Soil Particulate Organic Matter Research Articles

Minimisation of organic matter disruption during particle-size fractionation of grassland epipedons

In order to be able to avoid artefacts during fractionation of soil into primary particles, knowledge is required about the soil organic matter (SOM) behaviour when dispersing soil aggregates from different environments. This study was designed to investigate dispersion behaviour of macro- and microaggregates of native grassland epipedons from different climatic regions. Samples were collected from eight grassland epipedons to a depth of 10 cm along a climosequence in the prairie from Central Saskatchewan, Canada, to Southern Texas, USA. The samples were sieved to 2 mm and ultrasonically dispersed at 0, 1, 5, 15, 25, and 75 kJ, with partial dispersion also at 2 and 3 kJ. Carbon and nitrogen analysis was performed on the <20 μm, 20–250 μm and 250–2000 μm size separates of all dispersion treatments. Dispersion of macroaggregates (250–2000 μm) was achieved at 1 kJ ultrasonic energy for most of the sites whereas 3 kJ were needed for dispersing macroaggregates at wet extremes in the prairie. More ultrasonic energy (≥5 kJ) was required to disperse microaggregates (20–250 μm). Intense dispersion (>5 kJ), however, disrupted particulate organic matter. Ultrasonic dispersion at 75 kJ was different for different ultrasonic instruments, suggesting that the calorimetric calibration was not very suitable for standardisation of ultrasonic energy input. For minimisation of artefacts, it is suggested that particulate SOM should be removed after weak dispersion (≥3 kJ), and, prior to subsequent aggregate disruption at 22 kJ, input energy should be checked at intervals by ultrasonic pressure measurements to determine whether the tip of the probe has deteriorated.

Molecular Modelling of Humic Substances, Soil Organic Matter and Soil Particles: Potential and Limits

The combination of pyrolysis-field ionization mass spectrometry (Py-FIMS) and Curie-point pyrolysisgas chromatography/mass spectrometry (Py-GC/MS) together with a wide variety of analytical (chemical, thermal, spectroscopic) techniques has been utilized in an integrated approach to model molecular structures for humic acids (HA), fulvic acids (FA), total humic substance (THS), dissolved organic matter (DOM), soil organic matter (SOM) and soil particles. Personal computer-based molecular modelling of humic-xenobiotic complexes and computational chemistry (molecular-mechanics calculations; geometry optimization; determination of binding sites; transport-, energy-balances, quantitative structure-activity relationships) are reported. Examples for atrazine-, hydroxyatrazine-, DDT-, mecoprop-, metribuzin-, pentachlorophenol-, trinitrotoluene-, and dioctylphthalate-interactions with THS, DOM, SOM and soil particles are given and illustrate the processes of trapping and binding anthropogenic substances in soil and water (Schulten et al., 1998). Hypothetical structural models for humic substances such as HA monomer (Schulten et al., 1991) Schulten and Schnitzer, 1993) and oligomeric humic acids (Schulten, 1996) were proposed in an integrated approach using a wide variety of analytical methods. The HA model is important because humic substances constitute 70-80% of soil organic matter (SOM) and all chemical reactions of the latter (large surface, voids, high adsorption capacity, good metal complexer, good medium for microorganisms, can store nutrients and especially water) can be explained on the basis of HAs. From the analytical aspect, main emphasis was put on pyrolysis-mass spectrometry and pyrolysis-gas chromatography/mass spectrometry. Modem personal computer-based techniques of molecular modelling (Hypercube | Inc., 1115 NW 4th Street, Gainesville, Florida, 32601-4256, U.S.A.) were employed to develop three-dimensional structures of humic acids (Schulten, 1995; Schulten and Schnitzer, 1993) and organomineral soil complexes (Schulten, 1996). In particular trapping and bonding of biological (peptides, carbohydrates) and anthropogenic substances such as pesticides and plastizisers in the voids of the geometrically optimized structures were investigated. Basis of the presented work were improved structural concepts of SOM and soils (Schulten and Schnitzer, 1997) and the development to model humic xenobiotic complexes in water (Schulten et al, 1998). In a recent survey on the role and structure of organic nitrogen in soils, the combination of analytical pyrolysis proved to be particularly powerful when the results of sophisticated analytical methods could be supported and by computer modelling (Schulten et al., 1998). Despite an inspiring dispute on the value or nonsense of three-dimensional models of humic substances, in the meantime excellent textbooks on environmental soil chemistry cite and display these model structures.

