Implications of Free and Occluded Fine Colloids for Organic Matter Preservation in Arable Soils.

Colloidal organo-mineral associations contribute to soil organic matter (OM) preservation and mainly occur in two forms: (i) as water-dispersible colloids that are potentially mobile (free colloids) and (ii) as building units of soil microaggregates that are occluded inside them (occluded colloids). However, the way in which these two colloidal forms differ in terms of textural characteristics and chemical composition, together with the nature of their associated OM, remains unknown. To fill these knowledge gaps, free and occluded fine colloids <220 nm were isolated from arable soils with comparable organic carbon (Corg) but different clay contents. Free colloids were dispersed in water suspensions during wet-sieving, while occluded colloids were released from water-stable aggregates by sonication. The asymmetric flow field-flow fractionation analysis on the free and occluded colloids suggested that most of the 0.6-220 nm fine colloidal Corg was present in size fractions that showed high abundances of Si, Al, and Fe. The pyrolysis-field ionization mass spectrometry revealed that the free colloids were relatively rich in less decomposed plant-derived OM (i.e., lipids, suberin, and free fatty acids), whereas the occluded colloids generally contained more decomposed and microbial-derived OM (i.e., carbohydrates and amides). In addition, a higher thermal stability of OM in occluded colloids pointed to a higher resistance to further degradation and mineralization of OM in occluded colloids than that in free colloids. This study provides new insights into the characteristics of subsized fractions of fine colloidal organo-mineral associations in soils and explores the impacts of free versus occluded colloidal forms on the composition and stability of colloid-associated OM.

Thermal induced changes of rice straw phytolith in relation to arsenic release: A perspective of rice straw arsenic under open burning.

On-site open burning is a common practice for handling rice straw, but its negative impacts, e.g., biomass loss and air pollution, are largely debated worldwide. To address the negative effects of open burning, many efforts have been made to 'ignite' worldwide bans. However, these bans are likely based on a singular view in which some positive aspects of open burning are overlooked. In this study, we aimed to determine the thermal-induced changes of straw and straw arsenic (As) under open burning and heat-treatments (in the temperature range from 300 to 900 °C). It was found that silica phase in rice straw (so-called phytolith) can encapsulate As in its structure. Open burning or heat-treatment of straw resulted in a tighter association of As and phytolith, thereby reducing dissolution of As. We proposed an opinion that open burning causes air pollution, but it can increase the activity of phytolith in sequestrating As, enabling delayed As cycle in rice ecosystems. The combat of on-site open burning of rice straw to reduce air pollution will alter straw handling routines, thereby changing the cycle of straw phytolith and the route of straw As.

Shifting Boundaries: Ecological and Geographical Range extension Based on Three New Species in the Cyanobacterial Genera Cyanocohniella, Oculatella, and, Aliterella.

The polyphasic approach has been widely applied in cyanobacterial taxonomy, which frequently led to additions to the species inventory. Increasing our knowledge about species and the habitats they were isolated from enables new insights into the ecology of newly established genera and species allowing speculations about the ecological niche of taxa. Here, we are describing three new species belonging to three genera that broadens the ecological amplitude and the geographical range of each of the three genera. Cyanocohniella crotaloides sp. nov. is described from sandy beach mats of the temperate island Schiermonnikoog, Netherlands, Oculatella crustae-formantes sp. nov. was isolated from biological soil crusts of the Arctic Spitsbergen, Norway, and Aliterella chasmolithica originated from granitic stones of the arid Atacama Desert, Chile. All three species could be separated from related species using molecular sequencing of the 16S rRNA gene and 16S-23S ITS gene region, the resulting secondary structures as well as p-distance analyses of the 16S-23S ITS and various microscopic techniques. The novel taxa described in this study contribute to a better understanding of the diversity of the genera Cyanocohniella, Oculatella, and Aliterella in different habitats.

Size, shape, and stability of organic particles formed during freeze-thaw cycles: Model experiments with tannic acid.

