Phosphorus associated to forest soil colloids

Dissertation, RWTH Aachen University, 2018; Aachen 1 Online-Ressource (IX, 105 Seiten) : Illustrationen, Diagramme, Karte (2018). = Dissertation, RWTH Aachen University, 2018
Natural soil colloids (1 nm – 1 µm) and specifically their subset nanoparticles (1 nm – 100 nm) are known to associate phosphorus (P). This affects the P mobilisation and transfer in soils and hence the availability of P for plants and microorganisms. However, in the most soil P studies the presence of low nm-sized particles has been neglected. One reason is the overlap in size ranges between colloids and the standard filter size of 450 nm widely used to define 'dissolved compounds'. In particular, the colloids present in forest soils and their relevance for the P association were hardly investigated. In this thesis the size and composition of natural forest soil colloids and their characteristics regarding P association and P transfer were investigated. Field-flow fractionation (FFF) separation methods were developed to separate water dispersible soil colloids (WDC) and leached soil colloids (LC) by size in the present study. The FFF was coupled online to various detectors, e.g. to a UV-vis detector, an inductively coupled plasma mass spectrometer (ICP-MS), an organic carbon detector (OCD) and to a dynamic light scatter device (DLS) to quantify the size related element composition of WDC and LC. The OCD was used for the first time for soil colloid analysis. Furthermore, the identification of different inorganic and organic P species was conducted by liquid state 31P-nuclear magnetic resonance spectroscopy (31P-NMR). The nanoparticles and colloids were visualised by transmission-electron microscopy (TEM) and their composition additionally quantified by energy-dispersive X-ray spectroscopy (EDX). Five German beech dominated forest soil profiles of varying bulk soil P content were studied. The WDC < 500 nm were isolated from the five forest soil profiles and for the first time the WDC composition was investigated by FFF and evaluated regarding their P content, association and storage. The FFF method separated the WDC into three size fractions. The size fractions showed comparable element compositions between the five forest sites but the proportions of the different WDC size fractions were characteristic for the specific soil horizon. The P concentration in the overall WDC was up to 16-fold higher than in the bulk soil. Nanoparticles < 25 nm were rich in organic carbon (Corg) and were mainly present in the organic layers and surface soils. They were of great relevance for P binding in the organic surface layers. The nanoparticle content decreased with increasing soil depth in all five forest soil profiles. Fine colloids between 25 nm–400 nm, mainly composed of Corg, Fe and Al, probably as associations of Fe- and Al-(hydr)oxides and organic matter were mainly present in the upper mineral soil. Medium-sized colloids of 240 nm–500 nm, rich in Corg, Fe, Al and Si, indicated the presence of phyllosilicates in association with organic matter and potentially with metal(hydr)oxides. In the mineral soil the fine and medium-sized colloids were of great relevance for the P association. The fine and medium-sized colloids showed a local maximum in the mineral topsoil due to soil acidification. Variant forest site characteristic distributions of fine and medium-sized colloids were observed in the subsoil. Regardless of the bulk soil P content the colloids appeared to be highly relevant as P carriers in the forest soils studied in particular in the topsoils. The 31P-NMR analysis showed that soil WDC were enriched with P compared to the bulk soil, particularly the phosphate diesters were more dominant in the colloidal fraction. The colloidal phosphate diester to phosphate monoester ratios were also two to three times higher in the colloidal fraction than the bulk soil. In contrast, relatively large inorganic P proportions were found in the electrolyte phase but only small dissolved organic P proportions. Forest mesocosm artificial rain experiments were subsequently designed to simulate field conditions for P leaching and colloid facilitated P leaching. Three beech and spruce dominated forest sites with varying soil P concentrations were selected for investigation. The study demonstrated that significant proportions of P leached from acidic forest topsoil were associated to colloids with a maximum size of 400 nm. By FFF the leached colloids (LC) were separated into three size fractions. The size and composition of LC were found to be characteristic for the forest sites. The composition of leached colloids of all size classes was dominated by Corg and these colloids contained 12–91% of the leached P depending on the forest soil. Organic P, beside phosphates was also leached associated to colloids. The study showed that the percentage of colloid–associated leached P decreased with increasing total P concentrations within the leachate. The dissolved and colloid associated P leaching concentrations were related to the soil texture It was found that total and colloid associated P leaching from the forest surface soils soil did not increase with increasing bulk soil P concentrations and that they were not related to tree species. LC and WDC concentrations of the three forest sites showed the same colloid concentration gradient. However, LC were enriched in Corg and P compared to WDC suggesting that the LC are a subunit of the WDC rich in Corg and P. Comparison with forest stream colloids showed that stream and leached colloids are highly comparable in size and composition. The study showed that colloid associated P can be of higher relevance for the P leaching from forest surface soils than dissolved P and should not be neglected in soil water flux studies. The investigations of this thesis gained a deeper insight in the colloids present in forest soils and raised the current knowledge about colloid associated P forms, enrichment and transport in soils. The results clearly and univocally pointed out the previously unrecognized importance of nanoparticles and colloids for the P dynamics and storage in forest soils.
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