A model of ammonia volatilization from applied urea. I. Development of the model

Urea application to soil raises the pH and ammonium concentration, thus providing ideal conditions for ammonia volatilization. A mechanistic model is presented, which combines the process of ammonia volatilization with the simultaneous transformation and movement of urea and its products in soil, for predicting the concentration profiles of urea, ammoniacal-nitrogen and soil pH, and ammonia losses, following application of urea. The model consists of continuity equations describing the diffusion and reaction of urea, ammoniacal-nitrogen and soil base; it takes into account the volatilization of ammonia and the concurrent acidification of the soil surface; and considers a variable P_Co2 profile due to soil respiration and urea hydrolysis. The derivation of the continuity equations and their boundary conditions, calculations of ammonia volatilization, and appropriate methods for numerical solutions are described.     

Calcium chloride and ammonium thiosulfate as ammonia volatilization inhibitors for urea fertilizers

Abstract

Surface‐applied urea fertilizers are susceptible to hydrolysis and loss of nitrogen (N) through ammonium (NH₃) volatilization when conditions favorable for these processes exist. Calcium chloride (CaCl₂) and ammonium thiosulfate (ATS) may inhibit urease activity and reduce NH₃ volatilization when mixed with urea fertilizers. The objective of this study was to evaluate the effectiveness of CaCl₂ and ATS as urea‐N loss inhibitors for contrasting soil types and varying environmental conditions. The proposed inhibitors were evaluated in the laboratory using a closed, dynamic air flow system to directly measure NH₃ volatilization. The initial effects of CaCl₂ on ammonia volatilization were more accentuated on an acid Lufkin fine sandy loam than a calcareous Ships clay, but during volatilization periods of ≥ 192 h, cumulative N loss was reduced more on the Ships soil than the Lufkin soil. Calcium chloride delayed the commencement of NH₃ volatilization following fertilizer application and reduced the maximum N loss rate. Ammonium thiosulfate was more effective on the Lufkin soil than the Ships soil. For the Lufkin soil, ATS reduced cumulative urea‐N loss by 11% after a volatilization period of 192 h. A 20% (v/v) addition of ATS to urea ammonium nitrate (UAN) was most effective on the coarse textured Lufkin soil whereas a 5% addition was more effective on the fine textured, Ships soil. Rapid soil drying following fertilizer application substantially reduced NH₃ volatilization from both soils and also increased the effectiveness of CaCl₂ but not ATS. Calcium chloride and ATS may function as limited NH₃ volatilization inhibitors, but their effectiveness is dependent on soil properties and environmental conditions.     

Reducing ammonia loss from urea and improving soil-exchangeable ammonium retention through mixing triple superphosphate, humic acid and zeolite

Ammonia losses from soil following fertilization with urea may be large. This laboratory study compared the effect of four different, urea–triple superphosphate (TSP)–humic acid–zeolite, mixtures on NH₃ loss, and soil ammonium and nitrate contents, with loss from surface‐applied urea without additives. The soil was a sandy clay loam Typic Kandiudult (Bungor Series). The mixtures significantly reduced NH₃ loss by between 32 and 61% compared with straight urea (46% N) with larger reductions with higher rates of humic acid (0.75 and 1 g kg^?1 of soil) and zeolite (0.75 and 1 g kg^?1 of soil). All the mixtures of acidic P fertilizer, humic acid and zeolite with urea significantly increased soil NH₄ and NO₃ contents, increased soil‐exchangeable Ca, K and Mg, and benefited the formation of NH₄ over NH₃ compared with urea without additives. The increase in soil‐exchangeable cations, and temporary reduction of soil pH may have retarded urea hydrolysis in the microsite immediately around the fertilizer. It may be possible to improve the efficiency of urea surface‐applied to high value crops by the addition of TSP, humic acid and zeolite.     

