THE HIGH- AND LOW-ENERGY PHOSPHATE ADSORBING SURFACES IN CALCAREOUS SOILS

The two-surface Langmuir equation was used to study P adsorption by 24 calcareous soils (pH 7.2-7.6; 0.8-24.2 per cent CaCO₃) from the Sherborne soil series, which are derived from Jurassic limestone. High-energy P adsorption capacities (x″_m) ranged from 140–345 μg P/g and were most closely correlated with dithionite-soluble Fe. Hydrous oxides therefore appear to provide the principal sites, even in calcareous soils, on which P is strongly adsorbed (x″_m 6–51 ml/μg P). The low-energy adsorption capacities (x″_m) ranged from 400–663 μg P/g and were correlated with organic matter contents and the total surface areas of CaCO₃ but not with per cent CaCO₃, pH, or dithionite-soluble Fe. Total surface areas of CaCO₃ in the soils ranged from 4.0 to 8.5 m²/g soil. Low-energy P adsorption capacities agree reasonably with values (100 pg P/m²) for the sorption of phosphate on Jurassic limestones but phosphate was bonded much less strongly by soil carbonates (k″= 0.08–0.45 ml/μg P) than by limestones (k～10.0 ml/μg P). Low-energy P adsorption in these soils is tentatively ascribed to adsorption on sites already occupied by organic anions (and probably also by bicarbonate and silicate ions) which lessen the bonding energy of co-adsorbed P.     

EFFECTS OF CATION EXCHANGE MATERIAL ON ZINC ADSORPTION BY SOILS

The adsorption of Zn by soils which are different in their major cation-exchange materials was measured at equilibrium Zn concentrations up to 10^?2 M in 10^?2 to 10^?3 M CaCl₂. The results are interpreted on K^Zn_Ca[Zn]_soil plots, where K^Zn_Ca is the selectivity coefficient defined by the equation All natural samples except those containing halloysite exhibited no or very small specific Zn adsorption. All Ca-saturated samples exhibited specific Zn adsorption dependent on cation-exchange materials. The cation-exchange sites with high selectivities for Zn (K^Zn_Ca > 10) constitute more than 40 per cent of the total exchange sites in soils containing allophane, imogolite, and halloysite, whereas those with moderate to low selectivities for Zn (K^Za_Ca < 10) predominate in montmorillonitic, vermiculitic, and humic soils. Differences in the contribution of the respective cation-exchange materials to specific Zn adsorption are discussed relating to differences in the origin of their negative charge.     

ADSORPTION OF PHOSPHATE BY VARIOUS OXIDES: THEORETICAL TREATMENT OF THE ADSORPTION ENVELOPE

An explanation is put forward for the shape of adsorption envelopes found for phosphate adsorption by various metallic oxides. The equation x_m= C₁ (μH₃PO⁴⁺μ∑anions) is proposed, where μH₃pO₄ is the chemical potential of undissociated H₃PO₄; μ∑anions is the chemical potential of all phosphate anions considered as one. component; C₁ is a constant that includes influences of surface charge, chemical affinity of the metal for phosphate, specific surface area, etc., and x_mis the calculated Langmuir maximum adsorption of P at each pH. The dependence of C₁ on the metal present in the oxide is shown.     

Effects of rainfall pattern on carbon and nitrogen dynamics in soil amended with biogas slurry and composted cattle manure

Soil moisture affects the degradation of organic fertilizers in soils considerably, but less is known about the importance of rainfall pattern on the turnover of C and N. The objective of this study was to determine the effects of different rainfall patterns on C and N dynamics in soil amended with either biogas slurry (BS) or composted cattle manure (CM). Undisturbed soil cores without (control) or with BS or CM, which were incorporated at a rate of 100 kg N ha^–1, were incubated for 140 d at 13.5°C. Irrigation treatments were (1) continuous irrigation (cont_irr; 3 mm d^–1); (2) partial drying and stronger irrigation (part_dry; no irrigation for 3 weeks, 1 week with 13.5 mm d^–1), and (3) periodic heavy rainfall (hvy_rain; 24 mm d^–1 every 3 weeks for 1 d and 2 mm d^–1 for the other days). The average irrigation was 3 mm d^–1 in each treatment. Cumulative emissions of CO₂ and N₂O from soils amended with BS were 92.8 g CO₂‐C m^–2 and 162.4 mg N₂O‐N m^–2, respectively, whereas emissions from soils amended with CM were 87.8 g CO₂‐C m^–2 and only 38.9 mg N₂O‐N m^–2. While both organic fertilizers significantly increased CO₂ production compared to the control, N₂O emissions were only significantly increased in the BS‐amended soil. Under the conditions of the experiment, the rainfall pattern affected the temporal production of CO₂ and N₂O, but not the cumulative emissions. Cumulative NO leaching was highest in the BS‐amended soils (9.2 g NO ‐N m^–2) followed by the CM‐amended soil (6.1 g NO ‐N m^–2) and lowest in the control (4.7 g NO ‐N m^–2). Nitrate leaching was also independent of the rainfall pattern. Our study shows that rainfall pattern may not affect CO₂ and N₂O emissions and NO leaching markedly provided that the soil does not completely dry out.     

