Soil Microaggregates Research Articles

Chemical and Spectroscopic Investigation of Different Soil Fractions as Affected by Soil Management

The interaction of organic matter with the finest soil fractions (<20 μm) represents a good way for its stabilization. This study investigates the effects of conventional (CT), minimum (MT), and no (NT) tillage, fertilization, and non-fertilization, and soil depth (0–30, 30–60, and 60–90 cm) on the amount of organic carbon (OC) in four soil fractions. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) was performed to obtain information about the OC quality and the mineralogical composition of these fractions. The CT shows the highest amount of the finest fraction while the fertilization enhances the microbial community with the increase of soil micro-aggregates (250–53 μm). The coarse fraction (>250 μm) is highest in the upper soil layer, while the finest fraction is in the deepest one. The greatest OC content is observed in the topsoil layer and in the finest soil fraction. DRIFT analysis suggests that organic components are more present in the finest fraction, calcite is mainly localized in the coarse fraction, quartz is in micro-aggregates and 53–20 μm fraction, and clay minerals are in the finest fraction.

Composition and fractal features of soil micro-aggregates in microsites of different types of treefall gaps

Through field survey and laboratory analysis, we examined the composition and fractal features of soil micro-aggregates in different types of treefall gaps and microsites (pit bottom and mound top) in broad-leaved Korean pine forest and spruce-fir-Korean pine forest. Results showed that the contents of soil microaggregates under the classes of 0.25-2 mm and 0.05-0.25 mm were higher in both forest types, ranging from 25.7% to 50.7% and from 27.0% to 42.8%, respectively, and that of <0.002 mm was the lowest, ranging from 4.4% to 8.9%. In the pit bottom and mound top of gaps, soil bulk density was higher in both forest types. Soil nutrient content in mound top was higher than that in pit bottom and was higher in broad-leaved Korean pine forest than spruce-fir-Korean pine forest. Soil microaggregates of <0.002 mm had no correlation with soil physical and chemical properties, whereas that of 0.25-2 mm and 0.002-0.02 mm had significantly positive and negative correlation with soil non-capillary porosity, total porosity, aeration porosity, organic matter, total phosphorus, total nitrogen and organic carbon, respectively. On the whole, soil fractal dimension (D) and the proportion of characteristic soil micro-aggregates (PCM) in broad-leaved Korean pine forest were larger than those in spruce-fir-Korean pine forest, and the ratio of soil microaggregates diameter (RMD) in mound top and pit bottom was increased in two forest types. Soil D and PCM had no significant correlation with soil physical and chemical properties, while RMD was negatively correlated with capillary porosity, total porosity, soil bulk density and aeration porosity. In two forest types, the formation of mound and pit microsites could decrease the larger size micro-aggregates and the stability of soil micro-aggregate, increase soil D and PCM, and signifi-cantly increase RMD. RMD could be used as a quantitative index of soil physical and chemical properties in pit and mound microsites of forest.

On Modeling the Connection of Soil Structure with the Content of Organic Matter and Exchangeable Ca2+ and Mg2+ Cations

This paper proposes a method for evaluation the relationship between organic matter (humus) and cation content in the soil adsorbing complex and the number of microaggregates determining the soil structure. The method is based on the use of statistical analysis of combined data of water extracts and the amount of microaggregates in soils (serozems and chernozems). The method is expected to enable a more complete assessment of the impact and effects of irrigation on the structure of soil layers.

