Kinetics of field oxidation of elemental sulfur in New Zealand pastoral soils and the effects of soil temperature and moisture

The effectiveness of elemental sulfur (S°) as a fertilizer is governed by its rate of microbial oxidation in soil to the sulfate form for absorption by plants. Some 80 field oxidation rate experiments were conducted under grazing throughout New Zealand for one year by measuring the residual S° in soil at approximately two-month intervals. The S° was applied as particles 75 to 150µm in diameter at the rate of 30 kg ha^–1. The simple cubic oxidation rate model (no allowance for temperature or moisture changes) accounted for more than 80% of the variance at nearly 70% of sites.The mean annual soil temperature (10-cm depth) was the most important factor found affecting the mean annual oxidation rate constants, accounting for 38% of the variance through the Arrhenius equation. Other factors, including rainfall, soil moisture, pH and soil groups accounted for only a further 2% of variance. The factor most likely to account for the balance of variance among the sites is the oxidizing efficiency of the associated microorganisms.The rate constant versus temperature relationship for field oxidation in the five New Zealand climate regions was consistent with mean rate constants of soil groups from a similar set of 47 soils incubated at 25°C and field capacity moisture for 10 weeks or more. Field soils, therefore, had an average moisture for the oxidation rate equivalent to that at field capacity. This would explain the lack of contribution of soil moisture to the variance, and would support the use of the mean annual oxidation rate constant (from the mean annual soil temperature) for calculating the optimum particle size range of S° fertilizer. In confirmation, S° particle size recommendations from field rate constants for pastoral fertilizer were consistent with those from earlier agronomic experiments.     

Temperature effects on soil organic sulphur mineralization and elemental sulphur oxidation in subtropical soils of varying pH

The increasing sulphur (S) deficiency in soils of several parts of world has led to the use of fertilizer S, an important factor in enhancing the production and quality of crops. Very limited information is available on the use of elemental sulphur (S⁰) as a fertilizer, its oxidation into SO4^2- and transformation into organic S in semiarid subtropical soils. We studied the impact of three temperature regimes on the mineralization of soil organic S, and the oxidation and immobilization of S⁰ in acidic (pH 4.9), neutral (pH 7.1) and alkaline (pH 10.2) subtropical soils of north-western India. Repacked soil cores were incubated under aerobic conditions (60% water-filled pore space) for 0, 14, 28 and 42 d with and without incorporated S⁰ (500 g g-1 soil). Temperature had profound effects on all three soils processes, the rates of mineralization of native soil organic S, oxidation of applied S⁰ and transformation of S⁰ into soil organic S being greatest at 36 °C, irrespective of soil pH. Mineralization of native soil organic S (without added S⁰) resulted in the accumulation of 39, 66 and 47 g SO4^2-–S g-1 soil in acidic, neutral and alkaline soil in 42 d period at 36 °C. Of the total mineralization, the majority (62 – 74%) occurred during the first 14 d period. Oxidation rate of added S⁰ during initial 14 d period at 36 °C was highest in alkaline soil (292 g S cm-2 d-1), followed by neutral soil ((180 g S cm-2 d-1) and lowest in acidic soil (125 g S cm-2 d-1). Of the applied 500 g S⁰ g-1 soil, 3.2 – 10.0%, 6.8 – 15.4% and 10.0 – 23.0% oxidized to SO4^2-, and 13.4 – 28.6%, 16.0 – 29.0% and 14.6 – 29.0% were transformed into organic S in 42 d period in acidic, neutral and alkaline soil, respectively. The results of our study suggest that in order to synchronize the availability of S with plant need, elemental S may be applied well before the seeding of crops, especially in acidic soil and in regions where temperature remains low. Substantial mineralization of native soil organic S in the absence of applied S⁰ and immobilization of applied S⁰ into organic S suggest that the role of soil biomass as source and sink could be exploited in long term S management.     

