Nitrogen transformations near urea in soil: Effects of nitrification inhibition,nitrifier activity and liming

A laboratory incubation experiment was conducted to gain a better understanding of N transformations which occur near large urea granules in soil and the effects of dicyandiamide (DCD), nitrifier activity and liming. Soil cores containing a layer of urea were used to provide a one-dimensional approach and to facilitate sampling. A uniform layer of 2 g urea or urea + DCD was placed in the centre of a 20 cm-long soil core within PVC tubing. DCD was mixed with urea powder at 50 mg kg^–1 urea and enrichment of soil with nitrifiers was accomplished by preincubating Conestogo silt loam with 50 mg NH ₄ ⁺ -N kg^–1 soil. Brookston clay (pH 5.7) was limited with CaCO₃ to increase the pH to 7.3. The cores were incubated at 15°C and, after periods of 10, 20, 35 and 45 days, were separated into 1-cm sections. The distribution of N species was similar on each side of the urea layer at each sampling. The pH and NH ₄ ⁺ (NH₃) concentration were very high near the urea layer but decreased sharply with distance from it. DCD did not influence urea hydrolysis significantly. Liming of Brookston clay increased urea hydrolysis. The rate of urea hydrolysis was greater in Conestogo silt loam than limed Brookston clay. Nitrite accumulate was relatively small with all the treatments and occurred near the urea layer (0–4 cm) where pH and NH ₄ ⁺ (NH₃) concentration were high. The nitrification occurred in the zone where NH ₄ ⁺ (NH₃) concentration was below 1000µgN g^–1 and soil pH was below 8.0 and 8.7 in Brookston and Conestogo soils, respectively. DCD reduced the nitrifier activity (NA) in soil thereby markedly inhibiting nitrification of NH ₄ ⁺ . Nitrification was increased significantly with liming of the Brookston soil or nitrifier enrichment of the Conestogo soil. There was a significant increase in NA during the nitrification of urea-N. The (NO ₂ ^– + NO ₃ ^– )-N concentration peaks coincided with the NA peaks in the soil cores.A practical implication of this work is that large urea granules will not necessarily result in NO ₂ ^– phytotoxicity when applied near plants. A placement depth of about 5 cm below the soil surface may preclude NH₃ loss from large urea granules. DCD is a potential nitrification inhibitor for use with large urea granules or small urea granules placed in nests.     

Effects of lanthanum on nitrification and ammonification in three Chinese soils

The effects of lanthanum on nitrification and ammonification in three Chinese soils were evaluated through an incubation experiment. Soils were collected from experimental plots under rice/rape rotation in Yingtan, Jiangxi province (red soil), under rice/wheat rotation in Wuxi, Jiangsu province (paddy soil), and under corn/wheat rotation in Fengqiu, Henan province (Fluvo-aquic soil). Soil nitrification was stimulated slightly by La at lower concentrations, and the stimulation rate reached about 20% in red soil at 150 mg La kg^–1 dry soil, and 14% in fluvo-aquic soil at 300 mg La kg^–1 dry soil. When more La was added in soils, nitrification was inhibited, with a maximum inhibition rate of 42, 44 and 66% in red soil, fluvo-aquic soil, and paddy soil, respectively. Soil ammonification was not significantly different between control and up to 600 mg La kg^–1 dry soil in red soil, but it was significantly reduced in doses of 900 and 1200 mg La kg^–1 dry soil. Significant reduction in soil ammonification was also found in doses from 60 to 1200 mg La kg^–1 dry soil except for 600 mg La kg^–1 dry soil in fluvo-aquic soil. In contrast the ammonification in paddy soil was strongly stimulated by La, reaching about 25 times that of control at 900 mg La kg^–1 dry soil. We assumed that application of La accelerates the transformation of nitrogen in soils at low dosage, and the currently applied dosage in agriculture in China cannot inhibit soil nitrification and ammonification even after long term successive application.     