HYPOTHESIS: Freeze-thaw cycles (FTC) in soils can cause the aggregation of dissolved organic matter but controlling factors are little understood. EXPERIMENTS: In freeze-thaw experiments with tannic acid (TA) as model substance, we studied the effect of TA concentration, pH, electrolytes (NaCl, CaCl2, AlCl3), and number of FTC on particle formation. Tannic acid (0.005 to 10 g L-1) was exposed to 1-20 FTC at pH 3 and 6. The size and shape of particles was determined by confocal laser scanning microscopy. Particle stability was deduced from the equivalent circle diameter (ECD) obtained in dry state and the hydrodynamic diameter measured in thawing solutions. FINDINGS: Tannic acid particles occurred as plates and veins, resembling the morphology of ice grain boundaries. Low pH and presence of electrolytes favored the formation of large particles. The freeze-concentration effect was most intense at low TA concentrations and increased with the number of FTC. While ECD of particles formed at low TA concentrations were smaller than at high concentrations, it was vice versa in the thawed state. At low TA concentrations, higher crystallization pressure of ice caused enhanced stability of large particles. We conclude that FTC can strongly alter the physical state of dissolved organic matter, with likely consequences for its bioavailability.

Effects of solution chemistry on conformation of self-aggregated tannic acid revealed by laser light scattering.

Inorganic soil solution constituents can alter the charge, size, and conformation of dissolved organic molecules, thus affecting their environmental behavior. Here, we investigated how pH, cation valence and activities induce conformational changes and aggregation-sedimentation reactions of organic polyelectrolytes. For that we determined the hydrodynamic diameter of the model compound tannic acid by laser light scattering at concentrations of 1-30 g L-1 in the pH range from 3 to 10 and with electrolyte additions of CaCl2 and hydroxyl-Al cations. Charge properties were quantified by polyelectrolyte titration and zeta potential measurements. After dispersion by sonication, aggregation was determined in time sequences up to 60 min and suspension stability was traced in sedimentation experiments. Tannic acid was present in ultrapure water in a self-aggregated state. At pH <3 as well as >7.5, its hydrodynamic diameter increased. Whereas at high pH this behavior could be assigned to unfolding of molecular conformations, at low pH it is likely that charge neutralization decreased repulsive forces and facilitated aggregation. At pH 5 and ionic strengths of up to 5 mM, CaCl2 did not affect aggregation state of tannic acid and results resembled those obtained in ultrapure water. Addition of hydroxyl-Al cations broke-up the self-aggregated tannic acid structures under formation of Al-organic coprecipitates. Strong aggregation only occurred at mixing ratios where opposite surface charges were completely balanced. Under natural conditions, self-aggregation of tannic acid can be expected only at higher solution concentrations. However, at acidic pH, hydroxyl-Al cations and tannic acid may form discrete colloidal particles already at low tannic acid concentrations, resulting in the destabilization of suspensions. Our data emphasize that the soil solution composition strongly modifies the physical state of tannic acid, and likely also of other biopolymers, and thus their interactions within environmental matrices.

Effects of CO2 and temperature on phytolith dissolution.

Phytoliths, silica structures derived from plant residues in silicon (Si)-accumulating plant species, have recently been recognized as a sink and source of nutrients and a hosting phase for carbon sequestration in soil. While the solubility of phytoliths in relation to their respective nature and solution chemistry has been intensively studied, the combined effects of CO2 and temperature, two highly variable parameters in soil, have not been fully understood. We hypothesized that changes in CO2 and temperature may affect the dissolution rate, thereby resizing the soil phytolith pool. Rice straw phytoliths were obtained from either open burning or controlled heating of straw from 300 to 900 °C and used to determine their batch incubation kinetics in a closed chamber at CO2 concentrations of 0 to 15% vol. and a temperature range of 20 to 50 °C for six days. The results revealed a contrasting effect in which temperature and CO2 were correspondingly found to accelerate or decelerate the dissolution rate of phytoliths. Under the most dissimilar conditions, i.e., 0% vol. CO2 and 50 °C and 15% vol. CO2 and 20 °C, the discrepancy in solubility was approximately six-fold, indicating a high vulnerability of phytoliths to CO2 and temperature changes. This finding also suggests that the soil phytolith pool can be diminished in the case of either increasing soil temperature or decreasing CO2 flux. Calculations based on these data revealed that the dissolution rate of phytoliths could be increased by an average of 4.5 to 7.3% for each 1 °C increase in temperature. This finding suggests a possible impact of current global warming on the global biogenic silica pool, and more insight into the relationship between this pool and climate change is, therefore, necessary to maintain the function of the phytolith phase in soil.

