Since the 1900s, the link between soil biotic activity, soil organic matter (SOM) decomposition and stabilization, and soil aggregate dynamics has been recognized and intensively been studied. By 1950, many studies had, mostly qualitatively, investigated the influence of the five major factors (i.e. soil fauna, microorganisms, roots, inorganics and physical processes) on this link. After 1950, four theoretical mile-stones related to this subject were realized. The first one was when Emerson [Nature 183 (1959) 538] proposed a model of a soil crumb consisting of domains of oriented clay and quartz particles. Next, Edwards and Bremner [J. Soil Sci. 18 (1967) 64] formulated a theory in which the solid-phase reaction between clay minerals, polyvalent cations and SOM is the main process leading to microaggregate formation. Based on this concept, Tisdall and Oades [J. Soil Sci. 62 (1982) 141] coined the aggregate hierarchy concept describing a spatial scale dependence of mechanisms involved in micro- and macroaggregate formation. Oades [Plant Soil 76 (1984) 319] suggested a small, but very important, modification to the aggregate hierarchy concept by theorizing the formation of microaggregates within macroaggregates. Recent research on aggregate formation and SOM stabilization extensively corroborate this modification and use it as the base for furthering the understanding of SOM dynamics. The major outcomes of adopting this modification are: (1) microaggregates, rather than macroaggregates protect SOM in the long term; and (2) macroaggregate turnover is a crucial process influencing the stabilization of SOM. Reviewing the progress made over the last 50 years in this area of research reveals that still very few studies are quantitative and/or consider interactive effects between the five factors. The quantification of these relationships is clearly needed to improve our ability to predict changes in soil ecosystems due to management and global change. This quantification can greatly benefit from viewing aggregates as dynamic rather than static entities and relating aggregate measurements with 2D and 3D quantitative spatial information.

A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics

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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.

Aggregate stability and assessment of soil crustability and erodibility: I. Theory and methodology

The long-term stabilization of soil organic matter (SOM) in tropical and temperate regions is mediated by soil biota (e.g. fungi, bacte- ria, roots and earthworms), soil structure (e.g. aggregation) and their interactions. On average, soil C turnover was twice as fast in tropical com- pared with temperate regions, but no major differences were observed in SOM quality between the two regions. Probably due to the soil mineralogy dominated by 1:1 clay minerals and oxides in tropical regions, we found a higher aggregate stability, but a lower correlation between C contents and aggregate stability in tropical soils. In addition, a smaller amount of C associated with clay and silt particles was observed in tro- pical versus temperate soils. In both tropical and temperate soils, a general increase in C levels (≈ 325 ± 113 kg C·ha -1 ·yr -1 ) was observed under no-tillage compared with conventional tillage. On average, in temperate soils under no-tillage, compared with conventional tillage, CH4 uptake (≈ 0.42 ± 0.10 kg C-CH4·ha -1 ·yr -1

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Soil organic matter, biota and aggregation in temperate and tropical soils - Effects of no-tillage

This review discusses the causes and consequences of 'non-equilibrium' water flow and solute transport in large structural pores or macropores (root and earthworm channels, fissures and interaggregate voids). The experimental evidence suggests that pores larger than c. 0.3 mm in equivalent cylindrical diameter allow rapid non-equilibrium flow. Apart from their large size and continuity, this is also due to the presence of impermeable linings and coatings that restrict lateral mass exchange. Macropores also represent microsites in soil that are more biologically active, and often more chemically reactive than the bulk soil. However, sorption retardation during transport through such pores is weaker than in the bulk soil, due to their small surface areas and significant kinetic effects, especially in larger macropores. The potential for non-equilibrium water flow and solute transport at any site depends on the nature of the macropore network, which is determined by the factors of structure formation and degradation, including the abundance and activity of soil biota such as earthworms, soil properties (e.g. clay content), site factors (e.g. slope position, drying intensity, vegetation) and management (e.g. cropping, tillage, traffic). A conceptual model is proposed that summarizes these effects of site factors on the inherent potential for non-equilibrium water flow and solute transport in macropores. Initial and boundary conditions determine the extent to which this potential is realized. High rain intensities clearly increase the strength of non-equilibrium flow in macropores, but the effects of initial water content seem complex, due to the confounding effects of soil shrinkage and water repellency. The impacts of macropore flow on water quality are most significant for relatively immobile solutes that are foreign to the soil and whose effects on ecosystem and human health are pronounced even at small leached fractions (e.g. pesticides). The review concludes with a discussion of topics where process understanding is still lacking, and also suggests some potential applications of the considerable knowledge that has accumulated in recent decades.