Soil microaggregates store phytoliths in a sandy loam

Phytoliths are fine silt-sized amorphous silica particles that form in living plant tissues. Once deposited in soils through plant debris, they may dissolve and increase the fluxes of silicon (Si) towards the biosphere and hydrosphere, thus enhancing positive Si impacts on e.g., plant health and carbon fixation by marine diatoms. Here we analyzed the role of soil aggregates in phytolith protection against dissolution. We investigated the distribution of phytoliths in the size fractions of a sandy loam topsoil subjected to two long-term treatments: conventional (CT) and no tillage (NT). The topsoil size fractions were separated through wet sieving and named, respectively: macroaggregates (250–2000 µm), free microaggregates (50–250 µm), and silt + clay (<50 µm). Protected microaggregates held within macroaggregates were extracted using a constant and continuous water flow process. We assessed the pool of phytoliths through heavy liquid separation and alkaline dissolution (Na2CO3), and we assessed the bioavailability of Si using CaCl2 extraction. As expected, NT generates larger amounts of aggregates than CT. Concentrations and stocks of phytoliths determined by heavy liquid separation are ten times higher than those measured by Na2CO3 in bulk soil and size fractions. Soil microaggregates contribute for over 60% to the pool of phytoliths, which control bioavailable Si. The release of bioavailable Si from microaggregates is slower than that from the silt + clay fraction, suggesting that soil microaggregates can trap phytoliths and protect them from rapid dissolution. No-tillage and associated conservation agricultural practices may thus promote the stabilization of phytoliths in soils and decrease their dissolution rate. We propose that the entrapment of phytoliths in soil aggregates may be one of the processes favoring the persistence of phytoliths in soils and sediments. We expect that this persistence should be enhanced in strongly aggregated soils.

Quantification and molecular characterization of organo-mineral associations as influenced by redox oscillations

Organo-mineral association is one of the most important stabilization mechanisms of soil organic matter. However, few studies have been conducted to assess the retention, transformation, and transportation of colloids (1–1000 nm) and associated organic carbon (OC) in soil. Given the particularly significant role that wetland soils play in carbon storage and cycling, we quantified the dynamics of organo-mineral association within colloidal size range by conducting three consecutive 35-day redox (reduction-oxidation) oscillation experiments using a wetland soil. Molecular compositions of natural nanoparticle (NNP, 2.3–100 nm), fine colloid (100–450 nm), and particulate (450–1000 nm) fractions were measured using isotope ratio mass spectrometry (IRMS) and x-ray photoelectron spectroscopy (XPS). Results showed that NNP and fine colloids constituted up to 8.94 ± 0.50% and 22.19 ± 7.52% of bulk C concentration (2.3–1000 nm), respectively; indicating substantial contributions of these two fractions to the operationally defined “dissolved” (<450 nm) fraction. There was significant enrichment in heavier δ13C isotopes (p < 0.001) with size: NNP (−29.64 ± 0.32‰) < fine colloid (−28.81 ± 0.31‰) < particulate (−28.34 ± 0.25‰) fractions. NNP had the highest percentages of carbonyl/carboxyl C (C=O); while fine colloid and particulate fractions contained more reduced aromatic or aliphatic C (C–C, C=C, C–H). OC became more enriched (‰) in microbial-derived C (higher δ13C) with increasing particle size as well as with repeated redox oscillations. Our findings clearly demonstrate limitations of using the operationally defined “dissolved” fraction (<450 nm) to assess C cycling in ecosystems such as wetlands. Increase in colloid and OC concentrations and presence of more microbial-derived C in larger size fractions additionally imply that redox oscillations promote the formation of molecularly diverse sub-colloid sized organo-mineral associations. Being a composite unit of soil microaggregates, organic-mineral associations can thus influence the overall stability of OC in wetland soils that undergo frequent redox oscillations.

Application of a Cellular Automaton Method to Model the Structure Formation in Soils Under Saturated Conditions: A Mechanistic Approach

Soil functions are closely related to the structure of soil microaggregates. Yet, the mechanisms controlling the establishment of soil structure are diverse and partly unknown. Hence, the understanding of soil processes and functions requires the connection of the concepts on the formation and consolidation of soil structural elements across scales that are hard to observe experimentally. At the bottom level, the dynamics of microaggregate development and restructuring build the basis for transport phenomena at the continuum scale. By modeling the interactions of specific minerals and/or organic matter, we aim to identify the mechanisms that control the evolution of structure and establishment of stationary aggregate properties. We present a mechanistic framework based on a cellular automaton model to simulate the interplay between the prototypic building units of soil microaggregates quartz, goethite, and illite subject to attractive and repulsive electrostatic interaction forces. The resulting structures are quantified by morphological measures. We investigated shielding effects due to charge neutralisation and the aggregate growth rate in response to the net system charge. We found that the fraction as well as the size of the interacting oppositely charged constituents control the size, shape, and amount of occurring aggregates. Furthermore, the concentration in terms of the liquid solid ratio has been shown to increase the aggregation rate. We further adopt the model for an assessment of the temporal evolution of aggregate formation due to successive formation of particle dimers at early stages in comparison to higher order aggregates at later stages. With that we show the effect of composition, charge, size ratio, time and concentration on microaggregate formation by the application of a mechanistic model which also provides predictions for soil aggregation behaviour in case an observation is inhibited by experimental limitations.

