J.M. Oades

Soil organic matter and structural stability: mechanisms and implications for management

SummaryThe stability of pores and particles is essential for optimum growth of plants. Two categories of aggregates macro- (> 250 μm) and micro- (<250 μm) depend on organic matter for stability against disruptive forces caused by rapid wetting. Dispersion of clay particles from microaggregates is promoted by adsorption of complexing organic acids which increase the negative charge on clays. The acids are produced by plants, bacteria and fungi. However, the dispersibility of clay in microaggregates is offset by the binding action of polysaccharides, mainly mucilages produced by bacteria, but also by plant roots and fungal hyphae. The stability of microaggregates is also enhanced by multivalent cations which act as bridges between organic colloids and clays. Macroaggregates are enmeshed by plant roots, both living and decomposing, and are thus sensitive to management, and increase in number when grasses are grown and the soil is not disturbed. Lack of root growth,i.e. fallow, has the opposite effect. Various implications for management of soil structure are discussed.

The retention of organic matter in soils

The turnover of C in soils is controlled mainly by water regimes and temperature, but is modified by factors such as size and physicochemical properties of C additions in litter or root systems, distribution of C throughout the soil as root systems, or addition as litter, distribution of C within the soil matrix and its interaction with clay surfaces.Soil factors which retard mineralization of C in soils are identified from correlations of C contents of soils with other properties such as clay content and base status. The rate and extent of C mineralization depends on the chemistry of the added organic matter and interaction with clays of the microbial biomass and metabolites.The organomineral interactions are shown to depend on cation bridges involving mainly Ca in neutral to alkaline soils, Al in acid soils and adsorption of organic materials on iron oxide surfaces. The various organomineral interactions lead to aggregations of clay particles and organic materials, which stabilizes both soil structure and the carbon compounds within the aggregates.

The role of biology in the formation, stabilization and degradation of soil structure

Abstract Soil structure is defined as the arrangement of particles and associated pores in soils across the size range from nanometres to centimetres. Biologic influences can be demonstrated in the formation and stabilization of aggregates but it is necessary to distinguish clearly between those forces or agencies which create aggregations of particles and those which stabilize or degrade such aggregations. The formation of soil structure involves the physical forces of shrinking and swelling created by changes in water status of soils, freezing and thawing, tillage, or by movement of the larger biota in soils. Expansive properties of soils are controlled by the clay content. Thus changes of structural organisation are minimal in sands and maximal in clays. Plant roots, earthworms and other macrofauna large enough to move soil particles create pores recognisable by cylindrical shapes and smooth curved surfaces. Various visual and microscopic techniques aided by dyes are available to demonstrate the extent of biovoids in soils. Biology plays a major role in stabilization of soil structure. The major factors vary depending on the scale of soil structure. At larger scales plant roots and associated hyphae can be seen to enmesh soil particles by acting as a “sticky string bag”. At the microscale the influence of mucilages from roots, hyphae, bacteria and fauna such as earthworms can be shown by a range of microscopic techniques to be involved in stabilizing smaller aggregates and the linings of biopores. Techniques include optical and fluorescence microscopy, scanning electron microscopy including EDAX, transmission electron microscopy using heavy metals or other electron dense staining techniques for specific chemical compounds, and computer aided tomography. The microscopic techniques can be used on individual aggregates, stabilized soils, sections or separates of soils. Both microflora and fauna are involved in the degradation of stabilizing agents. Fauna may comminute roots and hyphae which stabilized larger aggregates and microorganisms utilize mucilaginous stabilizing agents as an energy source resulting in a slow breakdown of structural stability. Such effects can be established by combinations of studies of aggregation including microscopy. Further destruction of structure is caused by tillage and compaction by vehicles and animals.

The chemistry and nature of protected carbon in soil

The nature of organic carbon in the < 2, 2–20, 20–53, 53–200, and 200–2000 mu m fractions of four surface soils was determined using solid state 13C nuclear magnetic resonance (n.m.r.) spectroscopy with cross polarisation and magic angle spinning (CP/MAS). Analyses were repeated after high energy ultraviolet photo-oxidation was performed on the three finest fractions. All four soils, studied contained appreciable amounts of physically protected carbon while three of the soils contained even higher amounts of charcoal. It was not possible to measure the charcoal content of soils directly, however, after photo-oxidation, charcoal remained and was identified by its wood-like morphology revealed by scanning electron microscopy (SEM) together with a highly aromatic chemistry determined by solid state 13C n.m.r. Charcoal appears to be the major contributor to the 130 ppm band seen in the n.m.r. spectra of many Australian soils. By using the aromatic region in the n.m.r. spectra, an approximate assessment of the charcoal distribution through the size fractions demonstrated that more than 88% of the charcoal present in two of the soils occurred in the < 53 µm fractions. These soils contained up to 0.8 g C as charcoal per 100 g of soil and up to 30% of the soil carbon as charcoal. Humic acid extractions performed on soil fractions before and after photo-oxidation suggest that charcoal or charcoal-derived material may also contribute significantly to the aromatic signals found in the n.m.r. spectra of humic acids. Finely divided charcoal appears to be a major constituent of many Australian soils and probably contributes significantly to the inert or passive organic carbon pool recognised in carbon turnover models.