Phosphatsorption einiger Böden in Bayern

Phosphate adsorption of some Bavarian soils . Phosphate adsorption isotherms were established in buffered (pH 4.6, 5.5, 7.0) and some in unbuffered (0.01 M CaCl₂) systems for 9 soil profiles, 6 of which represent loess soils in various stages of development. In the P-equilibrium concentration range of 0–10^?4 mole/l two to three ranges can be identified, each of which obeys the Langmuir equation. In the lowest concentration range (0–0.2·10^?4 mole/l) adsorption is linearly related to equilibrium concentration. For the different Langmuir ranges the adsorption maxima (b) increase and the adsorption coefficients (k) decrease with increasing equilibrium concentration. The maxima depend on buffer-pH in the order 5.5<4.6<7.0. A single point method similar to the one recently proposed by Bache and Williams (1971) was succesfully applied to obtain b from a single measurement using a significant correlation between b from complete isotherms and the ratio of P adsorbed to that in solution after the addition of a fixed amount of P (0.8–1,6 mg P/g of soil). The b-values in the equilibrium concentration range of 1–20·10^?4 mole/l depend mainly on soil pH (negative) and clay (positive) (multiple correlation coefficient r = 0.858). The common reason for this appears to be the amount of exchangeable Al which is also significantly correlated with b. During soil development the depth function of b changes due to decalcification, drop of pH and migration of clay and iron oxides.     

A new method for the estimation of total dissolved salts in saturation extracts of soils from electrical conductivity

We have determined electrical conductivity (E_e) and total dissolved salts (S) in saturation extracts from 39 soil samples from the Baza basin (Province of Granada, south-east Spain). E_e ranged from 2.8 to 110dS m^?3, and S from 2 to 444 8 dm^?3. The relationship between S and E_e was not linear. When the saturation extracts were diluted with progressively larger quantities of distilled water and their electrical conductivity calculated (E_ec) with the equation where E_d and E_w are the conductivity of the diluted extract and the distilled water and f is the dilution factor, the relationship between S and E_ec tended to become linear. The highest linear correlation coefficient relating S (mg dm^?3) and E_ec (dS m^?1) was reached when E_ec, values were calculated for dilutions with a conductivity (E_d) between 0.1 and 0.3 dS m^?1 (E*_ec). The regression equation was S= 490 E*_ec with r²= 0.999. This relationship can be used in all saturation extracts, regardless of the concentration and type of ions present.     

Das Verhalten von Phosphat in eisenoxidreichen Kalkgleyen der Münchener Schotterebene

Behaviour of phosphate in iron oxide-rich, calcareous gleys of the Münchener Schotterebene Iron oxide-rich calcareous humic gleys of the Münchener Schotterebene (a calcareous gravel plain of the Isar river), induce phosphate deficiency symptoms particularly on corn. Phosphate adsorption on these soils increases with increasing Fe-oxide concentration and shows a strong slow rate component where log (P equilibrium concentration) is linearly correlated with log (time). The Fe-oxide in these soils is a goethite consisting of needles approximately 50 nm in length and 5 nm in width with a surface area of approximately 100 m²/g. On this surface ～ 2 μmol P/m² can be adsorbed in the presence of calcite. Adsorption was measured in various electrolytes and follows the order 0.003 M CaCl₂ > 0.009 M KCl > H₂O.     

