Showing papers in "Soil Science Society of America Journal in 1982"

Changes in Inorganic and Organic Soil Phosphorus Fractions Induced by Cultivation Practices and by Laboratory Incubations

Abstract: Changes in inorganic and organic phosphorus (P) fractions resulting from 65 years of cropping in a wheat-wheat-fallow rotation were studied using a sequential extraction technique. Total P content of the cultivated soil was 29% lower than that of the adjacent permanent pasture; the major loss of P (74% of total P lost) was organic P and residual P. Of the total P lost, 22% was from the extractable organic P forms, whereas 52% originated from stable P.Incubation studies were used to study seasonal P transformations during simulated fallow with and without residue incorporation and P fertilization. Nine monthly additions of cellulose (765 µg C · g−1 soil) with and without P (9 µg · g−1 soil) significantly altered levels of total extractable organic P and inorganic P in incubated soils. Evidence is provided for microbial activity playing a major role in redistributing P into different forms in the soil

Trace metal chemistry in arid-zone field soils amended with sewage sludge: I. Fractionation of Ni, Cu, Zn, Cd, and Pb in solid phases

Abstract: The surface horizons of two arid-zone fields soils that had received amendments of either liquid or dried, anaerobically digested sewage sludge for 4 years were sampled to determine the forms of selected trace metals in the solid phase. The soils had been amended with sludge twice annually at rates of 0, 22.5, 45.0, or 90.0 tons . ha/sup -1/ . year/sup -1/. Barley and sorghum had been grown on the soils in randomized experimental plots. The soil samples were analyzed for total Ni, Cu, Zn, Cd, and Pb and were fractionated by sequential extraction to estimate the quantities of these metals in ''exchangeable,'' ''sorbed,'' ''organic,'' ''carbonate,'' and ''sulfide'' forms. The total contents of the five metals in the two field soils were governed by the total content of the metals in the sludges applied and by the rate of sludge application. The accumulation of metals in the surface horizons of field plots receiving liquid sludge was less than that in the plots receiving composted sludge, possibly because of a lesser reduction in soil bulk density resulting from sludge applications. The percentage of the total metal content in exchangeable and sorbed forms was very low, averaging between 1.1 and 3.7% formore » all of the metals regardless of the type of soil, the form of sludge applied, or the sludge application rate. The application of sludge tended to reduce the sulfide fraction and to increase the organic and carbonate fractions of all five trace metals. At the highest rate of sludge application, the predominant forms of the metals were: Ni, sulfide; Cu, organic; and Zn, Cd, and Pb, carbonate.« less

Effects of Nutrient Accumulation by Aspen, Spruce, and Pine on Soil Properties

Abstract: Nutrient analysis was done for adjacent, 40-year-old stands of pure quaking aspen (Populus tremuloides Michx.), white spruce (Picea glauca Moench Voss), red pine (Pinus resinosa Ait.) and jack pine (Pinus banksiana Lamb.) on two soils in Minnesota to determine the effects of tree species on soil properties. On both soils, aspen and spruce stands accumulated more of most nutrients than did pine stands, and these species differences were reflected in the litterfall. The weight of the forest floor did not differ among species, but nutrient accumulation and pH were greatest under aspen and spruce. Calcium content was about twice as high under aspen and spruce as under the pines. In the mineral soil, phosphorus (P) and potassium (K) did not differ among species; organic matter and nitrogen (N) tended to be lowest under aspen, and calcium (Ca) was much lower under aspen and spruce than under the pines. Soil pH and cation exchange capacity were highest under the pines; this was directly related to soil Ca contents. Mineral soil differences related to species were most pronounced in the top 10 cm; few differences occurred below 25 cm. The large species differences in the N, Ca, and Mg contents of vegetation, forest floor, and mineral soil show a redistribution of these nutrients, but their amounts in the entire ecosystem do not differ by species. In contrast, P and K in the ecosystem decrease in the order aspen>spruce>pines, and this is largely a reflection of their accumulation in the vegetation since their differences in the soil are minimal. Additional Index Words: calcium, litterfall, pH, species effects. View complete article To view this complete article, insert Disc 4 then click button8