Re-utilization by perennial ryegrass (Lolium perenne L.) of labelled fertilizer sulphur incorporated in field-grown tops and roots of pasture plants added to soils

³⁵S-labelled gypsum fertilizer was incorporated under field conditions into pastures which were separated into white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) tops and roots. These were added to four soils from improved and unimproved pastures. The re-utilization of labelled fertilizer sulphur (S) was assessed under growth cabinet conditions (20°C day, 13°C night, daylength 16 h, light intensity 120–170 lx) by growing perennial ryegrass plants for 23 weeks.Mean recoveries of labelled fertilizer S varied from 7 to 20% depending on soil type, form amount and kind of plant residue added. Greater recovery was obtained from clover roots (9.5–16.2%) than grass roots (6.7–12.5%), and from grass tops (13.1–19.7%) than clover tops (9.7–17.9%). These results are related to contents of labelled S, total S, C/N, C/S and N/S ratios in plant residues which also accounted for their relative rates of decomposition. Ground (< 1 mm) and chopped (3 mm) roots increased labelled fertilizer S recovery by about 30% compared with whole roots. Additions of unlabelled fertilizer S influenced the recovery of labelled fertilizer S. This effect depends on the amounts of labelled grass roots and unlabelled fertilizer added.The significance of the findings is discussed with the aid of results from previous field experiments conducted on these soils.     

Adsorption and desorption of phosphate in some semi-arid tropical Indian Vertisols

Adsorption and desorption of phosphorus in soils are among the key processes governing its availability to crops. There have been very few studies on the phosphorus adsorption and desorption characteristics of Vertisols. The P adsorption and desorption characteristics of four Vertisols belonging to three agriculturally important soil series were studied. The amounts of P adsorbed by the soils at 0.2µg ml^–1 equilibrium solution P concentration was low and ranged from 34.3 to 79.5µg g^–1 soil. The phosphate adsorption was very well described by Langmuir and Freundlich isotherms. The P adsorbed by a Vertisol (BR-1) fertilized with different rates of P in the previous season (0, 10, 20 and 40 kg P ha^–1) was similar (34.3–41.3µg g^–1 soil) indicating little effect of fertilization on P adsorption. The correlation studies indicated that the DTPA-extractable Fe was the most important factor accounting for P adsorption in these soils. Clay and CaCO₃ content were found to be relatively less important factors affecting P adsorption in the soils studied.The capacity of the two extractants and EUF (electro-ultrafiltration) to desorb the adsorbed P followed the order: EUF (400V, 80°C)>sodium bicarbonate>EUF (200V, 20°C)>calcium chloride. The average amounts of P desorbed from the four Vertisols using these methods were 74, 63, 50, and 3% respectively of the adsorbed P. In the Begamganj soil, the amount of P desorbed by EUF (400V, 80°C) exceeded 100%, indicating that all of the adsorbed P was desorbable including some native P.In conclusion the results of our study show that the Vertisols studied have low phosphate adsorption capacity and that the P they adsorbed is easily desorbable.Approved for publication as Journal Article No. 983 by International Crops Research Institute for the Semi-Arid Tropics (ICRISAT).     

Phosphate sorption isotherms in relation to P nutrition of wheat (Triticum aestivum)

The phosphate sorption isotherms are needed to explain differential plant responses to P fertilization in soils. Laboratory and greenhouse experiments investigated the use of phosphorus sorption isotherms in relation to P fertilizer requirement of wheat in ten benchmark soils of Punjab, India. The modified Mitscherlich Equation (3) was used to describe plant response observed in different soils. Maximum obtainable yield (MOY) ranged from 11.6 g pot^–1 in Gurdaspur (I) sandy clay loam to 7.0 g pot^–1 in Nabha sandy clay loam. Response to P applied @ 25 mg P kg^–1 soil was maximum (77%) in Bathinda sand and minimum in Chuharpur clay loam (33%). The response curvature varied from 3.74 × 10^–2 in Nabha sandy clay loam to 4.43 × 10^–2 in Kanjli sandy loam. The soil solution P required to produce optimum yield (90% MOY) varied from 1.61 µg ml^–1 in Bathinda sand to 0.10 µg ml^–1 in Sadhugarh clay. Dry matter yield obtained at 0.2 µg ml^–1 solution P concentration ranged from 55% in Bathinda sand to 85% of MOY in Gurdaspur (II) clay loam. At the same solution P concentration (0.1 µg P ml^–1), dry matter yield was 91% in Sadhugarh clay, 80% in Gurdaspur (II) clay loam and, 43% of MOY in Bathinda sand and eventually coincided with the decreasing maximum buffer capacity (MBC) in these soils. At the same level of sorbed P (100 mg P kg^–1 soil) the yield was observed to be inversely proportional to MBC. The study, therefore, concludes that, soils should be grouped according to their P sorption characteristics and MBC before using critical soil solution P as a criterion for obtaining optimum yields.     