The effect of increasing application rate of granular calcium ammonium nitrate on net nitrification in a laboratory study of grassland soils

Soil incubation studies were undertaken in controlled environment cabinets at 15°C to investigate the effect of increasing application rates of calcium ammonium nitrate (CAN) on net nitrification in two grassland soils. Granular CAN was applied to the surface of freshly collected, moist soil, at a rate equivalent to 0, 100, 200, 400, 800 and 1600µg NH ₄ ⁺ -N and NO ₃ ^- -N per gram of oven dry soil. In half the treatments finely ground CaCO₃ was incorporated into the moist soil to raise the starting pH. Changes in soil mineral N and pH were measured at weekly intervals up to six-weeks. The most probable number (MPN) technique was used to enumerate the NH ₄ ⁺ -N and NO ₂ ^- -N oxidizers at the beginning and end of the incubation.At low rates of CAN application there was considerable NH ₄ ⁺ -N oxidation to NO ₃ ^- -N during the incubation of both soils. Lime stimulated this N transformation. At high application rates (i.e. 800 and 1600 ppm) there was little change in NH ₄ ⁺ -N or NO ₃ ^- -N on either soil during the 6 week incubation, in the presence or absence of lime. The rate of NO ₃ ^- -N produced peaked at 5.6 and 3.8 mg NO ₃ ^- -N kg^–1 d^–1 on soil 1 and 2 respectively, in the presence of lime. Above a level of 400 ppm CAN (equivalent to 38 kg N ha^–1) the rate of NO ₃ ^- -N produced decreased. The higher rate of net nitrification in soil 1 compared with soil 2 was probably due to a higher number of nitrifying bacteria. Although high rates of CAN decreased the nitrifying activity of both soils there was little difference between treatments in the actual numbers of NH ₄ ⁺ -N and NO ₂ ^- -N oxidizers determined by the MPN technique.The results showed that the rate of granular CAN applied to the soil surface can influence the local activity of nitrifying bacteria and subsequent N transformations. At application rates of CAN generally used agriculturally for grass production, it is likely that net nitrification of the NH ₄ ⁺ -N in the fertilizer granule will be inhibited.     

Nitrogen leaching and uptake from calcium cyanamide in comparison to urea and calcium ammonium nitrate in an ultisol from the humid tropics

Leaching loss of N applied as calcium cyanamide (CaCN₂ — 19% N), urea and calcium ammonium nitrate (CAN — 26% N) to a coarse textured, kaolinitic ultisol profile was studied in the laboratory using undisturbed soil columns. The soil columns were leached with an amount of water equivalent to the annual rainfall of the sampling site (2420mm) using a rainfall simulator over a period of 42 days. The leachability of the three N fertilizers differed greatly and followed the order of CAN > urea > CaCN₂. Most of the N lost through leaching was in NO₃ form. Calcium cyanamide lost only 3% of applied N. Breakdown of CaCN₂ to NH₄ was incomplete (64%) and nitrification in the soil was inhibited resulting in negligible leaching loss. Nitrogen retained in the soil columns after the leaching cycle was mainly in ammoniacal form irrespective of source of N used.Effectiveness of CaCN₂ as a N source was also studied in a greenhouse experiment with maize (Zea mays) and upland rice (Oryza sativa) as testing crops. Calcium cyanamide applied one week before sowing of crops was as effective as CAN and urea under conditions of no N leaching. When applied at the time of planting and two or more weeks before planting gave lower dry matter yields and N uptake than CAN and urea.IITA Journal Paper no. 351     

Interdependence of ammonia volatilization and nitrification in arid soils

The effect of applied-N (urea) on interdependence of ammonia volatilization and nitrification was studied in twelve arid soils varying mainly in soil texture and CaCO₃. Ammonia volatilization from applied urea was observed only above a threshold N concentration in soil (V_i). Values of V_i ranged from 50 in sandy soils to 250 g N g^-1 in loamy and clay loam soils. Soils with higher CaCO₃ showed lower values of V_i. Concentration of applied-N in soils, in relation to V_i determined its transformation pathway(s). Below V_i, all of the applied-N was nitrified in all soils with a delayed nitrification period ranging from 0 to 5 days. Above V_i, ammonia volatilization was first to start. This reduced NH₄ ⁺ concentration in soil and nitrification started later after the delay period was over. Duration of delay period increased with applied-N, sand content and CaCo₃. Often 80% or more of the total ammonia, volatilized during the delay period. NH₄ ⁺ concentration in soil at which volatilization ended was generally higher than V_i. It was concluded that both ammonia volatilization and nitrification were interdependent only if concentration of applied N was more than V_i. Above V_i only one process predominated at a time as volatilization stopped soon after the start of nitrification even if NH₄ concentration in soil was sufficient to sustain both processes.     