Soil Bulk Density Analysis in Three Dimensions by Computed Tomographic Scanning

Abstract: Current soil bulk densitometric techniques lack the precision to detect three-dimensional changes in density in a nondestructive manner. The x-ray transmission computed tomography (CT) scanner was evaluated as a tool to determine soil bulk density. This scanner is an advanced tool in diagnostic radiology used to obtain a nondestructive cross-sectional representation of the human body. Typical accuracy and precision for CT scanners are known for materials like human tissue with a linear attenuation coefficient near water. Machine response was evaluated near the upper limit of the measurement range where denser materials, such as soil, are located. Scanner analyses of soil and glass bead-air filled sphere samples that varied in bulk density from 0.14 to 1.64 g/cm³ revealed that a positive linear response occurred with increasing density. For the Metea sandy loam soil (Arenic Hapludalfs), this CT scanner was found to have precision in the order of 19 mg/cm³. Spatial resolution or the ability to distinguish between two objects in the scanning plane was found to range from 1.25 by 1.25 by 2 mm³ to 6.4 mm in diameter by 2 mm. The greater resolution was obtained when density within the sample varied greatly. Errors in data can occur as a result of certain machine artifacts; however, many of these can be avoided by relatively simple methods. It was concluded that the CT scanner can be used to determine soil bulk density with good three-dimensional spatial resolution. This is a potentially promising tool for research in the areas of compaction, soil management, and cultivation.

Separating Soil Iron- and Manganese-Oxide Fractions for Microelement Analysis

Abstract: Since sodium dithionite (Na₂S₂O₄) is often contaminated with zinc (Zn) and can form metal sulfide precipitates, it is not suitable for solubilizing iron (Fe) oxides in fractionation schemes for soil microelements. The objective was to find an alternate method to solubilize crystalline Fe oxides that would fit into a scheme including extractants for amorphous Fe and manganese (Mn) oxides. Three soils were extracted with seven reagents designed to remove amorphous and/or crystalline Fe or Mn oxides [0.1M Na₄P₂O₇, pH 10.0; 0.2M (NH₄)₂C₂O₄ in 0.2M H₂C₂O₄, pH 3.0 (oxalate); 0.1M NH₂OH-HCl, pH 2.0; 1.0M NH₂OH-HCl in 25% acetic acid; 0.1M ascorbic acid in the oxalate solution; 0.1 g SnCl₂ per gram of soil in the oxalate solution; and 1.0 g dithionite per gram of soil in citrate buffer]. The Na₄P₂O₇, an extractant for elements associated with the organic fraction, extracted amounts of Fe similar to that for the oxalate solution. The two NH₂OH-HCl extractants solubilized very little Fe (<1% total), but NH₂OH-HCl alone solubilized as much Mn as most of the other extractants indicating that it is specific for Mn oxides. The ascorbic acid-oxalate and SnCl₂-oxalate experimental methods extracted amounts of Fe similar to the dithionite method and amounts of Al higher than the dithionite method. Of the amorphous Fe-oxide extractants, the oxalate solution solubilized the most Zn and Cu, whereas of the crystalline Fe-oxide extractants, the ascorbic acid-oxalate solubilized the highest amounts of Zn and Cu. The extractants suggested for a fractionation scheme are NH₂OH-HCl for Mn oxides, oxalate solution shaken with the soil in the dark for amorphous Fe oxides, and ascorbic acid-oxalate for crystalline Fe oxides.

