The relationship between soil structure and the ability of soil to stabilize soil organic matter (SOM) is a key element in soil C dynamics that has either been overlooked or treated in a cursory fashion when developing SOM models. The purpose of this paper is to review current knowledge of SOM dynamics within the framework of a newly proposed soil C saturation concept. Initially, we distinguish SOM that is protected against decomposition by various mechanisms from that which is not protected from decomposition. Methods of quantification and characteristics of three SOM pools defined as protected are discussed. Soil organic matter can be: (1) physically stabilized, or protected from decomposition, through microaggregation, or (2) intimate association with silt and clay particles, and (3) can be biochemically stabilized through the formation of recalcitrant SOM compounds. In addition to behavior of each SOM pool, we discuss implications of changes in land management on processes by which SOM compounds undergo protection and release. The characteristics and responses to changes in land use or land management are described for the light fraction (LF) and particulate organic matter (POM). We defined the LF and POM not occluded within microaggregates (53–250 μm sized aggregates as unprotected. Our conclusions are illustrated in a new conceptual SOM model that differs from most SOM models in that the model state variables are measurable SOM pools. We suggest that physicochemical characteristics inherent to soils define the maximum protective capacity of these pools, which limits increases in SOM (i.e. C sequestration) with increased organic residue inputs.

Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils

http://www.tinread.usarb.md:8888/tinread/fulltext/lal/soil_structure.pdf

Soil structure and management: a review

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

I. CONTEXT * The Ecosystem Concept * Earth's Climate System * Geology and Soils * II. MECHANISMS * Terrestrial Water and Energy Balance * Carbon Input to Terrestrial Ecosystems * Terrestrial Production Processes * Terrestrial Decomposition * Terrestrial Plant Nutrient Use * Terrestrial Nutrient Cycling * Aquatic Carbon and Nutrient Cycling * Trophic Dynamics * Community Effects on Ecosystem Processes * III. PATTERNS * Temporal Dynamics * Landscape Heterogeneity and Ecosystem Dynamics * IV. INTEGRATION * Global Biogeochemical Cycles * Managing and Sustaining Ecosystem * Abbreviations * Glossary * References

Principles of Terrestrial Ecosystem Ecology

Organic acids, such as malate, citrate and oxalate, have been proposed to be involved in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. A full assessment of their role in these processes, however, cannot be determined unless the exact mechanisms of plant organic acid release and the fate of these compounds in the soil are more fully understood. This review therefore includes information on organic acid levels in plants (concentrations, compartmentalisation, spatial aspects, synthesis), plant efflux (passive versus active transport, theoretical versus experimental considerations), soil reactions (soil solution concentrations, sorption) and microbial considerations (mineralization). In summary, the release of organic acids from roots can operate by multiple mechanisms in response to a number of well-defined environmental stresses (e.g., Al, P and Fe stress, anoxia): These responses, however, are highly stress- and plant-species specific. In addition, this review indicates that the sorption of organic acids to the mineral phase and mineralisation by the soil's microbial biomass are critical to determining the effectiveness of organic acids in most rhizosphere processes.

Organic acids in the rhizosphere: a critical review

Knowledge of plant root responses to both favorable and unfavorable soil conditions is fundamental to our understanding of the complex root soil interface. One of the greatest hindrances to the frequent measurements of the morphological and physiological responses of plant roots to soil environmental conditions has been the absence of an inexpensive method for quantitatively separating the soil from roots and other biological materials. The objectives of this study were to develop an inexpensive and quantitative method for separating roots from soils of field and greenhouse experiments and to determine the influence of soil type, pretreatment and plant type on the efficiency of separation. A mechanized system which separated roots from soil materials was constructed which combines the kinetic energy of pressurized spray jets and the low energy of air flotation. Root elutriation systems were fabricated from commercially available components at a cost of less than $25 per unit excluding labor. A manifold of these units was assembled to increase operator efficiency. Quantitative separation of roots is achieved by a closed system of mechanical separations using water and air to isolate and deposit roots on a submerged sieve. Washing times range from 3 to 10 min and is a function of soil texture, plant species, concentration of the dispersing agent and soaking time. Utilizing a manifold of nine separation units and two personnel, an average of 60 samples could be separated per hour. Water consumption per unit ranges from 2.1 to 3.4 liters per minute. This method, in combination with a powered soil sampler, appears to provide a rapid, quantitative and inexpensive approach for measuring plant root responses to soil biological, chemical, and physical stresses. Please view the pdf by using the Full Text (PDF) link under 'View' to the left. Copyright © . . Quantitative Separation of Roots from Compacted Soil Profiles by the Hydropneumatic Elutriation System 1

Quantitative Separation of Roots from Compacted Soil Profiles by the Hydropneumatic Elutriation System1

The dynamics of soil organic carbon (SOC) play an important role in long-term ecosystem productivity and the global C cycle. We used extended laboratory incubation and acid hydrolysis to analytically determine SOC pool sizes and fluxes in US Corn Belt soils derived from both forest and prairie vegetation. Measurement of the natural abundance of 13 C made it possible to follow the influence of continuous corn on SOC accumulation. The active pools (Ca) comprised 3 to 8% of the SOC with an average field mean residence time (MRT) of 100 d. The slow pools (Cs) comprised 50% of SOC in the surface and up to 65% in subsoils. They had field MRTs from 12‐28 y for C4-C and 40‐80 y for C3-derived C depending on soil type and location. Notill management increased the MRT of the C3-C by 10‐15 y above conventional tillage. The resistant pool (Cr) decreased from an average of 50% at the surface to 30% at depth. The isotopic composition of SOC mineralized during the early stages of incubation reflected accumulations of labile C from the incorporation of corn residues. The CO2 released later reflected 13 C characteristic of the Cs pool. The 13 C of the CO2 did not equal that of the whole soil until after 1000 d of incubation. The SOC dynamics determined by acid hydrolysis, incubation and 13 C content were dependent on soil heritage (prairie vs. forest), texture, cultivation and parent material, depositional characteristics. Two independent methods of determining C3 pool sizes derived from C3 vegetation gave highly correlated values. # 2000 Elsevier Science Ltd. All rights reserved.