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.     

Improvement of the N fertilizer efficiency with dicyandiamide (dcd) in citrus trees

Nitrification inhibitors such a dicyandiamide (DCD) help to reduce leaching losses by retaining applied N in the ammoniacal form. Research objectives were to evaluate dicyandiamide added to ammonium sulphate-nitrate (ASN) as a nitrification inhibitor in cultivated soils (Xeropsamments) and its effect on N uptake by citrus (Citrus sinensis (L.) Osbeck). Under field conditions, fertilization of adult trees with ASN (600 g N tree^–1) either with or without DCD (2% DCD-N) was compared (ASN+DCD and ASN, respectively). The NH ₄ ⁺ -N concentrations in plots fertilized with ASN+DCD were significantly higher than ASN plot in the 0-15 cm layer during 5–105 day period. Nitrification started immediately after N application in both treatments (ASN and ASN+DCD). In all three soil layers analyzed, NO^– ₃-N concentrations were higher in the ASN plots than in the ASN+DCD during the first 20 days. This indicates that nitrification of NH⁺ ₄ from ASN was more rapid in the absence of DCD. On the other hand, fertilization with ASN+DCD kept higher levels of NO^– ₃-N in soils than ASN during the rest of experience period (40–160 days). Addition of DCD to ASN showed a higher N concentration in the spring-flush leaves with respect to the trees fertilized with ASN, during the growth cycle. These results suggest that the use of a nitrification inhibitor permitted a more efficient utilization of fertilizer N by citrus trees. The plants treated with DCD added to ASN showed a higher yield in number of units per tree and a better fruit colour index than those treated with ASN alone.     

Significance of soil depth on nitrogen transformations in flooded and nonflooded systems under laboratory conditions

The influence of different depths of repacked soil cores on changes in N transformation processes was studied with a subtropical semi-arid soil amended with 100 mg N kg^-1 of Sesbania green manure (GM) or fertilizer (NH₄)₂SO₄ for 35 days under flooded and nonflooded conditions. Shallow soil depth enhanced the rate of nitrification, particularly where aeration was impeded in flooded soils. However, the opposite occurred for denitrification as the relative predominance of underlying anoxic zone increased with increase in soil depth. Nitrate produced in the thin oxic surface soil layer and overlying water in flooded soils was subsequently lost via denitrification, more rapidly where carbon was supplied by added GM. Decomposition of GM was rapid and apparent recovery of applied 100 mg GM-N kg^-1 soil as mineral N after 35 days in flooded soils was 8, 26, 30 and 38% in 1.25-, 2.5-, 5.0- and 7.5-cm deep soil cores, respectively. Soil ammonium-N declined rapidly after an initial rise during decomposition of GM in soil in the shallow soil depth. In contrast, no such decline in NH ₄ ⁺ -N was observed in deep soil cores. In conclusion, the use of shallow soil depths during laboratory incubations can lead to variable results under flooded conditions. To simulate field conditions for obtaining reliable and quantitative information regarding N transformations in soils under flooded conditions, soil depths of 7.5 cm or greater should be used for laboratory incubations and growth chamber studies.     

Effect of encapsulated calcium carbide and urea application methods on denitrification and N loss from flooded rice

Poor N fertilizer use efficiency by flooded rice is caused by gaseous losses of N. Improved fertilizer management and use of nitrification inhibitors may reduce N losses. A microplot study using¹⁵N-labelled urea was conducted to investigate the effects of fertilizer application method (urea broadcast, incorporated, deep-placed) and nitrification inhibitor [encapsulated calcium carbide (ECC)] treatments on emission of N₂+N₂0 and total loss of applied N on a grey clay near Griffith, NSW, Australia. Both incorporation and deep placement of urea decreased N₂+N₂O emission compared to urea broadcast into the floodwater. Addition of ECC significantly (P < 0.05) reduced emission of N₂+N₂0 from incorporated or deep-placed urea and resulted in increased exchangeable ammonium concentrations in the soil in both treatments. Fifty percent of the applied N was lost when urea was broadcast into the floodwater. Total N loss from the applied N was significantly (P < 0.05) reduced when urea was either incorporated or deep placed. In the presence of ECC the losses were reduced further and the lowest loss (34.2% of the applied N) was noted when urea was deep-placed with ECC.     