Toxicity of Aluminum to Coffee in Ultisols and Oxisols Amended with CaCO3, MgCO3, and CaSO4·2H2O

Abstract: A greenhouse experiment was conducted with six acid soils from southern Brazil to investigate the effect of available Al on growth and mineral nutrition of coffee (Coffea arabica L.) seedlings. Coffee seedlings were grown for 7 months in pots containing soil treated with varying amounts of CaCO/sub 3/ up to twice the lime equivalent, and amounts of MgCO/sub 3/ and CaSO/sub 4/ x 2H/sub 2/O equal to the lime equivalent. Leaf samples were collected immediately before harvesting the seedlings and analyzed for Ca and Al. At this time, soil was collected from each pot and analyzed for exchangeable cations and soluble ions. The chemical composition of the soil solution was used as input data for a computer program (GEOCHEM) to chemically speciate Al in the soil solutions. Shoot and root weights were correlated with KCl-exchangeable Al of soil, percent Al saturation of soil, the concentrations of total Al (Al/sub t/) and Al/sup 3 +/ (calculated), and the activity of Al/sup 3 +/ (calculated) in the soil solution. Growth reductions of the seedlings correlated best with the Al/sup 3 +/ activity value. The toxicity threshold for the Al/sup 3 +/ activity was approximately 4.0 x 10/sup -6/. Leaf Al concentrations likewisemore » correlated best with Al/sup 3 +/ activity. Threshold leaf Al concentrations of approximately 62 and 100 ..mu..g/g, respectively, were observed for reduction in root and shoot growth.« less

Phosphorus Accumulation and Toxicity in Leaves in Relation to Zinc Supply1

Abstract: The relationship of zinc (Zn) deficiency to phosphorus (P) toxicity was examined in 'Emerald' okra [Abelmoschus esculentus (L.) Moench] grown for 21 days (D21) in complete nutrient solution and then transferred to treatments with two levels of P (P1, P2 = 250, 2,000 P) and four levels of Zn (Zn1, Zn2, Zn3, Za4 = 0, 0.25, 1, 2 added Zn). No symptoms of "little leaf" or "resetting" developed in any plants. "Mottle leaf" symptoms developed as chlorotic and necrotic patches in the interveinal areas of the older leaves of Zn, plants at P1 and in Zn1, Zn2, and Zn3 plants at P2. Thus, increasing the level of P induced symptoms which were eliminated by adding Zn. Treatments had little effect on dry matter at D45 and D63: at D111, low Zn depressed dry weight of P1 tops and even more markedly depressed dry weight of P2 tops, roots, and fruits. Increasing solution P increased the concentration and amount of P in all plant parts at all harvests. At D45 it also depressed Zn concentrations in all parts of Zn1 plants but in no other Zn treatment and in no treatment at later harvests. At Zn1, leaves had relatively low concentrations of Zn and high concentrations of P. Increasing levels of Zn enhanced their Zn concentrations and depressed their P concentrations; at D45 and D63 they also depressed the concentrations and total amounts of P in tops and whole plants but increased them in roots. Thus Zn deficiency markedly enhanced P absorption by roots, transport to tops, and accumulation in leaves. Concentrations of magnesium in plant organs responded to treatments in the same way as did P but to a smaller degree, particularly in the leaves. No other element measured (Ca, K, Na, Fe, Cu, Mn) responded in the same way. The development and intensity of symptoms in old leaves correlated closely with P concentrations. Leaves with faint symptoms had 0.8% P; those with marked symptoms had > 1.5% P; and dead leaves had nearly 5% P. The intensification of symptoms is attributed to accumulation of P to toxic levels in leaves. It is suggested that Zn deficiency interferes with P metabolism enhancing the amounts of P absorbed by roots and transported to tops: under conditions of high P supply, P accumulates to toxic levels in leaves inducing or accentuating symptoms resembling Zn deficiency. This effect of Zn on P metabolism in roots seems to explain previously puzzling observations in which P treatments enhanced symptoms of Zn deficiency without any reduction in Zn contents of plant tops: there is no need to invoke any effect of P in inactivating Zn in leaves. Additional Index Words: P-Zn interactions, Zn deficiency, Zn requirements, Abelmoschus esculentus (L.) Moench. View complete article To view this complete article, insert Disc 4 then click button8

