Mass Distribution Of Aggregates Research Articles

Soil Carbon, Glomalin, And Aggregation in Onion Crop Under No-Tillage with Cover Crops or Conventional Tillage Systems for Eight Years

Crop systems using cover crops affect soil physical, chemical, and biological attributes, including aggregate formation. This work aims to evaluate winter cover crop species' effect on soil total organic carbon, glomalin, and aggregation in areas with onion crops in a no-tillage vegetable production system (NTVS) and conventional tillage system (CTS) for eight years. The experiment treatments were: control, with natural vegetation(NV); black oats (Avena strigosa); rye(Secale cereale);oilseed radish(Raphanus sativus);intercropped black oats and oilseed radish; intercropped rye and oilseed radish; and a conventional tillage systems area. A 33-year old adjacent secondary forest was evaluated as a reference for undisturbed conditions. We assessed soil total organic carbon, total glomalin, and easily extractable glomalin in three soil layers (0-5, 5-10, and 10-20 cm depth). Undisturbed samples were used to quantify soil aggregate stability, aggregation indexes (weighted mean diameter; geometric mean diameter), aggregate mass distribution (macroaggregates, mesoaggregates), and macroaggregate carbon contents. The conventional tillage areas had the lowest weighted mean soil aggregate diameter, geometric mean diameter, and macroaggregate mass. Those areas also had the lowest bulk soil and aggregate organic carbon contents and the lowest total and easily extractable glomalin. Winter cover crops' use resulted in a 10% higher aggregate weighted mean diameter and geometric mean diameter. Areas with cover crops had 13% higher organic carbon contents in aggregates and 17% higher macroaggregate mass than conventional tillage areas. The highest values of total and easily extracted glomalin occurred in plots with black oats. Winter cover crops, single or intercropped, improved physical attributes of soils with onion crops under not-tillage compared to conventional tillage areas.

Aggregation, carbon, nitrogen, and natural abundance of 13C and 15N in soils under no-tillage system fertilized with injection and surface application of pig slurry for five years

Injection of pig slurry (PS) into soils under no-tillage system (NTS) is more efficient for improving soil chemical and physical attributes, and reducing C and N emissions, than surface applications. This study evaluated the effect of using injection and surface application of PS, compared to NPK and control treatments, on the soil aggregate, C and N contents, and isotopes 13C and 15N. The NTS consisted of rotations of summer (maize) and winter (black oat and wheat) grasses from 2011 to 2015. The treatments were PS injected into the soil (PSI), PS applied on the soil surface (PSS), chemical fertilization (NPK), and control (CTRL). The following soil properties were evaluated in the 0–5, 5–10, and 10–20 cm layers: aggregate stability (geometric mean diameter – GMD; aggregate mass distribution); total organic carbon (TOC) and total nitrogen (TN) in macroaggregates (8.0–0.25 mm), microaggregates (<0.25 mm), and bulk soil (<2.0 mm); and isotopes 13C and 15N in macro and microaggregates. The application of PSI improved the soil physical attributes, presenting higher GMD (0–5 cm) than the PSS, NPK, and CTRL treatments. In the 5–10 cm layer, the PSI treatments were more efficient in increasing the GMD and macroaggregate mass than the NPK. PSI also was more efficient in increasing TOC and TN when compared to PSS, and generated a higher GMD, which are protectors of these elements in the soil. The natural abundance of 15N denoted the lower soil organic matter decomposition in the PSI treatment when compared to the PSS. The natural abundance of 13C showed less-negative values in macroaggregates than in microaggregates, denoting that the soil management practices and crops used (grasses) affected positively the abundance of 13C. After seven applications of PS in maize-oat-wheat rotation in NTS, the application of PSI was more efficient in improving the soil physical and chemical attributes than the application of PSS.

