Bioaccessibility and turnover of phosphorous in forest subsoils

本项目旨在验证以下假设：随着表层土壤中磷缺乏的增加，底土对树木获得磷的贡献越来越大。对土壤磷酸盐氧同位素组成的追踪表明，表层土壤磷酸盐δ 18 O值与生物循环磷处于平衡状态，而深层土壤磷酸盐δ 18 O值与生物循环磷不平衡。特别是，我们的数据表明，在非常深的底土（几米）的P有限的网站，这是不是在土壤中的情况下，P不受限制的基岩P的生物转化增加。有趣的是，我们的数据还表明，底土磷的利用可能在很大程度上受到额外的C和N源的供应，而不是由特定的P键合在稳定的池限制。因此，在该优先计划的第二阶段，我们的目标是通过以下方式证实这些发现：i）将磷酸盐氧同位素特征的变化与29 m土壤深度的磷结合形式的变化联系起来，ii）测试（深层）底土P的微生物周转率通过额外的C和N供应而增强的假设，iii）评估底土矿物对微生物和非生物磷酸盐18 O交换率的作用，并建立在我们的专业知识，从第一阶段使用iv）的氧同位素组成的磷酸盐在木质部汁液作为一个指标，磷吸收从底土在磷有限的网站。为了实现这些目标，我们进行现场土壤和木质部汁液监测以及异位培养实验，从不同的剖面深度与18 O-标记的水，我们将继续监测土壤磷酸盐中的氧同位素比同位素比质谱，二次离子质谱在纳米级，和拉曼光谱使用近红外和深紫外激光激发。

This project has been designed to test the hypothesis that subsoils increasingly contribute to the P acquisition of trees with increasing P deficiency in the surface soil. Tracing the oxygen isotope composition of HCl extractable soil phosphates showed that their delta18O values were in equilibrium with biological cycled P in topsoils, but not in deeper subsoils. In particular, our data pointed to an increased biological transformation of bedrock P in very deep subsoils (several m) of P limited sites, which was not the case in soils where P was not limited. Intriguingly, our data also suggested that the utilization of subsoil P might be largely restricted by the supply of additional C and N sources rather than by specific P bondings in stable pools. In the second phase of this priority program, we thus aim at substantiating these findings by i) relating changes in phosphate oxygen isotope signatures down to 29 m soil depth to changes in P bonding forms, ii) testing the hypothesis that microbial turnover of (deep) subsoil P is enhanced by additional supply with C and N, iii) assess the role of subsoil minerals on microbial and abiotic phosphate 18O exchange rates, and build on our expertise from the first phase to use iv) the oxygen isotope composition of phosphate in xylem sap as an indicator for P uptake from the subsoil in P limited sites. To achieve these aims, we perform onsite soil and xylem sap monitoring as well as ex-situ incubation experiments from different profile depths with 18O-labelled water, and we will continue to monitor oxygen isotope ratios in soil phosphates by Isotope Ratio Mass Spectrometry, Secondary ion mass spectrometry at nanoscale, and Raman Spectroscopy using near infrared and deep ultraviolet laser excitation.