Influence of Salinity on Soil Enzyme Activities

Abstract: Efforts were made to assess the levels of soil enzyme activities that have a specific role in the N, C, P, and S cycles of saline soils. Fieldmoist soil samples were treated with four rates of CaCl₂, NaCl, and Na₂SO₄ solutions applied to produce electrical conductivity readings of saturation extracts (ECₑ) ranging up to 22 mmho/cm. The range of ECₑ values included threshold salinity levels associated with reduced yields of agronomic crops. After 7 d of equilibration, the following soil enzymes were assayed: amidase, urease, acid phosphatase, alkaline phosphatase, phosphodiesterase, inorganic pyrophosphatase, arylsulfatase, rhodanese, α-glucosidase, α-galactosidase, dehydrogenase, and catalase. Soil enzyme activities decreased with increasing ECₑ; however, the degree of inhibition varied among the enzymes assayed and the nature and amounts of salts added. Dehydrogenase activity was severely inhibited by salinity, whereas, the hydrolases showed a much lesser degree of inhibition. Generally, the inhibition of soil enzyme activities by the salt solutions decreased in the following order when compared at the same ECₑ level: NaCl > CaCl₂ > Na₂SO₄. Reduced enzyme activities in saline soils may be due to the osmotic desiccation of microbial cells releasing intracellular enzymes which become vulnerable to attack by soil proteases, a “salting-out” effect modifying the ionic conformation of the active site of the enzyme-protein, and specific ion toxicities causing nutritional imbalances for microbial growth and subsequent enzyme synthesis.

Toxicity of Aluminum to Coffee Seedlings Grown in Nutrient Solution

Abstract: A solution culture experiment was conducted with coffee seedlings (Coffea arabica L.) to determine the combined effect of ionic strength and aluminum (Al) concentration of nutrient solutions on Al availability. The treatments consisted of three Al concentrations: 0.0, 0.037, and 0.148 mmol dm⁻³. These were combined with five levels of ionic strength produced by diluting a complete nutrient solution to relative concentrations of 0.5, 0.25, 0.10, and 0.01. The phosphorus (P) concentration of each solution was adjusted to 0.1 mmol dm⁻³. The pH of the solutions was maintained at 4.0 ± 0.2. Shoot and root growth decreased progressively under the Al treatments. The lateral roots of Al-stressed plants were thicker, shorter, and fewer in number than those of the control plants. The younger leaves of Al-stressed plants were small, curled along the margins, and frequently chlorotic along the leaf margin. At a constant Al concentration, seedling growth was reduced with dilution of the nutrient solution. This decrease in growth was best correlated with an increase in the activity of Al³⁺. An Al³⁺ activity value of 1.2 × 10⁻⁵ was associated with a 10% reduction of coffee seedling growth.

Dynamics of Ammonia Volatilization during Furrow Irrigation of Maize

Abstract: In irrigated agriculture, supplemental nitrogen (N) is commonly applied by dissolving ammonia (NH₃) gas in the irrigation water. The operation is convenient and labor-saving, but because NH₃ is volatile, the potential for loss is high. A micrometeorological method was used to measure losses of NH₃ gas during applications of water-run NH₃ to short (0.9 m) and tall (2.1 m) maize crops. The equilibrium NH₃ vapor pressure of the irrigation water and wind speed both affected the loss, but the influence of the latter was very marked, volatilization increasing with the approximate square of the wind speed. Average volatilization losses were 7% of the N present in the irrigation water per hour from the short crop, and 1% from the tall crop, the difference arising from the greater attenuation of wind in the tall crop. Results for both crops were reconciled by considering transfer through an air layer close to the ground. Volatilization rate could then be expressed as the product of a wind-dependent exchange coefficient and the NH₃ vapor pressure difference between the water surface and 0.3 m. Volatilization losses resulted in very uneven applications of N. The nitrogen content of the irrigation water decreased by 84% over a distance of 400 m along the furrow in the short crop, and by 59% in the tall crop. Some practical remedies are suggested, including irrigating at night. In these experiments, night-time volatilization rates were only one-half those observed by day for similar aqueous NH₃ concentrations.

