Bioaccessibility and turnover of phosphorous in forest subsoils

该项目旨在验证以下假设：随着表层土壤缺磷量的增加，地下土壤对树木获取磷的贡献越来越大。对HCl可提取土壤磷酸盐的氧同位素组成示踪表明，表层土壤的δ 18o值与生物循环磷平衡，而深层土壤的δ 18o值与生物循环磷平衡。特别是，我们的数据指出，在磷限制场地的极深底土（几米）中，基岩磷的生物转化增加，而在磷不限制的土壤中则不是这样。有趣的是，我们的数据还表明，底土P的利用可能在很大程度上受到额外C和N源供应的限制，而不是受到稳定库中特定P结合的限制。因此，在这个优先项目的第二阶段，我们的目标是通过以下方式来证实这些发现：1)将29 m土壤深度下磷氧同位素特征的变化与磷键形式的变化联系起来；2)验证（深层）地下磷的微生物更替会因额外的C和N供应而增强的假设；3)评估地下矿物对微生物和非生物磷18O交换率的作用。并在第一阶段的专业知识基础上，使用iv)木质部汁液中磷酸盐的氧同位素组成作为磷含量有限的地点从底土吸收磷的指标。为了实现这些目标，我们进行了现场土壤和木质部液的监测以及不同剖面深度的18o标记水的离地孵育实验，我们将继续使用同位素比质谱法、纳米级二次离子质谱法和近红外和深紫外激光激发的拉曼光谱法监测土壤磷酸盐中的氧同位素比例。

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.