Nitrogen movement and uptake by rice fertilized with urea supergranules in two contrasting Mollisols

In a glasshouse experiment, the periodic movement, loss and uptake of N by lowland rice fertilized with point-placed urea supergranule (USG) was studied in two soils differing in texture. Movement of urea-N, NH ₄ ⁺ -N and NO ₃ ^- -N was significantly faster in Patharchatta sandy loam (Typic Hapludoll) than in Beni silty clay loam (Aquic Hapludoll) and was mostly downward with peak concentration near the placement site.Nitrogen in leachate was higher in Patharchatta sandy loam than in Beni silty clay loam. About 60–70% of leaching of urea-N took place within 2 days of USG placement. The leaching of NH ₄ ⁺ -N and NO ₃ ^- -N increased till 14 and 21 days of USG placement in Patharchatta sandy loam and Beni silty clay loam, respectively. Nitrogen leached through urea, NH ₄ ⁺ and NO ₃ ^- forms was, respectively, 64, 25 and 25% higher from sandy loam. During 49 days, 49 and 32% of the applied N was recovered by rice plants from silty clay loam and sandy loam, respectively.     

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.     

Importance of soil type on the release of nonexchangeable NH 4 + and availability of fertilizer NH 4 + and fertilizer NO 3 -

Nitrogen uptake from non-exchangeable NH ₄ ⁺ byLolium multiflorum and availability of fertilizer NH ₄ ⁺ and fertilizer NO ₃ ^- were studied in pot experiments with three different soil types. The luvisol derived from loess released considerable amounts of non-exchangeable NH ₄ ⁺ when cropped. In this soil fertilizer NH ₄ ⁺ was only weakly fixed and was as available to the crop as fertilizer NO ₃ ^- . The recovery of fertilizer NH ₄ ⁺ was even higher than the recovery of fertilizer NO ₃ ^- . In the fluvisol (alluvial soil) and in the cambisol (brown earth from basalt) N recovery was higher from NO ₃ ^- fertilizer than from NH ₄ ⁺ fertilizer. In these soils NH ₄ ⁺ fertilizer was strongly fixed by 2:1 clay minerals and thus less available to the grass. Particularly in the basaltic soil the content of non-exchangeable NH ₄ ⁺ was low and so was the release of nonexchangeable NH ₄ ⁺ . At the same time this soil showed the strongest fixation of fertilizer NH ₄ ⁺ . Release and refixation of fertilizer NH ₄ ⁺ in the loess soil appears to be an important feature of this soil type with a beneficial effect on soil nitrogen turnover and availability.     

Movement and distribution of fertilizer nitrogen as affected by depth of placement in wetland rice

Experiments were conducted to monitor the movement and distribution of ammonium-N after placement of urea and ammonium sulfate supergranules at 5, 7.5, 10, and 15 cm. By varying depths of fertilizer placement, it is possible to determine the appropriate depth for placement machines. There were no significant differences in grain yields with nitrogen placed 5 and 15 cm deep. However, grain yields were significantly higher with deep placement of nitrogen than with split application of the fertilizer. The lower yields with split-applied nitrogen were due to higher nitrogen losses from the floodwater. The floodwater with split application had 78–98µg N ml^–1 and that with deep-placed nitrogen had a negligible nitrogen concentration.Movement of NH ₄ ⁺ -N in the soil was traced for various depths after fertilizer nitrogen application. The general movement after deep-placement of the ammonium sulfate supergranules was downward > lateral > upward from the placement site. Downward movement was prevalent in the dry season: fertilizer placed at 5–7.5 cm produced a peak of NH ₄ ⁺ -N concentration at 8–12 cm soil depth; with placement at 15 cm, the fertilizer moved to 12–20 cm soil depth. Fertilizer placed at 10 cm tended to be stable. In the wet season, deep-placed N fertilizer was fairly stable and downward movement was minimal.A substantially greater percentage of plant N was derived from¹⁵N-depleted fertilizer when deep-placed in the reduced soil layer than that applied in split doses. The percent N recovery with different placement depths, however, did not vary from each other. The results suggest that nitrogen placement at a 5-cm soil depth is adequate for high rice yields in a clayey soil with good water control. In farmers' fields where soil and water conditions are often less than ideal, however, it is desirable to place nitrogen fertilizer at greater depths and minimize NH ₄ ⁺ -N concentration in floodwater.     

