W.B. McGill

Nitrification in three Alberta soils: Effect of temperature, moisture and substrate concentration

Abstract The objectives of this study were to determine the effect of temperature on relative rate of nitrification in soils and by comparing it with published results from other climates to see if it supported the hypothesis that temperature relations of nitrification vary with the climate of the region; to determine if the effect of NH + 4 -N concentration on nitrification rate in unperfused soil could be represented by an extremely simple model such as a Michaelis-Menten (rectangular hyperbola) expression and if so, calculate the constants; and to determine the effect of soil moisture potential on nitrification in these soils. The optimum temperature for nitrification in soils from central Alberta was 20°C and at 30° activity had almost ceased. An adaptation of soil nitrifiers to soil climatic is indicated. Nitrification in these soils could be represented by a Michaelis-Menten expression with calculated maximum velocities of 9.0–9.9 μg NH + 4 -N oxidized g −1 soil day −1 . Half saturation ( K n ) values were calculated as 154–186 μg NH + 4 -N g −1 soil. This same expression was found to fit only about half the reported results examined. Nitrification was rapid at 200 μg NH + 4 -N g −1 but was inhibited at 300μg NH + 4 -Ng −1 . The depressing influence of NH + 4 -N on nitrification in the present soils seems to be the combined effect of low pH and increase in salt content with increased NH + 4 -N from 200 to 300 μg N g −1 . Results suggested that soil moisture content and temperature in late fall and during winter, under snow cover, in central Alberta or in locations with a similar climate may be high enough to result in nitrification during late fall and winter.

Elucidation of the source and turnover of water soluble and microbial biomass carbon in agricultural soils

Understanding the dynamics of soil C is key to managing soil organic matter to enhance soil quality and ecosystem functioning, and reduce trace gas emissions from soils. Our objective was to determine the source and turnover of C pools in some agricultural soils in eastern Canada. Soils from five field experiments under continuous maize cropping for 4–37 yr were sampled, and the organic C content and stable C isotope (13C) composition of whole soil and water soluble and microbial biomass fractions determined. The 13C results showed a clear distinction between the water soluble organic C and microbial biomass C, with the water soluble organic C more like the whole soil and the microbial biomass more like the maize residues. A simple linear model was used to explore the relationship among the soil organic constituents and evaluate the turnover of these carbon pools. Even though the water soluble organic C had a higher turnover rate than the microbial biomass C, the proportion of C4-derived C in the biomass was about 2.5 times greater than that in water soluble organic C. Apparently the large amount of native soil C, the small amount of water soluble organic C, and its equilibrium with the native soil C, cause humus to dominate the isotopic composition of water soluble organic C even though the water soluble C is very active. Our results suggest that the quantity, as well as the turnover rate, of soil organic matter constituents that are in equilibrium influence the isotopic composition of such constituents.

Assessing soil water repellency using the molarity of ethanol droplet (MED) test

The molarity of ethanol droplet (MED) test is a popular rapid method for assessing soil water repellency under field and laboratory conditions. This paper reviews the theoretical basis of the MED test, discusses controllable and uncontrollable sources of error affecting its results, and proposes a detailed protocol for its standardization. Soil water repellency is a function of soil surface chemistry. More specifically, it is a function of the free energy of the solid/gas interface in soil (γSG). Because γSG is not directly measurable in the laboratory, soil water repellency must be assessed using thermodynamically related parameters such as the initial advancing contact angle (&thgr;) or the work of wetting (WW). The MED test can be used to determine &thgr;, and in turn WW, but only if some simplifications are accepted and the test is performed under standard controlled conditions. Wetting theories exclude the dissolution or swelling of the solid by the liquid or chemical reactions between the liquid, solid, and gas phases that change system composition. Consequently, for MED tests to give valid &thgr; estimates, system composition must not change measurably during 10 s of solid/liquid contact. We discuss system conditions that should be controlled before and during MED testing to improve the validity of test results. Finally, we propose a detailed MED testing protocol for possible adoption by commercial analytical laboratories and the soils research community.

Soil aggregate dynamics and the retention of organic matter in laboratory-incubated soil with differing simulated tillage frequencies

It is generally accepted that aggregate dynamics are a significant control on the dynamics of organic C and that aggregate dynamics differ under cultivated and uncultivated (or no-till) conditions. Cultivation may alter soil organic matter (SOM) dynamics by changing its position within the soil matrix, either releasing organic materials from within aggregates during disruption or occluding materials during aggregate formation. The goal of this study was to observe aggregate dynamics under differing regimes of simulated tillage and relate these to organic matter dynamics. The experiment specifically examined the mechanism of physical protection by inducing different rates of soil aggregate turnover without changes in environmental conditions. Soil samples were incubated with dysprosium-labeled tracer spheres and finely ground corn residues for 8 weeks under imposed rates of aggregate turnover: no simulated tillage, three simulated tillage events, or five simulated tillage events. Total soil respiration and evolution were used to determine the relative degree of physical protection afforded by the induced rates of aggregate turnover. Increased frequency of simulated tillage increased the incorporation of tracer spheres into stable macroaggregates, and reduced the total amount of CO2 evolved during the experiment. We propose that organic matter retention in tilled samples was achieved through a reduction of the priming effect afforded by the increased aggregate turnover and the disruption of the microbial biomass decomposing the added POM and native organic matter. While there appears to be a disparity between short-term tillage-enhanced organic matter protection and the long-term decrease in organic matter content observed in cultivated soils, the results suggest that there may be threshold rates of aggregate turnover that will protect rather than release organic C. We propose that the physical protection available under differing rates of soil aggregate turnover will differ for incoming organic materials versus previously protected organic matter, and that soil C sequestration is maximal at an intermediate aggregate turnover rate.