Microaggregate and Macroaggregate of Andisol Affected by Arbuscular Mycorrhizal Fungi and Rhizobacteria

.The formation of soil microaggregate and macroaggregate involves different microbes. A Screenhouse experiment to determine the effect of Arbuscular Mycorrhizal Fungi (AMF) and rhizobacteria on soil microaggregate stability and soil macroaggregate stability (soil microaggregate and macroaggregate stability) had been carried out. The treatments included AMF (control and Glomus sp.) and rhizobacteria (Pseudomonas diminuta, P. diminuta + Bacillus alvei, P diminuta + P malei, P malei). The results showed that inoculation Glomus sp. and rhizobacteria didn’t increase the stability of microaggregates (0.2 mm – 0.3 mm and < 0.2 mm). Inoculation Glomus sp and rhizobacteria increased water stable aggregate (2,83 mm – 4,8 mm, 0,50 mm – 1,00 mm, and 0,30 mm - 0,50 mm) and inoculation Glomus sp increased water stable aggregate (2,00 mm – 2,83 mm dan 1,00 mm – 2,00 mm). The experiment indicated that rhizobacteria didn’t effect soil microaggregate due to silt fraction domination and Glomus sp was involved in the formation of macroaggregate

Soil microaggregate size composition and organic matter distribution as affected by clay content

Aggregation assembles different size mixtures of soil particles into a larger architecture. Such mixtures impede resolving which particles build aggregates and how these control the accumulation of soil organic matter (OM). Here we present an approach to differentiate the size distributions of soil fractions in the size range of microaggregates (<250 μm) from their dispersible particle-size distribution using dynamic image analysis. This approach enabled us to differentiate the magnitude and preferential size ranges of aggregates and non-aggregated particles. Wet sieving was used to isolate free microaggregate-sized fractions. Larger soil structures >250 μm were sonicated to isolate occluded size fractions <250 μm. To investigate the impact of soil texture, we analyzed topsoil samples of an arable site on Cambisol soils with a gradient in clay content of 16–37% and organic carbon concentrations of 10–15 g kg−1. Our results demonstrate how soil texture governs aggregate size distributions: most water-stable microaggregates were found to be of approximately 30 μm diameter, independent of the clay content gradient. High-clay soils contain more water-stable macroaggregates (>250 μm) and larger microaggregates in the 50–180 μm size range. The low-clay soils, on the other hand, contained more non-aggregated sand-sized particles >100 μm which probably hampered the buildup of larger aggregates. The size distribution of particles <100 μm in size fractions <250 μm showed a similar prevailing soil texture pattern, with approximately 24% clay, 59% silt, and 17% sand-sized particles at all clay contents. In contrast to the prevailing texture pattern along the clay content gradient, 4% more clay-sized particles helped build up water-stable macroaggregates. In the low-clay soils, the aggregates were smaller and the size fractions <53 μm had higher OM concentrations. This indicates that the low-clay soils held most of their OM in smaller microaggregates. Such arrangement of OM in smaller microaggregates demonstrates that soil texture may control OM stabilization mostly indirectly via the distribution of OM in different aggregate fractions. The occlusion of microaggregates in larger structures led to lower alkyl:O/N-alkyl ratios in 13C nuclear magnetic resonance (NMR) spectroscopy, indicating increased preservation.

Soil erosion affects variations of soil organic carbon and soil respiration along a slope in Northeast China

