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The removal of 2,4-dichlorophenol (DCP) using calcined and modified forms of hydrotalcite (HT) was studied. HT was calcined at 550 ° C for 3 h. The calcined form (HTC) was modified under argon atmosphere using sodium stearate in an amount equivalent to 100% of the theoretical anion exchange capacity of HT. The 3 forms of HT were subjected to X-ray diffraction analysis. It was determined that the peak with characteristic basal spacing d = 7.62 Å corresponding to interlayer carbonate anion disappeared in HTC and a peak with d = 32 Å confirming the adsorption of stearate anion appeared in organo-HT. Adsorption equilibria of DCP on HT and organo-HT were measured using batch equilibration. Adsorption behavior was modeled by the modified Freundlich equation. It was determined that HTC and organo-HT have maximum efficiencies corresponding to certain initial concentrations, i.e. 2.76 mmol/l for HTC and 3.68 mmol/l for organo-HT. At these concentrations, HTC and organo-HT can adsorb 61% and 17% of DCP, respectively. [ABSTRACT FROM AUTHOR]

Natural soils have frequently been considered to decrease the mobility of engineered nanoparticles (NPs) in comparison to quartz sand due to the presence of colloids that provide additional retention sites. In contrast, this study demonstrates that the transport and release of silver nanoparticles (AgNPs) in sandy clay loam and loamy sand soils were enhanced in the presence of soil colloids that altered soil grain surface roughness. In particular, we found that the retention of AgNPs in purified soils (colloid-free and acid-treated) was more pronounced than in raw (untreated) soils or soils treated to remove organic matter (H 2 O 2 or 600 °C treated). Chemical analysis and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy demonstrated that the grain surfaces of raw and organic matter-removed soils were abundant with metal oxides and colloids compared to purified soil. Column transport and release experimental results, SEM images, and interaction energy calculations revealed that a significant amount of concave locations on purified soils hindered AgNP release by diffusion or ionic strength (IS) reduction due to deep primary energy minima. Conversely, AgNPs that were retained in soils in the presence of soil colloids were more susceptible to release under IS reduction because the primary minimum was shallow on the tops of convex locations created by attached soil colloids. Additionally, a considerable fraction of retained AgNPs in raw soil was released after cation exchange followed by IS reduction, while no release occurred for purified soil under the same conditions. The AgNP release was highly associated with soil colloids and co-transport of AgNPs and soil colloids was observed. Our work is the first to show that the presence of soil colloids can inhibit deposition and facilitate the release and co-transport of NPs in soil by alteration of the soil grain surface morphology and shallow primary minimum interactions. [Display omitted] • Soil colloids enhance NP transport/release by altering grain surface morphology. • Surface roughness dominates over chemical heterogeneity for NP transport/release. • Soil colloids promote NP release by grain surface modification and co-transport. • NPs release from shallow primary minimum when soil colloids create protrusions. • NP retention at valleys on grain is irreversible due to deep primary minimum. Transport and release of nanoparticles can be enhanced by soil colloids via modification of grain surface morphology that alters the interaction energy and co-transport. [ABSTRACT FROM AUTHOR]

• Soil colloids promote the stability of microaggregate occlusion in macroaggregates. • Microaggregate reaggregation is affected by colloidal iron and organic carbon. • Less microaggregate reaggregation with reduced organic carbon concentrations in Luvisols. • Smaller-sized microaggregate structures in presence of colloidal-sized iron. 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; <20 µm) from larger macroaggregate units (>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. [ABSTRACT FROM AUTHOR]

• Low δ 18O HCl-P values were constant with parent material at the driest sites. • δ 18O HCl-P values increase at less arid sites. • δ 18O HCl-P values trace past biological activity in the Atacama Desert. The Atacama Desert (Chile) is one of driest places on Earth, with a hyper-arid climate and less than 2 mm yr−1 precipitation; nevertheless, it has experienced rare periods of sporadic rainfall. These periods shortly enhanced vegetation growth and microbial activity, which must have utilized major nutrients such as phosphorus (P). However, any biological cycling of P involves an oxygen exchange with water, which should now reside in the hyperarid soils as tracer of life. In order to identify such evidences, we performed sequential P fractionation and analyzed the oxygen isotope composition of HCl-extractable phosphate (δ 18O HCl–P) in the surface soil (0–15 cm) of a climatic gradient along the rising alluvial fans of the Central Depression to the Precordillera, Chile. At the driest sites, the δ 18O HCl-P values were constant with depth and deviated from biologically-driven isotopic equilibrium. In contrast, we observed a considerable increase of δ 18O HCl-P values below the soil surface at less arid sites, where some isotope values were even within the range of full isotopic equilibrium with biologically cycled phosphate. For the latter sites, this points to most efficient biological P cycling right below the uppermost surface of the desert. Critically, the absolute concentrations of this biologically cycled P exceeded those of P potentially stored in living microbial cells by at least two orders of magnitude. Therefore, our data provides evidence that δ 18O HCl-P values trace not recent but past biological activity, making it a powerful tool for assessing the existence, pathways and evolution of life in such arid ecosystems on Earth and, thus, potentially on other planets such as Mars. [ABSTRACT FROM AUTHOR]