A model of ammonia volatilization from applied urea. III. Sensitivity analysis, mechanisms, and applications

A sensitivity analysis of the model described in Part I showed that the proportion of N lost as ammonia from surface applied urea is very sensitive to the initial pH of the soil, its pH buffer capacity, the rate of urea application, and the soil urease activity. Under the conditions tested, the diffusion of bicarbonate ion to the soil surface, to neutralize the acid generated when NH₄⁺ is volatilized as NH₃, appeared to be the main process controlling the rate of ammonia volatilization. The amount of ammonia volatilized was not very sensitive to the value of the transfer coefficient between the soil surface and the atmosphere, nor to the soil moisture status if this was around field capacity. Adsorption of ammoniacal-nitrogen was less important than the soil pH buffer capacity in influencing the ammonia volatilization. Further applications and extensions of the model are discussed.     

Assay of urease enzyme activity in an ammonium‐fixing soil

Abstract

Rates of substrate disappearance and product formation were compared as measures of urease enzyme activity in an NH₄‐fixing and in a non‐fixing soil under tris‐, borate‐ or non‐buffered assay conditions over 4h at 37°C. Tris‐buffered urease activity of the NH₄‐fixing soil was 119 μg urea‐N hydrol./g/h or 116 μg (KCl‐extractable) NH₄‐N/g/h indicating prevention of NH₄ fixation by the buffer; without tris, NH₄ production rates amounted to only 35% of coresponding urea hydrolysis rates. Equal rates of urea disappear‐ ance and NH₄ formation occurred in the non‐fixing soil irrespective of buffer amendment.

Tris‐inhibition of NH₄ fixation during 4h incubation at 37°C, however, depended on NH₄ Cl rate and buffer strength. 0.025–0.10 M tris (pH 9.0) reduced NH₄ fixation to negligible amounts at < 0.03 M NH₄C1 whereas, at 0.06–0.24 M NH₄Cl, substantial NH₄ fixation occurred in the presence of 0.05 M tris; NH₄ fixation in unbuffered soil, however, always exceeded that in tris‐buffered soil. Borate buffer (0.06M, pH 10) did not influence the extent of NH₄ fixation.

Tris significantly enhanced urea hydrolysis in the slightly acid, non‐fixing soil but not in the moderately alkaline NH₄ ‐fixing soil indicating an effect of soil type on pH optima of urease enzyme activity. The urease activities of both soils in borate were considerably lower than in tris, possibly because of the combined effects of excess alkalinity and high substrate concentration.     

Presubmergence and green manure affect the transformations of nitrogen‐15‐labeled urea under lowland soil conditions

The effect of presubmergence and green manuring on various processes involved in [¹⁵N]‐urea transformations were studied in a growth chamber after [¹⁵N]‐urea application to floodwater. Presubmergence for 14 days increased urea hydrolysis rates and floodwater pH, resulting in higher NH₃ volatilization as compared to without presubmergence. Presubmergence also increased nitrification and subsequent denitrification but lower N assimilation by floodwater algae caused higher gaseous losses. Addition of green manure maintained higher NH₄⁺‐N concentration in floodwater mainly because of lower nitrification rates but resulted in highest NH₃ volatilization losses. Although green manure did not affect the KCl extractable NH₄⁺‐N from applied fertilizer, it maintained higher NH₄⁺‐N content due to its decomposition and increased mineralization of organic N. After 32 days about 36.9 % (T₁), 23.9 % (T₂), and 36.4 % (T₃) of the applied urea N was incorporated in the pool of soil organic N in treatments. It was evident that the presubmergence has effected the recovery of applied urea N.     

Solubility of soil phosphorus as influenced by urea

The objective of this study was to investigate possible ways of mobilizing residual fertilizer P as a result of local pH elevation caused by urea hydrolysis. The response of water-soluble P (P_w) and dissolved organic C (DOC) to urea hydrolysis was monitored in three cultivated soils and at two P levels for up to 127–135 d and compared with corresponding changes in soils limed with Ca(OH)₂. Hydrolysis of urea was complete in 8–15d during which soil pH increased by 1–1.5 units at the maximum. Subsequently, the pH decreased to or below the original level owing to nitrification. Mobilization of soil P was enhanced substantially in parallel with the increase in pH, the peak P_w occurring simultaneously with the highest pH value. In all urea-treated soils, P_w remained at an elevated level for at least 60d. As compared to urea, elevation of soil pH with Ca(OH)₂ had only a minor and inconsistent influence on P_w. In mobilization of soil P, the urea-induced increase in pH and a simultaneous production of NH₄⁺ ions proved to be superior to liming with Ca(OH)₂. It was hypothesized that when an acid soil is amended with urea, phosphate is first displaced by OH^? ions, resulting in elevated solution P concentrations. A simultaneous dissolution of organic matter contributes to the persistence of high P concentration by competition for sorption sites on Fe and Al oxides, and thus retards the resorption of P.     

