The leak in the phosphorus cycle - exploring the mechanisms and controls of phosphorus leaching in soils of acquiring and recycling forest ecosystems

在生态系统发展过程中，森林生态系统逐渐从获取岩石磷向循环利用有机磷转变，正磷酸盐和溶解有机磷(DOP)在有机层中释放，并部分淋溶到矿物土壤中。我们在主体工程第一阶段获得的结果表明，随着森林生态系统循环趋势的增加，正磷酸盐从有机层中的淋溶减少。相比之下，DOP下降幅度不大。大多数DOP不是酶水解性的，因此生物利用度很低。矿质土壤中正磷酸盐随深度的增加而减少，DOP的下降幅度较小。DOP很可能对所有森林生态系统的稳定磷流失有很大贡献，但正磷酸盐也不能完全保留在矿物土壤中。然而，控制获取和再循环森林生态系统有机层中溶解磷差异产生的因素尚未得到解决。本研究旨在探讨森林生态系统中磷的活化和淋失的机制和控制，通过在SPP施肥试验和不同质量有机质的微宇宙中测定磷形态的释放和DOP的生物有效性来评估有机层中磷的活化。此外，我们还将追踪13C标记化合物的命运，以确定DOP周转和动员过程中的限速步骤。通过测定不同化学计量比(C：P：N比)的土壤和土壤中磷的淋溶情况，我们可以推断DOP的产生是由P的有效性驱动的，还是土壤有机质循环的副产品。在气候室中，我们将估计磷动员过程的温度依赖性，这是磷循环中的一个关键因素。溶解磷迁移的一个可能原因是胶体运输。因此，我们将通过超速离心法分析土壤溶液中的胶体。与胶体的结合也可以解释DOP较差的酶水解性。土壤基质结合的有机磷化合物释放DOP的可能性将通过分离富P组分和贫磷组分，结合14C分析来测试。溶解磷的表征将用31P-核磁共振进行，这往往受到底土溶液中少量磷的限制。因此，我们将利用银阳极激发的XPS来测定小样品中的磷结合形式。该项目的一部分将致力于这一新方法的开发。为了推广所获得的结果，我们将在一些额外的森林地点进行零星采样的土壤水中溶解磷的测定。最后，我们将在主要项目内综合与运输和通量有关的结果，这是模拟森林生态系统中P循环的先决条件。

During ecosystem development, forest ecosystems gradually change from the acquisition of rock P to the recycling of organic P. Orthophosphate and dissolved organic P (DOP) are released in the organic layers and partly leached into the mineral soil. Our results obtained during phase 1 of the main project indicate decreasing leaching of orthophosphate from organic layers with increasing recycling tendency of the forest ecosystems. In comparison, DOP does not decrease much. Most DOP is not enzymatic hydrolysable, and consequently bioavailability is little. Orthophosphate decreases with depth in the mineral soil; the decline in DOP is less strong. Likely, DOP contributes substantially to the steady P loss from all forest ecosystems, but also orthophosphate is not retained completely in the mineral soil. Yet, the factors controlling the differential production of dissolved P in organic layers of acquiring and recycling forest ecosystems are not resolved. Also, the causes for the steady leaching of dissolved P are not fully understood.The proposed work aims at exploring the mechanisms and controls of the mobilization and leaching of P in forest ecosystems.Mobilization of P in organic layers will be assessed by measuring the release of P forms and DOP bioavailability in the SPP-fertilization experiment and in microcosms with organic matter of different quality. Also, we will trace the fate of 13C-labelled compounds to determine DOP turnover and the rate limiting steps in the mobilization process. By measuring P leaching with and without nutrient fertilization and from substrates with differing stoichiometry (C:P:N ratios), we can deduce if DOP production is driven by P availability or is rather a byproduct of soil organic matter cycling. In climate chambers, we will estimate the temperature dependency of P mobilization processes, which is a key factor in the P cycle.One possible cause for the mobility of dissolved P is colloidal transport. We, thus, will analyze soil solutions for colloids by ultracentrifugation. Binding to colloids can also explain the poor enzymatic hydrolyzability of DOP. The possible release of DOP from soil matrix-bound organic P compounds will be tested by separating DOP in P-rich and P-poor fractions, combined with 14C analyzes. Characterization of dissolved P will be carried out with 31P-NMR, which is often limited by the small amounts of P in subsoil solutions. Therefore, we will employ XPS with Ag anode excitation to determine P binding forms in small samples. Part of the project will be dedicated to the development of this new method.In order to generalize the obtained results, we will determine dissolved P in soil waters sampled sporadically at a number of additional forest sites. Finally, we will contribute to the synthesis on transport- and flux-related results within main project, which is a prerequisite for the modeling of the P cycle in forest ecosystems.