A review of non-equilibrium water flow and solute transport in soil macropores: principles, controlling factors and consequences for water quality

The supplies of O₂ and water and the mechanical impedance of soil are determined by soil structural form and soil water content. The range in soil water content in which limitations for plant growth associated with matric pressure, aeration, and mechanical resistance are minimal was defined as the least limiting water range (LLWR). This study was carried out to evaluate the LLWR as an index of the structural quality of soils. Undisturbed soil cores were taken from the 5- to 10-cm depth of a silt loam and a loamy sand, cropped to corn (Zea mays L.) and red clover (Trifolium pratense L.). Soil water retention, soil resistance, air-filled porosity, and bulk density (Db) were measured. Water contents at critical limits associated with field capacity (−0.01 MPa), wilting point (−1.5 MPa), air-filled porosity (10%), and soil resistance (2.0 MPa) were predicted and the LLWR calculated for each measured Db. The natural variation in Db on both soils gave rise to a wide variation in LLWR. Values of LLWR varied from 0 to 0.14 cm³ cm⁻³ for the silt loam soil and from 0.05 to 0.13 cm³ cm⁻³ for the loamy sand soil. At Db above 1.36 g cm⁻³ for the silt loam and 1.43 g cm⁻³ for the loamy sand, the LLWR declined sharply with increasing bulk density. Further research relating LLWR to crop response is required before LLWR can be recommended as a soil structural quality index for crop production.

Characterization of the least limiting water range of soils

Pores and organic matter take a multitude of forms in soil and their characteristics change in space and time following a change in tillage practices as a new “steady state” is approached. Information on the variation with depth (stratification) in the characteristics of pores and organic matter and the rates of change in these characteristics is vital to interpreting the short- and long-term impacts of a reduction of tillage on the productivity and hydrology of agricultural soils. This information is also of value in estimating the effect of a reduction in tillage on the sequestration of carbon in agricultural soils. Recent literature comparing conventional tillage (CT) with no-till (NT) in temperate agroecosystems with varying soil textures and climates was reviewed for the purpose of assessing rates of change in the magnitude and stratigraphy of bulk density, porosity, pore size classes, organic matter content and organic matter fractions. The influence of tillage on bulk density, macroporosity and organic matter content was found to be documented more extensively than the effects on pore size distribution, soil organic matter fractions and their interactions at different soil depths. Many of the reports documenting tillage-induced changes in soil porosity and organic matter were based on measurements at a specific time after initiating the tillage trial. Results obtained by different investigators were found to be most consistent when measurements were made ≥15 years after initiating the tillage trial. Data from different studies were used to generalize trends in pore and organic matter characteristics with depth and time. However, few studies provided measurements that permitted accurate prediction of either the rates of change or the maximum change that will occur following a change from CT to NT. Future research must enhance our ability to make these predictions if we wish to garner a better understanding of the effects of NT on the quality and productivity of agricultural soils and their ability to sequester carbon.

Conservation tillage and depth stratification of porosity and soil organic matter

Fractals are spatial and temporal model systems generated using iterative algorithms with simple scaling rules. This paper reviews the literature on spatial fractals as it applies to soil and tillage research. Applications of fractals in this area can be grouped into three broad categories: (i) description of soil physical properties; (ii) modeling soil physical processes; (iii) quantification of soil spatial variability. In terms of physical properties, fractals have been used to describe bulk density, pore-size distribution, pore surface area, particle-size distribution, aggregate-size distribution, ped shape and soil microtopography. In terms of physical processes, fractals have been used to model adsorption, diffusion, transport of water and solutes, brittle fracture and fragmentation. In terms of spatial variability, fractals have been applied to quantify distributions of soil properties and processes using semivariograms, power spectra and multifractal spectra. Further research is needed to investigate the specificity of different fractal models, to collect data for testing these models, and to move from the current descriptive paradigm towards a more predictive one. Fractal theory offers the possibility of quantifying and integrating information on soil biological, chemical and physical phenomena measured at different spatial scales.

Applications of fractals in soil and tillage research: a review

The least limiting water range, LLWR, is the range of soil water content within which plant growth is least limited by water potential, aeration, and mechanical resistance. Little information is available on the influence of soil property or management practice on LLWR. The LLWR calculation is based on the water release curve (WRC) and the soil resistance curve (SRC). This study tested the hypothesis that pedotransfer functions (PTF) that describe the influence of soil properties and tillage on the WRC and the SRC could be used to assess the influence of these factors on the LLWR. Thirty-two paired sampling sites were located along two parallel transects in a side by side comparison of no-till and conventional tillage. The transects crossed three soil types: Aquic Hapludalf, Psammentic Hapludalf, and Typic Hapludalf. Clay content (CLAY) varied from 5.8 to 37.4%, organic carbon (OC) varied from 9 to 39 g kg -1 , and the bulk density (D b ) varied from 0.96 to 1.71 g cm -3 . Multiple regression analyses showed that WRC was related (R 2 = 0.94) with CLAY, OC, and D b whereas the SRC was related with CLAY and OC (R 2 = 0.86). Tillage had no independent effect in either of the two functions. Values of LLWR, calculated from the PTF, varied from 0 to 0.3093 cm 3 cm -3 . The LLWR was negatively related with CLAY and D b and positively related with OC. The analyses supported the hypothesis illustrating the value of PTF in assessing the sensitivity of LLWR to soil properties.

B. D. Kay

Papers

Characterization of the least limiting water range of soils

Conservation tillage and depth stratification of porosity and soil organic matter

Applications of fractals in soil and tillage research: a review

Estimating the Least Limiting Water Range of Soils from Properties and Management

Factors Influencing Soil Structural Stability within a Growing Season