Effect of phosphate on the sorption,desorption and plant-availability of selenium in soil

Phosphate, applied at 5µg P cm^–3, decreased selenite sorption by from 30–70% in three soils studied. Both maximum sorption (X_m) and the binding-energy of sorption as indicated by the binding-energy related constant (k) or the molar free energy (G) of the sorption reaction derived from the Langmuir equation were considerably decreased. On the other hand, phosphate sorption was decreased by increasing concentration of selenite from 0.2µg Se cm^–3 to 1.0µg Se cm^–3 in the initial solution. The competitive sorption of phosphate with selenite was likely the main mechanism involved in the P-Se interactions. The competitively sorbed selenite exhibited much larger desorption in 0.01M CaCl₂ solution, more readily extractable to 0.5M NaHCO₃ and significantly higher isotopic exchangeability compared to that sorbed without the competing anion. Results from pot trial using ryegrass indicated that phosphate application increased more efficiently the plant-availability of applied fertilizer Se than that of indegeneous Se in soil.     

Studies on the redox behaviour of La1.867Th0.100CuO4 and its catalytic performance for NO decomposition

La_1.867Th_0.100CuO₄ was prepared by means of the citric acid complexing method. The reduction–oxidation (redox) properties of this composite oxide have been investigated by using the XRD, TGA, EPR, TPD, and SEM methods. The fresh (non-reduced) La_1.867Th_0.100CuO₄ catalyst is single phase with tetragonal K₂NiF₄-type structure. There were three reduction steps observed over La_1.867Th_0.100CuO₄ in the temperature ranges of 25–100, 100–300, and 300–500 °C, respectively. After reduction at 300 °C, the material still retained its original single phase but there were oxygen vacancies generated in the lattice. After reduction at 500 °C, it decomposed to a mixture of oxides. In the course of reduction, trapped electrons were generated. During the oxidation of the reduced sample, O ₂ ^– was detected. Apparently, oxygen vacancies are able to stabilise O ₂ ^– on the surface of the -1ptcatalyst. NO adsorption on both the fresh and reduced La_1.867Th_0.100CuO₄ samples generated NO radicals and O ₂ ^– species. On a La_1.867Th_0.100CuO₄ sample reduced at 300 °C, [O₂NO₂]^2– was generated in NO adsorption and decomposed to N₂ and O^2– at ca. 730 °C. After reduction, the O ₂ ^– inside the La_1.867Th_0.100CuO₄ lattice became more mobile and participated in the decomposition of [O₂NO₂]^2–. The fresh (non-reduced) La_1.867Th_0.100CuO₄ sample with cation defects in its lattice shows higher NO decomposition activity than the fresh La₂CuO₄ sample in which there are no cation defects. The 300 °C-reduced La_1.867Th_0.100CuO₄ with cation defects and oxygen vacancies is more active than the fresh one for NO decomposition. The redox action between Cu⁺ and Cu²⁺ is an essential process for NO decomposition.     

An attempt to evaluate phosphorus fertilizer requirements of western Nigeria savannah soils

A greenhouse fertilizer trial was carried out on 60 surface soils of the western Nigeria savannah derived from basement complex rocks. Bray's P₁ available P in the soils varied between 1 and 112µg ml^–1. There was maize response to P addition and a critical P level of 12.7µg ml^–1 was calculated for the soils.For 22 of the soils, a laboratory incubation technique was used in evaluating changes in Bray's P₁ extractable P at various rates with time. The initial rapid decline in soil available P was completed between 28 and 84 days of incubation. A fertilizer factor, calculated from extracted P in treated and untreated soils varied between 1.5 and 16.7µg ml^–1 and was significantly correlated with soil pH and citrate-dithioniteextractable oxides of Fe and Al.Fertilizer rates based on critical soil P, available soil P and fertilizer factor, correlated significantly with greenhouse estimates for optimum yield obtained with the linear response plateau model (r = 0.91,p < 0.001). At ten field locations varying in available P content, response was only to P applications lower than 60 kg ha^–1 and the calculated P rates using a mean fertilizer factor of 3.0µg ml^–1 corresponded to P rates at which maximum yields were obtained in the sites.     