Carbon and nitrogen contents and aggregation index of soil cultivated with onion for seven years using crop successions and rotations

Onion is usually cultivated using conventional tillage system (CTS), with excessive soil turning, leaving it with low or no vegetation cover. This favors erosive processes and impacts negatively diverse edaphic attributes. Adopting soil management systems with conservationist bases that use permanent soil coverage and crop rotation can maintain or improve these attributes (for example, soil aggregation and soil organic matter). The objective of this work was to evaluate the total organic carbon (TOC) and total nitrogen (TN) contents, aggregation index, and aggregate mass distribution of a Humic Cambisol cultivated with onions in succession or rotation with other species in no-tillage system (NTS) and CTS. The treatments were: maize/onion (NTS-T1); cover plants (winter)/onion (NTS-T2); maize/winter grasses/onion (NTS-T3); velvet bean/onion (NTS-T4); millet/cover plants (winter)/onion (NTS-T5); velvet bean/rye/onion (NTS-T6); maize/onion (CTS-T7); intercrops cover plants (summer)/onion (NTS-T8). Seven years after the implementation of the experiment, the weighted mean diameter (WMD) of the aggregates, distribution of macroaggregates, mesoaggregates, and microaggregates, and TOC, and TN contents of the soil (0–0.05, 0.05–0.10 and 0.10–0.20 m layers) were evaluated. Periodic soil turning (CTS) in the succession of maize, and onion (T7) reduces TOC and TN contents in the soil surface layer, compared to succession and rotation systems with onion crops in NTS. This negative effect on soil quality is connected to the reduction of aggregate stability, especially the decrease in the amount of macroaggregates. The use of grasses, especially winter grasses in rotation with maize (T3), preceding onion crops in NTS increases TOC content in the soil surface layer. Higher TN accumulation in the surface layer is found in areas with more soil cover plant species in rotation or succession with onion in NTS (T2, T3, T5, T6 and T8). The use of NTS for onion crops generates high soil aggregate stability, with predominance of macroaggregates, regardless of the crop succession or rotation system used. Treatments with no winter soil cover plants (T1, T4 and T7) reduce soil TOC contents and the mass of water-stable macroaggregates and increase the amount of microaggregates in the soil surface layer when compared to the other treatments.

Large macroaggregates determine distribution of soil amidohydrolase activities at different landscape positions

Microbial activities have been shown to be regulated by soil aggregation. However, it is unknown how landscape-induced changes in the soil structure shape microbial activity in the soil ecosystems. This study aims to analyze the spatial distribution of amidohydrolase activities within the soil aggregates obtained from different landscape positions. Soil samples were collected from five landscape positions (summit, shoulder, backslope, footslope, and toeslope) at two pasture sites with contrasting climatic condition. Soil organic C (SOC) content, total N (TN), urease (URS), l-glutaminase (LGL), and l-asparaginase (LAS) activities were measured in five sizes of soil aggregates (4–2, 2–1, 1–0.5, 0.5–0.25 and 0.25–0.05 mm). The results showed that the landscape position significantly influenced the mass distribution of aggregates. This was observed, in particular, for larger macroaggregates (4–2 mm). By increasing the SOC and TN contents along both the landscapes studied, the macroaggregate mass fractions (4–2; 2–1; 1–0.5 mm) increased. The results also showed that the SOC and TN contents as well as amidohydrolase activities were heterogeneously distributed within soil aggregates. Macroaggregates (generally 4–2 mm) exhibited greater SOC and TN contents as well as amidohydrolase activities compared to microaggregate, though the 4–2 mm aggregates constituted a small component of most soils. Our findings also revealed that proportional distribution of amidohydrolase activities within the soil aggregates was influenced by the landscape position. With increasing soil aggregation along the landscapes, the amidohydrolase activities tended to increase into macroaggregate fractions. Moreover, SOC, TN and, the enzyme activities in larger macroaggregates are more sensitive to landscape-induced changes than other size of aggregates.

Species and root traits impact macroaggregation in the rhizospheric soil of a Mediterranean common garden experiment

We evaluated the influence of plant species and life forms on soil aggregate distribution among size-classes, total macroaggregate mass and aggregate mean weight diameter (MWD), and examined how specific root traits were related to these aggregation variables. We analyzed the soil attached to the roots (i.e., rhizospheric soil) under 13 Mediterranean species grown in monocultures in a common garden experiment for four years, and compared it to a bare soil. The mass distribution of aggregates in six size-classes and aggregate MWD were calculated, both on a rhizospheric soil and root biomass basis. Compared to bare soil, macroaggregate mass increased by an average of 13% in the presence of plants, with a strong effect of species and life forms (both P < 0.0001); some species such as Sanguisorba minor showing increases of up to ~40%. Although the soil under graminoids had a greater macroaggregate mass, their MWD was lower than under non-woody dicots. Large (2000–1000 μm) and intermediate (1000–500 μm) macroaggregate mass increased with root mass and length density and decreased with root lignin concentration, while very large macroaggregate (6000–2000 μm) mass and the MWD increased with root soluble compound concentration. Species and life forms differently influenced the distribution of macroaggregates among size-classes and aggregate MWD. Easily-decomposable roots with traits related to resource acquisition (i.e., high fine root length, high water-soluble compound concentration) are more favorable for the development of water-stable macroaggregates than roots traits related to resource conservation (high lignin concentration, thick roots).

