Phosphorus mobilization in acid forest soils as affected by interactions of water regime, fertilization and growth of beech

在天然结构性土壤中，土壤水分是主要的环境因素，其固有的小尺度非均质性。与有机碳的有效性一起，它共同决定了微生物热点和时刻的出现，从而也决定了更大范围内的分解速率和养分循环。因此，土壤水分，特别是干湿交替影响淋溶、微生物固定和植物吸收磷。土壤水分的极端变化，可能是气候变化的结果，可能会增加森林土壤中磷的流失。此外，土壤水分控制着与铁化合物和磷结合的氧化还原敏感的非生物反应，特别是在强聚合亚土壤中。我们的总体目标是调查不同的水分状况如何与酸性森林土壤中磷的活化和固定过程相互作用，以及这将如何影响磷的淋溶和有效性。在第一个实验中，我们将解决不同有机质矿化导致的表层土壤中磷的活化问题。我们建议用取自SPP三个核心站点(Lüss、Vessertal、Bad Brucenau)的自然结构性土壤进行柱状实验，并种植来自相同站点的幼年山毛榉。中温层将受到不同的水分状况的影响，包括三个水平的恒定土壤湿度和再湿脉冲。我们将评估这些不同的制度是如何(I)影响山毛榉对磷的淋溶和吸收，(Ii)影响微观尺度上磷动员潜力的异质性，以及(Iii)控制微生物和山毛榉之间对磷的竞争。在第二个实验中，我们将研究由于土壤团聚体中不同的氧化还原条件，底土中磷的活化和固定。大孔道流动对集料表面的对流和补给，以及它们从集料内部和向集料内部的扩散补给和排放将在连续渗流实验中确定。此外，我们将在我们的专栏实验中集成中央SPP田间实验的部分肥料变体。通过将氮肥作为附加变量纳入表土试验，我们将检验这样一种假设，即持续的高氮沉降对磷素活化的影响取决于土壤水分状况。在底土试验中，团聚体内磷库的对流-扩散补给模拟了从表土淋失的可能的磷输入。最后，通过在两个水分水平的微尺度上的磷素动员潜力的田间测量，我们将有助于评估普通施肥试验检验的氮磷添加效应。

In naturally structured soils with their inherent small-scale heterogeneity, soil moisture is the chief environmental factor. Together with the availability of organic carbon it co-determines the occurrence of microbial hot spots and moments and thereby, also decomposition rates and nutrient cycling on a larger scale. Therefore, soil moisture, and in particular the alternation of periods with dry and wet conditions, influences leaching, microbial immobilization, and plant uptake of P. More extreme variations in soil moisture, as a possible consequence of climate change, may increase the P loss from forest soils. In addition, soil moisture governs redox-sensitive abiotic reactions involving iron compounds and P bound to them, in particular in strongly aggregated subsoils.Our general objective is to investigate how different water regimes interact with P mobilization and immobilization processes in acid forest soils, and how this will affect leaching and availability of P. In a first experiment, we will address P mobilization in the topsoil as a consequence of differing organic matter mineralization. We propose column experiments with naturally structured soils taken from three core sites of the SPP (Lüss, Vessertal, Bad Brückenau) that are planted with juvenile beeches taken from the same sites. The mesocosms will be subjected to different water regimes involving three levels of constant soil moisture combined with rewetting pulses. We will assess how these different regimes (i) influence P leaching and P uptake by beech, (ii) affect the heterogeneity of the P mobilization potential at the microscale, and (iii) govern the competition for P between microbes and beech. In a second experiment with soils from the aggregated B horizon in Vessertal, we will study the P mobilization and fixation in the subsoil, as a consequence of differing redox conditions in soil aggregates. The convective discharge and recharge of aggregate surfaces by macropore flow, as well as their diffusive recharge and discharge from and to the inside of the aggregates will be determined in a sequential percolation experiment. In addition we will integrate part of the fertilizing variants of the central SPP field experiment in our column experiments. By including N fertilization as an additional variant in the topsoil experiment, we will test the hypothesis that the effect of continuing high N deposition on P mobilization depends on the soil water status. In the experiment with subsoil, the convective-diffusive recharge of intra-aggregate P pools simulates the possible P input with leaching from the topsoil. Finally, with field measurements of P mobilization potential on the microscale at two moisture levels, we will contribute to the assessment of N and P addition effects tested with the common fertilization experiment.