Temporal variation of C and N turnover in soil after oilseed rape straw incorporation in the field: simulations with the soil-plant-atmosphere model DAISY

The Soil Organic Matter submodel of the soil-plant-atmosphere model DAISY was evaluated using data from one year field trials with and without incorporation of 8 t ha −1 rape straw into the soil. Periodic measurements of soil microbial biomass (C and N), mineral N and light particulate soil organic matter in the top 15 cm of the soil, and of soil CO 2-evolution were made. The simulation of the temporary pattern of soil microbial biomass and mineral N was improved markedly by systematic modification of the default turnover rate coefficients, the substrate utilization efficiencies and the initial levels of soil microbial biomass. Metabolic quotients ( qCO 2) turnover rates of microbial biomass and substrate utilization efficiencies derived from the parametrization of the model were evaluated against literature data. The soil microbial biomass seems to be associated with the production of temporarily protected microbial residual products. The production of these residuals might be responsible for the nitrogen immobilization observed after incorporation of rape straw. In the early stage of rape straw decomposition, measured carbon in light particulate soil organic matter seemed to be represented by one of the added organic matter pools simulated in the DAISY model. We propose that turnover rate coefficients of microbial biomass and added organic matter obtained by fitting a model to measured values may be used as a tool to characterize the physiological state of microbial populations in their natural environment.

Soil aggregate stability and soil organic matter fractions under agropastoral systems established in native savanna

An area of native savanna on an Oxisol in the Eastern Plains of Colombia was opened and sown to various rotations of grass or grass-legume pasture with rice. After 4.5 years, the soil was sampled for studying the effect of land conversion on soil aggregation and on the distribution of total and particulate soil organic matter across the aggregate size classes. The size distribution of undisturbed aggregates did not vary among treatments. Five different methods were used to measure wet aggregate stability (WAS). The choice of method affected the WAS average across treatments as well as the differences among treatments. The only consistent observation was the lower WAS under monocropped rice compared with the other treatments. Inclusion of a legume in a pasture hardly affected aggregate stability. In contrast to the WAS measurements, which were carried out with soil aggregates of 1-2 mm, wet sieving of whole-soil samples revealed additional differences among treatments: large macroaggregates (&gt;2 mm) proved less stable under those treatments that involved soil disturbance through ploughing and harvesting. Total soil C and N content did not vary among treatments, despite considerable differences in plant production levels. The C concentration, but not the N concentration, declined with decreasing aggregate size. The distribution of whole-soil C and N content across aggregate size classes depended more on the amount of soil in a certain size class than on the size class's C or N concentration. Those treatments that involved frequent soil disturbance had a smaller fraction of large macroaggregates (&gt;2 mm) and, as a consequence, less C and N in the large macroaggregate fraction. The particulate organic matter (POM) fraction accounted for only 6.2-8.5% of total soil carbon. The small size of this pool makes it unlikely that POM can serve in these Oxisols for estimating the amount of soil organic matter with medium turnover rate, as suggested by others.

Organic and Conventional Management Effects on Biologically Active Soil Organic Matter Pools

AbstractSpecific impacts of organic management practices on soil organic matter characteristics have not been documented. This study tested how organic management practices influence soil fertility by investigating whether 10 yr of organic or conventional management generated differences in biologically active soil organic matter (SOM) pools at the Rodale Institute Research Center's long‐term Farming Systems Trial experiment (FST). The experiment included an organically managed rotation that was animal based, an organic treatment that was cash‐grain based, and a conventional cash‐grain‐based rotation. The biologically active SOM matter pools of the three FST treatment soils were compared through characterization of soil CO2 evolution, available inorganic N pools and N mineralization rates, water‐dispersible organic carbon (WDOC), and particulate organic matter (light fraction). Soils receiving the organic treatments accumulated biologically active C. Accumulated organic matter in the manureamended soil was the most labile whereas the cover‐cropped soil accumulated the most organic matter overall. In the cover‐cropped soil, higher total C and N, particulate SOM, and reduced WDOC contents indicated that its SOM was more stable than SOM in the other two treatment soils. The conventionally managed soil had the lowest biological activity (N supply and soil respiration rates) and did not accumulate SOM during the 10‐yr experiment. Assays that characterize particulate organic matter emerged as the best indices of biologically active SOM because they documented important quality (i.e., biological lability) and quantity aspects of SOM character in the Rodale FST soils.