Methane Emission from Rice Fields at Cuttack, India

Methane (CH₄) emission from rice fields at Cuttack (State of Orissa, eastern India) has been recorded using an automatic measurement system (closed chamber method) from 1995–1998. Experiments were laid out to test the impact of water regime, organic amendment, inorganic amendment and rice cultivars. Organic amendments in conjunction with chemical N (urea) effected higher CH₄ flux over that of chemical N alone. Application of Sesbania, Azolla and compost resulted in 132, 65 and 68 kg CH₄ ha^–1 in the wet season of 1996 when pure urea application resulted in 42 kg CH₄ ha^–1. Intermittent irrigation reduced emissions by 15% as compared to continuous flooding in the dry season of 1996. In the wet season of 1995, four cultivars were tested under rainfed conditions resulting in a range of emissions from 20 to 44 kg CH₄ ha^–1. Application of nitrification inhibitor dicyandiamide (DCD) inhibited while Nimin stimulated CH₄ flux from flooded rice compared to that of urea N alone. Wide variation in CH₄ production and oxidation potentials was observed in rice soils tested. Methane oxidation decreased with soil depth, fertilizer-N and nitrification inhibitors while organic amendment stimulated it. The results indicate that CH₄ emission from the representative rainfed ecosystem at the experimental site averaged to 32 kg CH₄ ha^–1 yr^–1.     

Effect of source and nest size of N fertilizers and temperature on nitrification in a coarse textured, alkaline soil

Nitrification occurring in an alkaline sandy loam soil fertilized with urea, ammonium sulphate (AS) and ammonium chloride (AC) was studied in the laboratory at 20°C and 40°C for 30 days. Nitrogen fertilizers were applied as nest of sizes 0.2, 0.5, 1.0 and 2.0 g. Unfertilized control and soil mixed with 50 mg N kg^-1 were also included as treatments.Nitrification in all the fertilizer treatments decreased markedly with increasing nest size. At 20°C, differences among the three N sources were not significant at 5 days after incubation but marked differences appeared thereafter. All the N was nitrified by 30 days in case of fertilizers mixed into the soil. In nest placement, nitrification ranged from 30.1 to 75.5%, 28.3 to 74.6% and 35.3 to 88.7% for urea, AC and AS, respectively. When equal amounts of fertilizers were placed in a nest, nitrification occurred at a slower rate with urea than with AC and AS. Rates of nitrification were significantly higher at 40°C than at 20°C. At 20 days, nitrification from different nest sizes ranged from 8.4 to 64.9% and from 24.9 to 87.0% at 20°C and 40°C, respectively. The difference in nitrification at two temperatures were more pronounced at higher nest sizes than at smaller nest sizes. While nitrification with the three N sources decreased linearly with increase in N concentration (nest size) in soil at 40°C, it showed a quadratic relationship at 20°C. At equal N concentration, the highest rate of nitrification occurred with urea and the lowest with AC. At the same rate of applied N (50–2000 mg kg^-1), AC and AS increased electrical conductivity of soil by 1.3–9 times that of urea. Apparent mineral N recovery of applied N decreased with the increase in nest size.     