Effect of Long-term Land Use Management Practices on Distribution of C and N Pools in Water Stable Aggregates in Alfisols

Soil aggregates enhance carbon (C) sequestration by physically protecting it from the microbial activity. The position of soil organic carbon (SOC) in the aggregates and its chemical nature affects the rate of its decomposition. Thus, a study was conducted using soils of a 16-year-old tillage experiment and the adjoining forest on a Crosby silt loam soil at the Waterman Agricultural and Natural Resource Laboratory of the Ohio State University, Columbus, Ohio to assess the effect of different management practices on the aggregate size distribution, the C and N concentrations in the aggregates, and the C and N pools associated in these aggregates. The data showed that the forest and the cultivated soils were statistically at par with respect to the aggregate mass distribution. In comparison with the cultivated soil, the forest soil had significantly higher C and N concentrations in bulk soil (117 and 98.7%), water stable aggregates (WSA) (80.9 and 68.9%), particulate organic matter (POM) (118.3 and 94.5%) and the mineral associated organic matter (MAOM) (69.1 and 62%). It also contained higher C (59%) and N (53.5%) pools than the cultivated soil. However, there was no significant difference between the conventional tillage and no-till system with respect to the C and N concentrations in the bulk soil, WSA, POM, MAOM and the C and N pools. The aggregate mass and C and N concentrations in the WSA, POM and MAOM and the C and N pools associated with the large macro-aggregates were significantly higher than in the other aggregate fractions. Total N, C:N ratio, particulate organic C, particulate organic N, C:N ratio of POM, mineral associated C, mineral associated N and C:N ratio of the MAOM accounted for 99.9% variation in the total soil C.

Soil aggregate isolation method affects measures of intra-aggregate extracellular enzyme activity

Within soil aggregates, binding of organic matter is known to occlude it from microbial attack. Within aggregate fractions of different sizes, microbial communities and activities have also been shown to differ. As a result the soil physical structure, organic inputs and microbial activity together impact the rate at which organic matter is decomposed and stored within soil. However, methods developed for isolating soil aggregates may affect subsequent biological assays. In this study, we sought to understand how enzyme activity within soil aggregates is influenced by aggregate isolation methodology, including wet, dry, and ‘optimal moisture’ sieving procedures within two contrasting ecosystems (a corn agroecosystem and a 2-yr old, diverse planted tallgrass prairie). Mass distribution of aggregates from wet-sieving was skewed toward small macroaggregates (250–1000 μm) and microaggregates (<250 μm), but the distribution of dry and optimal moisture aggregates was highly skewed toward large macroaggregates (>2000 μm). Wet-sieved macroaggregates (>1000 μm) had greater aggregate potential enzyme activity (nmol substrate h−1 g−1 dry aggregate) than smaller aggregate fractions and whole soil, particularly for C-cycling enzymes cellobiohydrolase and β-glucosidase. Also, wet-sieved aggregates from corn systems had higher potential cellobiohydrolase and β-glucosidase activity than aggregates isolated from prairie. Neither of these relationships was observed in dry and optimal moisture aggregates, suggesting that elevated activities are characteristic of water-stable aggregates and possibly stimulated by soil rewetting. The proportional contribution to total enzyme activity observed in water-stable microaggregates accounted for 46–62% of whole soil activity; although water-stable large macroaggregates (>2000 μm) had greater aggregate enzyme activity, they contributed a minority of overall soil activity. In contrast, the proportional contribution of large macroaggregates comprised 70–78% of whole soil activity when dry sieved and 38–66% under optimal moisture sieving. Wet-sieving soil aggregates is most useful to examine long-term changes in soil organic matter and microbial activity between soil types. Optimal moisture and dry sieved aggregates may be useful alternatives to more closely capture short-term in situ measures of seasonal and intra-annual soil microbial activity.

