，边材的磷浓度随高度增加而增加，而心材的浓度保持不变。这将产生更能代表整个树干的结果。此外，该方法使我们能够跟踪同一生长环内磷浓度的高度相关变化，从而为计算和模拟木本树木生物量的磷储量和吸收提供了一种新的方法。此外，将利用优先规划新建立的施肥试验，在三个磷效价不同的地点，评估林根肥料对磷的短期固定化潜力。在这里，我们假设P在边材和树皮中的固定化在P循环中比在P获取系统中更强。因此，我们的分析对于优先计划中分析的生态系统的磷预算至关重要，并解决了其主要假设之一，即磷循环系统的磷回收效率高于磷获取系统。

Negative trends in foliar P concentrations at long-term monitoring sites have indicated that P tree nutrition in many European forests has declined over the last 2-3 decades. Possible reasons for this phenomenon comprise soil acidification, nutrient imbalances as a consequence of high N deposition and improved tree growth, soil liming, and increasing frequencies of droughts and mast (high seed production) years. Analysis of element concentrations in growth rings offer the opportunity to detect long-term trends in nutrition of P as well as other nutrients which may be related to soil acidification (Ca, Al, in tree rings), eutrophication (N), and other environmental fluctuations. In addition, tree ring profiles of P in stem cross-sections permit assessment of the recycling efficiency of P within the woody biomass and thus help to address one of the core questions of the DFG Priority Programme Ecosystem Nutrition. In the first phase of the Priority Programme, we have established that the Laser Ablation - Inductively Coupled Plasma - Mass Spectrometry (LA-ICP-MS) approach is suited to measure P and other elements in growth rings. The method needs to be developed further, however, to capture the spatial heterogeneity of nutrients within growth rings. Preliminary results for tree cores from P. abies taken at 1.3 m height show that the P recycling efficiency is not higher in a P-recycling than a P-acquiring system, and that P content in hardwood at the poorest site is unrelated to ongoing soil acidification. Further analyses for more sites and F. sylvatica are currently underway. Here we propose to expand this analysis to include tree rings and bark from upper tree heights and thus stem regions with higher physiological activity. We hypothesize that within tree ring P concentrations in sapwood increase with height whereas those in heartwood remain constant. This will yield results that are more representative of entire tree stems. In addition, this approach allows us to track height-related changes in P concentrations within the same growth ring and thus offers a new method to calculate and model P stocks and uptake into woody tree biomass. In addition, the short-term immobilization potential for P from fertilizer will be assessed for F. sylvatica using the newly established fertilization trial of the Priority Programme at three sites differing in P availability. Here we hypothesize that immobilization of P into sapwood and bark is stronger in P-recycling than in P-acquiring systems. Our analyses are therefore essential for P budgets of the ecosystems analyzed in the priority program and to address one of its main hypotheses that recycling efficiency of P is higher in P-recycling than in P-acquiring systems.