Organic matter and water-stable aggregates in soils
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.
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TL;DR: A review of the literature reveals a significant number of early studies on biochar-type materials as soil amendments either for managing pathogens, as inoculant carriers or for manipulative experiments to sorb signaling compounds or toxins as mentioned in this paper.
Abstract: Soil amendment with biochar is evaluated globally as a means to improve soil fertility and to mitigate climate change. However, the effects of biochar on soil biota have received much less attention than its effects on soil chemical properties. A review of the literature reveals a significant number of early studies on biochar-type materials as soil amendments either for managing pathogens, as inoculant carriers or for manipulative experiments to sorb signaling compounds or toxins. However, no studies exist in the soil biologyliterature that recognize the observed largevariations ofbiochar physico-chemical properties. This shortcoming has hampered insight into mechanisms by which biochar influences soil microorganisms, fauna and plant roots. Additional factors limiting meaningful interpretation of many datasets are the clearly demonstrated sorption properties that interfere with standard extraction procedures for soil microbial biomass or enzyme assays, and the confounding effects of varying amounts of minerals. In most studies, microbial biomass has been found to increase as a result of biochar additions, with significant changes in microbial community composition and enzyme activities that may explain biogeochemical effects of biochar on element cycles, plant pathogens, and crop growth. Yet, very little is known about the mechanisms through which biochar affects microbial abundance and community composition. The effects of biochar on soil fauna are even less understood than its effects on microorganisms, apart from several notable studies on earthworms. It is clear, however, that sorption phenomena, pH and physical properties of biochars such as pore structure, surface area and mineral matter play important roles in determining how different biochars affect soil biota. Observations on microbial dynamics lead to the conclusion of a possible improved resource use due to co-location of various resources in and around biochars. Sorption and therebyinactivation of growth-inhibiting substances likelyplaysa rolefor increased abundance of soil biota. No evidence exists so far for direct negative effects of biochars on plant roots. Occasionally observed decreases in abundance of mycorrhizal fungi are likely caused by concomitant increases in nutrient availability,reducing theneedfor symbionts.Inthe shortterm,therelease ofavarietyoforganic molecules from fresh biochar may in some cases be responsible for increases or decreases in abundance and activity of soil biota. A road map for future biochar research must include a systematic appreciation of different biochar-types and basic manipulative experiments that unambiguously identify the interactions between biochar and soil biota.
3,612 citations
Cites background from "Organic matter and water-stable agg..."
...Biochar “aggregates” may provide similar functions such as protection of organic matter, habitat for soil biota, or retention of soil moisture and nutrients as described for aggregates made from minerals and organic matter (Tisdall and Oades, 1982)....
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TL;DR: 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 as discussed by the authors.
Abstract: 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.
3,301 citations
Cites background from "Organic matter and water-stable agg..."
...Physical protection by aggregates is indicated by the positive influence of aggregation on the accumulation of SOM (e.g. Edwards and Bremner, 1967; Elliott, 1986; Jastrow, 1996; Tisdall and Oades, 1982; Six et al., 2000a)....
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...These larger silt-sized aggregates have more C per unit material because additional C binds the primary organomineral complexes into silt-sized aggregates (Tisdall and Oades, 1982)....
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TL;DR: In this paper, soil organic carbon (SOC), biota, ionic bridging, clay and carbonates are associated with aggregation by rearrangement, flocculation and cementation.
3,241 citations
Cites background from "Organic matter and water-stable agg..."
...Polysaccharides are readily mineralizable and act as transient binding agents , inert carbon; CL, labile fraction; CR, recalcitrant C. initiating aggregation, but may not have long-term stability (Kay, 1998; Tisdall and Oades, 1982)....
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...Lower bacteria/ fungi ratio in macroaggregates than microaggregates suggests that bacterial activity may dominate in microaggregation while fungal activity dominates in macroaggregate formation (Schutter and Dick, 2002; Tisdall and Oades, 1982)....
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...Aggregates are formed in stages, with different bonding mechanisms dominating at each stage (Tisdall and Oades, 1982)....
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...These are often grouped by size: macroaggregates (>250 Am) and microaggregates ( 250 Am) with these groups being further divided by size (Tisdall and Oades, 1982)....
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...These are often grouped by size: macroaggregates (>250 Am) and microaggregates ( < 250 Am) with these groups being further divided by size (Tisdall and Oades, 1982)....
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TL;DR: In this article, 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.
Abstract: 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.
3,134 citations
TL;DR: In this paper, the authors present a modeling approach aimed at seasonal resolution of global climatic and edaphic controls on patterns of terrestrial ecosystem production and soil microbial respiration using satellite imagery (Advanced Very High Resolution Radiometer and International Satellite Cloud Climatology Project solar radiation), along with historical climate (monthly temperature and precipitation) and soil attributes (texture, C and N contents) from global (1°) data sets as model inputs.
Abstract: This paper presents a modeling approach aimed at seasonal resolution of global climatic and edaphic controls on patterns of terrestrial ecosystem production and soil microbial respiration. We use satellite imagery (Advanced Very High Resolution Radiometer and International Satellite Cloud Climatology Project solar radiation), along with historical climate (monthly temperature and precipitation) and soil attributes (texture, C and N contents) from global (1°) data sets as model inputs. The Carnegie-Ames-Stanford approach (CASA) Biosphere model runs on a monthly time interval to simulate seasonal patterns in net plant carbon fixation, biomass and nutrient allocation, litterfall, soil nitrogen mineralization, and microbial CO2 production. The model estimate of global terrestrial net primary production is 48 Pg C yr−1 with a maximum light use efficiency of 0.39 g C MJ−1PAR. Over 70% of terrestrial net production takes place between 30°N and 30°S latitude. Steady state pools of standing litter represent global storage of around 174 Pg C (94 and 80 Pg C in nonwoody and woody pools, respectively), whereas the pool of soil C in the top 0.3 m that is turning over on decadal time scales comprises 300 Pg C. Seasonal variations in atmospheric CO2 concentrations from three stations in the Geophysical Monitoring for Climate Change Flask Sampling Network correlate significantly with estimated net ecosystem production values averaged over 50°–80° N, 10°–30° N, and 0°–10° N.
