Richard E. Farrell

RECYCLING OF THE NATURALLY-OCCURRING 15N IN AN ESTABLISHED STAND OF LEUCAENA LEUCOCEPHALA

Abstract The natural 15 N abundance method was used to trace the sources of N in a stand of Leucaena leucocephala and its understorey species. Changes in the δ 15 N of various above-ground parts of the L. leucocephala trees and understorey vegetation were monitored for 6 yr. The resulting data provide the first direct evidence of internal N cycling between a N 2 -fixing tree and non-N 2 -fixing understorey species within an agro-ecosystem. The δ 15 N of all tree parts and understorey vegetation decreased with time. One year after the plantation was established, the non-N 2 -fixing understorey species exhibited a significant enrichment in 15 N relative to the L. leucocephala . At 4 and 6 yr after planting, however, the δ 15 N value of the understorey vegetation had decreased significantly and was nearly identical to that of the L. leucocephala . The decline in δ 15 N values of the understorey species was attributed to net N-mineralization of plant parts shed by L. leucocephala , and to root exudation and decay. That is, a portion of the N 2 fixed during the early stages of tree growth was made available to the understorey species through the decomposition and subsequent incorporation into the available soil-N pool of the abscissed L. leucocephala parts. This indicates that there was a tight coupling between N 2 fixation and the availability of net mineralized N. The natural 15 N abundance method appears to provide a suitable means by which the fate of fixed-N in a forest ecosystem can be monitored, provided the difference between the initial (δ 15 N value of the N 2 -fixing and non-N 2 -fixing (reference) species is of sufficient magnitude. The rate of change in the δ 15 N value of the understorey species can provide valuable insights into the intensity of net N-mineralization and N-cycling.

Effects of cultivation on the activity and kinetics of arylsulfatase in Saskatchewan soils

Abstract Arylsulfatases play an important role in the biochemical mineralization of organic ester sulfates in soil. The effects of clearing and cultivation of native North America prairie grassland and forest soils on the activity and kinetics of arylsulfatase were studied. Arylsulfatase activities ranged from 89 to 829 μg p -nitrophenol g −1 soil h −1 . Clearing and cultivation of the native soils resulted in significant decreases in enzyme activity in both the grassland and forest soils. Long-term cultivation (69 yr) of the native grassland soil resulted in a 66% reduction in arylsulfatase activity. Likewise, cultivation of the forest soil resulted in 63% decrease in enzyme activity after 5 yr and an 88% decrease in activity after 40 yr. In contrast, clearing the forest soil and leaving it fallow for 5 yr produced only a 30% reduction in arylsulfatase activity. In general, the soil arylsulfatases exhibited typical Michaelis-Menten kinetics. Variations in the kinetic parameters, however, presumably reflected changes in the enzymes brought about by the different land management schemes. Long-term cultivation of the soils resulted in a 74% reduction in the V max of the grassland soil and 90% reduction in the V max of the forest soils. Values of Michaelis constant ( K m ) ranged from 1.72 to 9.38 mm and decreased as the intensity (duration) of cultivation increased. Variations in the K m , values obtained for the native soils suggest that the origins of the arylsulfatases in the grassland and forest soils were different. Variations within each cultivation sequence, however, indicate that management had a significant effect on the nature and location of the enzyme. These results demonstrate the important influence of land management on enzymatic processes in soil.

Ability of Cold-Tolerant Plants to Grow in Hydrocarbon-Contaminated Soil

Phytoremediation of hydrocarbons in soil involves plants and their associated microorganisms. Differences in environmental conditions and restrictions on species importation mean that each country may need to identify indigenous plants to use for phytoremediation. Screening plants for hydrocarbon tolerance before screening for degradation ability may prove more economical than screening directly for degradation. Thirty-nine cold-tolerant plants native, or exotic and naturalized, in western Canada were assessed for their ability to survive in crude oil-contaminated soil. Four naturalized grasses (i.e., Agropyron pectiniforme, Bromus inermis, Phleum pratense, and Poa pratensis), three naturalized legumes (i.e., Medicago sativa, Melilotus officinalis, and Trifolium repens), two native forbs (i.e., Artemisia frigida and Potentilla pensylvanica), one native grass (i.e., Bromus ciliatus) and two native legumes (i.e., Glycyrrhiza lepidota and Psoralea esculenta) exhibited phytoremediation potential, based on survival. We determined the effect of increasing crude oil concentrations on total and root biomass, and relative growth rate of those species with the highest survival. The addition of 0.5%, 1%, and 5% (crude oil wt/fresh soil wt) crude oil to soil significantly decreased both the total biomass by at least 22% of the control and the relative growth rate of all species except P. esculenta. Root biomass significantly decreased by at least 22% with crude oil addition in all species except P. esculenta and A. frigida. Total biomass production in contaminated soil had a significant negative correlation with the relative growth rate in uncontaminated soil.