Despite numerous studies about the change of soil biogeochemical properties by climate alternation driven by elevation, latitude, etc. in arid to hype-arid regions, little is known about soil colloids (1-1000 nm) and nanoparticles (1-100 nm) as nutrient carriers and which events are driving this in such environments. We investigated the size and composition of water dispersible colloids (WDCs) to quantify colloidal bound C and P from three depth profiles in an altitudinal transect in the Atacama Desert (Paposo region) using asymmetric field flow field fractionation (A-FFFF) coupled online to various detectors (ICP-MS, organic carbon detector (OCD) and UV). One of the profiles (A; 200 cm) was located in a semi-arid part and the two others (70 cm depth) in an arid area in the vicinity of each other (B on an abandoned alluvial fan, and C on the more active part of the fan). Three size categories of WDCs were detected based on the FFF results, including nanoparticles from 0.6 to 24 nm, fine colloids from 24 to 210 nm, and medium colloids from 210 to 500 nm. Nanoparticles and soil colloids accounted for up to 60% of the bioavailable P concentration (Olsen P) at the surface layer of profile C. Moreover, the contribution of colloidal Corg to total Corg was 5% at the surface of profile A, with a maximum of 11% at depth of 115 cm, and maximum 1% at the surface layer of both profiles B and C. The size distribution of colloids and their elemental content was significantly different between B and C especially at upper 20 cm. Our results, showing pronounced local differences in soil colloidal properties, suggest that colloidal matter (here mainly C and P) preserved in the Atacama deserts is terrestrial proxy record of both local (e.g. intermittent drainage events) and larger scale (periods of dust input) of environmental changes which have occurred in this part of South America. [ABSTRACT FROM AUTHOR]

The functions of soils are intimately linked to their three‐dimensional pore space and the associated biogeochemical interfaces, mirrored in the complex structure that developed during pedogenesis. Under stress overload, soil disintegrates into smaller compound structures, conventionally named aggregates. Microaggregates (<250 µm) are recognized as the most stable soil structural units. They are built of mineral, organic, and biotic materials, provide habitats for a vast diversity of microorganisms, and are closely involved in the cycling of matter and energy. However, exploring the architecture of soil microaggregates and their linkage to soil functions remains a challenging but demanding scientific endeavor. With the advent of complementary spectromicroscopic and tomographic techniques, we can now assess and visualize the size, composition, and porosity of microaggregates and the spatial arrangement of their interior building units. Their combinations with advanced experimental pedology, multi‐isotope labeling experiments, and computational approaches pave the way to investigate microaggregate turnover and stability, explore their role in element cycling, and unravel the intricate linkage between structure and function. However, spectromicroscopic techniques operate at different scales and resolutions, and have specific requirements for sample preparation and microaggregate isolation; hence, special attention must be paid to both the separation of microaggregates in a reproducible manner and the synopsis of the geography of information that originates from the diverse complementary instrumental techniques. The latter calls for further development of strategies for synlocation and synscaling beyond the present state of correlative analysis. Here, we present examples of recent scientific progress and review both options and challenges of the joint application of cutting‐edge techniques to achieve a sophisticated picture of the properties and functions of soil microaggregates. [ABSTRACT FROM AUTHOR]

[Display omitted] • Different deposition behaviors of co-transport were observed in column and QCM-D. • Ca2+ exhibited a more significant impact on co-transport and deposition than K+. • Divalent cation enhanced the non-DLVO interactions of MWCNTs with BNPs or GNPs. • Ca2+ enhanced straining and competitive blocking in the co-transport in QS. • Ca2+ exhibited a much stronger impact on MWCNTs and GNPs than MWCNTs and BNPs. This work investigates the influence of cation type (Ca2+ as compared to K+) on the co-transport and deposition of functionalized multi-walled carbon nanotubes (MWCNTs) with negatively (bentonite nanoparticles, BNPs) or positively (goethite nanoparticles, GNPs) charged natural nanoparticles under the same ionic strength (IS = 1 mM) conditions. In packed column tests with quartz sand (QS), the co-transport of MWCNTs was slightly increased by BNPs and inhibited by GNPs in both CaCl 2 and KCl solutions, whereas the co-transport of BNPs or GNPs was facilitated by MWCNTs in both solutions. However, the co-transport of MWCNTs in the presence of BNPs or GNPs exhibited a different dependency on the cation type in quartz crystal microbalance with dissipation (QCM-D) tests. No deposition of MWCNTs and BNPs was observed in QCM-D studies in the presence of KCl, while enhanced deposition of MWCNTs and GNPs occurred in this same solution. In contrast, the deposition of both MWCNTs and BNPs or MWCNTs and GNPs increased in QCM-D in the presence of CaCl 2. In both column and QCM-D studies, Ca2+ exhibited a more significant impact on co-transport than K+, especially for MWCNTs with GNPs than BNPs. Results from molecular dynamic simulations, aggregation studies, and interaction energy calculations indicate that non-DLVO interactions (e.g. , H-bonding and cation-π interaction) between two nanoparticles (MWCNTs with BNPs or GNPs) played a non-negligible role in interpreting the deposition and co-transport in the presence of the divalent cation Ca2+. Mathematical modeling of breakthrough curves and retention profiles and aggregation results suggest that both straining and competitive blocking were enhanced and played dominant roles in the co-transport of MWCNTs in the presence of BNPs or GNPs in QS, especially in the presence of Ca2+. This work sheds novel insights on the contribution of divalent cations to interactions between two colloids and their co-transport in porous media. This information is needed to assess the environmental fate and risks of engineered nanoparticles (ENPs) and natural nanoparticles in aquatic and soil environments with abundant divalent cations. [ABSTRACT FROM AUTHOR]