Evaluation of ammonium thiosulfate as a soil urease inhibitor

Interest in use of ammonium thiosulfate (ATS) in conjunction with urea as a fertilizer has been stimulated by recent reports that this compound retards hydrolysis of urea by soil urease and thereby reduces volatilization of urea N as ammonia from soils fertilized with urea. We evaluated ATS as a soil urease inhibitor by studying its effects on urea hydrolysis, seed germination, and early seedling growth in soil. We found that ATS significantly retarded urea hydrolysis only when applied at rates as high as 2,500 or 5,000µg g^–1 soil, whereasN-(n-butyl) thiophosphoric triamide (NBPT) (a patented inhibitor of urea hydrolysis in soil) caused substantial retardation of urea hydrolysis when applied at rates as low as 1µg g^–1 soil. We also found that ATS had an adverse effect on germination of corn or wheat seeds in soil when applied at the rate of 2,500 or 5,000µg g^–1 soil and caused a dramatic reduction of early seedling growth of corn or wheat when applied at the rate of 1,000, 2,500, or 5,000µg g^–1 soil. These findings indicate that ATS has little, if any, potential value for retarding hydrolysis of urea fertilizer in soil.     

Assessment of the nitrogen requirements of maize (Zea mays L.) in Southern Guinea Savanna agroecology of Nigeria

During the growing seasons (May to October) of 1987 and 1988 respectively five and four different rates of N were tested on maize (Zea mays L.) at 12 different field sites across the Southern Guinea Savanna of Nigeria. Nitrogen was applied through granular urea (size +14 mesh), ordinary prilled urea (–35 mesh) and calcium ammonium nitrate. Marked differences existed among experimental sites in maize grain yield response to N with Yelwa and Ta-Hoss in Plateau State having the highest response at 60 and 90 kg N ha^–1 respectively.During 1988, at five experimental sites the yield was maximized with 120 kg N ha^–1, while at three other sites the yield maximization occurred at 90 kg N ha^–1. During 1987, the corresponding number of sites was two and six with 120 and 90 kg N ha^–1 rates, respectively. Plant height and cob number exhibited a linear relationship with yield. Differences in yield in response to application of different N sources were non-significant.Contribution from the Nationally coordinated fertilizer use programme funded by Federal Government of Nigeria     

Sintering and recrystallization of high-purity magnesia-alumina spinel during hot pressing

Conclusions A study was made of the densification and recrystallization of magnesia — alumina spinel of high dispersion during hot pressing in the temperature range 1200–1600°C, a pressure of 60–300 cm², and a holding time of 10–30 min.The relative density of 92–98% theoretical was achieved at 1300–1400°C, a pressure of 300 kg/cm², and a soaking of 10 min. The spinel of theoretical density with a finely crystalline homogeneous structure was obtained by hot pressing at 1450–1600°C.Intensive recrystallization of the spinels during hot pressing occurs in the temperature range ensuring maximum rate of densification (1450–1600°C).The rate of recrystallization of the spinel grains during hot pressing at 1400°C and a pressure of 300 kg/cm² and soakings of 10 and 30 min, has the order of 10^–6 cm/sec and the maximum for the grains of diameter from 2 to 6.Report read at a symposium on pure oxide sintering, Khar'kov, 1968.Deceased.Translated from Ogneupory, No.6, pp. 32–36, June, 1970.     

Chemical evolution of CO2 by electrocatalytic oxidation of vanadium-impregnated coke anode material

Baked carbon containing impregnated vanadium may be electrochemically oxidized to CO₂ in 1 M H₂SO₄ at 80–90% current efficiency during prolonged electrolysis (>20 h) at 70°C under an applied potential of 1.0 V versus saturated calomel electrode (SCE). The vanadium is electrocatalytically maintained in the highest oxidation state with an activation energy of 44–80 kJ mol^–1 at temperatures up to 80°C.     

