Aggregate stability and assessment of soil crustability and erodibility: I. Theory and methodology
TL;DR: In this paper, a unified framework for the measurement of aggregate stability is proposed to assess a soil's susceptibility to crusting and erosion, which combines three treatments having various wetting conditions and energies (fast wetting, slow wetting and stirring after pre-wetting).
Abstract: Summary
Crusting and erosion of cultivated soils result from aggregate breakdown and the detachment of soil fragments by rain, and the susceptibility of soil to these processes is often inferred from measurements of aggregate stability. Here, theories of aggregate breakdown are reviewed and four main mechanisms (i.e. slaking, breakdown by differential swelling, mechanical breakdown by raindrop impact and physico–chemical dispersion) are defined. Their relative importance depends on the nature of the rain, as well as on the soil's physical and chemical properties. The relations between aggregate breakdown, crusting and water erosion are analysed, and existing methods for the assessment of aggregate stability are reviewed. A unified framework for the measurement of aggregate stability is proposed to assess a soil's susceptibility to crusting and erosion. It combines three treatments having various wetting conditions and energies (fast wetting, slow wetting, and stirring after pre-wetting) and measures the resulting fragment size distribution after each treatment. It is designed to compare different soils, or different climatic conditions for a given soil, not to compare time-dependent changes in that soil.
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TL;DR: This paper reviews the different methods of measurement of soil aggregate stability used in the literature, paying attention to the conditions of sample collection in the field and sample preparation and treatmen's conditions.
Abstract: Soil aggregate stability is a crucial soil property affecting soil sustainability and crop production A broad outline of the processes and agents of aggregate formation and aggregate stabilization are presented and discussed in this review Aggregate stability is difficult to quantify and interpret The aim of aggregate stability tests is to give a reliable description and ranking of the behavior of soils under the effect of water, wind and management Numerous methods have been used to determine aggregate stability with varying success The different methodologies complicate the comparison among aggregate stability data It is also difficult to obtain a consistent correlation between aggregate stability and other important soil properties such as soil erodibility or crusting potential This paper reviews the different methods of measurement of soil aggregate stability used in the literature, paying attention to the conditions of sample collection in the field and sample preparation and treatmen
942 citations
TL;DR: The concept that the soil-microbe system is self-organized is introduced and new priorities for research based on an integrative approach that combines biochemistry and biophysics are suggested.
Abstract: Soil is the most complicated biomaterial on the planet. As with any material, the physical habitat is of prime importance in determining and regulating biological activity. However, until recently the opaque nature of soil has meant that any interrogation of its interior architecture has been relatively rudimentary, restricted to simple qualitative expressions of the physical heterogeneity that fail to relate to any specific function. However, new techniques and insights into the biophysical and biochemical processes of this inner space are leading to the developments of theoretical frameworks and experimental approaches that will allow us to sustainably manage Earth's most important resource. We introduce the concept that the soil-microbe system is self-organized and suggest new priorities for research based on an integrative approach that combines biochemistry and biophysics.
781 citations
TL;DR: In this paper, the contribution of organic matter to aggregate stability by decreasing aggregate wettability and the specific role of clay-associated organic matter was assessed by measuring water drop penetration times on individual 3-to 5-mm aggregates.
Abstract: Soil organic matter is thought to increase aggregate stability by lowering the wettability and increasing the cohesion of aggregates. In southwest France, thick humic loamy soils (Vermic Haplubrepts) have been intensively cropped for 40 yr, decreasing the soil organic pool and lowering the soil agregate stability. This study assessed (i) the contribution of organic matter to aggregate stability by decreasing aggregate wettability and (ii) the specific role of clay-associated organic matter. Soil samples with a C content of 4 to 53 g kg -1 were sampled and soil aggregate stability was measured. Aggregate wettability was assessed by measuring water drop penetration times on individual 3-to 5-mm aggregates. The <2-μm fractions were extracted without organic matter destruction and their wettability was determined by measuring contact angles of water on clay deposits. Aggregate stability against slaking was correlated to soil C content (r 2 = 0.71 for fast wetting). Water drop penetration time increased with C contents from 1 to 32 s and was very heterogeneous among individual aggregates from a given soil. The contact angle of water on the clay fraction increased linearly with the C content (r 2 = 0.86). This change in clay wettability could partly explain the higher water stability of soils rich in C.
677 citations
TL;DR: In this paper, the authors extend the validity of relationships between soil aggregation and erosion through comparisons of topsoil aggregate stability and field-assessed susceptibility to runoff and erosion, and confirm that simple laboratory determination can provide data closely correlated with those resulting from field investigations.