Abiotic uptake of gases by organic soils

Methodological and experimental studies of the abiotic uptake of gaseous substances by organic soils were performed. The static adsorption method of closed vessels for assessing the interaction of gases with the solid and liquid soil phases and the dynamic method of determining the sorption isotherms of gases by soils were analyzed. The theoretical substantiation of the methods and their practical implementations on the basis of a PGA-7 portable gas analyzer (Russia) were considered. Good agreement between the equilibrium sorption isotherms of the gases and the Langmuir model was revealed; for the real ranges of natural gas concentrations, this model can be reduced to the linear Henry equation. The limit values of the gas sorption (Langmuir monolayer capacity) are typical for dry samples; they vary from 670–4000 g/m³ for methane and oxygen to 20 000–25 000 g/m³ for carbon dioxide. The linear distribution coefficients of gases between the solid and gas phases of organic soils (Henry constants) are 8–18 units for poorly sorbed gases (O₂, CH₄) and 40–60 units for CO₂. The kinetics of the chemicophysical uptake of gases by the soil studied is linear in character and obeys the relaxation kinetic model of the first order with the corresponding relaxation constants, which vary from 1 h ^?1 in wet samples to 10 h ^?1 in dry samples.     

The solid solution equilibria of lead and cadmium in polluted soils

A method for the measurement of Pb and Cd in equilibrium soil solutions involving soil equilibration with a dilute Ca electrolyte, centrifugation and filtration to <0.2 μm was evaluated. The procedure was subsequently used for the analysis of 100 Pb- and 30 Cd-contaminated soils. Solutions were analysed for Pb- and Cd using graphite-furnace AAS and the concentrations of Pb²⁺ and Cd²⁺ were estimated using standard speciation calculations. The concentrations of Pb and Cd found in the soil solutions were in the range 3.5–3600 μg dmp ^?3 and 2.7–1278 μg dm ^?3 respectively; both ranges represented less than 0.1% of the total metal concentration in the soils. Depending on solution pH, Pb ⁺² accounted for between 42–78% of Pb in solution while about 65% of Cd in solution was present as Cd⁺². The concentrations of Pb²⁺ and Cd²⁺ in solution suggested that the soil solutions were undersaturated with respect to the solid phases PbC0₃ and CdC0₃ but supersaturated with respect to Pb₅(P0₄)₃Cl and, for some samples, Cd₃(P0₄)₂ respectively. However, for both metals, a good empirical relationship was obtained between the total metal concentration in soil (mol kg^?1), free metal concentration in solution (mol dm^?3) and solution pH. The relationships took the general form of a pH-dependent Freundlich adsorption equation: For both lead and cadmium relationships, the values ofn and K₁ were close to unity, so that the distribution coefficient could be estimated from pH and a single metal-dependent constant, K₂. The algorithms appeared to be valid over a metal concentration range of four logarithmic units and pH range of 3.5–7.5.     

土壤吸附砷酸盐动力学的初步研究

本研究目的是了解陕西省的几种土壤吸附和解吸附砷酸盐的速率和过程,以及其吸附能量。Kuo和Lotse导出的双常数速率公式拟合试验资料优于一级、二级、三级反应公式,抛物线扩散和Elovich公式等五个公式。用双常数速率公式(C=k·C₀·t^1/m)分两段拟合能进一步提高拟合优度。根据Arrhenius公式计算出的吸附活化能是0.70—3.40千卡/克分子。低的活化能表明,供试土壤对砷酸盐的吸附作用是一种完全不同于真溶液条件化学反应的物理学过程。土壤吸附和解吸附砷酸盐的速度和容量受作用时间、温度、溶液∶土壤比率,加入的砷量和浓度,以及土壤特性的影响。粘土的吸附反应常数(k),吸附量比沙壤土大。而沙壤土有高的解吸附反应速度常数(k_-1^'),砷酸盐容易被解吸附而释放出来。     

Constraining the conditions conducive to dissimilatory nitrate reduction to ammonium in temperate arable soils

Here we offer the first assessment of conditions conducive to dissimilatory nitrate reduction to ammonium (DNRA) in temperate arable soils, through an examination of the potential for this process to occur in a range of soils of contrasting characteristics. NH₄¹⁵NO₃ (6.2 g N m⁻², 25 atom % excess ¹⁵N) was applied, and recovery of ¹⁵N in the pool taken as indicative of occurrence of DNRA. Up to 5% of applied ¹⁵N was recovered in the pool 2 d after addition of N, glucose (44.6 g C m⁻²) and l-cysteine (7.7 g m⁻², 0.9 g N m⁻², 2.3 g C m⁻²). concentrations were positively correlated with soil pH, ratio, bulk density, sand content and concentration, but negatively correlated with soil C and organic N content. Our results demonstrate the potential for DNRA to contribute to N cycling in temperate arable soils, but its detection and significance is likely to depend on the provision of a low molecular weight C source.     