Breakup of small aggregates driven by turbulent hydrodynamical stress

The breakup of small solid aggregates in homogeneous and isotropic turbulence is studied theoretically and by using direct numerical simulations at high Reynolds number, Reλ =/~ 400. We show that turbulent fluctuations of the hydrodynamic stress along the aggregate trajectory play a key role in determining the aggregate mass distribution function. The differences between turbulent and laminar flows are discussed. A definition of the fragmentation rate is proposed in terms of the typical frequency at which the hydrodynamic stress becomes sufficiently high to cause breakup along each Lagrangian path. We also define an Eulerian proxy of the real fragmentation rate, based on the joint statistics of the stress and its time derivative, which should be easier to measure in any experimental setup. Both our Eulerian and Lagrangian formulations define a clear procedure for the computation of the mass distribution function due to fragmentation. Contrary, previous estimates based only on single point statistics of the hydrodynamic stress exhibit some deficiencies. These are discussed by investigating the evolution of an ensemble of aggregates undergoing breakup and aggregation.

Agregação, carbono e nitrogênio em agregados do solo sob plantio direto com integração lavoura-pecuária

O objetivo deste trabalho foi avaliar a estabilidade dos agregados, o conteúdo de matéria orgânica leve do solo (MOL), a distribuição dos teores de carbono orgânico total (COT) e nitrogênio (N) e a abundância natural de 13C e 15N dos agregados de um Latossolo Vermelho argiloso sob sistema plantio direto (SPD) com integração lavoura-pecuária (ILP), em comparação à uma área em SPD sem ILP e uma área de Cerrado natural em Montividiu, GO. Foram coletadas amostras de solo nas camadas de 0-5 e 5-10 cm, em delineamento inteiramente casualizado. Foram avaliados distribuição da massa dos agregados, diâmetro médio ponderado (DMP), diâmetro médio geométrico (DMG) e MOL, e foram quantificados os teores de COT, N, δ13C e δ15N dos agregados. A área de Cerrado apresentou os maiores valores de DMP, DMG e MOL. O SPD-ILP apresentou valores maiores de DMP, DMG, COT, N e MOL na camada 5-10 cm do que o SPD, sem braquiária. O SPD-ILP promoveu aumentos nos índices de agregação do solo (0-5 e 5-10 cm), nos teores de MOL (5-10 cm), COT e N (0-5 cm) e na formação de agregados estáveis em água (5-10 cm), em comparação ao SPD, sem braquiária.

Competition between aggregation and migration processes of a multi-species system

We propose a solvable multi-species aggregation–migration model, in which irreversible aggregations occur between any two aggregates of the same species and reversible migrations occur between any two different species. The kinetic behaviour of an aggregation–migration system is then studied by means of the mean-field rate equation. The results show that the kinetics of the system depends crucially on the details of reaction events such as initial concentration distributions and ratios of aggregation rates to migration rate. In general, the aggregate mass distribution of each species always obeys a conventional or a generalized scaling law and for most cases at least one species is scaled according to a conventional form with universal constants. Moreover, there is at least one species that can survive finally.

Precursor Aggregate Distributions for Particulate Gels

We analyze the aggregation process leading to gel formation in colloidal dispersions under quiescent conditions in order to identify possible precursor aggregate distributions for particulate gels. The aggregation process is described with an experimentally validated population balance model, which allows us to calculate how the aggregate mass distribution evolves in time. When the cumulative occupied volume fraction of fractal aggregates reaches a certain limit, the random cluster−cluster aggregation regime crosses over to an interconnection regime, and a gel network forms. The aggregate distribution at the end of the aggregation regime is the precursor for the subsequently formed particulate gel. We demonstrate that, when aggregation proceeds fully in the reaction-limited regime, the precursor aggregate distribution depends only on the solid volume fraction while being insensitive to both the primary particle size and the stability ratio. However, the stability ratio is an important operating parameter controlling the overall aggregation and gelation time. The situation can be significantly changed by entering the diffusion-limited aggregation regime. The interplay among these various factors is illustrated through an example of the optimal design of the precursor aggregate distribution. Primary particle size, solid volume fraction, and stability ratio are used as design parameters, and the objective function is formulated in terms of specific requirements on the precursor cluster mass distribution. These requirements were determined on the basis of qualitative arguments about the relation between the precursor structure and the gel mechanical properties.