Use of nitrification inhibitors to increase fertilizer nitrogen recovery and lint yield in irrigated cotton

This paper describes field experiments designed to evaluate the effectiveness of several nitrification inhibitors to prevent loss of fertilizer nitrogen (N) applied to cotton. The usefulness of nitrapyrin, acetylene (provided by wax-coated calcium carbide), phenylacetylene and 2-ethynylpyridine to prevent denitrification was evaluated by determining the recovery of N applied as¹⁵N labelled urea to a heavy clay soil in 1 m × 0.5 m microplots in north western N.S.W., Australia. In a second experiment, the effect of wax-coated calcium carbide on lint yield of cotton supplied with five N levels was determined on 12.5 m × 8 m plots at the same site.The¹⁵N balance study showed that in the absence of nitrification inhibitors only 57% of the applied N was recovered in the plants and soil at crop maturity. The recovery was increased (p < 0.05) to 70% by addition of phenylacetylene, to 74% by nitrapyrin, to 78% by coated calcium carbide and to 92% by 2-ethynylpyridine.In the larger scale field experiment, addition of the wax-coated calcium carbide significantly slowed the rate of NH ₄ ⁺ oxidation in the grey clay for approximately 8 weeks. Lint yield was increased (p < 0.05) by the addition of the inhibitor at all except the highest level of N addition. The inhibitor helped to conserve the indigenous N as well as the applied N.The research shows that the effectiveness of urea fertilizer for cotton grown on the heavy clay soils of N.S.W. can be markedly improved by using acetylenic compounds as nitrification inhibitors.     

Chemical interactions between soil N and alkaline-hydrolysing N fertilizers

Chemical interactions between soil N and alkaline-hydrolysing N fertilizers labelled with¹⁵N were studied in the laboratory using twelve-irradiated soils. Fertilizer was recovered in the soil organic N fraction via the process of NH₃ fixation. NH₃ fixation at day 7 varied from 1.8 to 4.6% of the N added as aqua ammonia at 1000 mg kg^–1 soil. The amount of NH₃ fixed increased with increasing rates of application of NH_3(aq) and urea. The rate of NH₃ fixation decreased with time, with more than 55% of the total NH₃ fixation in 28 days occurring in the first week following application of 2000 mg urea-N kg^–1 soil. Soil pH and NH₃ fixation varied in response to N source, and increased in the order of di-ammonium phosphate 3 fixation, resulting in the release of unlabelled ammonium (deamination) and a real added nitrogen interaction in all but two of the soils studied. The release of NH ₄ ⁺ initially increased up to a pH of 7.5, was inhibited between pH 8.5 and 9.0, but increased thereafter. The balance (N_bal) between NH₃ fixation and deamination was either positive or negative, depending on the pH of the fertilized soil, which was directly related to N source and concentration for a given soil.     

Effects of soil fumigation and N source on soil inorganic N and tomato growth

Soil fumigation, commonly used in vegetable production, may alter the rate of nitrification, affecting availability of N for crop use. The objective of this research was to examine effects of soil fumigation and N fertilizer source on tomato growth and soil NO₃–N and NH₄–N in field production. Experiments 1 and 2 included application of methyl bromide at 420 kg ha^-1 to a Norfolk sandy loam (fine loamy siliceous thermic Typic Kandiudult) in combination with preplant applications of calcium nitrate, ammonium nitrate, and ammonium sulfate at 144 kg N ha^-1. An additional fumigant, metam-sodium, was included in the second experiment at 703 L ha^-1 (268 kg sodium methyldithiocarbamate ha^-1). Experiment 3 included methyl bromide and metam-sodium, with ammonium sulfate as the sole source of N applied at 144 kg N ha^-1. In the first two studies, fumigants had little or no effect on soil NH₄–N or NO₃–N concentration. Tomato plants were larger and fruit yield was greater in fumigated plots, but there were few growth or yield responses to N source. In the third experiment, fumigants increased concentration of soil NO₃–N and NH₄–N at 16 days after fumigation (DAF), however, there was no effect on nitrification owing to fumigants. It appears that N source selection to overcome inhibition of nitrification is not necessary in plant production systems that involve fumigation     

The effects of a nitrification inhibitor on leaching losses and recovery of mineralized nitrogen by a wheat crop after ploughing-in temporary leguminous pastures