The unsaturated zone plays a significant role in protecting groundwater resources from microbial contaminants. The mechanisms governing bacteria transport and retention in aggregated soils are still not fully understood and quantified. This study aims to investigate Pseudomonas fluorescens transport and retention in undisturbed calcareous soils under both saturated and unsaturated conditions. Series of transport experiments with well-controlled suctions and flow rates were performed. The breakthrough curves of bacteria and a nonreactive tracer (Cl-) were obtained. The bacteria were then quantified in different layers of soil. HYDRUS-1D two-site depth-dependent deposition model was employed to simulate transport and retention of bacteria. HYDRUS-1D two-site depth-dependent deposition model fit the observed data well (R2=0.99). Results revealed that the average ratio of detachment to attachment rate was 0.028 and 0.016 under saturated and unsaturated flow conditions, respectively, demonstrating nearly irreversible attachment of cells. Increasing depth dependent-deposition coefficient (k 2) in unsaturated soils was attributed to straining and film straining. The majority of bacteria were deposited nearby the inlet of soil columns, and the rate of retention diminished with depth. Bacterial removal rate was 2.5 times higher under unsaturated than under saturated condition. High removal rate of bacteria under unsaturated flow was attributed to retention of bacteria to film straining, air-water interface, and also attachment to CaCO 3 minerals that contained preferred sites for negatively charged cells. [Display omitted] • We study transport and deposition of bacteria in saturated and unsaturated undisturbed soil. • Transport and retention were well described by two-site kinetic deposition model. • Strong attachment of bacteria was attributed to soil calcium carbonate. • High removal rate of bacteria under unsaturated condition due to straining. [ABSTRACT FROM AUTHOR]

The concentration of nonionic surfactants like Triton X-100 (TX100) can influence the transport and fate of emerging contaminants (e.g. , carbon nanotubes) in porous media, but limited research has previously addressed this issue. This study investigates the co-transport of functionalized multi-walled carbon nanotubes (MWCNTs) and various concentrations of TX100 in saturated quartz sand (QS). Batch experiments and molecular dynamics simulations were conducted to investigate the interactions between TX100 and MWCNTs. Results indicated that the concentration ratio of MWCNTs and TX100 strongly influences the dispersion of MWCNTs and interaction forces between MWCNTs and QS during the transport. Breakthrough curves of MWCNTs and TX100 and retention profiles of MWCNTs were determined and simulated in column studies. MWCNTs strongly enhanced the retention of TX100 in QS due to the high affinity of TX100 for MWCNTs. Conversely, the concentration of TX100 had a non-monotonic impact on MWCNT retention. The maximum transport of MWCNTs in the QS occurred at an input concentration of TX100 that was lower than the critical micelle concentration. This suggests that the relative importance of factors influencing MWCNTs changed with TX100 sorption. Results from interaction energy calculations and modeling of competitive blocking indicate that the predictive ability of interaction energy calculations and colloid filtration theory may be lost because TX100 mainly altered intermolecular forces between the MWCNT and porous media. This study provides new insights into the co-transport of surfactants and MWCNTs in porous media, which can be useful for environmental applications and risk management. ga1 • The concentration of TX100 had a non-monotonic impact on MWCNT retention. • TX100 influences interaction forces between MWCNTs and QS. • The interactions between MWCNTs and TX100 were calculated by molecular dynamics simulations. • MWCNTs strongly inhibited TX100 transport in QS. • Both breakthrough curves and retention profiles were determined and simulated by HYDRUS 1D. [ABSTRACT FROM AUTHOR]

Light expanded clay aggregates (LECA) have been increasingly used as substrate material for constructed wetlands given their phosphate removal capacity, mechanical strength, hydraulic conductivity and their plant rooting and biofilm growth supporting structure. This review summarizes the current literature on LECA-based constructed wetlands. Removal performances for main wastewater parameters phosphate, nitrogen species, suspended solids and oxygen demand are tabulated. Both, physical and biological water purification processes in LECA wetlands are discussed. Additional emphasis is on design and layout of LECA wetlands for different types of wastewater, under different climatic conditions and to improve treatment performance in general. LECA life cycle considerations include sourcing, production energy demand, reuse and recycling options for spent wetland substrates, for example as soil amendment. Research and development opportunities were identified for structural and compositional LECA modification to obtain tailored substrates for the use in water treatment and specific treatment tasks. Beyond traditional wastewater contaminants the fate of a wider range of contaminants, including organic trace contaminants, needs to be investigated as high Fe, Al and Ca oxides content of LECA substrates provide adsorptive sites that may facilitate further biological interactions of compounds that are otherwise hard to degrade. • Gap/strength analysis of LECA as a substrate material in constructed wetlands. • High Fe, Al, and Ca oxides content underpin LECA pollutant removal and adsorbance. • LECA has indirect effects on parameters controlling biochemical efficiency of CWs. • Technical aspects and optimal design considerations for LECA in different CWs evaluated. • Saturated LECA potential use as soil amendment and fertilizer reviewed. [ABSTRACT FROM AUTHOR]