Aggregate restructuring and its effect on the aggregate size distribution

The aggregation model is developed to take into account the restructuring of newly formed aggregates to form more compact structures. The effect of aggregate restructuring on the aggregate mass distribution is investigated. If the exponent characterizing the mass dependence of the aggregation activity (ω) increases with the fractal dimension of aggregates, restructuring broadens the aggregate mass distribution. The distributions follow then the general dependence on the ω-exponent, valid for non-fractal aggregates. Otherwise, restructuring narrows the aggregate mass distribution, but not along with the general dependence.

Kinetics of Reversible Gravitational Coagulation in a Spatially Uniform Disperse System

Spatially uniform disperse system where the processes of coagulation, fragmentation, and sedimentation of fractal aggregates occur simultaneously, was discussed. Kinetics of the passage of such system from the initial state, where the dispersed phase was already separated into fine and coarse fractions, to a new equilibrium state was studied. The enlargement of aggregates takes place both due to the capture of the particles of fine fraction and their adherence during collisions. Disintegration of aggregates occurs by the detachment of fragments from the aggregate surface under the action of hydrodynamic stresses arising during aggregate sedimentation. The solution of the coagulation–fragmentation equations indicates that, at the intermediate stage of a process, the aggregate mass distribution density is characterized by polymodal structure, which is degenerated into bimodal by the end of the passage.

Experimental measurement of the mass distribution of petroleum asphaltene aggregates using ultracentrifugation and small-angle X-ray scattering

Fractionation of petroleum asphaltene in toluene has been carried out using ultracentrifugation. The separated asphaltenes are examined by small-angle X-ray scattering (SAXS). The radii of gyration and molar mass of the separated asphaltenes range over a wide scale that provides a good indication of the self-similar structure of asphaltene aggregates. The relations between mass and radius of gyration ( M∝ R G D ) and between scattered intensity and wave vector ( I( q)∝ q − d ) were studied. Values of both D (=1.99) and d (=2.06) show a fractal dimension D f of the asphaltene structure close to 2. The mass distribution of these aggregates can be calculated from the knowledge of both the quantity of separated fractions and the mass M of every fraction. We found experimentally a mass distribution with a power-law shape ( N( M)∝ M − τ ) with exponent τ=1.66. The fractal dimension values and the mass distributions of asphaltene aggregates are similar to those predicted by the RLCA model.

A method of maximum entropy modeling the aggregation kinetics

It has been demonstrated how a method of maximum entropy of the aggregation probability distribution determines the aggregate mass distribution for the Brownian aggregation occurring in three-dimensional space. The resulting distribution corresponds to the experimental data. Using the equation describing an act of aggregation, which determines the fractal dimension of formed aggregate as dependent on the sizes and fractal dimensions of constituent clusters, it is also possible to calculate the fractal dimension of formed aggregates. The obtained value D=1.82 corresponds well to theoretical predictions and measured values. This method is then employed to determine the fractal dimension and size distribution of clusters produced by the two-dimensional Brownian aggregation, as well as by the aggregation in the free molecular regime occurring in three-dimensional space. The obtained result is also close to measured and calculated values. Hence, this work confirms the validity of the maximum entropy method in aggregation kinetics. The possibility to apply the reported method to other mechanisms of aggregation is discussed.

Perikinetic and Orthokinetic Aggregation of Hydrated Colloids

Perikinetic and orthokinetic aggregation in laminar tube flow of hydrophilic polystyrene latex particles was investigated to determine the aggregation characteristics of weakly unstable colloids in aqueous media. The weightSand numberNaverage sizes of colloids and the colloid mass frequencyc(n) were determined by particle counting. Destabilization of hydrated latex particles was found to be predicted by DLVO theories as far as the repulsive forces of the hydration layer were assumed to oppose the van der Waals attractive forces, the combined effects being expressed using an apparent low Hamaker constant. Deviation from the usual reaction-limited aggregation process could be established by comparing experimental and theoretical power laws for perikinetic processes. The development of orthokinetic processes depended on latex concentration and shear rate. At a given mean shear rate of 106 s−1the aggregation rate increased with the latex concentration below 0.3 g/L, while above this concentration the mass of aggregates collected at the tube exit strongly decreased. Hydrodynamic interactions similarly modified the collision efficiency of the smallest colloids, whereas the volume flow rate slightly modified the mass distribution of aggregates of intermediate and large sizes. The true collision efficiency in laminar tube flow could be established by taking into account the effective volume fraction of aggregates.