BackgroundAlthough soil erosion plays a key role in the carbon cycle, a holistic and mechanistic understanding of the soil erosion process within the cycle is still lacking. The aim of this study was therefore to improve our mechanistic understanding of soil organic carbon (SOC) and soil respiration dynamics through an experiment conducted in an eroding black soil farmland landscape in Northeast China.ResultsThe depositional profiles store 5.9 times more SOC than the eroding profiles and 3.3 times more SOC than the non-eroding profiles. A linear correlation between the SOC and 137Cs (Caesium-137) was observed in our study, suggesting that the SOC decreased with increased soil erosion. Furthermore, the fractions of intermediate C and the microaggregate C were lowest at the eroding position and highest at the depositional position. In the depositional topsoil, the input of labile materials plays a promotional role in soil respiration. Conversely, in the subsoil (i.e., below 10 cm), the potential mineralization rates were lowest at the depositional position—due to effective stabilization by physical protection within soil microaggregates. The field results of soil surface respiration also suggest that the depositional topsoil SOC is prone to be mineralized and that SOC at this depositional context is stabilized at subsoil depth. In addition, the high water contents at the depositional position can limit the decomposition rates and stabilize the SOC at the same time.ConclusionsThe findings from this study support that a majority of the SOC at footslope is stored within most of the soil profile (i.e., below 10 cm) and submitted to long-term stabilization, and meanwhile support that the depositional profile emits more CO2 than the summit due to its high amount and quality of SOC.

Short-term impacts of forest clear-cut on soil structure and consequences for organic matter composition and nutrient speciation: A case study.

Clear-cuts of forests severely affect soil structure and thus soil organic matter (SOM) and nutrient cycling dynamics therein, though with yet unknown consequences for SOM composition as well as phosphorus (P) and sulfur (S) chemical form within the soil microaggregate size fraction. To determine the effects of conventional clear-cutting on soil chemistry in a Cambisol of the Wüstebach Forest (northwestern Germany), we sampled the mineral A- and B-horizons prior to clear-cut as well as 10 and 24 month thereafter. We measured the SOM composition of soil microaggregates using pyrolysis field ionization mass spectrometry (Py-FIMS), as well as P and S chemical form and speciation using wet-chemical extractions and X-ray absorption near edge structure (XANES) spectroscopy. We found that clear-cut led to an increase of the microaggregate size fraction up to 6% due to break-down of macroaggregates and initially significantly increased total elemental concentrations (C, N, P, S) due to the introduction of slash-residues. The SOM of slash-residues consisted to a substantial amount of sterols and was generally found to be of low thermal stability and probably did not contribute to aggregate stability. Deterioration of the aggregate structure probably led to an exposure of originally inaccessible sites within aggregates to the attack by soil microorganisms and thus to an increased P and S turnover as reflected in a significantly reduction of available P proportions (4 to 7%) and a reduction of the most reduced S forms (5%). A probable increased microbial activity and contribution to SOM after clear-cut is also reflected in the significantly increasing hexose:pentose ratio by 0.25 between 10 and 24 month after clear-cut, significantly increasing the general thermal stability of SOM in the microaggregate size fraction and believed to contribute to aggregate stability. This indicated that a first deterioration of the aggregate structure after clear-cut might in the long-term be remediated with time.

Study on Improvement Effect of Saline-alkali Soil by Cationic Modified Biochar

The severe soda-alkali soil in the western Songnen Plain was used as the experimental object. Compared with the application of common biochar, the effects of modified biochar on the basic physical and chemical properties, agglomerate distribution, organic carbon and total nitrogen distribution of saline-alkali soil were analyzed. The experiment adopts the indoor culture method, and the use of common biochar (B) and iron modified biochar and calcium modified biochar is 0, 1.5%, 5% . A total of 7 treatments: CK, B1.5, B5, FBS1.5, FBS5, CBS1.5, CBS5. The results show: 1. Modified biochar significantly reduced soil pH with a maximum reduction of 12.2%. Both biochar and modified biochar significantly reduce soil ESP. The reduction effect of FB5 was the most significant in each treatment, which was 58.2% lower than CK, followed by FB5, which was 41.8% lower than CK. The introduction of cations and the significant reduction of ESP showed strong Na+ substitution and adsorption capacity, thereby alleviating soil salt accumulation and improving soil basic physical and chemical properties. 2. Both modified biochar and ordinary biochar increased the content of soil microaggregates (0.053-0.25mm). The modified biochar was significantly higher than ordinary biochar, and the function of improving soil structural stability was stronger. Among them, CBS improvement effect is better than FBS. The ratio of CBS5 treatment increased the microaggregate to the highest, with an increase of 66.4%; the increase of CBS1.5 treatment was 56.2%; the proportion of FBS5 and FBS1.5 microaggregates increased by 51.1% and 32.2% respectively. The geometric mean diameter (GMD) of all treatments was higher than that of CK treatment, in which CBS5 treatment significantly increased GMD by 18.6%; modified biochar treatment increased mean weight diameter (MWD), of which CBS5 significantly improved MWD. The increase was 8.8%.

