R. S. Swift

SEQUESTRATION OF CARBON BY SOIL

Soil carbon is a major component of the terrestrial carbon cycle. The soils of the world contain more carbon than the combined total amounts occurring in vegetation and the atmosphere. Consequently, soils are a major reservoir of carbon and an important sink. Because of the relatively long period of time that carbon spends within the soil and is thereby withheld from the atmosphere, it is often referred to as being sequestered. Increasing the capacity of soils to sequester C provides a partial, medium-term countermeasure to help ameliorate the increasing CO2 levels in the atmosphere arising from fossil fuel burning and land clearing. Such action will also help to alleviate the environmental impacts arising from increasing levels of atmospheric CO2. The C sequestration potential of any soil depends on its capacity to store resistant plant components in the medium term and to protect and accumulate the humic substances (HS) formed from the transformations or organic materials in the soil environment. The sequestration potential of a soil depends on the vegetation it supports, its mineralogical composition, the depth of the solum, soil drainage, the availability of water and air, and the temperature of the soil environment. The sequestration potential also depends on the chemical characteristics of the soil organic matter and its ability to resist microbial decomposition. When accurate information for these features is incorporated in model systems, the potentials of different soils to sequester C can be reliably predicted. It is encouraging to know that improved soil and crop management systems now allow field yields to be maintained and soil C reserves to be increased, even for soils with depleted levels of soil C. Estimates of the soil C sequestration potential are discussed. Inevitably HS are the major components of the additionally sequestered C. It will be important to know more about the compositions and associations of these substances in the soil if we are able to predict reasonably accurately the ability of any soil type to sequester C in different cropping and soil management systems.

Calibration of the Rothamsted organic carbon turnover model (RothC ver. 26.3), using measurable soil organic carbon pools

A fractionation scheme that provided the measurement of a labile pool (particulate organic carbon), a charcoal-carbon pool, and a humic pool by difference was tested as a means of initialising the Rothamsted organic carbon turnover model version 26.3. Equating these 3 fractions with the resistant plant material, inert organic matter, and humic pools of the model, respectively, gave good agreement between measured and modelled data for 2 long-term rotation trials in Australia using a soil depth of 30 cm. At one location, Brigalow Research Station in Queensland, there were 3 distinct soil types, two clays and a duplex soil, in a semi-arid, subtropical climate. At this site, continuous wheat with some sorghum was established after clearing land under brigalow (Acacia harpophylla) and continued for 18 years. The second location was near Tarlee, South Australia, and was established on existing agricultural land. One soil type (red brown earth) with 2 rotations (continuous wheat and wheat–fallow) were available over a period of 8 years. The modelled and measured data were in good agreement for both locations but the level of agreement was substantially improved when the resistant plant material decomposition rate was reduced from 0.3 to 0.15/year. No other modifications were required and the resulting values provided excellent agreement between the modelled and measured data not only for the total soil organic carbon but also for the individual pools. Using this fractionation scheme therefore provides an excellent means of initialising and testing the Rothamsted model, not only in Australia, but also in countries with similar soil types and climate. For the first time, the work reported here demonstrates a methodology linking measured soil carbon pools with a conceptual soil carbon turnover model. This approach has the advantage of allowing the model to be initialised at any point in the landscape without the necessity for historical data or for using the model itself to generate an initial equilibrium pool structure. The correct prediction of the changing total soil organic carbon levels, as well as the pool structure over time, acts as an internal verification and gives confidence that the model is performing as intended.

Method for determining the acid ameliorating capacity of plant residue compost, urban waste compost, farmyard manure, and peat applied to tropical soils

Abstract Plant residue compost, urban waste compost, farmyard manure, and peat can be used to ameliorate soil acidity. The diversity of these materials and their highly variable composition mean that their reliability in increasing the soil pH is uncertain because of lack of a method to test their acid ameliorating capacities. Incubation of a Spodosol from Sumatra, an Oxisol from Burundi and an Ultisol from Cameroon with 1.5% by weight of four composts, a farmyard manure and a sedge peat resulted in increased soil pH and decreased aluminum (Al) saturation measured at 14 days of incubation. The increased soil pH was directly proportional to the protons consumption capacity of the organic materials. This was measured by titrating the organic material from their natural pH values down to pH 4.0. This measure of acid neutralizing capacity provides a simple test method that was reliable across the variety of materials used. The final pH of the soil treated with organic material can be predicted with reasonable...