Effects of phosphorus addition and energy supply on acid phosphatase production and activity in soils

Abstract To find out how acid phosphatase activity and production in some Alberta soils may be related to soil properties and past fertilizer history, soils of varying organic matter content, extractable P and P fertilization history were assayed for acid phosphatase using p -nitrophenyl phosphate as substrate. The effect of solution P concentration during the phosphatase assay was examined. The effect of P on the production of new phosphatase was examined in soils incubated with an added energy supply or orthophosphate. Phosphatase activity was influenced by P fertilization practices during the 5 yr before sampling. In a Black Chernozemic soil (Malmo SiCL) with a high organic matter content and high initial phosphatase activity, P fertilization at 27 or 54 kg P ha −1 y −1 for 5 yr reduced phosphatase activity by about 20%. However, in a Grey Luvisolic soil (Cooking Lake L) with low organic matter and initial phosphatase, P fertilization at 54 kg P ha −1 y −1 for 5 yr tended to increase activity, probably by increasing plant root growth and organic matter additions. Assay solutions containing orthophosphate at 0.55 mM reduced activity by 25% and 47% in a Malmo SiCL and Maleb L (Orthic Brown Chernozem) soil respectively. Further increases of phosphate concentration to 5.5 mM reduced phosphatase activity by 50% and 76% in the Malmo and Maleb L soils respectively. Phosphatase activity was increased up to 6-fold by incubation of soil with glucose and NH 4 NO 3 . Addition of P to produce an added C: added P ratio of 20:1 completely prevented synthesis of phosphatase by proliferating organisms and had a slight inhibitory effect on phosphatase already present. Similarly, addition of P without C in a 6-week incubation had only a small effect on phosphatase activity and maintained P concentrations in the assay solutions slightly below 0.55 mM. It was concluded that the effect of phosphate on soil phosphatase operates more through its effect on phosphatase synthesis than on activity of existing phosphatase.

Review and Classification of Ten Soil Organic Matter (SOM) Models

This paper describes ten SOM models briefly, presents a scheme for classifying information about them and summarizes relationships among model attributes and among models. All are process-oriented multicompartment models. Few attributes are needed to distinguish among them and with few exceptions they are empirical in nature. There appears to be a trend to include inert organic matter but not to specify its nature nor rate of formation. Soil texture is increasingly included in SOM models both to protect or slow decomposition of soil organic components, and to regulate partitioning of C among compartments. About half the models represent a homogeneous soil unit. Concepts pertaining to the nature of litter are different from those pertaining to SOM, as indicated by inclusion of biomass as a SOM component but not a litter component.. Many models in the past have specified biochemical fractions to follow C and N through litter and SOM components, but in no case were biochemical separations applied to litter or SOM.

Cross-Correlation of Polarity Curves To Predict Partition Coefficients of Nonionic Organic Contaminants

Accurate sorption coefficients are important for models to predict fate and movement of organic chemicals in soils or sediments. We report here on a new method for predicting partition coefficients (K d ) of nonionic chemicals onto soils and geological materials. It includes properties of sorbents and of sorbates, thereby yielding more accurate organic carbon-normalized partition coefficients (K oc ) than a single value derived from an octanol-water partition coefficient (K ow ). A regression of log K oc , on log K ow and polarity index (PI: [(O+N)/C]) was established using benzene, toluene, and o-xylene as sorbates and using model organic polymers as sorbents

Simulation of carbon and nitrogen transformations in soil: Mineralization

Abstract If mathematical models of decomposition and transformation processes are to be rigorously validated, they should be tested against the microbial dynamics from which these processes arise. A mathematical model was constructed from kinetic equations for microbial activity reported in the literature and in earlier models, and was tested against C and N mineralization during incubation of labelled glucose, cellulose and crop residue on several soils. The model was able to reproduce temporal trends in the mineralization, immobilization and retention of labelled C and N to within 10% of recorded values over time scales of hours, days and years following soil amendments. By treating the humification and adsorption of microbial products as functions of soil clay content, the simulated mineralization of C amendments was reduced, and retention increased, to extents consistent with recorded data in soils with clay contents which varied from 4 to 34%. By allowing the decomposition of all soil organic matter to be determined by the total concentration of microbial biomass, the model also reproduced most of the increased mineralization of non-labelled C recorded from 14C-amended soils.

Sorption of phenol by selected biopolymers : isotherms, energetics, and polarity

The behavior of phenol in the terrestrial environment is strongly regulated by its reaction with soil components. We report here on the uptake of phenol by soil minerals (goethite, kaolinite, and montmorillonite) and by organics that may occur naturally in or be added to soil (two lignins, chitin, cellulose, collagen, and activated carbon). Our objectives were to determine the energetics and capacity for their uptake of phenol using batch equilibration, calorimetry, and CPMAS 13 C NMR and to evaluate the relation of organic carbon referenced sorption coefficient (K oc ) with the polarity of biopolymers. The biopolymers sorbed 2-45-fold more phenol than did the minerals

Variation of 1-naphthol sorption with organic matter fractionation: the role of physical conformation

Abstract Nonionic, hydrophobic organic contaminant sorption to natural organic matter is of continual concern due to the strong association between the contaminant and organic matter. To better understand the underlying mechanisms in sorption interactions, humic acids (HAs) and humin from soils and geologic samples with varying diagenetic properties were isolated and used in sorption studies with 1-naphthol. The organic carbon-normalized sorption coefficient (Koc) values for the fractions differed from those of the source samples and, in most instances, increased. The Black, Brown, and Peat HA 1-naphthol Koc values are statistically similar (p