Thermal battery cells utilizing molten nitrates as the electrolyte and oxidizer

The Ca/LiNO₃-LiCl-KCl (50-25-25 mol%) thermal battery cell can be activated at 160° C and operated over a temperature range of 250–450° C to produce 2.5–2.8 V at open-circuit and initial operating voltages above 2 V at 10 mA cm^–2. At operating temperatures between 250 and 350° C, this system shows promise for applications requiring a sixty-minute thermal battery. Cell lifetimes decrease at higher temperatures due to the accelerating reaction of calcium with the molten nitrate salt to form gaseous products. An experimental energy density value of 142 Whkg^–1 was obtained at 300° C during constant current discharge at 10 mAcm^–2. Effects of applied face pressure on cell discharge characteristics were small. At current densities above 20–30 mA cm^–2, the cell performance deteriorates due to polarization at the anode. This is probably caused by the precipitation of CaO which blocks the active sites at the anode.     

The effect of fertiliser type and application rate on denitrification losses from cut grassland in Northern Ireland

Denitrification losses were measured using the acetylene inhibition technique adapted for a coring procedure. Two soils under a cut ryegrass sward were used. One soil was a freely-drained clay loam receiving under 900 mm rainfall annually, the other soil being a poorly-drained silty clay receiving over 1100 mm rainfall annually. Swards at each site received up to 300 kg N ha^–1 yr^–1 of calcium ammonium nitrate (CAN), urea or a new fertiliser mixture GRANUMS (30% ammonium nitrate, 30% urea, 10% ammonium sulphate, 30% dolomite). For both soils the rate of denitrification exceeded 0.1 kg N ha^–1 day^–1 only when the air-filled porosity of the soil was < 30% v/v and soil nitrate was > 2 mg N kg^–1 in the top 10cm of the profile and when soil temperature at 10 cm was > 4°C. When the soils dried such that their air-filled porosity was > 30% v/v, denitrification rates decreased to < 0.08 kg N ha^–1 day^–1. Highest rates (up to 3.7 kg N ha^–1 day^–1) were observed on the clay soil following application of 94 kg N ha^–1 CAN to soil near field capacity in early summer 1986. Losses from CAN were approximately 3 times those from urea for a given application. Denitrification losses from the GRANUMS treatment were, overall, intermediate between those from CAN and urea but the daily losses more closely resembled those from the CAN treatment. The impeded drainage on the clay soil, where soil moisture contents remained close to field capacity throughout the year, showed denitrification losses roughly 3 times those observed on the more freely drained clay-loam for any given treatment. Over a 12-month period, N losses arising from denitrification were 29.0 and 10.0 kg N ha^–1 for plots receiving 300 kg N ha^–1 CAN and urea, respectively, on the well drained clay-loam and 79.0 and 31.1 kg N ha^–1 respectively, for identical plots on the poorly drained clay soil. Annual denitrification losses from control plots were < 1 kg N ha^–1 on both soils.     

Changes in the magnetic properties of chromite ore heated with scale

Conclusions Chrome spinel and gangue of Kimpersarski chromite ores has a low and scarcely distinguishable magnetic susceptibility which does not alter after heating at temperatures from 400° to 1300°C. The magnetic susceptibility of the ores fired with additions of iron oxide or scale at 1000–1300°C increases from (20–120)×10^–6 to (1650–3400)×10^–6 cm³/g. The increase in the magnetic properties of chromite ores after firing occurs on account of the increase in the magnetic susceptibility of the gangue from (50–120)×10^–6 to (6000–8000)×10^–6 cm³/g. Thus after firing of chromite ores with iron-containing additives the gangue acquires ferromagnetic properties. The magnetic susceptibility of the gangue is 50–115 times greater than chrome-spinel which permits us to use highly productive magnetic separators with low and medium magnetic field forces for their treatment.     

Electrodeposition of titanium from chloride melts

The kinetics of the electrodeposition and electrocrystallisation of titanium were studied in alkali chloride melts. Voltammograms showed two distinct anodic peaks for the oxidation steps Ti/Ti(II) and Ti(II)/Ti(III) at 70 mV and 300 mV, respectively, referred to titanium. The cathodic reduction Ti(III)/Ti(II) was very irreversible, showing an extended cathodic wave and a shoulder on the Ti(II)/Ti reduction peak. The Ti(II)/Ti reduction was found to be quasi reversible. The rate constant at 450'C was 5 × 10^–3 cm s^–1 . The diffusion coefficient in KCl-LiCl eutectic was 1 × 10^–5 cm²s^–1 at 450° C rising to 3.5 × 10^–5 cm²s^–1 at 650° C. Potential step measurements gave curves indicating slow instantaneous nucleation and growth (I against t ^1/2) followed by slow diffusion control (I against t ^–1/2). During constant current steady state deposition the nature of the deposit depended on the composition of the melt, the temperature and the cd. In KCl-LiCl melts coherent deposits were obtained for cds of 60–120 mA cm² while higher current densities gave dendritic and spongy deposits.     