2,398 citations
References
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TL;DR: In this article, data from the Rothamsted classical field experiments on the effects of long-continued cropping and manuring on the amount of organic matter in soil, on the age of this soil organic matter, and on the number of microbial biomass in the soil.
Abstract: Data are assembled from the Rothamsted classical field experiments on the effects of long-continued cropping and manuring on the amount of organic matter in soil, on the age of this soil organic matter, on the amount of microbial biomass in the soil, and on the rate at which plant residues d
1,157 citations
TL;DR: With improved techniques, very striking effects of inoculation on plant growth and phosphate uptake have been demonstrated beyond doubt, and this has led to studies of the uptake mechanism and the source of the extra phosphate.
Abstract: The study of vesicular-arbuscular (VA) mycorrhiza is expanding rapidly. Since Gerdemann's review in 1968, over a hundred papers have been pub lished. During previous five year periods numbers were: 14 (1930-4), 22 1935-9), 17 (1948-52), 43 (1953-7), 56 (1958-62) and 40 (1963-7). These are small numbers considering how long it has been known that VA mycorrhiza are probably the most widespread root infections of plants. With some justification they have been described as the "mal aimee des microbiolo gistes" (22). The increase in publications has been accompanied by a shift in subject matter. Most papers, until recently, described the anatomy and recorded the occurrence of VA mycorrhiza, and many efforts were made to culture the fungi; since 1968, 37 papers have dealt with effects of the infection on plant growth. Several factors probably account for the increased popularity of the sub ject. The long-standing speculation about the identity of VA endophytes (47, 56) has largely been resolved in favor of one or another species of Endogone (32,46,95). Very impure inocula consisting of infected roots or of soil con taining a normal population of other soil micro-organisms, have been re placed by Endogone spores, sporocarps, or "sterilized" soil inoculated with them in the presence of a host plant. Such inocula now regularly produce typical VA infections in experimental plants. With improved techniques, very striking effects of inoculation on plant growth and phosphate uptake have . been demonstrated beyond doubt, and this has led to studies of the uptake mechanism and the source of the extra phosphate. These results have also stimulated interest in the fungi themselves, their ecology and taxonomy. The technique of clearing roots in KOH before staining (24, 77, 120, 121) is now
657 citations
TL;DR: In this paper, a theory is proposed depicting microaggregate formation as a solid-phase reaction involving linkage of electrically neutral clay mineral and organic matter particles by polyvalent metals on exchange sites, and micro aggregates disruption by sonic vibration as a reversal of this reaction.
Abstract: Summary
Studies of the dispersion of compound soil particles by sonic vibration and cation-exchange resin techniques indicate that the difficultly dispersible particles in mineral soils of high base status are microaggregates (< 250μ diam) consisting largely of clay and humified organic material linked by polyvalent metals. The inter-particle bonds in these microaggregates can be disrupted by application of mechanical energy (sonic vibration or prolonged shaking with water), the amount of energy required for dispersion of clay-size mineral material being reduced by treatments that weaken or destroy these bonds (e.g. treatments leading to replacement of polyvalent metals by monovalent metals or to destruction of organic matter). A theory is proposed depicting microaggregate formation as a solid-phase reaction involving linkage of electrically neutral clay mineral and organic matter particles by polyvalent metals on exchange sites, and microaggregate disruption by sonic vibration as a reversal of this reaction. Experiments to evaluate this theory are described.
524 citations
TL;DR: In this paper, the bonding mechanisms involved in clay-organic complexes, the nature of some clayorganic complexes and reactions, and the nature and importance of clays-organic complex in soils.
Abstract: Publisher Summary Clays interact with many organic compounds to form complexes of varying stabilities and properties. These interactions are of great importance in nature and in industry. The clays in soils and sediments often have organic material with which they are intimately associated. This chapter describes the bonding mechanisms involved in clay-organic complexes, the nature of some clay-organic complexes and reactions, and the nature and importance of clay-organic complexes in soils. The dominant factors determining the nature of clay-organic interactions are the properties of the organic molecule, the water content of the system, the nature of the exchangeable cation on the clay surface, and the unique properties of the clay mineral structures. The exchangeable cations determine the surface acidity, and therefore the possibilities of protonation of the organic compound. Much remains to be learned regarding the clay-organic systems and the reactions taking place at this interface. In particular, it is quite likely that many reactions that have not yet been recognized are catalyzed by the clay minerals.
512 citations
TL;DR: The root system of ryegrass was more efficient than that of white clover in stabilizing aggregates of Lemnos loam because the hyphae were covered with a layer of amorphous material, probably polysaccharide, to which clay particles appear firmly attached.
Abstract: The root system of ryegrass was more efficient than that of white clover in stabilizing aggregates of Lemnos loam because ryegrass supported a larger population of vesicular-arbuscular mycorrhizal hyphae in the soil. Electron micrographs show that the hyphae were covered with a layer of amorphous material, probably polysaccharide, to which clay particles appear firmly attached.
479 citations