Lichens Bite the Dust - A Bioweathering Scenario in the Atacama Desert.

Bioweathering mediated by microorganisms plays a significant role in biogeochemical cycles on global scales over geological timescales. Single processes induced by specific taxa have been described but could rarely be demonstrated for complex communities that dominate whole landscapes. The recently discovered grit crust of the coastal Atacama Desert, which is a transitional community between a cryptogamic ground cover and a rock-bound lithic assemblage, offers the unique chance to elucidate various bioweathering processes that occur simultaneously. Here, we present a bioweathering scenario of this biocenosis including processes such as penetration of the lithomatrix, microbial responses to wet-dry cycles, alkalinolysis, enzyme activity, and mineral re-localization. Frequently occurring fog, for example, led to a volume increase of microorganisms and the lithomatrix. This, together with pH shifts and dust accumulation, consequently results in biophysical breakdown and the formation of a terrestrial protopedon, an initial stage of pedogenesis fueled by the grit crust.

Colloidal interactions of micro-sized biochar and a kaolinitic soil clay.

Fine-sized biochars and clay minerals co-present in various circumstances, e.g., agricultural land and water treatment. Because both of these materials are scavengers for nutrients, agrochemicals and other toxicants, their dispersibility and transportability have received much attention. However, little is documented about their colloidal interactions and to what extent biochar particles can stimulate the dispersion of clay minerals. Here, the effect of engineered micro-sized biochar amendment on the surface charge (SC) and colloidal dynamics of the clay fraction of a kaolinite-rich soil was determined. The engineered biochars showed distinctive SC and colloidal properties depending on their pyrolysis conditions (e.g., oxygen level and temperature) and solution chemistry (i.e., pH and cation type). Two types of biochars prepared under non-biochar-oriented pyrolysis (open heating, 'O-biochar') and biochar-oriented pyrolysis (N2-supported heating, 'N2-biochar') showed contrasting effects on the colloidal dynamics of clay. The O-biochars provoked aggregation due to their higher content of soluble salts, which increased ionic strength and provided multivalent cations, inducing bridging between negatively charged colloids. In contrast, the N2 biochars low in soluble salts and rich in negatively charged burned organic matter compounds favoured the dispersion of clay. The adjustment of biochar production methods can therefore be highlighted as the way to customize biochar for specific uses or to reduce the risk of clay loss from soils in the short term. In the long term, when soluble salts are removed by leaching, it is likely that dispersion is facilitated and the risk for erosion increases.

Experimental Microbial Alteration and Fe Mobilization From Basaltic Rocks of the ICDP HSDP2 Drill Core, Hilo, Hawaii.

The interaction of a single bacterial species (Burkholderia fungorum) with basaltic rocks from the ICDP HSDP2 drill core and synthetic basaltic glasses was investigated in batch laboratory experiments to better understand the role of microbial activity on rock alteration and Fe mobilization. Incubation experiments were performed with drill core basaltic rock samples to investigate differences in the solution chemistry during biotic and abiotic alteration. Additionally, colonization experiments with synthetic basaltic glasses of different Fe redox states and residual stresses were performed to evaluate their influence on microbial activity and surface attachment of cells. In biotic incubation experiments bacterial growth was observed and the release of Fe and other major elements from drill core basaltic rocks to solution exceeded that of abiotic controls only when the rock sample assay was nutrient depleted. The concentration of dissolved major elements in solution in biotic colonization experiments with synthetic basaltic glasses increased with increasing residual stress and Fe(II) content. Furthermore, the concentration of dissolved Fe and Al increased similarly in biotic colonization experiments indicating that their dissolution might be triggered by microbial activity. Surface morphology imaging by SEM revealed that cells on basaltic rocks in incubation experiments were most abundant on the glass and surfaces with high roughness and almost absent on minerals. In colonization experiments, basaltic glasses with residual stress and high Fe(II) content were intensely covered with a cellular biofilm. In contrast, glasses with high Fe(III) content and no residual stress were sparsely colonized. We therefore conclude that structurally bound Fe is most probably used by B. fungorum as a nutrient. Furthermore, we assume that microbial activity overall increased rock dissolution as soon as the environment becomes nutrient depleted. Our results show that besides compositional effects, other factors such as redox state and residual stress can control microbial alteration of basaltic glasses.