The role of biochar in retaining nutrients in amended tropical soils

This study investigated the effect of biochar amendments on the retention and availability of plant nutrients and Al in seven acidic tropical soils from Zambia and Indonesia. The experiments carried out investigated whether the adsorption capacity of NH$ _4^+ $ in the soils increased upon the addition of biochar and which effect biochar had on available concentrations of NO$ _3^- $ , K⁺, Mn²⁺, Mg²⁺ , PO$ _4^{3‐} $ , and Al³⁺. These nutrients were selected as they represent those important to plant growth and soil quality. No significant increases or decreases in aqueous NH$ _4^+ $ ‐N concentration with additions of biochar were detected. The Gaines–Thomas model was used in order to calculate selectivity coefficients for NH$ _4^+ $ exchange (K_gt values). Following the addition of biochar to soil, K_gt values decreased showing a reduction in the selective binding of NH$ _4^+ $ in the biochar amended soil compared to the control. The concentration of NO$ _3^- $ increased following the addition of biochar to the soils. The addition of 5 and 10% biochar to the Indonesian soil did not significantly alter (t‐test confidence level 0.05) the sorption of PO$ _4^{3‐} $ to the soil–biochar mixtures as compared to the soil alone. However, the addition of biochar to the soil from Zambia increased the sorption of PO$ _4^{3‐} $ compared to the soil alone. The concentrations of K⁺ and Mg²⁺ were significantly increased for almost all soils (t‐test at the 0.05 confidence level) following the addition of biochar. Addition of biochar to all but two soils significantly decreased (t‐test confidence level 0.05) Mn²⁺ concentrations. The concentration of Al³⁺ in the soils decreased exponentially significantly (t‐test confidence level 0.05) following the amendment of biochar in accordance with the increase in pH observed when biochar was added to the soil. These results show that biochar has the ability to release essential plant growth nutrients as well as alleviate Al toxicity in these soils.     

Nitrogen fertilization and nitrate leaching into groundwater on arable sandy soils

Nitrate leaching depending on N fertilization and different crop rotations was studied at two sites with sandy soils in N Germany between 1995 and 2000. The leaching of NO was calculated by using a numerical soil‐water and N model and regularly measured N_min values as input data. Also the variability of N_min values on the sandy soils was determined along transects. They reveal the high variability of the N_min values and show that it is not possible to confirm a significant N_min difference between fertilizer treatments using the normal N_min‐sampling intensity. Nitrate‐leaching calculations of five leaching periods showed that even strongly reduced N‐fertilizer applications did not result in a substantially lower NO leaching into the groundwater. Strong yield reductions of even more than 50%, however, were immediately measured. Mean NO concentrations in the groundwater recharge are >50 mg L^–1 and are mainly due to mineralization from soil organic matter. Obviously, the adjustment of the N cycle in the soil to a new equilibrium and a reduced NO ‐leaching rate as a consequence of lower N inputs need a much longer time span. Catch crops are the most efficient way to reduce the NO concentrations in the groundwater recharge of sandy soils. Their success, however, strongly depends on the site‐specific development possibilities of the catch crop. Even with all possible measures implemented, it will be almost impossible to reach NO concentrations <50 mg L^–1 in sandy soils. The only way to realize this goal on a regional scale could be by increasing areas with lower nitrate concentrations in the groundwater recharge like grassland and forests.     

Sulfate adsorption by variable charge soils: Effect of low-molecular-weight organic acids