Aspects of the chemical structure of soil organic materials as revealed by solid-state13C NMR spectroscopy

Solid-state cross-polarisation/magic-angle-spinning3C nuclear magnetic resonance (CP/MAS13C NMR) spectroscopy was used to characterise semi-quantitatively the organic materials contained in particle size and density fractions isolated from five different mineral soils: two Mollisols, two Oxisols and an Andosol. The acquired spectra were analysed to determine the relative proportion of carboxyl, aromatic, O-alkyl and alkyl carbon contained in each fraction. Although similar types of carbon were present in all of the fractions analysed, an influence of both soil type and particle size was evident.The chemical structure of the organic materials contained in the particle size fractions isolated from the Andosol was similar; however, for the Mollisols and Oxisols, the content of O-alkyl, aromatic and alkyl carbon was greatest in the coarse, intermediate and fine fractions, respectively. The compositional differences noted in progressing from the coarser to finer particle size fractions in the Mollisols and Oxisols were consistent with the changes noted in other studies where CP/MAS13C NMR was used to monitor the decomposition of natural organic materials. Changes in the C:N ratio of the particle size fractions supported the proposal that the extent of decomposition of the organic materials contained in the fine fractions was greater than that contained in the coarse fractions. The increased content of aromatic and alkyl carbon in the intermediate size fractions could be explained completely by a selective preservation mechanism; however, the further accumulation of alkyl carbon in the clay fractions appeared to result from both a selective preservation and anin situ synthesis.The largest compositional differences noted for the entire organic fraction of the five soils were observed between soil orders. The differences within orders were smaller. The Mollisols and the Andosol were both dominated by O-alkyl carbon but the Andosol had a lower alkyl carbon content. The Oxisols were dominated by both O-alkyl and alkyl carbon.A model describing the oxidative decomposition of plant materials in mineral soils is proposed and used to explain the influence of soil order and particle size on the chemical composition of soil organic matter in terms of its extent of decomposition and bioavailability.

Assessing the extent of decomposition of natural organic materials using solid-state 13C NMR spectroscopy

Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy has become an important tool for examining the chemical structure of natural organic materials and the chemical changes associated with decomposition. In this paper, solid-state 13C NMR data pertaining to changes in the chemical composition of a diverse range of natural organic materials, including wood, peat, composts, forest litter layers, and organic materials in surface layers of mineral soils, were reviewed with the objective of deriving an index of the extent of decomposition of such organic materials based on changes in chemical composition. Chemical changes associated with the decomposition of wood varied considerably and were dependent on a strong interaction between the species of wood examined and the species composition of the microbial decomposer community, making the derivation of a single general index applicable to wood decomposition unlikely. For the remaining forms of natural organic residues, decomposition was almost always associated with an increased content of alkyl C and a decreased content of O-alkyl C. The concomitant increase and decrease in alkyl and O-alkyl C contents, respectively, suggested that the ratio of alkyl to O-alkyl carbon (A/O-A ratio) may provide a sensitive index of the extent of decomposition. Contrary to the traditional view that humic substances with an aromatic core accumulate as decomposition proceeds, changes in the aromatic region were variable and suggested a relationship with the activity of lignin-degrading fungi. The A/O-A ratio did appear to provide a sensitive index of extent of decomposition provided that its use was restricted to situations where the organic materials were derived from a common starting material. In addition, the potential for adsorption of highly decomposable materials on mineral soil surfaces and the impacts which such an adsorption may have on bioavailability required consideration when the A/O-A ratio was used to assess the extent of decomposition of organic materials found in mineral soils.