The effect of a nitrification inhibitor on the accumulation of ammonium (NH ₄ ⁺ -N) and nitrate (NO ₃ ^- -N) in the profile was investigated in two field experiments in Canterbury, New Zealand after the ploughing of a 4-year old ryegrass/white clover pasture in early (March) and late autumn (May). Nitrate leaching over the winter, and yield and N uptake of a following wheat crop were also assessed.The accumulation of N in the soil profile by the start of winter was greater in the March fallow (76–140 kg N ha^–1) than in the May fallow treatment (36–49 kg N ha^–1). The nitrification inhibitor dicyandiamide (DCD) did not affect the extent of net N mineralization, but it inhibited nitrification when applied to pasture before ploughing, especially at its depth of incorporation (100–200 mm). Nitrification inhibition in spring was greater when DCD was applied in May rather than in March due to its reduced degradation over the winter.Cumulative nitrate leaching losses were substantial from the March fallow treatment in both years (about 100 kg N ha^–1). A delay in the cultivation of pasture and the application of DCD both reduced nitrate leaching losses. When leaching occurred early in the winter (in 1991), losses were less when pasture was cultivated in May (2 kg N ha^–1) than when DCD was applied to pasture cultivated in March (68 kg N ha^–1). When leaching occurred late in the winter (in 1992), similar losses were measured from pasture cultivated in May (49 kg N ha^–1) and from DCD-treated pasture cultivated in March (57 kg N ha^–1).Grain harvest yield and N uptake of the following spring wheat crop were generally unaffected by the size of the N leaching loss over the winter. This was due to the high N fertility of the soil after four years of a grazed leguminous pasture.     

Nitrous oxide formation potential of various humid tropic soils of Malaysia: A laboratory study

Nitrous oxide (N₂O) is formed mainly during nitrification and denitrification. Inherent soil properties strongly influence the magnitude of N₂O formation and vary with soil types. A laboratory study was carried out using eight humid tropic soils of Malaysia to monitor NH₄ ⁺ and NO₃ ^– dynamics and N₂O production. The soils were treated with NH₄NO₃ (100 mg N kg^–1 soil) and incubated for 40 days at 60% water-filled pore space. The NH₄ ⁺ accumulation was predominant in the acid soils studied and NO₃ ^– accumulation/disappearance was either small or stable. However, the Munchong soil depicted the highest peak (238 g N₂O-N kg^–1 soil d^–1) at the beginning of the incubation, probably through a physical release. While the Tavy soil showed some NO₃ ^– accumulation at the end of the study with a maximum N₂O flux of 206 g N₂O-N kg^–1 soil d^–1, both belong to Oxisols. The other six soils, viz. Rengam, Selangor, Briah, Bungor, Serdang and Malacca series, formed smaller but maximum peaks in an decreasing order of 116 to 36 g N₂O-N kg^–1 soil d^–1. Liming the Oxisols and Ultisols raised the soil pH, resulting in NO₃ ^– accumulation and N₂O production to some extent. As such the highest N₂O flux of 130.2 and 77.4 g N₂O-N kg^–1 soil d^–1 was detected from the Bungor and Malacca soils, respectively. The Selangor soil, belonging to Inceptisol, did not respond to lime treatment. The respective total N₂O formations were 3.63, 1.92 and 1.69 mg N₂O-N kg^–1 soil from the Bungor, Malacca and Selangor soils, showing an increase by 49 and 99% over the former two non-limed soils. Under non-limed conditions, the indigenous soil properties, viz. Ca⁺⁺ content, %clay, %sand and pH of the soils collectively could have influenced the total N₂O formation.     

The tolerance of a grass forage,grown in pots,to urea applied to calcareous soils under very hot,dry conditions

Proso grass was grown in pots in calcareous soils that received different levels of urea applied at seeding under very hot, dry conditions. Urea applications greater than 83 mg N kg^/1 in one experiment and 130 mg N kg^/1 in another caused severe toxicity to the grass seedlings. The toxicity corresponded to the period of active urea hydrolsis, 3–5 days from germination. At this time detectable NH₄ in a low-and a high carbonate soil was equally far below that applied. A level of 40 mg NH⁴-N per kg soil appeared critical to seedling toxicity.     

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

Recent research on the NH₃ volatilization, NO ₂ ^- 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 triamide (NBPT) is the most effective compound currently available for retarding hydrolysis of urea fertilizer in soil, decreasing NH3 volatilization and NO ₂ ^- 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.     