Initial microaggregate formation: Association of microorganisms to montmorillonite-goethite aggregates under wetting and drying cycles

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.

Maize root‐derived C in soil and the role of physical protection on its relative stability over shoot‐derived C

Evidence is accumulating that belowground crop residues contribute more efficiently to the build‐up and maintenance of native soil organic carbon (SOC) than aboveground plant parts. We corroborated previous testing of the hypothesis that root‐C would preferentially accumulate in soil microaggregates, where it is physically protected against microbially mediated decomposition. In three European field trials with C3 to C4 crop transitions, we compared the content of maize‐C (Zea mays L.) in soil from rotations with grain maize (MG) or silage maize (MS) (i.e. with incorporation of roots and shoots or roots only). After decades of maize cultivation, SOC content did not differ within three out of four MG–MS pairs, although obviously larger amounts of shoot biomass were added to soil in the case of MG. We found that relative contribution of roots was on average 3.5 times more than shoots to the build‐up of SOC per equivalent mass of residue C added to soils. Preferential occlusion of root‐C as silt‐sized intra‐microaggregate particulate organic matter (iPOM) was not observed. There were much larger effects from shoot incorporation on maize‐C in the &gt;53‐μm fraction and free silt and clay. Storage of root‐C as sand‐sized iPOM was not quantified here, but first estimates suggested that physical entrapment at this level could only partly explain the longevity of root‐C in soil. We reconfirm the relative stability of root‐C in soil, but do not conclude that this stems from preferential physical entrapment over shoot‐C. Future work should investigate the cause of preferential root‐C association with the clay‐sized fraction and if this occurs before or after microbial processing.Highlights The hypothesized preferential physical stabilization of root‐C was assessed. Aboveground biomass had a minor effect on SOC storage in European long‐term trials. Contribution of maize root‐C to maintenance of SOC was three‐fold that of shoot‐C inputs. There was no preferential occlusion of root‐C in silt‐sized microaggregates.

Improved extraction efficiency of natural nanomaterials in soils to facilitate their characterization using a multimethod approach

Characterization of natural nanomaterial (NNM) physicochemical properties – such as size, size distribution, elemental composition and elemental ratios - is often hindered by lack of methods to disperse NNMs from environmental samples. This study evaluates the effect of extractant composition, pH, and ionic strength on soil NNM extraction in term of recovery and release of primary particles/small aggregate sizes (i.e., <200 nm). The extracted NNMs were characterized for hydrodynamic diameter and zeta potential by dynamic light scattering and laser Doppler electrophoresis, natural organic matter desorption by UV–Vis spectroscopy, element composition by inductively coupled plasma-mass spectroscopy (ICP-MS), size based elemental distribution by field flow fractionation coupled to ICP-MS, and morphology by transmission electron microscopy. The extracted NNM concentrations increased following the order of NaOH ≤ Na2CO3 < Na2C2O4 < Na4P2O7. Na4P2O7 was the most efficient extractant and results in 2–12 folds higher NNM extraction than other extractants. The Na4P2O7 extracted NNMs exhibited narrower size distribution with smaller modal size relative to NaOH, Na2CO3, Na2C2O4 extracted NNMs. Thus, Na4P2O7 enhances the extraction of primary NNMs and/or smaller NNM aggregates (i.e., size <200 nm). Na4P2O7 promote soil microaggregates breakup and release of NNMs by reducing free multivalent cation concentration in soil pore water by forming metal-phosphate complexes and by enhancing NNM surface charge via phosphate sorption on NNM surfaces. Additionally, the extracted NNM concentrations increased with the increase in extractant concentration and pH, except at 100 mM where the high ionic strength might have induced NNM aggregation. The improved NNM-extraction will improve the overall understanding of the physicochemical properties of NNMs in environmental systems. This study presents the key properties of NNMs that can be used as background information to differentiate engineered nanomaterials (ENMs) from NNMs in complex environmental media.