The Mechanism of Raindrop Splash on Soil Surfaces

Abstract: From the results of high-speed photography of 4.6-mm-diam drops impacting various soil materials and from soil mechanics principles, a new concept in describing the mechanism of soil detachment from raindrops impacting on saturated soil surfaces is proposed. The impulsive loading caused by the impacting drop does not permit time for drainage; thus there is no change in total soil volume or bulk density. The soil surface is deformed under the impulsive load application of the drop; however, the vertical strain under the impact area is compensated by a bulge around the perimeter of the depression. The vertical force of the drop is transformed to lateral shear caused by radial flow of the impacting drop. Splash angle is determined by the depth of the cavity and the size of the bulge surrounding it. Splash angle was highly correlated with soil shear strength as measured by the fall-cone method. Low soil strength resulted in (i) a larger cavity and surrounding bulge, (ii) a greater detachment of soil particles due to the shear stress of the radial flow, and (iii) a greater splash angle with the horizon.

Organic and inorganic sulfur constituents of a forest soil and their relationship to microbial activity.

Abstract: Sulfur (S) constituents, microbial biomass, and sulfohydrolase activity were determined for each soil horizon at both hardwood and conifer sites in a Becket soil (Adirondack Mountains, New York). Drying of soil before analysis altered the S constituents. There was a threefold increase (p<0.05) in sulfate in the organic horizons. Total S was greatest in the O horizons with 2,010 and 1,690 ..mu..g S/g in conifer and hardwood solums, respectively. Mineral soil had a maximum S concentration in the B21h horizon. Sulfate concentrations were a small proportion (<15%) of total S in B horizons. Organic S was dominant (93% of total S) in all horizons. Carbon-bonded S and ester sulfate were 74 and 18% of total S, respectively. Microbial biomass was greatest in the O1 horizon of both hardwood and conifer solums (59 and 70 mg biomass C per 100 g/sup -1/ dry mass, respectively). The B21h horizon contained the greatest biomass in the mineral soil. Sulfohydrolase activity exhibited the same distribution. Total S, carbon-bonded S, and ester sulfate were all positively correlated (p<0.05) to percent organic matter in the soil horizons. Correlations between microbial biomass and sulfohydrolase activity with organic S indicate the potential for microbial S transformations. Sulfate formationmore » by mineralization may be more important than exogenous inputs. This has major implications for assessing the impact of atmospheric S deposition on soils.« less

An Improved Technique for Determining Soil Electrical Conductivity-Depth Relations from Above-ground Electromagnetic Measurements1

Abstract: An improved method has been developed for determining the distribution of bulk soil electrical conductivity, EC0, through the soil from electromagnetic measurements taken at the soil surface with the Geonics Limited EM-38 device. Induced electromagnetic conductivity readings taken with the EM-38 device's coil configuration oriented parallel and then perpendicular to the soil surface provided sufficient information, when used with equations derived from geophysical instrumentation data, to produce a soil electrical conductivity-depth profile. The simplicity of this method further enhances the praiticability of the newly developed electromagnetic technique for field measurements of salinity and for saline seep diagnosis. Additional Index Words: soil salinity, soil resistivity, electromagnetic conductivity. Convin, D. L., and J. D. Rhoades. 1982. An improved technique for determining soil electrical conductivity-depth relations from above-ground electromagnetic measurements. Soil Sci. Soc. Am. J. 46:517-520. R Rhoades and Corwin (1981) have shown that bulk soil electrical conductivity, ECfl, of incremental depth intervals within the soil profile can be obtained from above-ground electromagnetic measurements of apparent soil electrical conductivity, EM, using multiple regression coefficients which relate electromagnetic conductivity to ECa. This initial method required the solution of a complex system of simultaneous equations. The coefficients of these equations were determined by multiple regression analyses of EM readings taken at five incremental heights (0, 0.3, 0.6, 0.9, and 1.2 m) above the soil and of ECa values measured (using a four-electrode probe) at corresponding depths (0 to 0.3, 0.3 to 0.6, 0.6 to 0.9, and 0.9 to 1.2 m) in the soil. It is the purpose of this paper to describe a less complicated method for obtaining ECa-depth relations of any soil from only two EM measurements taken at the soil surface. In 1 Contribution from the U. S. Salinity Laboratory, USDA, Riverside, CA 92501. Received 9 Sept. 1981. Approved 27 Jan. 1982. 2 Soil Scientist and Supervisory Soil Scientist, respectively. 3 Geonics Limited, 1745 Meyerside Drive, Mississauga, Ontario. Canada. this approach the EM readings are related to ECa by a series of simple equations derived from instrumentation data provided by the manufacturer of the EM device (EM-38).