Humic materials from an organic soil: A comparison of extractants and of properties of extracts

Five extraction procedures and thirteen extracting reagents, which included dipolar aprotic solvents, organic chelating agents, pyridine, ethylenediamine, sodium hydroxide, ion-exchange resins and two salts (sodium pyrophosphate and ethylenediamine hydrochloride), were used to extract humic materials from an organic soil. Extractabilities increased in the general order: salts < organic chelating agents < dipolar aprotic solvents < pyridine < ethylenediamine = sodium hydroxide, and the amounts of the soil organic matter extracted by the reagents in the series ranged from 13 to 63%. Gel chromatography techniques indicated that extracts in dipolar aprotic solvents were predominantly of intermediate and low molecular-weight values, and it is suggested that the more highly oxidised soil humic materials were extracted in these. The more efficient solvents extracted materials with a range (high—low) of molecular-weight values. Data from elemental analysis and from E.S.R. measurements indicated that ethylenediamine altered the chemical nature and the composition of extracts. Dipolar aprotics, by the same criteria, were found not to alter the humic extracts, and can be regarded as mild reagents for the extraction of a less representative (of the total) fraction of soil organic matter. Sodium hydroxide in solution, despite its oxidation effects, was the best of the reagents tested for isolating extracts which were representative of a wide range of soil humic substances.

Mineralization of soil organic P induced by drying and rewetting as a source of plant-available P in limed and unlimed samples of an acid soil

The effects of lime and P applications and drying and rewetting cycles, and their interactions, on the extractability and plant-availability of native and applied P were investigated in a glasshouse experiment using an acid (pHwater, 5.3), P-deficient soil. As expected, liming increased soil pH, decreased exchangeable Al to negligible concentrations and also caused a decrease in Bray No. 1- and NaHCO3 - extractable P. Dry matter yields and P uptake by wheat were enhanced by both lime and P applications. Subjecting the soil to two drying and rewetting cycles increased the extractability of soil P. This effect was more marked when P had been applied but was equally evident in both limed and unlimed soils. In addition, subsequent dry matter yields and plant P uptake were markedly increased by the drying and rewetting of the soils and this effect was more evident for the limed treatments. A subsidiary experiment using a 32P-labelled inorganic P-source applied to the soil revealed that the specific activity of 32P in both Bray No. 1 and resin soil extracts was decreased by subjecting the soil to drying and rewetting cycles. This occurred despite there being an increase in extractable soil P, dry matter yields and plant P uptake. These results strongly suggest that drying and rewetting stimulated mineralization of soil organic 31P and thus increased the size of the plant-available pool of soil P and diluting the added 32P. In relation to the fertility of highly weathered, P-deficient soils, these findings are important since organic P often accounts for 50%, or more, of their total P content and therefore represents a substantial pool of potentially plant-available P.

Comparison of the particulate organic carbon and permanganate oxidation methods for estimating labile soil organic carbon

Forty-four soils from under native vegetation and a range of management practices following clearing were analysed for ‘labile’ organic carbon (OC) using both the particulate organic carbon (POC) and the 333 mm KmnO4 (MnoxC) methods. Although there was some correlation between the 2 methods, the POC method was more sensitive by about a factor of 2 to rapid loss in OC as a result of management or land-use change. Unlike the POC method, the MnoxC method was insensitive to rapid gains in TOC following establishment of pasture on degraded soil. The MnoxC method was shown to be particularly sensitive to the presence of lignin or lignin-like compounds and therefore is likely to be very sensitive to the nature of the vegetation present at or near the time of sampling and explains the insensitivity of this method to OC gain under pasture. The presence of charcoal is an issue with both techniques, but whereas the charcoal contribution to the POC fraction can be assessed, the MnoxC method cannot distinguish between charcoal and most biomolecules found in soil. Because of these limitations, the MnoxC method should not be applied indiscriminately across different soil types and management practices.