Construction and evaluation of a reference electrode assembly for use in monitoring in situ soil redox potentials

Abstract A simple procedure for the construction of inexpensive Ag,AgCl reference electrode assemblies is described. Disposable plastic syringes (60‐cm) were used to form the bodies of the electrode assemblies and small Ag,AgCl internal reference elements were made from silver wire. The internal reference elements were immersed in a solution of AgCl‐saturated KCl, which also served as the salt bridge. Contact between the salt bridge and test solution or soil was made through a ceramic microtensiometer cup. This procedure yields reference electrode assemblies that perform as well as commercially available reference electrodes in terms of their stability and reproducibility. Because of their rugged construction and ease of maintenance, these reference electrode assemblies are ideal for use in the field. Moreover, they proved to be particularly useful when in situ soil redox potentials were monitored for 84 h.

Calibration method at the N K-edge using interstitial nitrogen gas in solid-state nitrogen-containing inorganic compounds

The standard method of soft X-ray beamline calibration at the N K-edge uses the nu = 0 peak transition of gas-phase N(2). Interstitial N(2) gas trapped or formed within widely available solid-state ammonium- and amine-containing salts can be used for this purpose, bypassing gas-phase measurements. Evidence from non-nitrogen-containing compounds (KH(2)PO(4)) and from He-purged ammonium salts suggest that production of N(2) gas is through beam-induced decomposition. Compounds with nitrate or nitrite as anions produce coincident features and are not suitable for this calibration method.

Advances in Understanding Organic Nitrogen Chemistry in Soils Using State-of-the-art Analytical Techniques

Abstract During the past decade, soil and geochemists have adopted a variety of novel chemical–analytical methods to explore the chemistry of soil organic N (N org ). This chapter summarizes some of the more recent developments in the use of wet-chemical and instrumental methods to determine total N org concentrations as well as to speciate the N org in soils. A critical evaluation of 15 N nuclear magnetic resonance (NMR) spectroscopy found the technique to be wanting, in terms of its sensitivity and ability to identify classes of N org compounds in soils. Complementary mass spectrometric techniques are described briefly, and improved data evaluations based on broad applications of high-resolution pyrolysis-field ionization mass spectrometry are presented and discussed. A reassessment of older data sets using the new spectral evaluation algorithms provides strong evidence of fire- and management-induced changes in N org speciation. Isotope-ratio mass spectrometry, Fourier transform ion cyclotron resonance mass spectrometry, and nanoscale secondary ion mass spectrometry (Nano-SIMS) also are discussed, with the latter two techniques having potential to (1) identify N org compounds and (2) provide spatially resolved information on the molecular, elemental and isotopic composition of soil N org . The use of 15 N labeling techniques is discussed both from a methodological standpoint and in terms of tracking the fate of plant-derived (residue or rhizodeposit) N in the soil. Indeed, coupling 15 N labeling with analytical techniques such as 15 N NMR, Nano-SIMS and high- or ultrahigh-resolution mass spectrometry can provide information on how N is incorporated into soil organic matter. Analytical and instrumental innovations have resulted in new insights into the chemistry of N org —together with a revised summary of the relative amounts of the different N org compound classes present in soils (e.g. aliphatic amine and amide N, aromatic heterocyclic N), as well as their ecophysiological functions. Particular emphasis is given to the use of multitechnique analyses and the outstanding molecular–chemical diversity of biogenic heterocyclic N org compounds. Examples are given of the new insights obtained using multi-analytical research approaches to explore microbial utilization of heterocyclic N and organic–mineral interactions, as well as the ability of human and environmental intervention to alter the composition of soil N org . Finally, we examine future challenges and propose analytical approaches to tackle open questions regarding the basic chemistry and cycling of N org in soils, as well as the agronomic and environmental consequences associated with N transformations in agro-ecosystems.

Plant-assisted degradation of phenanthrene as assessed by solid-phase microextraction (SPME).