Diurnal Variability in Rate of Emission of Nitrous Oxide from Soils

Abstract: Diurnal variability in the rate of emission of nitrous oxide (N/sub 2/O) from Iowa soils was studied by using a chamber technique to measure the N/sub 2/O emission rates at sites on fertilized and unfertilized soils at 1- or 2-h intervals for periods of from 1 to 5 d. The coefficients of variation for the rates of N/sub 2/O emission observed at these sites within 24-h periods ranged from 12 to 66% and averaged 38%. As much as 90% of the diurnal variability observed could be attributed to diurnal N/sub 2/O emission patterns related to changes in soil temperature. Observations reported suggest that these patterns were caused largely by changes in the solubility of N/sub 2/O in soil water induced by diurnal changes in soil temperature, that the amplitudes of these patterns were determined largely by the amounts of water and N/sub 2/O in the surface soil, that the times of minima and maxima in these patterns were determined by the depth at which temperature-induced changes in the solubility of N/sub 2/O in water were significant, and that neither the amplitudes nor the times of minima and maxima in these patterns can be predicted solely from soil temperature. Data reportedmore » show that there is no short time during a 24-h period that is always satisfactory for assessing the amount of N/sub 2/O evolved during that period.« less

Field Measurement of Denitrification: III. Rates During Irrigation Cycles1

Abstract: Absolute amounts and rates of denitrification from a Yolo loam field profile at Davis, Calif., were studied in relation to the influence of irrigation frequency and soil incorporation of crop residue. Two different carbon (C) treatments were established by using plots to which no crop residues had been incorporated within 1 year prior to the experiment and plots to which 10 metric tons ha of chopped barley straw were incorporated into the top 10 cm of soil 2 months prior to fertilization. The same total amount of water was applied at frequencies of three irrigations per week, one irrigation per week, and one irrigation every 2 weeks to areas cropped with perennial ryegrass. Fertilizer was applied as KNO (≅ 285 kg N ha) enriched with 56 to 60% N to 1-m plots. The surface flux of denitrification gases was measured from the accumulation of nitrous oxide (NO) and N beneath airtight covers placed over the soil and from measurements of NO using the acetylene (CH) inhibition method. Small fluxes of denitrification gases were measured in this well-drained alluvial soil under normal cyclic applications of irrigation water. Total denitrification ranged from 0.7 to 5% for the least frequently irrigated (no straw) and most frequently irrigated (straw) plots, respectively. Surface denitrification gas fluxes were largest after the first irrigation, decreased to near zero values within 1 or 2 days after each irrigation, and generally decreased for subsequent irrigations. The amount of N produced was much greater than NO. The nitrous oxide flux at the soil surface varied between 5 and 27% of the total denitrification over a 40- to 50-day period. Nitrous oxide mole fractions tended to be smallest immediately after irrigation and increased as the soil water redistributed and the soil profile became less anoxic. The irrigation frequency of three irrigations per week gave higher soil NO concentrations within the root zone of the crop than those of the other two frequencies. Thus, frequent, small irrigations may result in less leaching losses than infrequent, large irrigations.

The Effect of Carbon Mineralization on Denitrification Kinetics in Mineral and Organic Soils