Release of potassium accompanying the dissolution of rice straw phytolith.

In rice, Si assimilated from the soil solution is deposited in inter- and intracellular spaces throughout the leaf and stems to form silicified structures (so-called phytoliths). Because K is also present in significant concentrations in rice stems and leaves, the question arises if K is immobilized in the mineralized silica during the precipitation of Si. This work determined whether desilification of the phytolith is a factor regulating K release by implementing batch experiments. Solubility of Si and K of the rice straw heated at different temperatures were examined to identify effect of pretreatment. Analyses of phytoliths using SEM-EDX and X-ray tomographic microscopy in conjunction with the results from batch experiments revealed that K might co-exist with occluded organic matter inside the phytolith structure. In the kinetic experiments, corresponding increases of K and Si concentrations in the supernatants were observed which suggested that desilification of the phytolith is a main factor regulating K release. The extent of heat pretreatment of the rice straw is of significant importance with respect to dissolution of the phytolith by affecting organic removal and surface modification. At temperatures lower than 600 °C, corresponding increases of the soluble Si and K with heating temperature have been obviously observed. In contrast, the solubility of Si and K gradually decreased at temperatures above 600 °C. This work provides insights into factors that control release of K and Si from phytolith and a practical recommendation for practices of burning rice straw that may maximize subsequent release of Si and K for crops.

Clarification of olive mill and winery wastewater by means of clay-polymer nanocomposites.

Highly polluted effluents from olive mills and wineries, among others, are unsuitable for discharge into standard sewage-treatment plants due to the large amounts of organic and suspended matter. Efficiency of all management practices for such effluents depends on an effective pretreatment that lowers the amount of suspended solids. Such pretreatments are usually based on three separate stages, taking a total of 2 to 6h: coagulation-neutralizing the colloids, flocculation-aggregating the colloids into larger particles, and separation via filtration or decanting. Previous studies have presented the concept of coagoflocculation based on the use of clay-polymer nanocomposites. This process adds a higher density clay particle to the flocs, accelerating the process to between 15 and 60 min. This study examined suitable nanocomposites based on different clays and polymers. The charge of the compounds increased proportionally to the polymer-to-clay ratio. X-ray diffraction (XRD) measurements indicated that in sepiolite-based nanocomposites there is no change in the structure of the mineral, whereas in smectite-based nanocomposites, the polymer intercalates between the clay layers and increases the spacing depending on the polymer-to-clay ratio. Efficiency of the coagoflocculation process was studied with a dispersion analyzer. Sequential addition of olive mill or winery effluents with a boosting dose of nanocomposites may yield a very efficient and rapid clarification pretreatment.

Low pore connectivity in natural rock.

As repositories for CO(2) and radioactive waste, as oil and gas reservoirs, and as contaminated sites needing remediation, rock formations play a central role in energy and environmental management. The connectivity of the rock's porespace strongly affects fluid flow and solute transport. This work examines pore connectivity and its implications for fluid flow and chemical transport. Three experimental approaches (imbibition, tracer concentration profiles, and imaging) were used in combination with network modeling. In the imbibition results, three types of imbibition slope [log (cumulative imbibition) vs. log (imbibition time)] were found: the classical 0.5, plus 0.26, and 0.26 transitioning to 0.5. The imbibition slope of 0.26 seen in Indiana sandstone, metagraywacke, and Barnett shale indicates low pore connectivity, in contrast to the slope of 0.5 seen in the well-connected Berea sandstone. In the tracer profile work, rocks exhibited different distances to the plateau porosity, consistent with the pore connectivity from the imbibition tests. Injection of a molten metal into connected pore spaces, followed by 2-D imaging of the solidified alloy in polished thin sections, allowed direct assessment of pore structure and lateral connection in the rock samples. Pore-scale network modeling gave results consistent with measurements, confirming pore connectivity as the underlying cause of both anomalous behaviors: imbibition slope not having the classical value of 0.5, and accessible porosity being a function of distance from the edge. A poorly connected porespace will exhibit anomalous behavior in fluid flow and chemical transport, such as a lower imbibition slope (in air-water system) and diffusion rate than expected from classical behavior.