Effects of Chinese milk vetch intercropped with rape under straw mulching on soil aggregate and organic carbon character

There are severe soil erosion, shallow soil, reduction of soil organic matter, and poor soil and water conservation in purple soil areas in Southwest China, which become the main limiting factors for the sustainable development of agriculture. A series of buckets and field experiments in the field were employed to explore the soil aggregate and soil organic carbon in response to Chinese milk vetch intercropped with rape under straw mulching, aiming to improve soil aggregate structure and increase organic carbon content. Results showed that intercropped Chinese milk vetch increased soil micro-aggregate content in rape rhizosphere, and reduced soil aggregate mean mass diameter. The change of soil macro-aggregate in rape rhizosphere was mainly caused by the change of content of soil aggregate on 10-5 mm and 5-2 mm, while the soil micro-aggregate was mainly caused by soil aggregate on 0.25-0.053 mm. Intercropped Chinese milk vetch and straw mulching significantly increased soil organic carbon content after corn growing season, with increasing the content of soil organic carbon on 10-20 cm and 20-30 cm. Though intercropped Chinese milk vetch and straw mulching had less effect on soil total organic carbon in rape season, more and more significant effect on 0-10 cm, 10-20 cm, and 20-30 cm with rape growing, especially in stem elongation stage, flowering stage, and harvest stage. Our results showed that the characteristic of soil aggregate in rape rhizosphere could be changed by intercropped Chinese milk vetch, and that the content of soil organic carbon could be increased by Chinese milk vetch intercropped with rape under straw mul-ching.

Formation of mineral–mineral and organo–mineral composite building units from microaggregate‐forming materials including microbially produced extracellular polymeric substances

Structures of colloidal compounds in soil, including organo–mineral and mineral–mineral associations, are considered as composite building units (CBUs) that may combine into soil microaggregates. Despite the ubiquitous occurrence of CBUs, the major formation mechanisms are rather obscure and little is known about whether they form primarily during weathering of the parent rocks or by aggregation processes from the soil suspension. We studied the formation of CBUs from suspensions composed of minerals and organic matter typical for temperate soils (i.e. quartz, goethite, illite and extracellular polymeric substances [EPS]). Without EPS, we found CBUs formed as mineral–mineral associations by hetero coagulation of illite and quartz that is bridged by goethite. The presence of EPS, in contrast, led to the formation of a stable suspension of clay‐sized CBUs with no involvement of quartz. We explained this by the rapid formation of organo–mineral CBUs made of EPS‐associated goethite and EPS‐associated illite. The sorption of EPS to goethite screened its surface charge, thereby reducing the electrostatic attraction between goethite and illite. This interaction effectively impeded the formation of mineral–mineral CBUs. Moreover, interactions of EPS with goethite resulted in a marked decrease of the phosphorus/carbon ratio in the suspension. This suggested a preferred adsorption of phosphorus‐containing EPS constituents to goethite and in turn to a compositional fractionation of EPS constituents between the solid and liquid phase as shown by Fourier‐transform infrared spectroscopy with attenuated total reflection (FTIR–ATR). Laser light diffraction measurements revealed a shift from the fine silt fraction to that of the fine sand that also supports the role of EPS as a ‘binding’ agent.Highlights Composite building units (CBUs) form in suspensions with different mineral and organic components. Both, hetero mineral–mineral and organo–mineral CBUs were formed. The initial composition of the suspension controls type and properties of resulting CBUs. Depending on the mineral surfaces, EPS may serve as a separation or binding agent.

The effect of vegetation change in C and N contents in litter and soil organic fractions of a Northern Iran temperate forest

The properties of litter and soil, which are probably influenced by vegetation traits, have an effect on the fractions of C and N and ultimately global warming. In this study, litter and soil samples were collected from representative forestlands, including a mixed natural forest dominated by hornbeam‑ironwood and tree plantations of ash, maple, cypress and pine which are situated in a temperate area of Northern Iran. Main litter characteristics, soil aggregation and fractions of C and N were studied. Pine had the highest values of litter thickness (14.46 cm), C (60.31%) and C/N ratio (81.77 and 20.28 for litter and soil, respectively). However, litter N (2.20%), micro aggregates of total soil (39.75%), aggregate stability (73.86%) and macro aggregates of total soil (54.87%) were found significantly higher under the natural stands. Total C (2.58%), C in macro and micro aggregates (0.31 and 0.33 respectively), POM-C (4. 68 g/kg), DOC (125.54 mg/kg) and C sequestration (45.34 Mg/ha) were considerably higher under pine (for POM-C, pine ≈ cypress) and the greater values of total N (0.34%), N in macro and micro aggregates (0.10 and 0.08 respectively), POM-N (0.47 g/kg), DON (33.80 mg/kg) and N sequestration (6.86 Mg/ha) were observed under natural stands. Under our conditions, conversion of natural forest is due to reduction of soil N fractions and C increased under pine tree plantations.

