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Experiments were designed to investigate the validity of proposed reaction‐diffusion models of denitrification. Brass respirometers were used to measure fluxes of oxygen (O2), carbon dioxide (CO2), and nitrous oxide (N2O) from a 5 × 10−3‐m layer of saturated soil incubated at 25°C. Gaseous fluxes were measured as a function of time after wetting, with and without 0.42 mol m−3of acetylene (C2H2) present in the headspace. The presence of C2H2in the headspace doubled O2fluxes 35 to 60 h after incubations commenced. When C2H2was not present, N2O evolution was negligible after 25 h of incubation. Nitrous oxide fluxes predicted by the models were compared to those observed experimentally. When C2H2was present, the models predicted N2O fluxes reasonably well (r2= 0.90 and 0.78) for initial nitrate (NO−3) N levels of 7.1 and 15.9 mg N kg−1soil, respectively. When C2H2was not present, the models predicted the experimentally observed initial flux of N2O if an 8 to 10 h lag in N2O reductase activity was included in the models. The best model for describing N2O fluxes in these experiments used differential rates of Michaelis‐Menten reduction without NO−3and nitrite (NO−2) inhibition of N2O reduction. Experimental techniques and parameter characterization, however, need considerable refinement. Independent measurements are needed to quantify relationships between reaction order and kinetic parameters. Still, given the general agreement between observed and predicted fluxes, there can be little doubt that the reaction‐diffusion models are essential to a better understanding of denitrification.

The interaction of fairy rings of Lycopodium tristachyumPursh. with the soil which they invade was examined by measurement of various soil enzymatic activities and extractable soil organic matter. The activities of cellulase, xylanase, inulinase, α‐galactosidase, invertase, peroxidase, chitinase, and polygalacturonase were enhanced in soil under the rings compared with soil just outside the rings. The activity of urease was lower under the ring while the activity of β‐glucosidase was unaffected. The amount of nitrogen, hexose, and phenolics extracted by 0.01MCaCl2during autoclaving was enhanced in soil under the ring. The level of reducing substances extracted by dilute buffer and the optical density of buffered extracts were similarly elevated. Extractable lipid, humic acid, and β‐humus were independent of soil location. Significant correlations between some readily extractable organic components and the activities of chitinase, invertase, xylanase, and α‐galactosidase in the soil were observed.

Lycopodium species (L. tristachyum, L. clavatum, and L. complanatum) sometimes occur as fairy rings—dense annuli of plants, 1 to 1.5 m wide, that grow radially outward at a rate of 20 to 50 cm/year and die within. The impact of these fairy rings on soil organic matter was investigated through studies of soil respiration and selected enzymatic activities. Respiration was enhanced in the soil under lycopodium rings compared with soil either outside or inside the rings. The activities of phosphatase, protease, dehydrogenase, catalase, polyphenol oxidase, amylase, and dextrinase were found to be independent of soil location. However, significant increases in cellulase, invertase, polygalacturonase, and peroxidase activities were observed in the soil under lycopodium rings. A model based on the infection of the lycopodium plant by its endophytic fungus and subsequent peroxidative degradation of soil organic matter is presented to explain both the increases in enzymatic activities and nutrient availability under lycopodium fairy rings.

The C2H2‐block method was used to measure denitrification in a maize (Zea maysL.) field over the 85‐d period between sidedressing with N fertilizer and harvest. Measurement locations were changed weekly in order to avoid problems associated with microbial utilization of C2H2and inhibition of nitrification. Nitrous oxide (N2O) fluxes were measured both with and without C2H2addition to soil. Without C2H2addition to soil, 0.3 kg N ha−1was evolved from both unfertilized and Ca(NO3)2treated soil, whereas 2.5 kg N ha−1was evolved from urea treated soil. This difference was attributed to formation of N2O during nitrification of NH+4liberated from urea. With C2H2addition to soil, 1.6, 2.0, and 2.4 kg N ha−1were evolved as the result of denitrification in the zero N, Ca(NO3)2, and urea treated sites, respectively. Total gaseous N loss from a particular N treatment was approximated by adding together the N2O‐N loss with and without C2H2addition to soil. The gaseous N loss associated with fertilizer addition was <3% of that applied. The ratio of N2/N2O evolved without C2H2addition to soil was estimated to be in the range of 4:1 to 7:1 for the zero N and Ca(NO3)2treatments and about 1:1 for the urea treatment. Denitrification was responsible for at least 80% of the measured gaseous N loss from the zero N and Ca(NO3)2treatments. About equal amounts of gaseous N loss from the urea treatment occurred via denitrification and during nitrification of liberated NH+4.

The effect of complexing agents on Fe mobility was investigated using a simulated root‐soil system that consisted of 0.24‐mm o.d. cellulose fibers embedded in a clay‐agar matrix. Complexing agents diffused into the clay‐agar matrix from a solution flowing through the fibers and their effect on Fe flux to the fiber surface was determined by following Fe concentration in the effluent solution from the fibers. Experiments with the chelating agents ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), ethylenediamine di‐(o‐hydroxyphenylacetic acid) (EDDHA), and ethylene glycol‐bis‐(2‐amino‐ethylether) tetraacetic acid (EGTA) demonstrated that solubilization of Fe qualitatively followed patterns predicted from known thermodynamic stability constant‐pH relationships. The concentration of Fe in the fibers 24 h after introduction of either EDTA, DTPA, or EDDHA was increased from close to 0 to 30 to 100 g Fe L−1, whereas EGTA had no measurable effect on Fe solubility. The measured fluxes of Fe were compared with calculations based on diffusion and reaction. An equilibrium model, where the complexing agent was assumed to equilibrate instantaneously with the soil (reaction rate constant Kr= 1), predicted concentrations of Fe 103times greater than those observed. A first‐order kinetic model, which was solved by numerical methods, gave results similar to those found experimentally when Krwas on the order of 10−7. The partial success of the thermodynamic stability constant approach was reconciled with the kinetic model by considering Krto combine both kinetic and equilibrium factors.

Gaseous N loss via denitrification can be measured with static flux chambers when acetylene (C2H2) is used to block nitrous oxide (N2) reduction. This paper describes a method for the introduction of C2H2at low concentrations into the soil by diffusion. The method utilizes an above‐ground manifold connected to diffusion tubes inserted in the soil and C2H2flows through the manifold at a rate of 0.33 mL s−1. Acetylene levels in the soil atmosphere within the flux chamber site ranged from 0.16 to 2.07 mol m−3, and the apparatus did not alter CO2fluxes as measured by a static chamber method.

The quantities of N mineralized when columns of Histosols were incubated at field capacity for periods up to 28 days corresponded to 500‐600 kg N/ha per year without correction for N lost by denitrification. An absorption train utilizing molecular sieve 5Å to collect N2O was used in denitrification experiments. Under an argon stream, 52‐53% of the added NO3‐‐N was recovered as N2O, whereas the maximum concentration of N2O accumulating in the headspace of closed vessels amounted to 65% of the added NO3‐‐N. The quantity of N2O either collected or accumulating in a headspace was reduced to 15‐41% of the NO3‐‐N denitrified when a second addition of NO3‐was made to the soils. Denitrification occurred in both drained and flooded soil columns amended with NO3‐. The N2O recovered ranged from 2.5‐9% and 9‐19% of the NO3‐‐N denitrified in the drained and flooded columns, respectively. From 78‐98% of the N2O was recovered in leachate from the flooded columns and the corresponding range for the drained columns was 11‐52%.