Fractionation of organo-mineral complexes by sedimentation and density techniques

Soil samples were fractionated by sedimentation in water and by flotation in heavy liquids to separate complexed and uncomplexed organic and inorganic components. Flocculation of clays in heavy organic liquids was delayed by addition of a surfactant. Heavy liquids and surfactants sorbed by soil components were removed by washing with acetone-water mixtures. In a sample of a red-brown earth, the organic carbon and nitrogen contents were highest in the finest separates. In samples of a ground-water rendzina and a chernozemic soil, the coarse clay and silt separates had the highest organic carbon and nitrogen contents. Organic matter was concentrated in low density fractions in all separates. Carbon/nitrogen ratios were lowest in the finer and heavier separates. Calcium, and to a lesser extent manganese, iron and phosphorus, were concentrated in low density fractions: thus these elements appear to be associated with organic matter and may be important in organo-mineral complex formation. Carbonates, titanium, iron, silicon and potassium were concentrated at the highest densities. Organic fractions < 2.06 g cm−3 from sand size separates were insoluble in alkali and had wide carbon/nitrogen ratios characteristic of plant debris. The light fractions from fine silt and coarse clay separates were more soluble in alkali but showed high ratios of humic to fulvic materials and high absorption at 280 nm. Such materials were considered to be microbial cell debris and were associated with high contents of disordered aluminium and iron oxides and expanding lattice silicates in 1 to 5 μm aggregates. Heavier fractions, particularly of finer clay separates, contained more fulvic and humic materials of a more aliphatic nature than those in 2.06 g cm−3. Such minerals plus quartz and feldspars were associated with minor amounts of organic matter or possibly were not involved in organo-mineral associations.

Physical factors influencing decomposition of organic materials in soil aggregates

Glucose or starch labelled with 14C was mixed thoroughly into slurried soils. Aggregates of different sizes were obtained from the soils as they dried. The labelled substrates were considered to be distributed in both micro- and macropores in the aggregates. Control samples (labelled substrates in macropores only) were prepared by adding the labelled carbohydrates after the formation of the aggregates. The various samples were sterilized by γ-irradiation and stored at −15°C. Samples were wetted to about −20kPa, inoculated with soil organisms, and incubated for 4 weeks at 28°C in closed systems, which enabled regular measurement of 14CO2 released. Based on the 14CO2 released, it was concluded that starch was protected from microbial attack when present in micropores in aggregates made from fine sandy loam. After incubation samples were dried and rewetted. The flush of 14CO2 released was twice as big for samples containing labelled starch compared with glucose, showing that disruption of aggregates, containing residual starch, and rearrangements of soil components are as important as chemical and biological factors in causing the flush of CO2 resulting from wetting a soil. Mechanical disruption of the aggregates resulted in a similar flush of 14CO2.

Input of carbon to soil from wheat plants

Abstract The growth of wheat plants and the distribution of labelled photosynthate from pulse-labelling with 14 CO 2 were measured periodically during the growing season in the field. During early growth there was approximately the same proportion of photosynthate translocated below ground and retained in the shoots. Of the 14 C below ground about a half was respired and a quarter each was in the soil and roots. This distribution changed exponentially during growth with an increasing proportion of 14 C remaining in the shoots and a corresponding decreasing proportion being translated below ground, which was only a few percent by flowering. From this information the total input of carbon to the soil from the crop was calculated to be 1305 kg Cha −1 . Comparison of these estimates with those from previous experiments suggest that differences do occur due to the stage of growth of the plant, the environmental conditions, soil type and microbial activity.

Microbial biomass formed from 14C, 15N-labelled plant material decomposing in soils in the field

Abstract The decomposition of 14C, 14N-labelled medic (Medicago littoralis) material and the net formation and decay of isotope-labelled biomass have been measured in four South Australian soils in the field over 4 yr. The field sites were in similar climatic zones but two sites received about twice as much rainfall as the others. The soils were calcareous and of similar pH, but differed in texture and organic matter content. The decomposition of the organic-14C and organic-15N residues were, for a given site, similar. Initially, the concentrations of labelled residues decreased rapidly, then very slowly. Decomposition rates in a heavy clay soil were significantly less than in the other soils during the first 16 weeks after incorporation of plant material, but thereafter, rates of decomposition in all soils were similar, despite differences in soil texture and climate. More than 50% of the medic-14C had disappeared from all soils after 4 weeks of decomposition and only 15–20% of the medic-14C remained as organic residues after 4 yr. Of the medic-15N 60–65% remained as organic residues after 32 weeks decomposition; the percentage decreased to 45–50% after 4 yr. The amounts of 14C, 14N-labelled biomass, formed from decomposing plant material, were maximal 4–8 weeks after incorporation of plant material into the soils. In samples taken at 8 weeks from the sandy Roseworthy soil, biomass-14C and -15N accounted for 14 and 22% respectively of the total organic-14C and -15N residues present. Thereafter in this soil, the concentrations of biomass-14C and -15N decreased, rapidly at first then more slowly. Nevertheless, throughout most of the decomposition the rates of decrease in the concentrations of biomass-14C and -15N exceeded those of the non-biomass, labelled organic residues. The proportions of 14C, 15N-labelled materials accounted for in the labelled biomass varied between soils. Soils of higher clay content generally retained higher proportions of residual organic-14C and -14N in the biomass, even though the net rates of decomposition of total labelled residues did not differ significantly between soils during most of the decomposition.