The role of natural nanoparticles and colloids for phosphorus binding in forested headwater catchments

Stream waters reflect the natural load of nutrients and minerals cycled within or released from ecosystems; yet, little is known about natural colloids (1-1000 nm) and especially nanoparticles (NNP, 1-100 nm) as nutrient carriers in the complex biogeochemical system of forested headwater catchments. NNP and colloids are recognized as ubiquitous components in natural aqueous phases and have the potential to encapsulate and bind nutrients, yet are often not included in the analysis of terrestrial nutrient cycling processes. The distribution of elements between the different physicochemical forms in solution is an important precursor to understand the mechanisms of ecosystem nutrition, especially for limiting nutrients like phosphorus (P). The size and composition of NNP and colloids in aqueous phases is therefore relevant for the transport of essential nutrients like P. Asymmetric Flow Field Flow Fractionation (AF4) was coupled online to a UV detector for approximation of organic C, a dynamic light scattering device for recording of the hydrodynamic particle diameter, a quadrupole inductively coupled plasma mass spectrometer with collision cell technology (ICP-MS) for elemental size-resolved detection and to an organic carbon detector (OCD) for high sensitive size-resolved organic carbon detection. Method development of hyphenated AF4 was performed whereas online P detection represented a specific challenge due to the low concentrations in many natural waters. Methodological considerations on the oxidation efficiency of OCD, the capability of ICP-MS to detect organic C and on a setup to be able to determine the bioavailability of NNP and colloid bound P were assessed. Stream waters of forested headwater catchments were sampled as representative medium for mobile components in ecosystems. To assess a more universally valid role of NNP and colloids, an upscaling approach of the catchment based analysis was chosen from regional to national to continental scale. The aim of the regional sampling study was to characterize NNP and colloidal bound P of distinct hydromorphological areas in stream water of the Wüstebach catchment. The NNP and colloidal P could be fractionated in two size fractions (2-20 nm and >20-300 nm), which constituted up to 100% of the total river P discharge depending on hydromorphology. For the small size fraction, variations in P concentrations followed the Al variations; in addition, a high Fe presence in both fractions was accompanied by high P concentrations. Moreover, organic C was approximated together with P in the presence of Fe and Al, suggesting that Fe and Al are potential carriers of P and associated with organic matter. Tracing the origin of NNP and colloid fractions revealed mixed inputs from soil and vegetation of the catchment. The data enables the inputs and source regions of NNP and colloidal fractions to be traced and conceptually defined for the first time within a small river of a headwater catchment.For the national sampling campaign it was tested if the majority of P is bound to NNP in forest streams but that their size and composition varies for different forested headwater systems. Five forested sites, which differ in total P content, were sampled for stream water during base flow conditions and analyzed for NNP and colloidal fractions. Through the refined AF4 method combined with exploratory data analysis, the results showed that the NNP and colloids of all sites could be distinguished into three distinct fractions (approx. 1 nm-20 nm, >20 nm-60 nm, >60 nm-300 nm), yet the elemental concentrations in the fractions were not homogenously distributed. Exploratory data analysis showed that each fraction had unique elemental signatures with different preferential P binding partners. P was preferentially associated to Fe in the smallest size fraction, with increasing contribution of organic C associated P as the hydrodynamic diameter of the fractions increased. The largest fraction was dominated by aluminosilicate minerals. The relative contribution of the NNP and colloidal fractions for ecosystem nutrient supply can be expected to rise as total P concentrations decline. Moreover, the stream water C to P ratio revealed that NNP and colloids are potentially capable of predicting the nutritional status of an ecosystem. The first flush effect is a potential major loss factor of nutrients bound to NNP and colloids but showed no significant effect on the identified fractions. The factors influencing NNP and colloid inputs to the stream were investigated in a first approach.On continental scale, a systematic variation with respect to size and composition of NNP and colloids across Europe was found. 96 stream water samples from 26 forested headwater catchments along two transects across Europe were simultaneously collected from base flow. Three fractions (approx. 1 nm-20 nm, >20 nm-60 nm, >60 nm-300 nm) of NNP and colloids were identified. NNP and colloids contributed up to 100% to total element concentrations, indicating a variable but potentially significant contribution of particles for element transport across different geographic regions. Two types of distribution patterns were found: org C, Fe and Al showed linear distribution patterns among particle bound to total concentrations, whereas Si, Mn, P and Ca was independent of total concentrations. Within the fractions, element patterns were able to discriminate sites according to pH value. These analyses suggested a geographical divide of NNP and colloid bound element transport at 45° and 57° latitude in Europe, corresponding to a categorization of streams into pH classes. Hence, NNP and colloids are a relevant component of element cycles across Europe. Site specific ecosystem parameters also showed to have relevant impacts on the composition of NNP and colloid fractions with a clear effect of dominant tree type (coniferous) and mean annual temperature.NNP and colloids play an important role in forest stream waters for P transport and thus P cycling by binding up to 100% of total P present in the stream. Three fractions of NNP and colloids, each with unique composition patterns and variable P binding, are present throughout European forested headwaters. The fractions follow predictive element specific patterns and compositions on all scales, also allowing a first assessment based on their ecological relevance. This work enhances the understanding of NNP and colloids for P transport and facilitates their inclusion into terrestrial ecosystem cycling processes

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

Field Flow Fractionation in Environmental Research: Characterization of natural nanoparticles in a forest stream

A high precision fractionation of particle size classes between 1nm and 1000nm is best applicable with the techniques of field flow fractionation (FFF). The separation is performed without a stationary phase in an open channel which is subject to a force acting perpendicular to the solvent flow and thus driving the fractionation. For the Asymmetric Flow Field Flow Fractionation (AF4), the sample runs over a membrane with a specific cut-off size perpendicular to which an additional solvent flow acts as the separation field. The fractionation occurs on behalf of diameter, density and diffusion rate of the particles. Every FFF can be coupled to a multitude of detectors serving for particle characterization. The exemplary study aims at characterizing the elemental composition of nanoparticle (NP) fractions in stream water from a forest site and identifying potential source regions. Through coupling of AF4 to ICP-MS, successful measurements of nanoparticle bound iron, aluminum and phosphorus were performed. Two distinct size fractions were observed for all sampling sites: an organic matter- and an iron-determined fraction. For both, concentrations are highest in the midst of the stream with similar low values in the headwaters and near the outlet of the catchment. Elemental ranges for the organic fraction are 0.1 - 5.2µg/l for phosphorus and aluminum and 25 - 680µg/l for iron; for the iron-determined fraction 2 - 26µg/l for P and Al and 340 - 1800µg/l for Fe, respectively. Moreover, between 10-100% of P and Fe concentrations in the samples were bound to particles, in contrast to ~1.5% for Al. Overall, through coupling of FFF and ICP-MS a first assessment of the source regions of nanoparticles in stream water and the distribution of size classes in accordance with the elemental composition could be done

Distribution of Phosphorous-Containing Fine Colloids and Nanoparticles in Stream Water of a Forest Catchment

Natural fine colloids and nanoparticles have the potential to encapsulate and bind nutrients. Their size range and composition is therefore relevant to understand the transport of essential nutrients like phosphorus in an aquatic ecosystem. The aim of the study was to characterize fine colloidal and nanoparticulate bound phosphorus of distinct hydromorphological areas in stream water samples from a forested experimental test site in a small headwater catchment. Asymmetric Flow Field Flow Fractionation (AF4) coupled online to inductively coupled plasma mass-spectrometry (ICP-MS) was applied for size resolved detection of phosphorus (P), iron (Fe), and aluminum (Al) in the fractions. Additionally, the dissolved organic matter (DOM) content was derived from the online UV signal. Two distinct fractions were detected and characterized. For the first size fraction, variations in P concentrations strongly correlated to the course of Al variations; in addition, high Fe presence in both fractions was accompanied by high P concentrations. Moreover, DOM was detected with P in presence of Fe and Al. Possibly, Fe and Al containing particles are carriers of P compounds and associated with organic matter. The study enables for the first time to trace and conceptually define the inputs and source regions of fine colloidal and nanoparticulate fractions within a small river of a headwater catchment. The stream water investigations will be extended to additional test sites and a broader range of elements

Fine colloidal and nanoparticulate P, Fe, Al and C distribution in stream water of a German mountainous forest catchment

Natural fine colloids and nanoparticles have the potential to encapsulate and bind nutrients. Their size range and composition is therefore relevant to understand the transport of essential nutrients like phosphorus in an aquatic ecosystem. The aim of the study was to characterize fine colloidal and nanoparticulate bound phosphorus of distinct hydromorphological areas in stream water from a forested experimental test site in a small headwater catchment. Asymmetric Flow Field Flow Fractionation (AF4) is a frequently used method when aiming at a separation and characterization of colloids in aquatic systems. It combines a large separation range (about 1 nm to 1 µm) with the possibility to couple various detection devices online. The separation is performed without a stationary phase in an open channel which is subject to a force acting perpendicular to the solvent flow and thus driving the fractionation. The fractionation occurs on behalf of diameter and diffusion rate of the particles. AF4 coupled online to ICP-MS was applied for size resolved detection of phosphorus (P), iron (Fe), and aluminum (Al) in the fractions. Special focus was on P detection which is present at low concentrations (few µg/L) in many natural waters. Two distinct fractions (mean d~8 nm and ~150 nm) were detected and characterized. For the small size fraction, variations in P concentrations strongly correlated to the course of Al variations; in addition, high Fe presence in both fractions was accompanied by high P concentrations. The developed methodology enables for the first time to trace and conceptually define the inputs and source regions of fine colloidal and nanoparticulate fractions within a small river of a headwater catchment

Distribution of Phosphorus-Containing Fine Colloids and Nanoparticles in Stream Water of a Forest Catchment

Natural fine colloids and nanoparticles have the potential to encapsulate and bind nutrients. Their size and composition is therefore relevant to understand the transport of essential nutrients like phosphorus in an aquatic ecosystem. The aim of this study was to characterize fine colloidal and nanoparticulate bound P of distinct hydromorphological areas in stream water from a forested test site in a small headwater catchment. Asymmetric flow field flow fractionation coupled online to inductively coupled plasma mass spectrometry was applied for size-resolved detection of P, Fe, and Al in the fractions. Online P detection was a challenge due to the low concentrations (in this study down to 0.1 μg/L) in many natural waters. Additionally, the “dissolved” organic matter (DOM) content was derived from the online UV signal. The colloidal P occurred in two size fractions (2–20 and 21–300 nm), which constituted up to 100% of the total river P discharge depending on hydromorphology. For the small size fraction, variations in P concentrations correlated with Al variations; in addition, a high Fe presence in both fractions was accompanied by high P concentrations. Moreover, DOM was detected with P in the presence of Fe and Al, suggesting that Fe and Al are carriers of P and associated with organic matter. The developed methodology enables the inputs and source regions of fine colloidal and nanoparticulate fractions within a small river of a headwater catchment to be traced and conceptually defined for the first time

Spatial distribution of hydroxylamine and its role to aerobic N2_{2}O formation in a Norway spruce forest soil

Hydroxylamine (HA) is potentially involved in soil N2O formation as a crucial intermediate in the oxidation of ammonium to nitrite. However, the determination of HA concentration in natural soil samples has not been reported until now. Here, we determined the HA concentrations in organic (Oh) and mineral (Ah) layers of 110 soil samples collected from a spruce forest (Wüstebach, Eifel National Park, Germany) using a novel approach, based on the fast extraction of HA from the soil at a pH of 1.7, the oxidation of HA to N2O with Fe3+, and the analysis of produced N2O using gas chromatography (GC-ECD). In a second step, N2O emission rates were determined by means of aerobic laboratory incubations of 3 g soil in 22-mL vials. Subsequently, the spatial distribution of soil HA concentrations and N2O emission rates in the Oh and Ah layers of the whole sampling area were analyzed using a geostatistical approach. The correlations among soil HA, N2O emission rate, pH, soil C, N, Fe, Mn and soil water content (SWC) were further explored. The HA concentrations ranged from 0.3–37.0 μg N kg-1 and 0.02–11.4 μg N kg-1 dry soil in the Oh and the Ah layer, respectively. The spatial distribution of HA was similar in both layers, with substantial spatial variability dependent on soil type, tree density and distance to a stream, e.g., HA concentration was greater at locations with a thick litter layer or at locations close to the stream. N2O emission rates showed a similar pattern as soil HA concentrations, with higher rates in the Oh layer than in the Ah layer. N2O emission rate exhibited the highest correlation with soil HA content in the Oh layer, while soil NO3- content explained N2O emissions best in the Ah layer, associated with SWC, Mn and C content. HA concentration was negatively correlated with pH and positively correlated with SWC in the Oh layer, while positively correlated with C and N as well as NO3- content in the Ah layer. Moreover, Mn content was the most important factor for HA recovery at the specific extraction conditions. The results demonstrated that HA is a crucial component for aerobic N2O formation and emission in spruce forest soils. Mn may also play a key role to the aerobic N2O emission due to the chemical reaction with HA. Further studies should focus to the relationships between HA, Mn and aerobic N2O emission in other ecosystems

Spatial distribution of hydroxylamine and its role to aerobic N2O formation in a Norway spruce forest soil

Hydroxylamine (HA) is potentially involved in soil N2O formation as a crucial intermediate in the oxidation of ammonium to nitrite. However, the determination of HA concentration in natural soil samples has not been reported until now. Here, we determined the HA concentrations in organic (Oh) and mineral (Ah) layers of 110 soil samples collected from a spruce forest (Wüstebach, Eifel National Park, Germany) using a novel approach, based on the fast extraction of HA from the soil at a pH of 1.7, the oxidation of HA to N2O with Fe3+, and the analysis of produced N2O using gas chromatography (GC-ECD). In a second step, N2O emission rates were determined by means of aerobic laboratory incubations of 3 g soil in 22-mL vials. Subsequently, the spatial distribution of soil HA concentrations and N2O emission rates in the Oh and Ah layers of the whole sampling area were analyzed using a geostatistical approach. The correlations among soil HA, N2O emission rate, pH, soil C, N, Fe, Mn and soil water content (SWC) were further explored. The HA concentrations ranged from 0.3–37.0 μg N kg-1 and 0.02–11.4 μg N kg-1 dry soil in the Oh and the Ah layer, respectively. The spatial distribution of HA was similar in both layers, with substantial spatial variability dependent on soil type, tree density and distance to a stream, e.g., HA concentration was greater at locations with a thick litter layer or at locations close to the stream. N2O emission rates showed a similar pattern as soil HA concentrations, with higher rates in the Oh layer than in the Ah layer. N2O emission rate exhibited the highest correlation with soil HA content in the Oh layer, while soil NO3- content explained N2O emissions best in the Ah layer, associated with SWC, Mn and C content. HA concentration was negatively correlated with pH and positively correlated with SWC in the Oh layer, while positively correlated with C and N as well as NO3- content in the Ah layer. Moreover, Mn content was the most important factor for HA recovery at the specific extraction conditions. The results demonstrated that HA is a crucial component for aerobic N2O formation and emission in spruce forest soils. Mn may also play a key role to the aerobic N2O emission due to the chemical reaction with HA. Further studies should focus to the relationships between HA, Mn and aerobic N2O emission in other ecosystems