Initial responses of maize and beans to decreased concentrations of monomeric inorganic aluminium with application of manure or tree prunings to an Oxisol in Burundi

Prunings of Calliandra calothyrsus, Grevillea robusta, Leucaena diversifolia and farm yard manure were applied each cropping season at 3 and 6 t dry matter ha−1 to an Oxisol in Burundi. The field plots also received basal applications of nitrogen (N), phosphorus (P) and potassium (K). Application of the tree prunings or farm yard manure decreased the concentration of monomeric inorganic aluminium (Al) in soil solution from 2.92 mg Al dm−3 in the control plots to 0.75 mg Al dm−3 in the plots receiving 6 t ha−1Calliandra prunings. The other organic materials also decreased the concentration of monomeric inorganic aluminium in the soil solution. The lowered Al concentration led to a corresponding decrease in the percentage Al saturation of the 0–10 cm soil layer from 80% to 68%. Grain yields of maize and beans were strongly inversely related to the percentage Al saturation of the soil. This confirms that soil acidity was the main constraint to maize and beans production. The yield improvement was mainly attributed to the ameliorating effects of the organic matter application on Al toxicity. The nutrient content had less effect presumably because of fertilizer use. In the best treatments, the yield of maize increased from 0.9 to 2.2 t ha−1 and the corresponding beans yield increased from 0.2 to 1.2 t ha−1. A C Borstlap Section editor

Measurement of the acid neutralizing capacity of agroforestry tree prunings added to tropical soils.

Laboratory incubation in the UK of an Oxisol from Burundi and an Ultisol from Cameroon with 3·1% by weight of prunings of young shoots of Calliandra calothyrsus, Cassia siamea, Flemingia congesta, Grevillea robusta, Gliricidia sepium, Leucaena diversifolia and Leucaena leucocephala resulted in increased soil pH and decreased exchangeable aluminium content. The greatest increase in pH and corresponding decrease in exchangeable aluminium occurred during the first 14 days of incubation and the decrease continued at a slower rate until 42 days incubation. The acid neutralizing effect decreased after 42 days but was still important at the last sampling time at 98 days. Polyphenol to nitrogen ratio was not well correlated with observed change in soil pH whereas the total base cation (calcium, magnesium and potassium) content proved to be a good predictor of these changes in the Ultisol, but not in the Oxisol. The proposed mechanism giving rise to acid neutralization is complexation of protons and aluminium by organic anions. The total base cation content of the prunings ranged from 0·94 to 2·25 mol c /kg and the buffer capacity of the Oxisol was 48 mmol OH − /pH/kg.

Investigation of the interaction between pesticides and humic sunstances using fluorescence spectroscopy

Abstract The interaction between humic substances and pesticides in solution has been investigated using the fluorescence spectra of the peat Na + -humate and Na + -fulvate. The pesticides studied include atrazine, 2,4-D, isoproturon and paraquat in solutions at pH 4, 8 with a background electrolyte concentration of 10 −1 M and below. Investigations using both excitation and emission modes show that sorbed pesticides affect the fluorescence spectra of humic substances. The effect was dependent on the nature of the pesticide and ranged from large changes in spectral characteristics in the presence of paraquat to insignificant changes in the presence of atrazine and 2,4-D. The extent of the change was dependent on the pH of the solution, and in some cases, dependent on the wavelength of excitation (e.g. paraquat), or the time of interaction (e.g. isoproturon). These fluorescence results were compared with results from sorption studies and dialysis experiments using solid Ca 2+ -humate and dissolved Na + -humate, respectively to compare the extent of sorption on solid humic substances relative to the amount of pesticide bound to dissolved humic substances. It is concluded that fluorescence spectroscopy is a useful technique in the study of the interaction between pesticides and humic substances in solution as it is able to provide information about the extent and possible mechanisms of the interaction.

The Influence of Biochar on Growth of Lettuce and Potato

Pot experiments were conducted in a glasshouse to determine the growth pattern of lettuce, true potato seedlings (TPS) and single node cuttings of TPS in response to biochar. The treatments were arranged in a randomized complete block design with 5 treatments (0, 10, 30, 50 and 100 t ha−1) of biochar from greenwaste with 5 replications in lettuce, 10 in TPS and 5 in single node cuttings of TPS. The observations recorded on growth parameters showed that biochar had significant effect on growth of lettuce but no consistent effect on growth of TPS and single node cuttings. Among the biochar rates, 30 t ha−1 had the greatest influence on overall growth of lettuce. The pH and electrical conductivity increased as the biochar rates increased in all experiments. These results provide an avenue for soil management system by using biochar as an amendment in horticultural crops. However, their verification in the field is important for specific recommendations.