Nutrient transformation in soil due to addition of organic manure and growing crops

Maintaining organic pools of nitrogen (N) in soil is important for providing a steady flux of N in soil solution. Bioslurry, which is the product obtained from anaerobically digested (methanised) farm yard manure (FYM), is an efficient source of organic manure with capability to supply nutrients, particularly N to crops. A study was conducted to see the equilibrium relationship between the inorganic and organic N fractions as affected by application of bioslurry and fertilizer N in a maize (Zea mays L.) — mustard (Brassica campestris) crop sequence. Results obtained revealed that 75.7 percent of the total soil N was in the hydrolyzable N fraction. Among the hydrolyzable fractions, aminoacid N, unidentified N and hydrolyzable NH ₄ ⁺ constituted 25.8, 25.7 and 18.6 percent of the total N, respectively. Ammonium fixed in clay lattice constituted 19.1 percent of the total N. Application of bioslurry @ 13.32 t ha^–1 under N-unfertilized conditions increased NO₃-N, fixed NH ₄ ⁺ , aminoacid N, hexosamine N and hydrolyzable NH ₄ ⁺ . The magnitude of increase in total hydrolyzable and inorganic N fractions was 31.4 and 15.2 percent, respectively. Growing crops decreased N in the inorganic fractions. Transformation reaction of organic N to inorganic N was evident after second crop in the sequence. Fertilizer N application encouraged build-up of N in organic fractions, particularly in aminoacid, hydrolyzable NH ₄ ⁺ and unidentified N fractions. Application of bioslurry maintained higher status of N in both organic and inorganic N fractions. Linear regression relationship between N content in different fractions and bioslurry applied both under fertilized and unfertilized conditions assisted in developing prediction models on the rate of bioslurry to be applied to arrive at the desired N content in different fractions. Significant intercorrelation coefficients (r²) between different fractions indicated free mobility between the N fractions under limited N conditions suggesting a dynamic equilibrium between them. Path coefficient analysis showed that exchangeable NH ₄ ⁺ and NO₃-N had substantial direct positive effect on N uptake by mustard with bioslurry application. Under untreated conditions exchangeable NH ₄ ⁺ , hexosamine and hydrolyzable NH ₄ ⁺ fractions had higher direct contribution to meet mustard N requirement. Most of the hydrolyzable N fractions contributed to N uptake by mustard by first transforming to exchangeable NH ₄ ⁺ and NO₃—N and thus setting an equilibrium condition for maintaining the steady flux of N to plants.Part of Ph.D. Thesis of the senior author     

Nitrogen dynamics during till and no-till pasture restoration sequences in Rondônia,Brazil

The clearing of tropical rain forest in the Amazon basin has created large areas of cattle pasture that are now declining in productivity. Practices adopted by ranchers to restore productivity to degraded pastures have the potential to alter soil N availability and gaseous N losses from soils. We examined how soil inorganic N pools, net N mineralization and net nitrification rates, nitrification potential and NO and N₂O emissions from soils of a degraded pasture responded to the following restoration treatments: (1) soil tillage followed by replanting of grass and fertilization, (2) no-till application of non-selective herbicide, planting of rice, harvest followed by no-till replanting of grass and fertilization, and (3) the same no-till sequence with soybeans instead of rice. Tillage increased soil NH₄⁺ and NO₃^? pools but NH₄⁺ and NO₃^? pools remained relatively constant in the control and no-till treatments. Cumulative rates of net N mineralization and net nitrification during the first 6 months after treatment varied widely but were hightest in the tilled treatment. Emissions of NO and N₂O fluxes increased with tillage and with N fertilization. There were no clear relationships among rates of N fertilizer application, net N mineralization, net nitrification, NO, N₂O and total N oxide emissions. Our results indicate that pasture restoration sequences involving tilling and fertilizing will increase emissions of N oxides, but the magnitude of the increase is likely to differ based on timing of fertilizer application relative to the presence of plants and the magnitude of plant N demand. Emissions of N oxides appear to be decreased by the use of restoration sequences that minimize reductions in pasture grass cover.