Phosphorus associated to forest soil colloids

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

Phosphorus forms in forest soil colloids as revealed by liquid-state 31P-NMR

Nanoparticles and colloids affect the storage and hence the availability of P in forest ecosystems. We investigated the fine colloids present in forest soils and their association with inorganic and organic P. To differentiate between the different P forms, we performed liquid-state 31P-nucelar magnetic resonance (31P-NMR) measurements on forest bulk soil extracts, on colloid extracts and on the electrolyte phase of their soil suspensions. The 31P-NMR spectra indicated that soil nanoparticles and colloids were more enriched with organic than with inorganic P forms compared to the electrolyte phase. The P concentration was enriched in the colloidal fraction in comparison to the bulk soil and the phosphate diesters were more dominant in the colloidal fraction when compared to the bulk soil. The colloidal P-diester to P-monoester ratios were 2 to 3 times higher in the colloidal fraction than in the bulk soil. In contrast, relatively large percentages of inorganic P were found in the electrolyte phas

Extending the capabilities of field flow fractionation online with ICP-MS for the determination of particulate carbon in latex and charcoal

There is a broad range of carbon based engineered particles including polymer latex particles and carbon black. Also in environmental systems particulate carbon such as humic acids and soot or coal (the latter two summarized as black carbon) is of great importance and is involved in nutrient storage and (re)cycling. Therefore, detailed characterisation of the size distribution and elemental composition of such particles is required to understand the material properties and their environmental relevance. Field flow fractionation (FFF) online with inductively coupled plasma mass spectrometry (ICP-MS) is routinely applied for the characterisation of metal containing particles. However, the far majority of FFF studies relies on UV detection for organic carbon while elemental detection of carbon has hardly been used. Our previous work demonstrated the capability of FFF-ICP-MS for the determination of carbon in fine particulate matter, focusing on humic acid in water samples. The current work investigates the feasibility of carbon detection and quantification in larger particles with sizes up to about 750 nm. For this purpose, latex particle size standards of 21 nm, 100 nm, 250 nm and 740 nm were analysed as well as extracts of charcoal spiked soil. Elemental analysis using combustion techniques was employed as reference for the total carbon content of the samples to establish a mass balance. Recoveries for FFF separation of latex particle standards were in the range from 69% to 83% and in the range from 78% to 104% in flow injection mode. Carbon mass balance calculated from FFF fractionation, ultrafiltration and total content for the extracts from soil and charcoal spiked soil achieved 76% to 105%. Variation of the sampling depth was investigated to check if increased dwell time of the particles in the plasma affects the carbon ionisation and quantification. No significant change of carbon recoveries was observed, yet the signal to noise ratio improved 3-fold. This study provides a method for the analyses of carbon containing particles via FFF-ICP-MS, which allows for the first time the simultaneous measurement of carbon and other nutrients and is hence more timesaving than other methods

Field flow fractionation online with ICP-MS as novel approach for the quantification of fine particulate carbon in stream water samples and soil extracts

Reliable and efficient analytical techniques are required for quantitative size-resolved carbon determination of nanoparticles and colloids in complex sample matrices due to the key role of carbon in biological and environmental processes. Field flow fractionation (FFF) online with inductively coupled plasma mass spectrometry (ICP-MS) is a powerful technique for identification and quantification of particle bound metals, but has not been applied for quantitative determination of particulate carbon, yet, due to several challenges. Therefore, our study explores the potential of online particulate carbon detection by ICP-MS to overcome limitations of previously used UV detection or offline total organic carbon measurements. A novel organic carbon detector (OCD) was used as independent sensitive carbon detector to validate the ICP-MS results. Basic validation of organic carbon detection by offline quadrupole and sector-field ICP-MS was performed for fresh water samples using OCD as reference achieving recoveries of 107 ± 16% with Q-ICP-MS and 122 ± 22% with SF-ICP-MS. Limits of detection were 0.6 mg L−1 for Q-ICP-MS, 0.3 mg L−1 for SF-ICP-MS and 0.04 mg L−1 for OCD. The main focus was on comparison of FFF-ICP-MS and FFF-OCD for quantification of particulate carbon in fresh water samples, soil extracts as well as in bovine serum albumin (BSA) as candidate reference standard. Recoveries obtained by FFF-Q-ICP-MS with a flow-injection calibration approach were in a range from 90 to 113% for replicate analyses of fresh water samples compared to FFF-OCD and from 87 to 107% with an alternative post-channel calibration strategy