A model of ammonia volatilization from applied urea. VI. The effects of transient-state water evaporation

Rachhpal-Singh & Nye's model of ammonia volatilization is further expanded to account for the effects of transient-state water evaporation when the soil surface dries significantly. Full details are given of the derivation and numerical solution of equations describing transient-state water movement, and the diffusion and convection in soil of urea and its hydrolysis products, and of acid generated by ammonia volatilization. For the wide range of soil hydraulic properties considered, the effects of a dry soil layer on the rate of volatilization supplement the effects of increased convective supply of NH⁺₄ and HCO^?₃ ions to the soil surface. The dry layer results in increased gaseous NH₃ diffusion through the soil, and thereby increases the flux of NH₃ across the soil surface and the neutralization of H⁺ ions generated by volatilization.     

Simultaneous nitrification and diffusion in soil. I. The effects of the addition of a low level of ammonium chloride

The nitrification of NH₄⁺ and the simultaneous diffusion of NH₄⁺, NO₃^? and H⁺ following the addition of ammonium chloride to a fine sandy loam soil was analysed experimentally and theoretically. Experimentally, the concentration profiles of mineral N and pH were analysed 140h and 284 h after the homogeneous addition of 11 μmoles NH₄Cl_cm^?3 of soil to one part of a composite soil column. The mathematical model presented includes a kinetic model of nitrifier growth and activity, the adsorption equilibria of NH₄⁺ and soil acidity with the soil solid phase and the influence of other ions on the diffusion characteristics of each diffusing ion. The predictions of the model were generated using parameters derived from independent experiments so that the predictions did not depend on data derived from the experimental concentration profiles. Good agreement was found between experimental and predicted profiles. The use of the model for predicting the penetration of NH₄⁺ and NO₃^? into the soil is demonstrated.     

Ammonia volatilization following urea application at maize fields in the East African highlands with different soil properties

Use of nitrogen (N) fertilizer is underway to increase in Sub-Saharan Africa (SSA). The effect of increasing N rates on ammonia (NH₃) volatilization—a main pathway of applied-N loss in cropping systems—has not been evaluated in this region. In two soils (Alfisols, ALF; and Andisols, AND) with maize crop in the East African highlands, we measured NH₃ volatilization following urea broadcast at six rates (0–150 kg N ha^?1) for 17 days, using a semi-open static chamber method. Immediate irrigation and urea deep placement were tested as mitigation treatments. The underlying mechanism was assessed by monitoring soil pH and mineral N (NH₄⁺ and NO₃^?) concentrations. More cumulative NH₃-N was volatilized in ALF than in AND at the same urea-N rate. Generally, higher urea-N rates increased proportional NH₃-N loss (percent of applied N loss as NH₃-N). Based on well-fitted sigmoid models, simple surface urea application is not recommended for ALF, while up to 60 kg N ha^?1 could be adopted for AND soils. The susceptibility of ALF to NH₃ loss mainly resulted from its low pH buffering capacity, low cation exchange capacity, and high urease activity. Both mitigation treatments were effective. The inhibited rise of soil pH but not NH₄⁺ concentration was the main reason for the mitigated NH₃-N losses, although nitrification in the irrigation treatment might also have contributed. Our results showed that in acidic soils common to SSA croplands, proportional NH₃-N loss can be substantial even at a low urea-N rate; and that the design of mitigation treatments should consider the soil’s inherent capacity to buffer NH₃ loss.     

Simultaneous nitrification and diffusion in soil. II. The effects at levels of ammonium chloride which inhibit nitrification

The physicochemical and microbiological changes occurring in a fine sandy loam soil following the application of ammonium chloride were followed experimentally and with a simulation model. Two levels of ammonium addition were used corresponding to application rates of 37 and 143 kg ha^?1. The measured concentration profiles of ammonium nitrate and pH, which developed in soil columns as a result of the diffusion and simultaneous nitrification of the added NH₄⁺, were measured at different incubation times. The measured profiles suggested that nitrification was inhibited at the site of application of the ammonium salt. This inhibition was attributed to an effect of increased osmotic pressure or chloride ion in the soil. A simulation model was developed to account for the inhibition by examining and testing two hypotheses about the response of nitrifiers to a fluctuating osmotic pressure. These were the irreversible inhibition model, which assumed that exposure to high osmotic pressures irreversibly inactivated a portion of the nitrifier population, and the reversible inhibition model, which assumed that the nitrifiers would recover after exposure to high osmotic pressures. The model included terms for the adsorption equilibria of NH⁴⁺ and soil acidity with the soil solid phase, and the influence of other ions on the rate of diffusion of each diffusing ion. The inputs to the model were based on parameters obtained independently of the diffusion experiments. Good agreement was found between experimental and predicted concentration profiles for both models although the reversible inhibition model gave the better simulation of the data.     

Transformation of urea and ammonium nitrate in an entisol and a spodosol under citrus production

Abstract

Chemical transformations of ammonium nitrate (NH₄NO₃) and urea‐nitrogen (N), at different rates of application, were studied in a Candler (Typic Quartzipsamment) and Wabasso (sandy, Alfic Haplaquod) sand by incubating fertilized surface soil (from 0 to 15 cm depth) samples at 10% moisture content (by weight) in the laboratory at 25±1°C. During the 7 d incubation, the percentage of transformation of NH₄‐N into NO₃‐N was 33 to 41 and 37 to 41% in the Candler fine sand and Wabasso sand, respectively, at application rates of 1.00 g N kg¹. In a parallel experiment, 85 to 96% of urea applied (equivalent to 0.25 to 1.00 g N kg^‐1soil) was hydrolyzed to NH₄‐N within 4 d in the Candler soil, whereas it required 7 d to hydrolyze 90 to 95% of the urea applied in the Wabasso soil. No nitrification was evident for 30 days in the Candler fine sand which received urea application equivalent to ≥ 0.50 g N kg^‐1. In the urea‐amended Wabasso sand, the formation of NO₃ decreased as the rate of urea‐N increased. Possible loss of N from NH₃ volatilization or inhibition of activity of nitrifiers due to elevated soil pH (8.7 to 9.2) during the incubation of urea amended soils may have caused very low nitrification.     

Urea nitricphosphate for cool‐season grass production

Abstract

Significant losses of nitrogen (N) can occur via volatilization of ammonia (NH₃) when non‐incorporated broadcast applications of urea or urea‐containing fertilizers are made. This study was conducted to determine the efficacy of urea nitricphosphate (UNP) as an N and phosphorus (P) source for cool‐season grasses and to evaluate NH₃ volatilization potential of UNP as compared to urea under laboratory conditions. A three‐year field study compared UNP to ammonium nitrate (AN) and urea at 56 and 112 kg N/ha for tall fescue (Festuca arundinacea Schreb.) and smooth brome (Bromus inermis Leyss.). Brome yields were significantly higher from UNP as compared to urea for one of the three years. No such differences occurred with fescue. Nitrogen uptake was significantly higher from UNP as compared to urea for one year each for brome and fescue. Phosphorus uptake by brome was significantly higher from UNP as compared to urea for two years. Laboratory incubation studies showed significantly lower NH₃ volatilization from UNP than from urea after seven days, but no significant differences after 14 days. The delay in NH₃ volatilization was due to the diffusion and subsequent hydrolysis of urea immediately below the soil zone initially influenced by the UNP. The reduction in NH₃ volatilization at the early time could partially be attributed to an inhibition of urea hydrolysis and significantly lower soil pH values for UNP as compared to urea in the upper 30 mm of soil cores. The general conclusion from the field and laboratory work was that UNP is a suitable N source for cool‐season grasses, with the primary potential benefit being delayed NH₃ volatilization as compared to urea.     

Effect of biuret content on transformation of urea nitrogen in soil

The effect of the biuret content (0.0, 1.0, 2.5, 5.0 and 10.0% of urea) on transformations of urea-N was studied in a sandy loam (pH 7.7). While biuret did not affect urea hydrolysis, it inhibited the conversion of NH⁺₄ to NO^?₂ and the subsequent oxidation of NO^?₂ to NO^?₃. This resulted in the accumulation of larger amounts of both NH⁺₄-N and NO^?₂-N in soil as compared to soil receiving urea alone. The results suggest that biuret impurity in urea fertilizer is likely to enhance nitrite toxicity.     

Chemical fixation of ammonia to soil organic matter after application of urea

Chemical fixation of NH₃ to soil organic matter was studied in two Swedish soils with different contents of organic matter: a clay soil with 2.3% C and an organic soil with 36.6% C. ¹⁵N‐labelled urea was applied at different rates to both sterilized and non‐sterilized soils. After 10 days, the soils were extracted and washed with K₂SO₄ and determined for total N and atom% ¹⁵N excess. Urea N was recovered as non‐extractable N in sterilized soil corresponding to 9.7% of supplied ^l5N‐labelled urea in the organic soil and 2.2% in the clay soil. Since no biological immobilization is thought to occur in the sterile soil, this non‐extractable N is suggested to be chemically fixed to soil organic matter. Owing to urea hydrolysis in the clay soil, pH increased from 6.3 to 9.3 and in the organic soil from 5.7 to 6.9 and 8.8, respectively, at the low and high urea supply.     

Ammonia volatilization following surface application of urea to tilled and no-till soils: A laboratory comparison

Broadcasting of urea to agricultural soils can result in considerable losses by NH₃ volatilization. However, it is unclear if the impact of this practice on NH₃ emissions is further enhanced when performed on no-till (NT) soils. The objective of this study was to compare NH₃ volatilization following broadcasting of urea to NT and moldboard plowed (MP) soils. Intact soil cores were taken shortly after harvest from NT and MP plots of three long-term tillage experiments in Québec (Canada) and stored for 4.5 months prior to incubation. Urea (14 g N m⁻²) was applied at the soil surface and NH₃ volatilization was measured for 30 d using an open incubation system. Mean cumulative NH₃ losses were greater (P < 0.001) in NT (3.00 g N m⁻²) than in MP (0.52 g N m⁻²). Several factors may have contributed to the higher emissions from the NT soils. Urease activity in the top 1 cm of soils was on average 4.2 times higher in NT than in MP soils. As a result, hydrolysis of urea occurred very rapidly in NT soils as indicated by enhanced NH₃ emissions 4 h after application of urea. The presence of crop residues at the surface of NT soils also decreased contact of the urea granules with the soil, possibly reducing adsorption of NH₄⁺ on soil particles. Lower volatilization on the MP soils may also have partly resulted from a fraction of urea granules falling into shallow cracks. Field trials are needed to confirm our finding that NT soils bear greater potential for NH₃ volatilization following surface application of urea than MP soils.     

The effect of soil pH and high urea concentrations on urease activity in soil

The rate of hydrolysis of urea in soil over the wide range of concentrations, up to 10 moles N per dm³ soil solution, found in fertilizer practice, was examined in Begbroke sandy loam adjusted to different pH values. On rewetting air-dry soil, urease activity increased rapidly, reached a maximum within the first 24 h and then decreased slowly to level off after about 4 days. Pretreatment of the soil with urea or ammonium had no effect on the urease activity. Urease activity increased with substrate concentration, reached an optimum value and then decreased with rising urea concentration. The results could be explained by substrate inhibition at higher urea concentrations, and the data are well described by a modified Michaelis-Menten equation involving three parameters, V_max, K_m and K_i where K_i is an inhibition constant. K_m decreased linearily with rise in pH whereas K_i increased slightly between pH 4.9 and 7.0 and steeply between 7.0 and 8.4. V_max increased with rise in pH, reached a maximum value at pH 6.0 and then declined at higher pHs. There was a further reaction, reaching a maximum rate at a urea concentration of about 0.2 molar N in the soil solution, that followed Michaelis-Menten kinetics. K_m for this high affinity reaction increased up to pH 7.2 and then decreased at higher pH values; V_max increased up to pH 6.8 and then decreased. The contribution of the high affinity reaction was small except at low concentrations of urea.     

Effect of biuret on the transformation of urea in two egyptian soils

In model experiments (60 % water holding capacity and 30°C) with a loamy clay (pH 8.3) and a calcareous sand (pH 7.9) the effect of biuret upon the breakdown of urea was studied. Ammonification and nitrification occured very quickly, in case of the calcareous sand at the beginning more NH₄ was evolved. Biuret in concentration from 0.4 to 8 % maximum did not influence urea hydrolysis, however it resulted in higher NO₂-figures at the beginning of the experiments demonstrating an initial marked inhibition of the process NO₂ → NO₃ especially in the calcareous sand. The total nitrification seemed to be a little retarded but did not differ very much finally.     

Soil properties affecting volatilization of ammonia from soils treated with urea

Abstract

The effects of various soil properties on ammonia (NH₃) volatilization from soils treated with urea were studied by measuring the NH₃ evolved when 20 soils selected to obtain a wide range in properties were incubated at ‐0.034 mPa soil moisture potential and 30°C for 10 days after treatment with urea. The nitrogen (N) volatilized as NH₃ from these soils represented from 0 to 65% of the urea‐N applied and averaged 14%. Simple correlation analyses showed that loss of NH₃ was negatively correlated (P<0.1%) with cation‐exchange capacity, silt content, and clay content and was positively correlated (P <0.1%) with sand content. Loss of NH₃ was also negatively correlated with total nitrogen content (P<1.0%), organic carbon content (P<1.0%), hydrogen ion buffering capacity (P<5.0%), and exchangeable acidity (P<5.0%), and was positively correlated with calcium carbonate equivalent (P <1.0%) and with soil pH after incubation with urea (P<1.0%), but was not significantly correlated with initial soil pH or soil urease activity. Multiple linear regression analyses indicated that the amount of urea N volatilized as NH₃ from the 20 soils studied increased with increase in sand content and decreased with increase in cation‐exchange capacity. They also indicated that soil texture and cation‐exchange capacity are better indicators of potential loss of urea N as NH₃ from soils fertilized with urea than are hydrogen ion buffering capacity or initial soil pH.     

Effect of ammonium sulfate pretreatment on ammonia volatilization after urea fertilization

Abstract

Urea applications to soil are subject to loss by ammonia (NH₃) volatilization, unless incorporated. It has been proposed that this loss can be reduced by stimulating populations of soil nitrifiers by an ammonium sulfate [(NH₄)₂SO₄] pretreatment two to four weeks before urea application. The objective of this laboratory trial was to evaluate this concept with five diverse soils, two North American Mollisols and three South American Oxisols. The soils were incubated untreated for two weeks, followed by pretreatment with 0 or 5 kg nitrogen (N) ha^‐1 as (NH₄)₂SO₄, on a soil surface area basis. After another two weeks of incubation, the soils were treated with the equivalent of 0 or 50 kg N ha^‐1 as urea. Ammonia loss was estimated after trapping into phosphoric acid (H₃PO₄). Ammonium sulfate pretreatment reduced NH₃ loss with the two Mollisols and a sandy Oxisol and increased the recovery of the urea application as mineral [ammonium (NH₄ ⁺) + nitrate (NO₃ ^‐)] N in these soils. Little NH₃ loss was detected from the two clay Oxisols, and (NH₄)₂SO₄pretreatment did not influence NH₃ loss or recovery of urea as mineral N. An example of a cropping system where this concept may have utility is discussed.