Effect of crop rotation and residue management on properties of cracking clay soils under irrigated cotton‐based farming systems of new south wales

AbstractThe effects of planting cereal or leguminous crops in rotation with irrigated cotton (Gossypium hirsutum L.) on the properties of cracking clay (swelling) soils in the Macquarie and Namoi Valleys of New South Wales, Australia were evaluated during the summer of 1992–3. The observations were made on commercial farmers' fields. The soil properties evaluated were the particle size distribution, the dispersion index, the plastic limit, the percentage of coarse (particle diameter 212–2000 μm) and fine (particle diameter 53‐212 μm) particulate soil organic matter, soil respiration rate, soil reactivity, soil aggregate density, pH, nitrate‐N and exchangeable Ca, Mg, K and Na. In general, the planting of rotation crops decreased the dispersion index, plastic limit and soil aggregate density, and increased the amount of coarse particulate organic matter. Planting rotation crops also resulted in significantly higher clay and lower silt contents in the Macquarie Valley, and significantly higher soil respiration in the Namoi Valley. Soil pH, nitrate‐N and exchangeable cation concentrations were not significantly affected by planting rotation crops in the Macquarie Valley, whereas exchangeable Na was increased in the Namoi Valley. The retention of crop residues in situ, compared with burning crop residues, decreased the dispersion index, plastic limit and aggregate density, and increased the amount of coarse particulate soil organic matter at all measured depths of the Macquarie Valley. The retention of crop residues in the Namoi Valley decreased the plastic limit and dispersion index only in the 0–50 mm depth range, whereas burning crop residues increased exchangeable K at all depths. In general, planting rotation crops and the retention of crop residues had greater beneficial effects on the soil physical properties in the Macquarie Valley than in the Namoi Valley, and in the topsoil compared with the subsoil. These differences are attributed to a shorter rotation interval in the Namoi Valley, smaller amounts of coarse particulate soil organic matter in the subsoil, and differing soil types in the two valleys. In the Namoi Valley the coarse organic matter produced by leguminous crops appeared to be more effective in promoting structural stability than that from non‐leguminous crops, although no such difference was observed in the Macquarie Valley.

Study of free and occluded particulate organic matter in soils by solid state 13C Cp/MAS NMR spectroscopy and scanning electron microscopy

A simple densimetric method for the separation of free and occluded particulate organic materials was developed and applied to five virgin soils. The free organic matter was isolated by suspending the soil in sodium polytungstate solution (d = 1.6 Mg m-3) and decanting the light material. The remaining soil was disaggregated by sonification for liberation of occluded organic materials. The free light fraction consisted of large, undecomposed or partly decomposed root and plant fragments. This fraction comprised 0.59-4.34% of soil dry weight and accounted for 6.9-31.3% and 5.9-22.1% of total soil carbon and nitrogen respectively. Identifiable components of the occluded fraction were small particles of incompletely decomposed organic residues, pollen grains, particles of plant tissue such as lignin coils and phytoliths. This fraction comprised 0.69-1.81% of soil dry weight and represented 9.2-17.5% and 6.2-14.1% of the total soil carbon and nitrogen. The proportion of soil organic carbon recovered as the occluded fraction was high in soils with high clay contents. The chemical composition of occluded and free organic materials was investigated by solid-state 13C CP/MAS NMR spectroscopy. Despite the differences in soils, environmental conditions and vegetation, the organic structure of the free light fraction was similar in four of the five soils. This fraction consisted of 55-63% O-alkyl C, 18-25% alkyl C, 14-18% aromatic C, and 5-7% carbonyl C. In the other soil, this fraction showed a higher proportion of alkyl C (31%) and lower O-alkyl C (46%). Most of the differences between soils were associated with organic materials contained in the occluded light fraction. The differences in chemical structure between the occluded light fraction and free light fractions were similar in all examined soils. The NMR data showed that the proportion of O-alkyl C was lower and alkyl C higher in the occluded light fractions than in the free light fractions. The proportion of aromatic and carbonyl carbon was higher in the occluded fractions of three soils while the percentage of these two types of carbon remained unchanged in the two other soils. It is considered that the occluded organic matter is an old pool of carbon that has been accreted within aggregates during decades of root growth and it is that pool which is lost due to cultivation.