/pdf/soil-carbon-pools-and-fluxes-in-long-term-corn-belt-2fuspfxxn8.pdf

Soil carbon pools and fluxes in long-term corn belt agroecosystems

Pore network geometries of intra-aggregate pore spaces are of great importance for water and ion flux rates controlling C sequestration and bioremediation. Advances in non-invasive three-dimensional imaging techniques such as synchrotron-radiation-based x-ray microtomography (SR-pCT), offer excellent opportunities to study the interrelationships between pore network geometry and physical processes at spatial resolutions of a few micrometers. In this paper we present quantitative three-dimensional pore-space geometry analyses of small scale (∼5 mm across) soil aggregates from different soil management systems (conventionally tilled vs. grassland). Reconstructed three-dimensional microtomography images at approximate isotropic voxel resolutions between 3.2 and 5.4 pm were analyzed for pore-space morphologies using a suite of image processing algorithms associated with the software published by Lindquist et al. Among the features quantified were pore-size distributions (PSDs), throat-area distributions, effective throat/pore radii ratios as well as frequency distributions of pore channel lengths, widths, and flow path tortuosities. We observed differences in storage and transport relevant pore-space morphological features between the two aggregates. Nodal pore volumes and throat surface areas were significantly smaller for the conventionally tilled (Conv. T) aggregate (mode ≈ 7.9 x 10 -7 mm 3 /≈ 63 μm 2 ) than for the grassland aggregate (mode ≈ 5.0 x 10 -6 mm 3 /≈ 400 μm 2 ), respectively. Path lengths were shorter for the Conv. T. aggregate (maximum lengths 600 μm). In summary, the soil aggregate from the Conv. Tsite showed more gas and water transport limiting micromorphological features compared with the aggregate from the grassland management system.

Three-Dimensional Quantification of Intra-Aggregate Pore-Space Features using Synchrotron-Radiation-Based Microtomography

The aim of this paper is to clarify the effect of soil aggregation on physical and chemical properties of structured soils both on a bulk soil scale, for single aggregates, as well as for homogenized material. Aggregate formation and aggregate strength depend on swelling and shrinkage processes and on biological activity and kinds of organic exudates as well as on the intensity, number and time of swelling and drying events. Such aggregates are more dense than the bulk soil. The intra-aggregate pore distribution consists not only of finer pores, but these are also more tortuous. Therefore, water fluxes in aggregated soils are mostly multidimensional and the corresponding water fluxes in the intra-aggregate pore system are much smaller. Furthermore, ion transport both by mass flow and diffusion are delayed, whereby the length of the follow path in such tortuous finer pores further retards chemical exchange processes. The rearrangement of particles by aggregate formation also induces an increased apparent thermal diffusivity as compared with the homogenized material. The aggregate formation also affects the aeration and the gaseous composition in the intra-aggregate pore space. Depending on the kind and intensity of aggregation, the intra-aggregate pores can be completely anoxic, while the inter-aggregate pores are already completely aerated. The possibility to predict physical properties on these various scales depends on the rigidity of the pore system. In general, this rigidity depends on the above-mentioned physical and chemical processes both with respect to intensity and frequency. Thus, aggregates like subangular block or plates are the most rigid systems, while physical processes in the seedbed cannot be modelled using existing approaches due to the low strength and consequent instability of the pore system.

Structure formation and its consequences for gas and water transport in unsaturated arable and forest soils

Root growth and soil water content were measured in a field experiment with wheat subjected to two periods of water deficit The first period was induced early in the season between the early vegetative stage (22 DAS) and late terminal spikelet (50 DAS), the second period at mid-season between terminal spikelet (42 DAS) and anthesis (74 DAS) Total root growth was reduced under water deficit by a reduction in the top 30 cm, while the root system continued to grow in the deeper soil profile between 30 and 60 cm Shortly after rewatering, the growth pattern reverted to fastest root growth rates in the shallow soil layers In relative terms, the total root system increased in relation to the above ground dry matter under water shortage The early-, the mid-season water deficit treatments, and the control treatment had total root length of 274, 194 and 306 km m-2, respectively, about 2 wk before maturity Evapotranspiration declined under water deficit, but water uptake in deeper layers increased Water uptake per unit root length was reduced with water deficit and was still low shortly after rewatering Remarkable was the increase in water uptake at 2–3 weeks after rewatering, both deficit treatments exceeded the control by almost 100% This increase in water uptake followed the burst of new root growth in the upper regions of the soil However, water uptake rates subsequently declined towards maturity, being between 015 L km-1 d-1 and 017 L km-1 d-1 for the early and mid-season water deficit treatments, slightly higher than the control, 012 L km-1 d-1 The results showed that the crop subjected to early water deficit could compensate for some of the reductions in root growth during subsequent rewatering, but the impact of the mid-season water deficit treatment was more severe and permanent

Alvin J. M. Smucker

Papers

Quantitative Separation of Roots from Compacted Soil Profiles by the Hydropneumatic Elutriation System1

Soil carbon pools and fluxes in long-term corn belt agroecosystems

Three-Dimensional Quantification of Intra-Aggregate Pore-Space Features using Synchrotron-Radiation-Based Microtomography

Structure formation and its consequences for gas and water transport in unsaturated arable and forest soils

Root growth and water uptake during water deficit and recovering in wheat