Sulfate (SO₄ ^2–) movement and transport in soils has received considerable attention in recent years. In most soils, SO₄ ^2– coexists with a variety of natural organic compounds, especially organic acids. Studies were conducted to assess the effect of low-molecular-weight organic acids (eight aliphatic and five aromatic acids) on SO₄ ^2– adsorption by variable charge soils from Chile and Costa Rica. The effects of type of organic acid, pH, type of soil, and organic acid concentration were investigated. In one experiment, a 1.0 g soil sample was equilibrated with 25 ml 0, 0.5, 1.0, 2.0, 4.0, or 6.0 mM K₂SO₄ in 1 mM NaCl in the presence or absence of 5 mM citric acid. In the second set of experiments, the adsorption of 2 mM SO₄ ^2– in soils at pH 4 or pH 5 in the presence or absence of one of 13 organic acids at a concentration of 2 mM or 5 mM was studied. Results showed that citric acid significantly decreased SO₄ ^2– adsorption by the two soils. Sulfate adsorption decreased with increasing pH of the equilibrium solution. Aliphatic acids, with the exception of cis-aconitic acid, decreased the amount of SO₄ ^2– adsorbed by the two soils, with oxalic, tartaric, and citric acid showing the greatest effect. The differences in pH values of the equilibrium solutions in the presence and absence of organic acids were significantly, but negatively, correlated with the amount of SO₄ ^2– adsorbed, suggesting chemisorption of SO₄ ^2– and the release of hydroxide ions. The ionization fraction values of the organic acids at the equilibrium pH were correlated with the amounts of SO₄ ^2– adsorbed, suggesting that the protonation of surface hydroxyl groups of the mineral phase increased as the strength of the ionization of the acid increased, thus creating more positively charged surfaces. Received: 12 February 1997     

THE MEASUREMENT AND MECHANISM OF ION DIFFUSION IN SOILS. VIII —A THEORY FOR THE PROPAGATION OF CHANGES OF pH IN SOILS

The way pH changes in soil are propagated by movement of acids and bases is described. In acid soils the H₃O⁺-H₂O acid-base pair is most important, while in alkaline soils the H₂CO₃-HCO₃^? pair is always dominant, its effect depending directly on the pressure of CO₂. In neutral and slightly acid soils, soluble organic matter and the H₂PO₄^?-HPO²₄^? pair may also contribute. A soil acidity diffusion coefficient is derived, and defined as: where v_l= the volume fraction of the soil solution, f_l= the impedance factor for the liquid diffusion pathway, b_HS= the pH buffer capacity of the soil, b _HB= the pH buffer capacity of each mobile acid-base pair, Dl _HB= the diffusion coefficient of each mobile acid-base pair in free solution, and the sum is taken over all mobile acid-base pairs. The soil acidity diffusion coefficient may be used to predict the course of pH equilibration in practical situations. It is high in acid and alkaline soil, and at a minimum in slightly acid soil. It is little affected by variation of the ionic strength of the soil solution at concentrations less than 0.01M. When the pH buffer capacity of the soil is constant, and only the H₃O⁺-H₂O and H₂CO₃-HCO₃^? pairs are important, the soil acidity diffusion coefficient varies as cosh{2.303(pH—pH₀)}, where pH₀ is the pH at which the soil-acidity diffusion coefficient is a minimum.     

Ionic composition of rainwater and atmospheric aerosols in Sardinia,southern Mediterranean

The ionic composition of 55 aerosol samples and 31 precipitation events collected in a coastal site in southern Sardinia (Capo Carbonara, 39°06 N; 09°31 E) were compared. The samples were collected during one year period (Oct'90/Oct'91) and showed high variability in composition according to meteorological conditions. Rain and soluble part of aerosol showed a strikingly similar ionic composition: most significant anions were chlorine and sulphate, and sodium is the principal cations, followed by magnesium and calcium. The acid events are associated with N-NW trajectories (anthropogenic influxes from N. Europe) with avg. pH=4.65, non sea salt (nss) Ca=60 eq/l and NO₃/_nssSO₄=0.6. Southern precipitations are influenced by Saharan dust alkaline effects, with avg. pH=6.75,_nssCa=271 eq/l and NO₃/_nssSO₄=0.4. Na/Cl ratio in rain is similar to sea water (0.87), whilst in aerosols there is a Cl loss (Na/Cl=1.10), probably due to reaction with nitric acid. Total fluxes of Ca, Mg, NO₃ and SO₄ were 104, 9, 64 and 113 g/cm², and wet deposition exceeded (65–90%) dry deposition. Scavenging ratios (SR) as defined by the equation:SR=[(Ci)rain/(Ci)air] ^*d, (d=1200g/m³) were calculated, using geometric means (Ci) of precipitation and aerosols collected concurrently during the period (a total of 23 samples). The SR values are Ca=3400, Cl=2400, Na, K, SO₄=1700, Mg=1000 and NO₃=750. These numbers could be useful to infer total fluxes by using simply rainwater ionic composition in Mediterranean semi-arid sites like Sardinia.     