Abstract: Rates of denitrification and organic carbon (C) mineralization were measured simultaneously in soil suspensions maintained at 30°C under anoxic conditions. Nine mineral and seven organic soils were used in the study. Disappearance of NO3~ and production of CO2 were measured at various times during the 12-day incubation. Labeled NO, was used to differentiate denitrification from immobilization and reduction to NH4-N. The rate of organic C mineralization followed first-order kinetics in all soils with the mineralization rate coefficient (kc) values varying from 0.075 to 0.405 day'. The denitrification rates in anaerobic soils were shown to be proportional to the concentration of the two substrates: NO, and available C. The denitrification rate coefficient (&„) value was essentially constant for the mineral soils [0.00147 ± 25% day" (fig C/mi)"], while kn values for the organic soils were somewhat more variable [0.00155 ± 65% day" (jig C/ml)"']. Significant correlations were observed between NO," consumption and CO, production. The molar ratio of NO, consumption to CO, production ranged from 0.6 to 1.8. Significant relationships were also observed between water-soluble C (WSC) and total organic C (TOC), maximum available C (C^,,), and WSC and Cmls, respectively. Watersoluble C represented 0.4 to 0.9% of TOC, while Cm represented about 0.6 to 1.4% of TOC. Results also showed that denitrification rates were influenced by the rate at which available C is mineralized and made available to the organisms. Additional Index Words: nitrate reduction, water-soluble carbon, first-order kinetics, anaerobic soil, flooded soil, Michaelis-Menton kinetics. Reddy, K. R., P. S. C. Rao, and R. E. Jessup. 1982. The effect of carbon mineralization on denitrification kinetics in mineral and organic soils. Soil Sci. Soc. Am. J. 46:62-68. T ROLE of organic carbon (C) as an electron donor in the denitrification process has been widely recognized. Several researchers (Wijler and Delwiche, 1954; Bremner and Shaw, 1958; Nommik, 1956; Bowman and Focht, 1974; Reddy et al., 1978) have shown increased denitrification rates following the addition of energy source (organic C) to a soil. Significant relationship was reported between NOf losses via denitrification and "available" C, as evaluated either by glucose-equivalent C (Stanford et al., 1975a), by watersoluble C (Burford and Bremner, 1975; Reddy et al., 1980), or by mineralizable C (Burford and Bremner, 1975). A considerable portion of available C would be used in normal oxidative respiration by denitrifiers and 1 Florida Agric. Exp. Stn. Journal Series no. 2,902. Received 15 Jan. 1981. Approved 31 Aug. 1981. 2 Assistant Professor, Agric. Res. and Ed. Cen., Univ. of Florida, Sanford, Fl 32771, and Assistant Professor and Scientific Programmer, 'respectively, Soil Sci. Dep., Univ. of Florida, Gainesville, FL 32611. other microorganisms (using O2 as an electron acceptor) until the system becomes anoxic. When O2 is depleted from the system, the denitrifiers use NO3~ as an electron acceptor during their oxidative respiration. Once the easily decomposible C is used by the denitrifiers, the rate of NO3~ loss would depend on the rate of soil organic C conversion to mineralizable organic C and to soluble organic C (Focht and Verstraete, 1977), while C loss in the same system may not depend on the availability of NO3~, because some organisms use Fe, Mn, and SO4 as their electron acceptors during C oxidation. Denitrification rates, expressed either as zero-order (Wiljer and Delwiche, 1954; Nommik, 1956; Patrick, 1960; Reddy et al., 1978) or first-order (Stanford et al., 1975a; Reddy et al., 1980) rate coefficients, and which do not account for the soluble soil organic C availability will be specific to the soil and a given set of environmental conditions, and not of general value in modeling the denitrification process. Also, most researchers (Stanford et al., 1975a; Bowman and Focht, 1974; Burford and Bremner, 1975; Kohl et al., 1976) measured denitrification rates under static conditions where NO3~ was present in the overlying water and the underlying soil. Since denitrification occurs only in the anaerobic soil layer, diffusion of NO3~ from the floodwater to the anaerobic soil layer also controls the rate of denitrification (Reddy et al., 1978). No data are available to date on the kinetics of available C mineralization coupled with the kinetics of denitrification measured under diffusion nonlimiting conditions. The objectives of the present investigation were (i) to determine simultaneously the rates of denitrification and C mineralization in several mineral and organic soils; (ii) to develop a relationship between the kinetics of denitrification and organic C mineralization; and (iii) to obtain a denitrification rate coefficient as a function only of available C (i.e., independent of soil type). REACTION RATE EQUATIONS In this study, the rate of available C mineralization during denitrification was assumed to follow first-order kinetics: