Fertilizer Research 42: 321-329. 1995. © 1995 Kluwer Academic Publishers.

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Recent research on problems in the use of urea as a nitrogen fertilizer

1. M. Bremner Department of Agronomy, Iowa State University, Ames, IA 5001 I, USA

Key words: urea fertilizer, phytotoxicity, ammonia volatilization, nitrite accumulation, urea hydrolysis, urease inhibitors

Abstract

Recent research on the NH3 volatilization, NOz accumulation, and phytotoxicity problems encountered in the use of urea fertilizer is reviewed. This research has shown that the adverse effects of urea fertilizers on seed germination and seedling growth in soil are due to NH3 produced through hydrolysis of urea by soil urease and can be eliminated by addition of a urease inhibitor to these fertilizers. It also has shown that the leaf burn commonly observed after foliar fertilization of soybean with urea results from accumulation of toxic amounts of urea in soybean leaves rather than formation of toxic amounts of NH3 through hydrolysis of urea by leaf urease. It further showed that this leaf burn is accordingly increased rather than decreased by addition of a urease inhibitor to the urea fertilizer applied. N-(n-butyl)thiophosphoric triarnide (NBPT) is the most effective compound currently available for retarding hydrolysis of urea fertilizer in soil, decreasing NH3 volatilization and NOz accumulation in soils treated with urea, and eliminating the adverse effects of urea fertilizer on seed germination and seedling growth in soil. NBPT is a poor inhibitor of plant or microbial urease, but it decomposes quite rapidly in soil with formation of its oxon analog N-(n-butyl) phosphoric triamide, which is a potent inhibitor of urease activity. It is not as effective as phenylphosphorodiamidate (PPD) for retarding urea hydrolysis and ammonia volatilization in soils under waterlogged conditions, presumably because these conditions retard formation of its oxon analog. PPD is a potent inhibitor of urease activity but it decomposes quite rapidly in soils with formation of phenol, which is a relatively weak inhibitor of urease activity. Recent studies of the effects of pesticides on transformations of urea N in soil indicate that fungicides have greater potential than herbicides or insecticides for retarding hydrolysis of urea and nitrification of urea N in soil.

Introduction

The use of urea as a N fertilizer has increased dramati­cally during the past 25 years and urea is now the most important N fertilizer in world agriculture. There is a clear and urgent need, therefore, to find methods of reducing problems encountered in use of urea fertilizer, which include damage to seeds, seedlings and young plants, NOz toxicity, phytotoxicity of foliar-applied urea, and volatilization of urea N as NH3. Gaseous loss of urea fertilizer N as NH3 is of particular con­cern because it can exceed 50% of the N applied [87]. The purpose of this article is to review and summarize recent work to account for some of these problems and to find methods of reducing them.

Approaches to reduction of problems

Several approaches have been explored for reduction of the problems encountered in the use of urea fertil­izer. They include (a) addition of a urease inhibitor to the fertilizer; (b) coating of the fertilizer with sulfur or other materials to slow its rate of dissolution; (c) acidulation of the fertilizer with inorganic acids such as phosphoric or nitric acid; (d) treatment of the fer­tilizer with inorganic salts such as CaCh or KCI; (e) use of urea supergranules. These approaches have been discussed in numerous articles during the past decade [36,42,47,76,80,89,91,92,101]. All have proved use­ful under some circumstances, but the urease inhibitor approach has received the most attention during the past 10 years and it has shown the most promise for

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reduction of NH3 volatilization and other problems encountered in the use of urea fertilizer.

Ammonia volatilization

It is generally accepted that the NH3 volatilization problem results largely, if not entirely, from the nor­mally rapid enzymatic hydrolysis of urea to NH3 and C02 by soil urease (NH2CONH2 + H20 -+ 2NH3 + C02) and the resulting rise in pH and accumulation of NHt. To minimize this problem and the adverse effects of urea fertilizers on seed germination and seedling growth, farmers have been advised not to surface-apply urea fertilizer without incorporating it into the soil soon after application and not to band-place urea with or near seed. As noted by Hauck [47], however, fertilizer and crop management systems now being adopted by farmers make such restrictions on urea use impracti­calor inconvenient. The risk of gaseous loss of urea N as NH3 is particularly high in reduced tillage agri­culture in which urea is often surface-applied without incorporation.

The approach currently favored for reduction of the NH3 volatilization problem is to find compounds that will inhibit soil urease activity and thereby retard urea hydrolysis when applied to soils in conjunction with urea fertilizer. This approach has received considerable attention during the past 25 years, and numerous com­pounds have been patented or proposed as inhibitors of urea hydrolysis. Early work at Iowa State Universi­ty to identify promising soil urease inhibitors showed that dihydric phenols and quinones such as hydro­quinone, catechol, and p-benzoquinone were the most effective of more than 100 compounds tested and indi­cated that hydroquinone had special promise because of its low cost [7,8,15,18,72,73]. Continuation of this work, however, showed that several phosphoroamides, notably N-(n-butyl) thiophosphoric triarnide (NBPT) and phenylphosphorodiamidate (PPD), are more effec­tive than hydroquinone or p-benzoquinone for retard­ing hydrolysis of urea fertilizer in soil [5,6,28,60]. It also showed that NBPT is markedly superior to PPD for retarding urea hydrolysis and decreasing NH3 volatilization and N02 accumulation in upland soils treated with urea [5,6,28]. In contrast to PPD, which is a potent inhibitor of jackbean urease activity but decomposes quite rapidly in soil with formation of phe­nol [12], which is a poor inhibitor of urease activity, NBPT has very little effect on jackbean urease activity but decomposes quite rapidly in soil with formation

of its oxon analog [N-(n-butyl) phosphoric triamide], which is a potent inhibitor of urease activity [65]. It is noteworthy that thiophosphoryl triamide (TPT), which has been under consideration by the Tennessee Valley Authority (TVA) as a dual purpose urease/nitrification inhibitor [76,77], is also a poor inhibitor of jackbean urease activity, but decomposes quite rapidly in soil with formation of its oxon analog, phosphoryl triarnide, which is a potent inhibitor of urease activity [66]. Oth­er workers have confirmed that the inhibitory effect of NBPT on urea hydrolysis in soil is due to its oxon analog [32] and have shown that, although NBPT is superior to PPD for retarding urea hydrolysis and NH3 volatilization in soils under aerobic conditions, PPD is better than NBPT for retarding these processes in soils under waterlogged conditions [24,25,57,89], presum­ably because aerobic conditions are required for the conversion ofNBPT to its oxon analog.

Studies of the effects of phosphoroamides and other urease inhibitors on seed germination and N transfor­mations in soils other than urea hydrolysis have been reported [11,13,20,93,94,100]. They have shown that NBPT and PPD do not inhibit nitrification or denitrifi­cation, do not retard mineralization of organic N when applied at the rate of 10 J-lg g-I soil [13], and do not affect seed germination even when applied at the rate of 250 J-lg g-I soil [11]. Bremner et al. [14] recent­ly studied the persistence of the inhibitory effects of NBPT, PPD, and TPT on urea hydrolysis in soils by measuring the ability of four soils to hydrolyze urea after they had been treated with 5J-lg phosphoroamide g-l soil and incubated at 15°C or 30 °C for 0, 3, 7, 14, or 28 days. The soils used differed markedly in pH, texture, and organic-matter content. The data obtained showed that the persistence of the effects of the phos­phoroamides studied decreased with increase in soil temperature from 15°C to 30 °C and that whereas the effect of PPD decreased with increase in the time of incubation, the effects of NBPT and TPT some­times increased before decreasing with increased time of incubation. These observations are in harmony with the finding that, although PPD is a potent inhibitor of urease activity, it decomposes in soils with formation of phenol, which is relatively weak inhibitor of urease activity, whereas NBPT and TPT have little or no effect on urease activity but decompose in soil with forma­tion of their oxon analogs, which are potent inhibitors of urease activity. The inhibitory effect of NBPT on urea hydrolysis was considerably more persistent than that of PPD or TPT and was significant even after incu-

bation of NBPT-treated soil at 15°C or 30 °C for 28 days.

Most of the information currently available con­cerning the potential value of NBPT, PPD and other phosphoroamides for reduction of NH3 volatilization and other problems encountered in the use of urea fer­tilizer has emerged from laboratory investigations, but a significant number of field and greenhouse studies have been performed to evaluate these compounds for urea fertilization of corn, wheat, rice, and grass­es [3,4,21,22,23,24,48,50,51,55,78,79,82]. The most extensive field studies have been with corn [48].

Although most recent research on urease inhibitors has been focused on phosphoroamides such as NBPT and PPD, hydroquinone (HQ) has received consider­able attention during the past decade, especially in China, where it has been formulated with urea. Sev­eral workers have reported that application of HQ with urea can increase crop yield and urea efficien­cy [26,56,79,104]. Recent work has indicated that the use of HQ to retard hydrolysis of urea fertilizer will not have adverse effects on human health or the envi­ronment and that the increase in crop yield and urea efficiency observed on application of HQ with urea is due to a beneficial physiological effect of HQ on the plant as well as the inhibitory effect of HQ on urea hydrolysis [97,102,103].

Nitrite accumulation

Soils fertilized with urea tend to accumulate N02 and this can lead to N02 toxicity problems [29,30,31]. There also has been concern that N02 accumula­tion following urea fertilization may lead to signifi­cant gaseous loss of urea N via chemo-denitrification (chemical decomposition ofN02) and to N02 pollu­tion of ground waters.

The N02 accumulation observed following appli­cation of urea to soils is believed to be due to the increase in soil pH and accumulation of NH3 resulting from hydrolysis of urea by soil urease, because there is evidence that the Nitrobacter bacteria responsible for oxidation of N02 to NO;- in soils are considerably more sensitive to NHt salts under alkaline conditions than are the bacteria responsible for oxidation of NHt to N02 [1,2].

Bundy and Bremner [19] found that nitrification inhibitors such as N-Serve (nitrapyrin) eliminated or greatly reduced N02 accumulation and greatly retard­ed NO;- formation in soils that accumulated consid-

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erable amounts of N02 when treated with urea. They also found that addition of nitrification inhibitors to soils that accumulated substantial amounts of N02 after treatment with urea had little, if any, effect on the recovery of urea N as (urea + exchangeable NHt + N02 + NO;- + NH3)-N after various times, which indicated that the 'N deficits' observed in studies of urea N transformations in soils may not be largely due to gaseous loss of urea N through chemo-denitrification and are at least partly due to volatilization and fixation of the NHt formed by urea hydrolysis in soils. Their work also indicated that although N-Serve and other nitrification inhibitors may prove useful for reduction of the N02 toxicity problem associated with the use of urea as a fertilizer, application of such inhibitors in conjunction with surface-applied urea fertilizer may promote gaseous loss of urea N as NH3. These obser­vations concerning the effect ofN-Serve on N02 accu­mulation and NH3 volatilization in soils treated with urea have been confirmed in recent work by Magalhaes and Chalk [59].

Since N02 accumulation in soils treated with urea is believed to result from the rapid hydrolysis of urea by soil urease, it should be reduced by application of a urease inhibitor to the urea. This expectation was con­firmed by a study of the effects of four urease inhibitors, namely, NBPT, PPD, DPCA (N-(diaminophosphinyl)­cyclohexylamine], and HQ (hydroquinone), on urea hydrolysis, NH3 volatilization, and N02 accumulation in five soils treated with urea [6]. This study showed that the ability of these urease inhibitors to retard urea hydrolysis, NH3 volatilization, and N02 accumulation in the five soils studied decreased in the order NBPT > DPCA » PPD > HQ. On average, the gaseous loss of urea N as NH3 and the accumulation of urea N as N02 were decreased from 53% to 5% and from 11 % to 1 %, respectively, by addition of NBPT at the rate of 10 /kg g-l soil (0.47 parts of NBPT per 100 parts of urea). There seems little doubt, therefore, that the N02 accumulation problem in use of urea fertilizer could be eliminated or greatly reduced by amending the fertilizer with NBPT.

Loss of urea N by denitrification

Recent work in Australia has indicated that NH3 volatilization, leaching, and run off are not impor­tant mechanisms for N loss from red-brown earths when urea is applied in irrigation water or broadcast onto the soil and washed in by sprinkler irrigation

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and that N loss from these soils is due principally to denitrification [84,85]. Support for these conclu­sions has been provided by studies showing that N loss from these soils via denitrification can be reduced, and that the recovery of urea N by irrigated wheat or cotton can be markedly increased, through applica­tion of nitrification inhibitors such as wax-coated CaC2

or 2-ethynylpyridine to retard formation of nitrate by nitrification of urea N [37,38,86].

Adverse effects of urea on seed germination, seedling growth, and early plant growth

Numerous investigations have shown that urea fer­tilizers can have adverse effects on seed germina­tion, seedling growth and early plant growth in soil [33,40,43,58,88]. Studies to account for these effects have given divergent results, and several explanations have been advanced for each effect [27,30,31,41,47,88]. For example, studies to account for the adverse effect of urea fertilizer on seed ger­mination have suggested that it is due to urea fertil­izer impurities such as biuret [40,88,96], to cyanate formed by isomerization of urea in aqueous solutions [81], to the high pH or high concentration of NHt ions resulting from hydrolysis of urea fertilizer by soil urease [31,32,95], to the NH3 produced by urea hydrolysis [31,32], or to N02" produced through nitri­fication of urea N by soil microorganisms [29,30,31]. But recent work [9,10] leaves very little doubt that the adverse effects of urea fertilizer on seed germination and seedling growth in soil are due to the NH3 formed through hydrolysis of urea by soil urease and are not due to urea itself, to urea fertilizer impurities such as biuret or cyanuric acid, or to N02" formed by nitrifica­tion of urea N. Support for this conclusion was obtained from (a) comparison ofthe effects on seed germination in soil of purified urea, urea fertilizers, urea fertilizer impurities, and compounds formed by enzymatic and microbial transformations of urea in soil (Tables 1 and 2); (b) studies showing that NH3 volatilized from soils treated with urea completely inhibited germination of seeds close to, but not in contact with, these soils; and ( c) experiments showing that the adverse effect of urea fertilizer on seed germination in soil was complete­ly eliminated when the soil was autoclaved to destroy urease or was treated with PPD or NBPT to inhibit soil urease activity before treatment with urea fertiliz­er (Table 3).

Table 1. Comparison of effects of purified urea, urea fertiliz­ers, and urea fertilizer impurities on germination of seeds in Dickinson soil. After [10].

Substance added to soil % Germination of seeds in 7 days

(2.5 mg g-l soil) Barley Com Rye Wheat

None (control) 94 86 95 92

Purified urea 0 0 0 0

Urea fertilizers 0 0 0 0

Biuret 95 74 93 36 Cyanuric acid 97 86 94 92

Table 2. Comparison of effects of different forms of N on seed germination in Dickinson soil. After [10].

Form of N added to soil % Germination of seeds in 7 days

(1.00 mg N g-l soil) Barley Com Rye Wheat

None (control) 96 85 94 93 Urea 0 0 0 0

Ammonium as (NH4hS04 94 84 96 91 Ammonium as (NH4hC03 27 75 6 10

Ammonium as NH40H 0 0 0 0

Nitrite as KN02 0 0 0 0

Nitrate as KN03 94 85 92 94

Phytotoxicity of foliar-applied urea

Foliar application of N and other plant nutrients has potential advantages over soil application for fertiliza­tion of crops in that it may increase the efficiency of fertilizer use and allow relief of physiological stress [41,45]. Interest in foliar fertilization of soybean was

Table 3. Effect of PPD on germination of seeds in Dickinson soil treated with purified and fertilizer urea. After [10].

% Germination of seeds in 7 days

Soil treatment" Barley Com Rye Wheat

None(control) 96 84 95 92 Purified urea 0 0 0 0

Purified urea + PPD 93 85 92 94 Fertilizer urea 0 0 0 0

Fertilizer urea + PPD 96 87 93 95

"Urea added at rate of 2.5 mg g-l soil, PPD at rate of 25 I'g-l soil.

greatly stimulated by work by Garcia and Hanway [39] indicating that foliar fertilization of this crop during seed development could lead to substantial increases in the yield. But most studies of foliar fertilization of soybean during seed development have given disap­pointing results and have indicated that such fertiliza­tion usually leads to a decrease in yield and to some degree ofleaf burn (leaf-tip necrosis) [44,45].

It is generally believed that leaf burn is at least partly responsible for the reduced yields observed after foliar fertilization [74] and that it is increased by low humidity and high temperature and by use of a too con­centrated fertilizer solution [39]. It is also believed that the burn observed depends upon the form ofN fertiliz­er used and that urea is less likely to cause foliage burn than other N fertilizers because it has a lower salt index and is more rapidly absorbed into the leaf [39,75]. But leaf burn has often been observed after foliar fertiliza­tion of plants with urea, and it has been reported that the leaf-burn observed with urea increases with leaf urease activity and is due to the NH3 produced from urea by this activity [46,49,90].

Because the leaf-tip necrosis often observed after foliar fertilization of plants with urea is usually attribut­ed to NH3 formed through hydrolysis of urea by plant urease, Krogmeier et al. [53] studied the possibility that this necrosis could be eliminated or reduced by adding a urease inhibitor (phenylphosphorodiamidate) to the urea fertilizer. They found that although addition of this urease inhibitor to foliar-applied urea increased the urea content and decreased the NH3 content of soy­bean leaves fertilized with urea, it increased the leaf-tip necrosis observed after fertilization. They concluded that this necrosis resulted from accumulation of tox­ic amounts of urea rather than accumulation of toxic amounts of NH3. This conclusion was supported by their finding that the necrotic areas of soybean leaves treated with urea or with urea and phenylphosphorodi­amidate contained much higher concentrations of urea than did the nonnecrotic areas.

Nickel (Ni) is an essential component of urease [34,35], and recent work has shown that plants defi­cient in Ni tend to be necrotic. For example, Brown et al. [17] observed that barley, wheat, and oat plants grown in Ni-deficient media were necrotic, and Shi­mada and Ando [83] noted that tomato and soybean plants having a low Ni content accumulated urea and developed necrotic leaf-tips when grown with urea as the sole source ofN. These observations led Krogmeier et al. [54] to study the effect of Ni deficiency in soy­bean plants on the phytotoxicity of foliar-applied urea,

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their rationale being that if this phytotoxicity is due to urea itself, it should be increased if the plants foliar­applied with urea have a low urease activity as a result of Ni deficiency. They measured the urea content, ure­ase activity, and leaf-tip necrosis of leaves of soybean plants treated with urea after growth of the plants in nutrient solutions containing different amounts of Ni. They found that the urease activity of these leaves decreased, and that their urea content and leaf-tip necrosis increased, with decrease in the Ni content of the nutrient solution. Besides supporting the con­clusion that the leaf-tip necrosis observed after foliar fertilization of soybeans with urea is due to accumu­lation of toxic amounts of urea in the soybean leaves, these observations indicate that Ni-deficient plants may be more susceptible to leaf burn when foliar-fertilized with urea than plants that are not deficient in Ni.

Joo et al. [50,51] have reported laboratory and field experiments indicating that, by reducing ammo­nia volatilization, NBPT and PPD have the potential to increase the N use efficiency of urea N applied as an aqueous fertilizer to Kentucky bluegrass turf. Their work indicated that NBPT is more effective than PPD for increasing the efficiency of urea N applied to tur­fgrass but that it can increase tip burn on fertilization with urea, presumably because its inhibitory effect on urease activity can lead to accumulation of toxic amounts of urea in the grass [52,53].

Effects of pesticides on transformations of urea N in soil

The use of pesticides in conjunction with urea fer­tilizer has increased dramatically during the past 25 years. There is a clear need, therefore, for informa­tion concerning the effects of pesticides on transfor­mations of urea N in soils. To meet this need, Martens and Bremner [61-64] studied the effects of 46 herbi­cides, 17 fungicides, and 15 insecticides (Table 4) on urea hydrolysis and nitrification of urea N in two fine­textured and two coarse-textured soils. They found that when the 46 herbicides tested were applied at the rate of 5 Jig g-I soil, none of them retarded hydrolysis of urea in the four soils used or retarded nitrification of urea N in the two fine-textured soils, but ten of them (amit­role, 2,4-D amine, chlorpropham, dinoseb, propanil, propham, acifluorfen, diclopfop methyl, fenoxaprop ethyl and tridiphane) retarded nitrification of urea N in the two coarse-textured soils. When the 15 insecticides tested were applied at the rate of 5 Jig g-I soil none

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Table 4. Pesticides studied

Herbicides Herbicides

Acifluorfen Ethalfuralin

Alachlor Fenoxaprop ethyl

Amitrole Fluazifop butyl

Atrazine Glyphosate

Bentazon Haloxyfop methyl

Bifenox Linuron

Bromoxynil Mefluidide

Butylate Metolachlor

Chloramben Methoxydim

Chloropropham Monuron

Cinmethylin Oryzalin

Cyanazine Paraquat

2,4-Damine PendimethaIin

2,4-D ester Picloram

DaIapon Prometryn

Dicamba Propachlor

Diclofop methyl Propanil

Dimethazone Propham

Dinitramine Siduron

Dinoseb Simazine

Diuron Tridiphane

DPX-602 Trifluralin

EPTC Vemolate

of them affected urea hydrolysis in the four soils used or nitrification of urea N in the two fine-textured soils, but five of them (carbaryl, lindane, trimethacarb, diazi­non and fenitrothion) retarded nitrification of urea N in the two coarse-textured soils. When the 17 fungicides studied were applied at the rate of 1 JI,g g-I soil, seven ofthem (anilazine, benomyl, chloranil, captan, maneb, mancozeb and thiram) retarded hydrolysis of urea in the two coarse-textured soils and one (maneb) retarded urea hydrolysis in all four of the soils used. All of the fungicides except benomyl, chloroneb, chlorothalonil, fenarimol and iprodione retarded nitrification of urea N in one of the coarse-textured soils when they were applied at the rate of 1 JI,g g-I soil, and terrazole retard­ed nitrification of urea N in all four of the soils used when applied at this rate. These studies indicate that fungicides have a greater potential than herbicides or insecticides for retarding hydrolysis of urea and nitri­fication of urea nitrogen in soil.

Since the NO; -N formed by nitrification of urea N in soil is susceptible to loss by denitrification, it is note­worthy that several workers have reported that deni-

Insecticides Fungicides

Aldicarb Anilazine

Carbaryl Benomyl

Carbofuran Captan

Chlorpyrifos Chloranil

Diazinon Chloroneb

Ethoprop ChlorothaIonil

Fenitrothion Fenaminosulf

Fenvalerate Fenarimol

Fonfos Folpet

Isofenfos Iprodione

Lindane Mancozeb

Malathion Maneb

Phorate MetaIaxyl

Terbufos Metham-sodium

Trimethacarb PCNB

Terrazole

Thiram

trification in soils is retarded by very small amounts of certain pesticides and nitrification inhibitors. For example, there are reports that denitrification is retard­ed by as little as 1 JI,g g-I soil of atrazine [69,70], simazine [69,70], nitrapyrin [67,68] and etridiazole [71]. These reports could not be confirmed, however, in studies of the effects of 33 pesticides and 30 nitrifica­tion inhibitors on denitrification in soil [16,98,99].

Conclusions

The work reviewed indicates that NBPT has consider­able potential as a fertilizer amendment for reduction of NH3 volatilization, NO;- accumulation, and other problems encountered in the use of urea as a fertiliz­er. It is not yet available commercially, but should be available in 1996. A recent summary by Hendrickson [48] of the results of 78 field trials with NBPT con­ducted over 5 years showed that addition of NBPT to urea fertilizer applied to corn increased grain yields by an average of 280 kg ha- I and gave grain yields

equivalent to those obtained with ca. 80 kg ha- 1 of additional unamended urea-No

As noted by Hauck [47], there is a clear need for a urea-based fertilizer that can be drilled directly with wheat and other cereal grain seeds. There seems little doubt that this need could be met by amending urea fertilizers with NBPT because studies using seeds of wheat, barley, oats, and rye have demonstrated that the adverse effects of urea fertilizer on seed germination, seedling growth, and early plant growth in soil can be eliminated or greatly reduced by amending the fertil­izer with as little as 0.01 % (wt/wt) of NBPT (9).

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