Abstract: The evaluation of soil susceptibility to runoff and water erosion in the field is often expensive or time-consuming. Several authors have reported that susceptibility is linked to aggregate stability, whose determination is far easier. However, this susceptibility has generally been deduced from rainfall simulation experiments on sieved soil samples, whose behaviour is not always representative of field-scale phenomena. Our aim was to extend the validity of relationships between soil aggregation and erosion through comparisons of topsoil aggregate stability and field-assessed susceptibility to runoff and erosion. Susceptibility to runoff and erosion was determined at several levels: first, on a southern French Regosol, through measurements of runoff and soil loss from 1-m2 microplots under simulated rainfall; second, from 100- to 800-m2 runoff plots on a Nitosol in Benin, a Ferralsol in Cameroon and a Regosol in Mexico (with additional data on Syria from the literature); and finally, soil susceptibility to erosion was determined through semi-quantitative assessment of the frequency of erosion features on vineyard hillsides in southern France. Aggregate stability was determined by immersion in water and wet-sieving of 2-mm sieved, air-dried 0- to 10-cm soil samples, which actually tests aggregate resistance to slaking. Under simulated rainfall, runoff depth and soil loss after 30 min were negatively correlated with topsoil content in stable macroaggregates (>0.2 mm). On runoff plots, 3-year runoff rate and soil loss were negatively correlated with topsoil aggregate stability, especially stable macroaggregate content; these correlations were improved when slope gradient and climate aggressiveness were considered in addition to aggregate stability. On vineyard hillsides, the frequency index of erosion features was negatively correlated with topsoil content in stable macroaggregates, especially in the absence of conservation practices. These results confirm that aggregate stability is a relevant indicator of soil susceptibility to runoff and erosion, especially in Mediterranean and tropical areas where intense rainfall is frequent. They also confirm that simple laboratory determination can provide data closely correlated with those resulting from field investigations.
640 citations
Cites background or methods or result from "Aggregate stability and assessment ..."
...Indeed, the water stability of macroaggregates is known to prevent detachment of easily transportable particles, and thereby surface clogging and runoff (Le Bissonnais, 1996)....
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...…resistance to slaking: indeed, slaking results from the compression of air entrapped inside rapidly wetted aggregates, and it is the main mechanism causing aggregates to disintegrate when dry soil is immersed (Le Bissonnais, 1996; Fox and Le Bissonnais, 1998), as was the case in our studies....
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...…aggregate breakdown, i.e. slaking, microcracking through differential swelling, mechanical breakdown by raindrop impact, and physicochemical dispersion, Le Bissonnais (1996) designed a method combining three treatments: fast wetting in water, slow wetting in water, and stirring after pre-wetting....
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...Aggregate stability was determined through immersion and subsequent wet-sieving of 2-mm air-dried soil samples into water, which actually tests aggregate resistance to slaking (Le Bissonnais, 1996)....
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...Le Bissonnais (1996) nevertheless considered fast wetting, i.e. immersion wetting, as the simplest way to check aggregate stability, and as a good way to compare the behaviour of large ranges of soils on fast wetting under dry conditions....
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TL;DR: In this paper, the potential of observable soil structural attributes to be used in the assessment of soil functions is evaluated and discussed from a methodological point of view and with respect to their relevance to soil functions.
602 citations
References
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TL;DR: In this article, the effectiveness of various binding agents at different stages in the structural organization of aggregates is described and forms the basis of a model which illustrates the architecture of an aggregate.
Abstract: Summary
The water-stability of aggregates in many soils is shown to depend on organic materials. The organic binding agents have been classified into (a) transient, mainly polysaccharides, (b), temporary, roots and fungal hyphae, and (c) persistent, resistant aromatic components associated with polyvalent metal cations, and strongly sorbed polymers. The effectiveness of various binding agents at different stages in the structural organization of aggregates is described and forms the basis of a model which illustrates the architecture of an aggregate. Roots and hyphae stabilize macro-aggregates, defined as > 250 μm diameter; consequently, macroaggregation is controlled by soil management (i.e. crop rotations), as management influences the growth of plant roots, and the oxidation of organic carbon. The water-stability of micro-aggregates depends on the persistent organic binding agents and appears to be a characteristic of the soil, independent of management.
5,389 citations
TL;DR: In this article, the macroaggregate-micraggregate conceptual model is applied to help explain accumulation of soil organic matter under native conditions and its loss upon cultivation in North American grassland soils.
Abstract: This work corroborates the hierarchical conceptual model for soil aggregate structure presented by Tisdall and Oades (1982), extends it to North American grassland soils, and elaborates on the aspects relating to the influence of cultivation upon losses of soil organic matter. Aggregate size distributions observed for our soils are very similar to those of Australian soils, indicating that the microaggregate-macroaggregate model may hold for a wide array of grassland soils from around the world. The use of two wetting treatments prior to sieving demonstrated that the native sod soil had the same general structural characteristics as cultivated soil but the macroaggregates were more stable. When slaked, native and cultivated soil present in the microaggregate size classes had less organic C, N, and P than the soil remaining as macroaggregates, even when expressed on a sand-free basis. Moreover, the C/N, C/P, and N/P ratios of microaggregates were narrower than those of macroaggregate size classes. Much more C and N was lost than P under the conditions of this study. Element-specific differences in microbial catabolic processes vs. extracellular enzyme activity and its attendant feedback controls are postulated to account for this difference. When the macroaggregates were crushed to the size of microaggregates, mineralizable C as a percent of the total organic C was generally greater for macro- than microaggregates early in the incubation for the cultivated soil and throughout most of the incubation for the native sod soil. Mineralizable N as a percent of the total organic N was greatest in macroaggregates even when the macroaggregates were not crushed. The macroaggregate-micraggregate conceptual model is applied to help explain accumulation of soil organic matter under native conditions and its loss upon cultivation.
2,034 citations
TL;DR: The physical nature of the erosion process was studied on carefully controlled field plats of Cecil clay located on several slopes, and losses from this strongly aggregated soil occurred primarily in the form of water stable aggregates.
Abstract: 1. The inherent weaknesses of the elutriation method of aggregate analysis are pointed out and the use of this method of aggregate analysis is questioned. 2. A mechanism is suggested to account for the slaking reaction of soils in the presence of excess water. 3. A direct method, with suitable apparatus for determining the water stable aggregate distribution in soils, is reported. 4. Several soils of the Cecil series with widely varying clay contents were found to have similar distribution of water stable aggregates. 5. Soils from different senses were found to be characterized by different distributions of water stable aggregates. 6. The physical nature of the erosion process was studied on carefully controlled field plats of Cecil clay located on several slopes. The losses from this strongly aggregated soil occurred primarily in the form of water stable aggregates. 7. Data are presented which show the effectiveness of winter legumes in controlling sheet erosion losses. The manner in which this type of "vegetative control" functions is reviewed. 8. Results on the use of various widths of strip crop for controlling sheet erosion are presented. The basic weakness of this type of "vegetative control" practice is pointed out.
1,300 citations
TL;DR: In this article, a range of disaggregating treatments from gentle to vigorous were applied to different particle sizes and the results indicated the potential role of roots and hyphae in the stabilization of larger aggregates, and for fragments of roots as nuclei for smaller aggregates.
Abstract: An Alfisol, a Mollisol and an Oxisol were fractionated into different particle sizes after a range of disaggregating treatments from gentle to vigorous. The Alfisol and the Mollisol appeared to break down in steps; macroaggregates >250 µm diameter breaking down to microaggregates 20-250 µm diameter before particles <20 µm were released. Vigorous disruption led to particle size distributions similar to those obtained by classical methods used to determine particle size distributions. The Oxisol was stable to rapid wetting treatments but when aggregate disruption was initiated by vigorous treatments particles <20 µm diameter were released and there was no evidence of aggregate hierarchy. Scanning electron microscopy of particles of different sizes showed distinctly single grain particles and aggregates. The microscopic studies indicated the potential role of roots and hyphae in the stabilization of larger aggregates, and for fragments of roots as nuclei for smaller aggregates. Plant debris was not visible in aggregates <20 µm but clay microstructure was evident. It is suggested that aggregate hierarchy occurs in Alfisols and Mollisols because organic materials are the dominant stabilizing agents in larger aggregates but in the Oxisol oxides are dominant stabilizing agents and prevent the expression of aggregate hierarchy caused by organic materials.
1,046 citations
TL;DR: The applicability of such results to conditions of natural rainfall has been thrown in doubt as discussed by the authors, and the results have been found to be affected by the drop-size and velocity of the artificial rains applied.
Abstract: Curiosity concerning the drop-size composition of natural rain has arisen from attempts to measure erodibility and infiltration-capacity by sprinkling small areas of land with artificial rain. The results have been found to be affected by the drop-size and velocity of the artificial rains applied, and the applicability of such results to conditions of natural rainfall has been thrown in doubt.
921 citations