Specification of phosphorus compounds in tangelhumus forest soils by novel NMR techniques

Wang, Liming; Klumpp, Erwin (Thesis advisor); Schäffer, Andreas (Thesis advisor); Adams, Alina Reghina (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

In Tangelhumus soils of Bavarian alpine forests, large amount of organic phosphorus (P) has been immobilized along with long-term accumulation of organic matters. Such large P stocks, however, remained barely understood. The most commonly used analytical methods involves NaOH-EDTA extraction following by NMR detection. Nevertheless, in forest soils with a diverse soil organic P (SOP) pool, signals were highly overlapped and unrecognizable with 1D NMR only. In addition, NaOH-EDTA hydrolyzed certain labile SOP species prior to NMR detection, thus biased the results of P studies. The scarcity and inaccuracy of P data precluded conclusive answers to fate of individual SOP compound in forest soils. In this thesis, 1D and 2D heteronuclear single quantum coherence (HSQC) NMR were firstly employed to study the Tangelhumus SOP pool, in order to understand SOP transformation over long-term Tangelhumus formation. A diverse SOP pool, composed primarily of monoesters together with diesters, and phosphonates was detected. All predominant P monoester signals were assigned as hydrolysis products from diesters. Myo-inositol hexakisphosphate, commonly present in soils, was below detection limit. The overall picture of SOP speciation manifested as the continuous degradation of organic matter and accumulation of monoesters in lower horizons. In addition, unlike the dramatic change in P composition in bulk soil with depth, composition of P fractions associated with water dispersible colloids (WDC-P) was consistent with depth and similar only with that of surface horizons. Elemental composition of water dispersible colloids (WDC) revealed by asymmetric field flow fractionation also didn’t show a depth-dependent change. These observations together indicated WDC in calcareous soil originated from surface layers. Ca was the dominating metallic element in WDC. The quantification results suggested that the WDC-P content was independent on stand P abundance, and elevated pH likely resulted in more colloidal P but less dissolved P.In this thesis diffusion ordered spectroscopy (DOSY) was applied for the first time for soil P study. This technique allowed virtual separation of individual SOP component in a second domain based on translational diffusion coefficient. Combining chemical shift and diffusion value led to more confident signal assignment than 1D NMR. Diffusion value correlated directly to SOP molecular size and interpretation of DOSY was more straightforward than HSQC. Current ubiquitous application of this novel technique is limited to soil with high SOP content. In order to track the hydrolysable SOP species, alkaline extraction was performed in 18O-enriched medium, thereby labeled resultant hydrolysis products with heavy isotope. This isotope shift of corresponding NMR resonances enabled an immediate identification and quantification of artificial degradation products produced by sample treatment. It turned out that 83 - 91 % of RNA was artificially hydrolyzed in Tangelhumus Of layer. Up to 93 % of detected β-glycerophosphate was from phosphatidylcholine hydrolysis, while α-glycerophosphate was mostly inherently present. Polyphosphate was also hydrolyzed to yield 18O-labeled orthophosphate. Adding up these artificial hydrolysis products to corresponding precursors allows for the very first time to mathematically restore the original P composition in environmental samples.

Institutions

• Department of Biology [160000]
• Chair of Environmental Biology and Chemodynamics [162710]