ABSTRACT The soil bacterium Sphingomonas yanoikuyae was isolated from a petroleum-contaminated soil and grown on mineral salts agar overlaid with the polycyclic aromatic hydrocarbon phenanthrene. The effect of white mustard, Sinapis alba, on phenanthrene degradation by S. yanoikuyae in artificially contaminated Redi-earth-sand was examined. Solid-phase-microextraction (SPME) gas chromatography-flame ionization detection (GC-FID) was used to quantify the concentration of phenanthrene in the gas phase of Magenta jars containing S. alba and S. yanoikuyae, each alone and with no additions. Gas chromatography–mass spectrometry (GC–MS) of Soxhlet extracts was used to determine the concentration of phenanthrene remaining in Redi-earth-sand. The gas phase concentration of phenanthrene in nonsterile Redi-earth-sand decreased by 99.7% in treatments with S. alba plus S. yanoikuyae, by 98.6% with S. alba, by 96.7% with S. yanoikuyae, and by 95.8% with no additions. Under gnotobiotic conditions, the gas phase concentration of phenanthrene in Redi-earth-sand decreased by 94% in treatments with S. alba plus S. yanoikuyae, by 77% with S. yanoikuyae, by 26% with S. alba, and 0% with no additions. The concentration of phenanthrene in Redi-earth-sand under gnotobiotic conditions decreased in treatments with S. alba plus S. yanoikuyae by 88%, by 67% with S. yanoikuyae, by 13% with S. alba, and 0% with no additions as measured in Soxhlet extracts. These results suggest that SPME–GC can be used to rapidly assess the potential of plants and microorganisms to reduce the level of unaged polyaromatic hydrocarbons such as phenanthrene in soil. This method provided results that were consistent with the more costly Soxhlet extraction method and was less time consuming.

Net Cd2+ flux at the root surface of durum wheat (Triticum turgidum L. var. durum) cultivars in relation to cultivar differences in Cd accumulation

Net Cd2+ flux and Cd accumulation for four durum wheat cultivars were measured to determine whether the net Cd2+ flux to the roots plays a role in cultivar differences in Cd accumulation. Plants were germinated for 4 d in the dark and then grown hydroponically for 2 d before being placed in a Petri dish with their roots immersed in a 1.0 µM Cd solution. Net Cd2+ flux to the root was measured at ten positions located 0.5 mm to 25 mm from the root tip. Cadmium concentrations were measured using Cd2+-selective microel ectrodes; Cd2+ flux to the root surface was then calculated using the MIFE™ system. The flux was greatest nearest the root tip (within 1.5 mm of the tip) and there were no significant differences in net Cd2+ flux among the various cultivars — likely because of the absence of a Casparian strip in the root tip cells. Net Cd2+ flux decreased with increasing distance from the root tip, with significant differences among cultivars. The mean Cd2+ flux to the roots in the 2–25 mm region increased in t...

Nitrous oxide emissions from ephemeral wetland soils are correlated with microbial community composition

Nitrous oxide (N2O) is a greenhouse gas with a global warming potential far exceeding that of CO2. Soil N2O emissions are a product of two microbially mediated processes: nitrification and denitrification. Understanding the effects of landscape on microbial communities, and the subsequent influences of microbial abundance and composition on the processes of nitrification and denitrification are key to predicting future N2O emissions. The objective of this study was to examine microbial abundance and community composition in relation to N2O associated with nitrification and denitrification processes over the course of a growing season in soils from cultivated and uncultivated wetlands. The denitrifying enzyme assay and N15O3− pool dilution methods were used to compare the rates of denitrification and nitrification and their associated N2O emissions. Functional gene composition was measured with restriction fragment length polymorphism profiles and abundance was measured with quantitative polymerase chain reaction. The change in denitrifier nitrous oxide reductase gene (nosZ) abundance and community composition was a good predictor of net soil N2O emission. However, neither ammonia oxidizing bacteria ammonia monooxygenase (bacterial amoA) gene abundance nor composition predicted nitrification-associated-N2O emissions. Alternative strategies might be necessary if bacterial amoA are to be used as predictive in situ indicators of nitrification rate and nitrification-associated-N2O emission.

Hydrocarbon Tolerance Correlates with Seed Mass and Relative Growth Rate

ABSTRACT The abundance and vegetation cover of plant species with different seed masses growing on crude oil contaminated and uncontaminated field plots were examined. In addition, seedling mass and relative growth rate of eight plants in four seed mass classes were compared in contaminated and uncontaminated soils. Species producing seed in the second largest seed class (1.0 to 9.9 mg) were more common on contaminated than uncontaminated field plots. Species with seed in the smallest seed class (< 0.1 mg) were less common on contaminated than uncontaminated plots. Species with seed masses between 1.0 and 9.9 mg formed 15% more vegetation cover, and those with masses between 0.1 and 0.9 mg formed nearly 15% less vegetation cover on contaminated compared to uncontaminated field plots. In the growth chamber, species with larger seeds produced approximately 25% less seedling mass and exhibited a 32% reduction in relative growth rate, when grown in contaminated compared to uncontaminated soil. Small-seeded species had reductions of 95% in seedling mass and 72% in relative growth rate. Species with the highest relative growth rates in uncontaminated soil had the lowest seedling mass in contaminated soil. Seed mass and relative growth rate were negatively correlated (r = 0.866).