Modeling the formation of soil microaggregates

The functions of soils are intimately linked to its aggregated structure. Microaggregates formed during pedogenesis from a vast variety of mineral, organic, and biotic materials are the smallest conceivable compounds at the basis of soil structural hierarchy. Qualitative hypotheses and concepts on how aggregates form are quite elaborate, but the consistent quantitative application within a mechanistic and physically rigorous framework is still missing. This is partly caused by the fact that such an endeavor requires the combination of synergistic concepts on the transport of particles and their interactions affected by shape, size, and surface properties in aqueous suspension. Here we present a novel quantitative approach for the formation of soil microaggregates in silico that integrates diffusion-limited cluster-cluster aggregation with a probabilistic attachment following the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of particle interactions. To represent universal mineral shapes found in soils, we implemented spherical, plate-like as well as rod-like particle morphologies inspired by weathered silicates, secondary clay minerals and pedogenic iron oxides. In this way, we developed a model that produces an explicit three-dimensional structure of aggregates emerging from hetero- as well as homo-aggregation in response to the chemical milieu. We then exploited the 3D morphology to assess functional properties of aggregates exemplified with the water retention curve and pore size distribution.

Insights into Pb(II) binding by Fe/Al hydroxide–microbe composite: XAFS spectroscopy and isothermal titration calorimetry study

Soil is a complex assemble of colloidal and particulate constituents, including minerals, organic matter and microbes. Minerals and organics are not separate entities; rather, they are constantly interacting with each other. Such mineral-organic interaction can greatly affect the sequestration of metallic elements at the soil-water interfaces. This study aims to explore the binding of Pb(II) in a model Fe/Al hydroxide-organo composite (Fe/Al-OM) made with Pseudomonas putida cells which, for the purposes of this study, can be considered to be an analogue of some natural soil microaggregates. We measure an enhanced Pb adsorption to the Fe/Al hydroxide-bacteria composite (Fe/Al-B) over pure Fe/Al hydroxide at pH < 5.5. We find that Fe/Al-B sorbs an intermediate amount of Pb between the end-member Fe/Al hydroxide and bacteria. We identify a bidentate edge-sharing Pb complex on the Fe/Al oxide surfaces with a Pb–Fe/Al bond distance of ~3.33 Å, as well as a monodentate carboxyl binding on the bacterial surfaces. We report that Pb binding is an exothermic, energetically favorable process accompanied by positive entropy. Moreover, Pb distribution shifts from the bacterial fraction to the mineral fraction in Fe/Al-B when pH increases from 4 to 6.5. Results from batch to spectroscopic and thermodynamic experiments will facilitate our understanding of Pb speciation, mobility and fate in soils and sediments.

Changes of paradigms in agriculture soil microbiology and new challenges in microbial ecology

At the beginning of the 20th century Louis Pasteur laid the foundations of classical microbiology, based on the cultivation of microorganisms. Microbiology ceased to be an exclusive concern in matters of health or food conservation and extended to almost all fields in Nature. Microbes were recognized as the catalysts of all the cycles of the elements in our planet. In this context, agricultural microbiology works as a catalog of curiosities that offers the possibility to isolate particular microorganism and convert them into modern additives of agriculture. However, this paradigm vanished not many years ago, after a big anomaly in bacterial counts. Biochemical tools allow us to see the new unculturable microbes and a new scenario of soil microbiology is under construction at present. The new concept of microbiome expands the complexity of the soil where any macroscopic form of life develops. Analyzing the soil microbiology at a soil microaggregates scale suggests that agricultural managements are the way of shaping soil microbiomes to improve soil quality and improve crop production and soil services without harming the environment. The integration of soil microbiomes knowledge into the other levels of the soil food web is the future of soil science and could be one of the clues to mitigate the global climate change.