A novel ambipolar‐resin membrane for extracting soil nutrients

We have synthesized a novel ambipolar membrane for the simple, rapid, and simultaneous extraction of key nutrients from soil. The membrane was made by adding an anion‐ and a cation‐exchange resin to a polyvinyl alcohol hydrogel in the presence of glutaraldehyde as a cross‐linking agent. The synthetic membrane was efficient in adsorbing (extracting) NO , PO , K⁺, Ca²⁺, and Mg²⁺ ions from soil simultaneously. The ion‐adsorption capacity of the membrane was related to the soil nutrient status, duration of membrane–soil contact, and soil water content. The importance of these factors followed the order: soil nutrient status > contact time > soil water content. Adsorption by the membrane of NO and Mg²⁺ ions from soil leveled off after 48 h of membrane–soil contact but uptake of Ca²⁺, PO , and K⁺ ions required a longer contact time for equilibrium to be established. When the soil water content exceeds 55% w/w, this factor ceased to influence ion adsorption by the ambipolar‐resin membrane. The synthetic membrane is potentially useful for the in situ assessment of the nutrient requirement of certain crops at a given point in time.     

Effects of Biochar and Biosolid on Adsorption of Nitrogen,Phosphorus, and Potassium in Two Soils

Increasing the retention of nutrients by agricultural soils is of great interest to minimize losses of nutrients by leaching and/or surface runoff. Soil amendments play a role in nutrient retention by increasing the surface area and/or other chemical processes. Biochar (BC) is high carbon-containing by-product of pyrolysis of carbon-rich feedstocks to produce bioenergy. Biosolid is a by-product of wastewater treatment plant. Use of these by-products as amendments to agricultural soils is beneficial to improve soil properties, soil quality, and nutrient retention and enhance carbon sequestration. In this study, the adsorption of NH₄-N, P, and K by a sandy soil (Quincy fine sand (QFS)) and a silty clay loam soil (Warden silty loam (WSL)) with BC (0, 22.4, and 44.8 mg ha^?1) and biosolid (0 and 22.4 mg ha^?1) amendments were investigated. Adsorption of NH₄-N by the QFS soil increased with BC application at lower NH₄-N concentrations in equilibrium solution. For the WSL soil, NH₄-N adsorption peaked at 22.4 mg ha^?1 BC rate. Biosolid application increased NH₄-N adsorption by the WSL soil while decreased that in the QFS soil. Adsorption of P was greater by the WSL soil as compared to that by the QFS soil. Biosolid amendment significantly increased P adsorption capacity in both soils, while BC amendment had no significant effects. BC and biosolid amendments decreased K adsorption capacity by the WSL soil but had no effects on that by the QFS soil. Ca release with increasing addition of K was greater by the WSL soil as compared to that by the QFS soil. In both the soils, Ca release was not influenced by BC amendment while it increased with addition of biosolid. The fit of adsorption data for NH₄-N, P, and K across all treatments and in two soils was better with the Freundlich model than that with the Langmuir model. The nutrients retained by BC or biosolid amended soils are easily released, therefore are readily available for the root uptake in cropped soils.     

Nitrate stability in loess soils under anaerobic conditions—laboratory studies

The objective of this laboratory study with six loess soils (three Eutric CambisoIs and three Haplic Phaeozems) incubated under flooded conditions was to examine the effect of a wide range of NO doses under anaerobic conditions on soil redox potential and N₂O emission or absorption. Due to the fact that loess soils are usually well‐drained and are expected to be absorbers during prevailing part of the season, the study aimed at determination of the conditions decisive for the transition from emission to absorption process. On the basis of the response to soil nitrate level, the two groups of soils were distinguished with high and low denitrification capacity. The soil denitrification activity showed Michaelis‐Menten kinetics with respect to soil nitrate content with K_M in the range 50–100 mg NO ‐N kg^–1. Percentage of nitrates converted to N₂O increased linearly with nitrate concentration in the range from 25 to 100 mg NO ‐N kg^–1 up to 43% and decreased linearly at higher concentrations reaching practically zero at concentrations about 600 mg NO ‐N kg^–1. No denitrification was observed below 25 mg NO ‐N kg^–1. Nitrous oxide absorption in soil occurred only at nitrate concentrations to 100 mg NO ‐N kg^–1 and in this concentration range was proportional to the denitrification rate. Nitrous oxide was formed at redox potentials below +200 mV and started to disappear at negative E_h values.