• In Ap horizons of arable Luvisols, colloids control the aggregation of small soil microaggregates (<20 μm) in wet-dry cycles. • The presence of <450 nm colloids favored the formation of 1–40 μm soil microaggregates. • In absence of <1 μm colloids, >40 μm soil micro- and macroaggregates were preferentially formed. • The particle size is decisive for the aggregate formation under the current experimental condition. In the hierarchical model of soil aggregates, small soil microaggregates (small SMA; <20 μm) are often considered to be fundamental building units at the micron scale. Below which, soil colloids (<1 µm) have recently been proposed as binding agents of (micro)aggregates. However, the way in which soil colloids contribute to the formation and stability of soil micro- and macroaggregates remains largely unknown. For clarification, we evaluated potential impacts of the colloidal content, particularly the <450 nm colloids, on the aggregation of small SMA. Free water stable small SMA and <450 nm colloids were isolated from Ap-horizons of Stagnic Luvisols under different management (cropped and bare fallow). The size-resolved elemental composition of the <450 nm colloids was analyzed by asymmetric flow field-flow fractionation in combination with an inductively coupled plasma mass spectrometer and an organic carbon detector. To vary the colloidal content in small SMA, (1) suspensions containing different amounts of <450 nm colloids were added in small SMA, or (2) <1 µm colloids were removed from small SMA by centrifugation. In the maximum colloidal addition treatment, the mass ratios of added colloids to small SMA were 3.0 and 5.1 wt% for the cropped and bare fallow soil samples, respectively. Aggregation of small SMA with different colloidal amounts was performed in three successive wet-dry cycles. Afterwards, the size distribution of the resulting aggregates was measured by laser diffraction. Our results indicated that, in wet-dry cycles, colloids were important binding agents for the formation of SMA. Their presence, especially those <450 nm, was likely to support the formation of solid bridges during drying at particle contacts of 1–10 µm small SMA, favoring hereby SMA build-up in a relatively small size range of 1–40 µm. In contrast, the absence of <1 μm colloids in small SMA led to a preferential generation of relatively large aggregates in wet-dry cycles, i.e., typically with sizes >40 μm up to 1700 μm in maximum. Our study on aggregation in wet-dry cycles revealed that the colloidal content has a controlling effect on the size distribution of resulting aggregates by acting as a binding agent and provides hereby new insights into the evolvement of aggregate hierarchy in soils. [ABSTRACT FROM AUTHOR]

There is an intimate relationship between microorganisms and the formation and stability of soil microaggregates, realized by the immobilization and occlusion of microorganisms. Little is known about the initial aggregate formation phase and the role of microorganisms in this process under the impact of environmental changes such as wetting and drying. We investigated this initial aggregate formation process of montmorillonite and goethite in combination with two bacterial strains, Pseudomonas protegens strain CHA0 and Gordonia alkanivorans strain MoAcy 2, in the presence or absence of stress conditions in form of wetting and drying cycles for up to eight days. Montmorillonite and goethite formed microaggregates instantaneously, the size of these aggregates being enhanced in the presence of microorganisms, resulting in up to twofold larger aggregates. This increase in aggregate size was strain-dependent. However, the aggregates that developed during the first 48 h broke into smaller structures later on. A microscopic analysis of the microaggregates revealed that notably the larger microaggregates harbored bacteria and that microaggregates had a sheltering effect on living cells, especially when exposed to desiccation stress. Additionally, aggregate formation was analyzed in the presence of a Pseudomonas protegens mutant strain (CHA211) with increased production capability of extracellular polymeric substances (EPS). About fivefold higher survival rates of culturable cells were observed after desiccation for this EPS overproducing mutant strain in comparison to the wild-type. Our results hint at an aggregate formation process characterized by a rapid occlusion of mineral compounds, and, after the addition of microorganisms, the bacterial colonization of small microaggregates, leading to an increase in aggregate size. The further development of the aggregate size distribution varied depending on the presence of microbial taxa and was modulated by environmental conditions like desiccation events. • The presence of bacterial cells increases the size of microaggregates. • Desiccation stress induces the formation of larger microaggregates. • Microaggregates support bacterial survival upon desiccation stress. • Extracellular polymeric substances support bacterial survival upon desiccation stress. [ABSTRACT FROM AUTHOR]

As soil microaggregates (<250 μm, SMA) usually withstand long-term tillage, we hypothesized that i) elevated clay contents in arable soil support aggregation already at microaggregate level, leading to ii) increasing organic carbon (SOC) enrichment in smaller SMA size fractions. To test these hypotheses we sampled the topsoil (5–20 cm) of Luvisols with a long history of agricultural management at the Scheyern experiment station (Germany) in quintuplicates from each of five subsites with different clay contents (19–34%). The field-fresh topsoil was fractionated into macroaggregates (8000–250 μm), large SMA (250–20 μm), and small SMA (<20 μm) and the mass distribution was recorded. In addition, the water stable macroaggregates (>250 μm) were dispersed ultrasonically to study occluded SMA and single building units. Finally, we analyzed the size distribution of the small SMA by laser diffraction analysis. The total mass distribution of free and occluded SMA grouped soils into those with small (19, 22, and 24%) and large (32 and 34%) clay contents. The finer textured soils exhibited larger portions of occluded SMA, with a gamma size distribution of small SMA peaking at 6 μm. Yet the occluded small SMA in the finer textured soils showed an additional enrichment of colloids <1 μm. The SOC was indeed enriched in finer fractions, but more in the small SMA of the coarse textured sites than in the finer textured ones, whereas the opposite was true for the large SMA. We conclude, therefore, that elevated contents of clay-sized particles promote SMA formation and stabilization, therewith shifting SOC enrichment from small to larger SMA. [ABSTRACT FROM AUTHOR]

The leaching of P from the upper 20 cm of forest topsoils influences nutrient (re-)cycling and the redistribution of available phosphate and organic P forms. However, the effective leaching of colloids and associated P forms from forest topsoils was so far sparsely investigated. We demonstrated through irrigation experiments with undisturbed mesocosm soil columns, that significant proportions of P leached from acidic forest topsoils were associated with natural colloids. These colloids had a maximum size of 400 nm. By means of Field-flow fractionation the leached soil colloids could be separated into three size fractions. The size and composition was comparable to colloids present in acidic forest streams known from literature. The composition of leached colloids of the three size classes was dominated by organic carbon. Furthermore, these colloids contained large concentrations of P which amounted between 12 and 91% of the totally leached P depending on the type of the forest soil. The fraction of other elements leached with colloids ranged between 1% and 25% (Fe: 1–25%; C org : 3–17%; Al: <4%; Si, Ca, Mn: all <2%). The proportion of colloid–associated P decreased with increasing total P leaching. Leaching of total and colloid-associated P from the forest surface soil did not increase with increasing bulk soil P concentrations and were also not related to tree species. The present study highlighted that colloid-facilitated P leaching 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. [ABSTRACT FROM AUTHOR]

Background and aims: Nanoparticles 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.Methods: We 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.Results: Three 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.Conclusion: The 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. [ABSTRACT FROM AUTHOR]

• Fan ages in the Atacama dated to ∼13.6 ka and ∼56.4 ka. • The younger fan with rough surfaces was dominated by nano and fine colloids. • The older fan showed smooth surface but heterogenous size distribution of colloids. • Soil depth distributions pointed to colloidal transport in the younger fan. • Plants enhanced accumulation of nanocolloids with elevated OC and Ca content. As located in one of the oldest and driest deserts on Earth, soils in the Atacama Desert are greatly affected by atmospheric dust deposited on soil surface and the related fate of water-dispersible colloids (WDCs, <300 nm). We hypothesize that formation and content of these WDCs change with topography and age of natural soils. To highlight the processes involved, we investigated a mid-sized and gently (∼5°) sloping alluvial fan system of multi-phase evolution at 1480 m a.s.l. in the Paposo region of the hyper-arid Atacama Desert, which is considered typical for this part of the Coastal Cordillera. Sampling was done along a topographic transect in 11 pits, and assessed the distribution and composition of WDCs by means of asymmetric flow field-flow fractionation (AF4). The younger fan section (optically stimulated luminescence (OSL)-age of ∼13.6 ka) exhibited a pronounced surface roughness and steep slopes. Here, WDCs from the top soils (0–1 cm) free of plants contained nearly 54 ± 7% of medium-sized colloids (MCs, 210–300 nm) with a dominance of Si and Al. The elevated concentrations of fine colloids (FC, 24–210 nm) and particularly nanocolloids (NCs, 0.6–24 nm) was shown in levelled surface soils near shrubs with predominance of organic carbon (OC) and Ca. With higher collodial OC and Ca content in soils near shrubs, more WDC-P was formed concomitantly through increased OC-Ca-P associations. Larger variations in total WDC content were detected in the surface soils of the older fan section, which was dated to ∼56.4 ka. Here, the peaking NC had almost disappeared and thus MC dominated, probably reflecting re-aggregation and wind erosion over longer periods of time across a relatively smooth land surface. The WDCs and WDC-P peaked at 5–10 cm depth in the older fan section, as here a solid mineral/salt layer was present, while in the younger fan section the WDCs were more likely to be translocated from 'permeable' surface into deeper layers, likely reflecting leaching with occasional heavy rainfall. Overall, forms and distribution of WDCs depended on both topographic position and sediment age, thus making colloids as unique tracers of soil development processes during myriad or more years. [ABSTRACT FROM AUTHOR]

The release and stability of soil water-dispersible colloids (WDC) in the soil structure are critical for colloid-facilitated soil organic carbon sequestration and contaminants transport. In this study, the potential effects of temperature and associated organic carbon (OC) on the release of WDCs in three silt loam topsoils with the same clay content (~ 20%) under different land uses were investigated. A soil fractionation method was used for simulating the release of colloids from the soil under environmental conditions where mobilization and sedimentation processes occur sequentially. The surface loading of OC has been characterized by the analysis of organic carbon content of WDC with the measurements of the specific surface area (SSA). The effects of fractionation temperature on colloidal properties (e.g., particle size and zeta potential) were systematically investigated and the aggregation kinetics of WDC in salt electrolyte influenced by temperature was assessed by dynamic light scattering (DLS). Experimental results demonstrated that the amount of extracted WDC from three soils decreased when the fractionation temperature increased. A more rapid sedimentation of WDC at higher temperatures outweighed the effect of temperature on WDC mobilization from bulk soil in the shaking step. The sedimentation of WDC at various temperatures indicated that the temperature dependence of the water viscosity ( η ) was a dominate parameter and caused lower efficiency of WDC mass gained at higher temperature according to the Stoke's law. After introducing the factor of η 7 °C / η T , the temperature effect only on WDC mobilization during shaking step could be described and the whole fractionation process could be successfully timely determined along the two shaking and sedimentation steps. Activation energies ( E a ) of about 10 kJ mol − 1 could be now calculated for the WDC mobilization processes from the three topsoils. The associated organic carbon contents of WDC (WDC(OC)) and the mineral surface of WDC blocked by organic carbon ( SSA OC - block ) after various shaking temperatures and shaking time were further determined in order to examine the WDC(OC) effect on the release of WDC from soil matrix. The results demonstrated that the escape of the mobile clay fraction ( F ) from soil at short shaking times is favored by the presence of effective surface loading by an OC layer ( SSA OC - block ), which is known to stabilize its colloidal state through electrosteric effects. The WDC(OC) surface concentration has been also used to estimate the clay-associated OC distribution in the three topsoils. In Ca 2 + solution, an increase of temperature favors the colloidal stability of WDC as measured from the shift of critical coagulation concentration ( CCC ) to higher concentrations of Ca 2 + . In total, the results from this study revealed that temperature and WDC(OC) distribution are critical parameters when considering soil WDC release and stability in natural bulk soils. [ABSTRACT FROM AUTHOR]

Transport and retention behavior of multi-walled carbon nanotubes (MWCNTs) was studied in mixtures of negatively charged quartz sand (QS) and positively charged goethite-coated sand (GQS) to assess the role of chemical heterogeneity. The linear equilibrium sorption model provided a good description of batch results, and the distribution coefficients (KD) drastically increased with the GQS fraction that was electrostatically favorable for retention. Similarly, retention of MWCNTs increased with the GQS fraction in packed column experiments. However, calculated values of KD on GQS were around 2 orders of magnitude smaller in batch than packed column experiments due to differences in lever arms associated with hydrodynamic and adhesive torques at microscopic roughness locations. Furthermore, the fraction of the sand surface area that was favorable for retention (Sf) was much smaller than the GQS fraction because nanoscale roughness produced shallow interactions that were susceptible to removal. These observations indicate that only a minor fraction of the GQS was favorable for MWCNT retention. These same observations held for several different sand sizes. Column breakthrough curves were always well described using an advective-dispersive transport model that included retention and blocking. However, depth-dependent retention also needed to be included to accurately describe the retention profile when the GQS fraction was small. Results from this research indicate that roughness primarily controlled the retention of MWCNTs, although goethite surfaces played an important secondary role. [ABSTRACT FROM AUTHOR]

Woody encroachment strengthens the retention of OC and P by soil colloids, consequently increasing overall C and P pools in savanna soils. [Display omitted] • P was associated with Ca in < 30 nm colloids, and with OC and Ca in 30–500 nm colloids. • Woody invasion increased colloidal OC and P and their proportions in < 30 nm colloids. • Higher proportions of orthophosphate and -diesters in colloids than bulk soil. • Heavy grazing and fire potentially accelerated the risk of colloidal P loss. • Vegetation type had larger effect on soil colloids than grazing or fire. Soil colloids (diameter < 1000 nm) are comprised mainly of clay minerals and organic matter, and play major roles in determining ion exchange capacity and in regulating key biogeochemical processes. Consequently, it is important to understand how soil colloids and their functions are influenced by land cover and anthropogenic disturbances. In grasslands, savannas, and other dryland ecosystems across the globe, woody plants are encroaching due to livestock grazing, fire suppression, elevated CO 2 concentrations, and climate change. These major land cover changes could influence soil colloidal properties, with implications for soil C, N, and P cycles. We assessed how woody encroachment, livestock grazing, and fire interact to influence soil colloidal properties in a juniper-oak savanna. Surface soils (0–10 cm) from the southern Great Plains (Texas, USA) were collected from long-term treatments differing in grazing intensity (none, moderate, and heavy) and fire history. Within each treatment, soil samples were taken under grass, juniper, and oak canopies. Water dispersible soil colloids (WDC, d < 500 nm) were isolated and analyzed by asymmetric flow field-flow fractionation and their P species by liquid-state 31P-nuclear magnetic resonance spectroscopy (31P NMR). Soil beneath oak and juniper canopies had smaller WDC and elevated colloidal organic carbon (OC) and P concentrations, especially in nanocolloid (<30 nm) and fine colloid (30–160 nm) size fractions. Woody encroachment enriched Ca, Fe, Al, Si and Mg in WDC in the ungrazed control, but not in any of the other grazed or burned areas. Colloidal soil P mainly occurred as orthophosphate and orthophosphate diesters, and was present as OC-Ca-P complexes in fine and medium colloid fractions (30–500 nm), while P in the nanocolloid fraction (<30 nm) was in direct association with Ca. Moderate grazing did not affect the retention of colloidal P, while heavy grazing potentially increased the loss risk of colloidal P. Fire accelerated soil P loss from colloid fractions only in woody areas. Our findings highlight that woody encroachment strengthens the retention of OC and P by soil colloids, consequently increasing overall C and P pools in savanna soils. [ABSTRACT FROM AUTHOR]

• P increased in all colloidal fractions with long-term cultivation management. • The nanoparticle fraction turned into a complex of OC-Ca-P after cropping management. • Cropping management generates more fine colloidal clays, oxides and associated P. Soil phosphorus (P) is usually hardly present in soluble forms as most of it is strongly bound to minerals. Therefore, transformations of soil colloidal P can play a key role in enhancing soil fertility. Here, we examined water extracts from a chronosequence of soils with 2000 years of paddy rice and 700 years of non-paddy cropping, and fractionated them into <1200 nm, <450 nm, and <3 kDa size fractions using sequential ultrafiltration. Asymmetric-flow field-flow fractionation (AF4) coupled to an organic carbon detector (OCD) and inductively coupled plasma mass spectrometry (ICP-MS) was used for organic carbon and elemental screening in the 3 kDa-450 nm colloidal size range. We found that P increased in all colloidal fractions both with long-term paddy rice and non-paddy managements. Furthermore, P in soil colloids of 3 kDa-450 nm size, as fractionated by AF4, peaked in three size ranges: nanoparticles (NP, ∼3 kDa-20 nm), fine colloids (FC, ∼20–225 nm), and medium-sized colloids (MC, ∼225–450 nm). The NP fraction originally contained mainly P and Ca for tidal wetland (TW), but turned into a complex of OC-Ca-P colloidal forms immediately after onset of arable management, with little temporal change thereafter. Fine colloidal P (FC-P) was detected only after > 50 years for the non-paddy and > 100 years under paddy management, associated with oxides, organic matter and fine clay fragments in the paddy soils and additionally Ca-bound P in the non-paddy ones, respectively. Increasing portions of medium-sized colloidal P appeared after 50 years paddy and non-paddy management, though not exceeding the concentrations of FC-P and with little changes during prolonged arable land use. Overall, after initial formation of FC-P during the first 50–100 years of management, longer term dynamics of colloidal P in these arable soils was fairly independent of paddy or non-paddy managements, but remained closely coupled to the increased presence or generation of fine colloidal clays, oxides and the associated P, thus potentially linking P fertility largely to the dynamics of fine colloids. [ABSTRACT FROM AUTHOR]

• Chronological sequence of aggregate formation processes. • Organic gluing agents initiate and accelerate aggregation process. • Minerals stabilize aggregates in the longer term. • Root-mediated processes further stabilize macroaggregates against ultrasonic dispersion. Microaggregates (<250 µm) are key structural subunits of soils. However, their formation processes, rates, and transformation with time are poorly understood. We took advantage of multiple isotope labelling of potential organic gluing agents and inorganic building units to unravel their role in soil aggregation processes being initiated with and without plant growth. We added 13C-labelled extracellular polymeric substances (EPS), 15N-labelled bacteria, 57Fe-labelled goethite, and 29Si-labelled montmorillonite to fine soil <250 µm of an Ap horizon from a Stagnic Luvisol, which was planted with Festuca heteromalla or kept bare in a climate chamber. Samples were taken after 4, 12, and 30 weeks, and separated into free (f) and occluded (o) microaggregates of different sizes (<20 µm, 53–20 µm, 250–53 µm), and in stable macroaggregates (>250 µm) that resisted 60 J mL−1 ultrasonic dispersion. Afterwards, we assessed the C, N, Fe, and Si stable isotope composition in each size fraction. After four weeks we found a rapid build-up of stable macroaggregates comprising almost 50 % of soil mass in the treatment with plants and respective soil rooting, but only 5 % when plants were absent. The formation of these stable macroaggregates proceeded with time. Soil organic carbon (SOC) contents were elevated by 15 % in the large macroaggregates induced by plant growth. However, the recovery of EPS-derived 13C was below 20 % after 4 weeks, indicating rapid turnover in treatments both with and without plants. The remaining EPS-derived C was mainly found in macroaggregates when plants were present and in the occluded small microaggregates (<20 µm) when plants were absent. The excess of bacterial 15N closely followed the pattern of EPS-derived 13C (R2 = 0.72). In contrast to the organic gluing agents, the goethite-57Fe and montmorillonite-29Si were relatively equally distributed across all size fractions. Overall, microaggregates were formed within weeks. Roots enforced this process by stabilizing microaggregates within stable macroaggregates. As time proceeded the labelled organic components decomposed, while the labelled secondary oxides and clay minerals increasingly contributed to aggregate stabilization and turnover at the scale of months and beyond. Consequently, the well-known hierarchical organization of aggregation follows a clear chronological sequence of stabilization and turnover processes. [ABSTRACT FROM AUTHOR]

• Nanocolloids dominated soil colloids in Acrisols. • Fine and medium-sized colloids dominated soil colloids in Terra Preta soils (TP). • Elevated pH and Ca2+ conc. correlated with SOM involvement in colloid formation in TP. • Enriched water-extractable and colloidal P in TP. Amazonian Dark Earths (or terra preta de índico) are known as highly fertile soils that can maintain elevated crop yields for centuries. While this fertility was frequently ascribed to the presence of black carbon, the availability and colloidal binding of major nutrients received limited attention. We examined the size distribution and the elemental compositions of water-dispersible colloids (WDC) in both forested and cultivated Terra Preta topsoils (0–10 cm, Anthrosols), as well as in their adjacent non-Terra Preta controls (Acrisols) via asymmetric flow field-flow fractionation (FFF). Liquid-state 31P-nuclear magnetic resonance (NMR) spectra, black carbon content, and scanning electron microscope (SEM) images were also obtained. We found that WDC in Terra Preta soils contained a significant proportion of organo-mineral associations in the size range 30–300 nm, whereas, in contrast, water-dispersible nanoparticles with a diameter < 30 nm were dominant in the adjacent Acrisols. The shifts to larger WDC sizes in the Terra Preta soils went along with elevated pH values, as well as with elevated contents of Si, Al, Fe, Ca and organic matter-containing particles. Also P concentrations were enriched in both the water-extractable phase (WEP) and WDC extracts of Terra Preta soils relative to the adjacent Acrisols. We assume that the higher pH values and Ca ion concentrations promoted the involvement of soil organic matter (SOM) into the formation of larger-sized colloids consisting of kaolinite-like clay minerals, iron oxides and Ca ions in the Terra Preta soils. The elevated content of Ca in Terra Preta soil colloids may also contribute to the retention of P, likely via bridging of anionic P like orthophosphate to SOM. Preventing soil acidification is thus not only to be recommended for Acrisols, but also for maintaining colloidal structures and related fertility in Terra Preta soils. [ABSTRACT FROM AUTHOR]

The hyperarid soils of the Atacama Desert, Chile, contain the largest known nitrate deposits in the world. They also represent one of the most hostile environments for microbial life anywhere in the terrestrial biosphere. Despite known for its extreme dryness, several heavy rainfall events causing localised flash flooding have struck Atacama Desert core regions during the last five years. It remains unclear, however, whether these soils can support microbial denitrification. To answer this, we sampled soils along a hyperaridity gradient in the Atacama Desert and conducted incubation experiments using a robotized continuous flow system under a He/O 2 atmosphere. The impacts of four successive extreme weather events on soil-borne N 2 O and N 2 emissions were investigated, i) water addition, ii) NO 3 − addition, iii) labile carbon (C) addition, and iv) oxygen depletion. The 15N–N 2 O site-preference (SP) approach was further used to examine the source of N 2 O produced. Extremely low N 2 O fluxes were detected shortly after water and NO 3 − addition, whereas pronounced N 2 O and N 2 emissions were recorded after labile-C (glucose) amendment in all soils. Under anoxia, N 2 emissions increased drastically while N 2 O emissions decreased concomitantly, indicating the potential for complete denitrification at all sites. Although increasing aridity significantly reduced soil bacterial richness, microbial potential for denitrification and associated gene abundance (i.e., napA , narG , nirS , nirK , cnorB , qnorB and nosZ) was not affected. The N 2 O15N site preference values based on two end-member model suggested that fungal and bacterial denitrification co-contributed to N 2 O production in less arid sites, whereas bacterial denitrification dominated with increasing aridity. We conclude that soil denitrification functionality is preserved even with lowered microbial richness in the extreme hyperarid Atacama Desert. Future changes in land-use or extreme climate events therefore have a potential to destabilize the immense reserves of nitrate and induce significant N 2 O losses in the region. • Denitrification was shown to occur in the hyperarid Atacama Desert. • Denitrification potential and associated gene abundance was not affected by aridity. • Increasing aridity reduced soil bacterial richness. • Fungal and bacterial denitrification co-contributed to N 2 O production. • Bacterial denitrification dominated N 2 O production with increasing hyperaridity. [ABSTRACT FROM AUTHOR]

• Dynamics and depth distribution of soil colloids in rice paddies are largely unknown. • Redox fluctuations lead to preferential release/formation of finer colloidal fractions. • Paddy topsoils are relatively depleted while subsoils are enriched in colloids. • Colloid illuvation and in situ colloid formation are plausible mechanisms. • Colloid mobility contributes to the high organic C and Fe stocks in paddy subsoils. Redox fluctuations in hydromorphic soils can influence ecosystem functions by altering the cycling of organic carbon (OC) and other elements in both the aqueous and solid phases throughout the soil profile. Most studies focusing on the mobility of dissolved OC in rice paddy soils have often disregarded the contribution of colloidal particles. We provide a detailed chemical and size-related characterization of water-dispersible soil colloids and their depth distribution in two soil profiles under long-term temperate paddy and non-paddy management, by asymmetric flow field-flow fractionation. Anoxic conditions enhanced colloid dispersion with a preferential release of the finer colloid fractions (5 and 4-fold increase in the < 30 nm and 30–240 nm fractions, respectively). The mobility of OC-rich, iron (Fe) and aluminium (hydr)oxide and aluminosilicate colloids along the soil profile was probably responsible for their depletion in the topsoil and a corresponding accumulation in the deeper illuvial horizons compared to the non-paddy soil. However, the release, percolation and subsequent reoxidation of Fe2+ was also shown to be a plausible mechanism leading to the formation of fine colloids in the subsoil. Redox-driven changes in colloid distribution were also linked to the differences in OC and pedogenetic Fe stocks in these two agro-ecosystems. [ABSTRACT FROM AUTHOR]

• Elevated pH promoted colloid formation in calcareous soils. • Ca acted as the bridging agent for colloidal formation. • Colloids in mineral soil layers originated from surface organic layers. • Retention of dissolved P accounted for P accumulation in deeper soil layers. • Colloids were more responsible than soil electrolytes for P loss to deeper soil horizons. 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. [ABSTRACT FROM AUTHOR]