Dissolved and colloidal phosphorus affect P cycling in calcareous forest soils

Dissolved and colloidal phosphorus (P) represent the mobile P fractions in soils, but their role in P cycling in forests is still largely unclear. In this study of four calcareous forest soil profiles, the elemental compositions of different size fractions of water dispersible colloids (WDC) were investigated by asymmetric field flow fractionation. Nuclear magnetic resonance spectroscopy (NMR) was applied to identify the organic P compounds in soils, WDC, and soil solutions. Carbon was the dominant element in WDC of all soil horizons, including mineral soils that were rich in Ca or Si. Although chemical composition of P varied dramatically with increasing depth, the colloidal P composition remained unchanged. This contrasting difference between mineral soil and its WDC fraction indicated that the colloids were not locally generated but originated from the overlying organic soil horizons. Carbonate minerals were unlikely involved in colloid formation under acidic condition. Instead, Ca2+ probably drove colloid formation by bridging organic matter, including P-containing compounds released from litter degradation. Colloid formation was influenced by climate, vegetation, and soil characteristics. No dissolved P was detected in deeper mineral soil horizons due to efficient retention by Ca minerals. Colloidal P was still present in deeper soil layers and thus of significance for potential P vertical transfer

Colloidal iron and organic carbon control soil aggregate formation and stability in arable Luvisols

Several beneficial soil functions are linked to aggregates, but how the formation and stability depend on the presence of colloidal- and nanosized (1000–1 nm) bulding blocks is still understood poorly. Here, we sampled subsites from an arable toposequence with 190 and 340 g kg−1 clay, and isolated small soil microaggregates (SMA; 250 µm) using an ultrasonic dispersion energy of 60, 250, and 440 J mL−1, respectively. We then allowed these small SMA to reaggregated after chemical removal of organic carbon (OC) as well as of Fe- and Al (hydr)oxides, respectively. The size distribution of the reaggregated small SMA and fine colloids (<0.45 µm) was analyzed via laser diffraction and asymmetric flow field-flow fractionation coupled to inductively coupled plasma mass spectrometry and organic carbon detection, respectively. We found elevated amounts of both finer colloids and stable SMA at subsites with higher clay contents. The size distribution of small SMA was composed of two distinctive fractions including colloids and larger microaggregates with an average size of 5 µm. The removal of Fe with Dithionite-Citrate-Bicarbonate (DCB) shifted the size of the small SMA to a larger equivalent diameter, while removal of OC with NaOCl reduced it. After three wetting and drying cycles, the concentration of colloids declined, whereas the small SMA without chemical pre-treatments reaggregated to particles with larger average diameters up to 10 µm, with the size depending on the clay content. Intriguingly, this gain in size was more pronounced after Fe removal, but it was not affected by OC removal. We suggest that Fe (hydr)oxides impacts the stability of small SMA primarily by being present in small-sized pores and thus cementing the aggregates to smaller size. In contrast, the effect of OC was restricted to the size of colloids, gluing them together to small SMAs within defined size ranges when OC was present but releasing these colloids when OC was absent

Phosphorus in water dispersible-colloids of forest soil profiles

Background and aimsNanoparticles and colloids affect the mobilisation and availability of phosphorus for plants and microorganisms in soils. We aimed to give a description of colloid sizes and composition from forest soil profiles and to evaluate the size-related quality of colloids for P fixation.MethodsWe investigated the size-dependent elemental composition and the P content of water-dispersible colloids (WDC) isolated from five German (beech-dominated) forest soil profiles of varying bulk soil P content by field-flow fractionation (FFF) coupled to various detectors.ResultsThree size fractions of WDC were separated: (i) nanoparticles <25 nm (NP) rich in Corg, (ii) fine colloids (25 nm–240 nm; FC) composed mainly of Corg, Fe and Al, probably as associations of Fe- and Al- (hydr)oxides and organic matter, and (iii) medium-sized colloids (240 nm–500 nm; MC), rich in Fe, Al and Si, indicating the presence of phyllosilicates. The P concentration in the overall WDC was up to 16 times higher compared to the bulk soil. The NP content decreased with increasing soil depth while the FC and MC showed a local maximum in the mineral topsoil due to soil acidification, although variant distributions in the subsoil were observed. NP were of great relevance for P binding in the organic surface layers, whereas FC- and MC-associated P dominated in the Ah horizon.ConclusionThe nanoparticles and colloids appeared to be of high relevance as P carriers in the forest surface soils studied, regardless of the bulk soil P content