Electrochemical production of cuprous oxide using the anode-support system

The preferred process for the production of cuprous oxide powder is by the anodic dissolution of copper in an alkaline solution of sodium chloride. The purpose of the present investigation was to develop a cuprous oxide process suitable for use on an industrial scale usiing the anode-support system, i.e. an anode comprising a titanium mesh basket loaded with small pieces of high-grade copper scrap. Laboratory investigations with this type of anode together with a titanium mesh cathode were conducted using cells having capacities up to 400 dm³. The recommended operating conditions based on 120 h runs using the 400 dm³ cell are as follows: NaCl: 250 g dm^–3; c.d.: 6 A dm^–2; CI: 0.37 A dm^–3; temperature: 80°C; pH 10. Of particular importance, especially as regards the quality of the product and cell scale-up, is the relationship between the current and the volume of the electrolyte, denoted as Cl and expressed as A dm^–3. The use of anode and cathode diaphragms of polypropylene obviated the need for additives to counteract copper redox reactions in the cell. The power yield was 0.8–0.9 kWh kg^–1. The product was well within ASTM specification D912-65 for Cu₂O for use in antifouling paints.     

Photoemission studies of rhenium disulfide oxidation: Altered core-level structure and reactivity of defect sites

X-ray photoemission was used to investigate the initial stages of rhenium disulfide oxidation and the altered reactivity and core-level electronic structure of defect sites produced by argon ion sputtering of single crystal and polycrystalline ReS₂-basal plane surfaces. Oxidation of polycrystalline-ReS₂ with O₂ at 100–300 ° C produced a mixture of surface oxides containing rhenium in the + 4 and + 7 oxidation states. Oxidation was facilitated by the presence of low coordination defect sites and was reversible upon resulfidization with H₂S/H₂ at 300 °C.     

Characterization of ethylene homo- and copolymers

Summary For the estimation of molecular mass distribution and average molecular masses of (norbornene ethylene) copolymers a size exclusion chromatography method is described. The validity of the universal calibration function of Benoit et al. based on polystyrene molecular mass standards was confirmed by comparing with the number average molecular masses obtained by osmometry. The constants of the Mark-Houwink equation are a = 0.535, K = 1.00·10^–1 (Xylene, 90 °C) and a=0.589, K=4.93·10^–2 (diethylbenzene, 120°C,[] in cm³g^–1).Part IV: Plaste u. Kautschuk 32, 12, 444 (1985)     

Ammonium transformation in paddy soils affected by the presence of nitrate

Coupled nitrification and denitrification is considered as one of the main pathways of nitrogen losses in paddy soils. The effect of NO₃ ^– on NH₄ ⁺ transformation was investigated by using the ¹⁵N technique. The paddy soils were collected from Wuxi (soil pH 5.84) and Yingtan (soil pH 5.02), China. The soils were added with either urea (18.57 mol urea-N enriched with 60 atom% ¹⁵N excess) plus 2.14 mol KNO₃-N (natural abundance) per gram soil (U+NO₃) or urea alone (U). The KNO₃ was added 6 days after urea addition. The incubation was carried out under flooded condition in either air or N₂ gas headspace at 25°C. The results showed that in air headspace, ¹⁵NH₄ ⁺ oxidization was so fast that about 10% and 8% of added ¹⁵N in the treatment U could be oxidized during the incubation period of 73 hours after KNO₃ addition in Wuxi and Yingtan soil, respectively. The addition of KNO₃ significantly inhibited ¹⁵NH₄ ⁺ oxidation (p<0.01) in air headspace, while it stimulated ¹⁵NH₄ ⁺ oxidation in N₂ gas headspace, although the oxidation was depressed by the N₂ gas headspace itself. Therefore, the accumulation of NO₃ ^– would inhibit further nitrification of NH₄ ⁺ at micro-aerobic sites in paddy soils, especially in paddy soils with a low denitrification rate. On the other hand, NO₃ ^– would lead to oxidation of NH₄ ⁺in anaerobic bulk soils.