Methane production from anaerobic soil amended with rice straw and nitrogen fertilizers

Laboratory experiments were conducted on the effects of rice straw application and N fertilization on methane (CH₄) production from a flooded Louisiana, USA, rice soil incubated under anaerobic conditions. Rice straw application significantly increased CH₄ production; CH₄ production increased in proportion to the application rate. Urea fertilization also enhanced CH₄ production. The maximum production rate was 17% higher, and occurred 1 week earlier, than that of soil samples which did not receive urea, possibly due to the increase in soil pH following urea hydrolysis. The increase in soil pH following urea hydrolysis may have stimulated CH₄-generating bacteria by providing more optimal soil pH conditions or contributed to the drop in redox potential (Eh). The significant decrease in both the production rate and the total amount of CH₄ by application of NH₄NO₃ was associated with increases in soil Eh after addition of this oxidant. Addition of 300 mg. kg^–1 NO ₃ ^- -N increased soil Eh by 220 mV and almost completely inhibited CH₄ production. However, this inhibitory effect was short-termed. Soon after the applied NO ₃ ^- -N was reduced through denitrification, CH₄ production increased. When (NH₄)₂SO₄ was applied, the inhibition of CH₄ production was not associated with an increase in soil Eh which did not change significantly. A direct inhibitory effect of sulphate on methanogenesis might have been more important.     

Movement of bromide as a tracer for nitrate in an Alfisol of the Indian Semi-Arid Tropics under rainfed condition

The variable responses of crops to added nitrogen (N) in Alfisols of the Indian semi-arid tropics are partly due to variable rainfall and partly due to variable losses of available-N. To measure the losses of N through leaching, which can be appreciable under some circumstances, a field experiment was conducted during the rainy season (June-September) of 1992, using bromide (Br) as a tracer for NO ₃ ^- . Bromide (as NaBr) was applied to bare fallow soil at a rate of 200 kg ha^–1 in microplots (2 m × 2 m) and its vertical movement was monitored periodically. Data on rainfall and Br^– distribution in the soil profile on different dates of soil sampling clearly indicated that the movement of Br^– was strongly dependent on rainfall. During the first month (15 June-15 July) after Br^– application, with scattered and light rainfall about 90% of the added Br^– remained in the soil profile (0.6 m). After continuous heavy rainfall in early August more than 90% Br^– had moved beyond 0.6 m depth. This indicates a very high risk of NO ₃ ^- leaching in this soil, and it is unavoidable without special measures to protect the applied N.     

Detectability of15N-depleted fertilizer N in soil and plant tissue during a corn-rye crop rotation

Use of¹⁵N-depleted fertilizer materials have been primarily limited to fertilizer recovery studies of short duration. The objective of this study was to determine if¹⁵N-depleted fertilizer N could be satisfactorily used as a tracer of residual fertilizer N in plant tissue and various soil N fractions through a corn (Zea mays L.) -winter rye (Secale cereale L.) crop rotation. Nitrogen as¹⁵N-depleted (NH₄)₂SO₄ was applied at five rates (0, 84, 168, 252, and 336 kg N ha^–1) to corn. Immediately following corn harvest a winter rye cover crop treatment was initiated. Residual fertilizer N was easily detected in the soil NO ₃ ^- -N fraction following corn harvest (140-d after application). Low levels of exchangeable NH ₄ ⁺ -N (<2.5 mg kg^–1) did not permit accurate isotope-ratio analysis. Fertilizer-derived N recovered in the soil total N fraction following corn harvest was detectable in the 0 to 30-cm depth at each N rate and in the 30 to 60 and 60 to 90-cm depths at the 336 kg ha^–1 N rate. Atom %¹⁵N concentrations in the nonexchangeable NH ₄ ⁺ -N fraction did not differ from the control at each N rate. Nitrogen recovery by the winter rye cover crop reduced residual soil NO ₃ ^- -N levels below the 10 kg ha^–1 level needed for accurate isotope-ratio analysis. Atom %¹⁵N concentrations in the soil total N fraction (approximately one yr after application) were indistinguishable from the control plots below the 168, 252, and 336 kg ha^–1 N rate at the 0 to 30, 30 to 60, and 60 to 90-cm depths, respectively. Recovery of residual fertilizer N by the winter rye cover crop was verified by measuring significant decreases in atom %¹⁵N concentrations in rye tissue with increasing N rates. The greatest limitation to the use of¹⁵N-depleted fertilizer N as a tracer of residual fertilizer N in a corn-rye crop rotation appears to be its detectibility from native soil N in the total N pool.Research partially supported by grants from the National Fertilizer and Environmental Research Center/TVA and the Virginia Division of Soil and Water Conservation.     

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.     

Transformation of calcium cyanamide and its inhibitory effect on urea nitrification in some tropical soils

Transformation of calcium cyanamide and its inhibitory effect on urea nitrification was studied in some tropical soils. Three soils, from Onne, Mokwa and Samaru, representing different agro-climatological zones of Nigeria were incubated with calcium cyanamide, urea or a mixture of both for eight weeks at 30 °C and at field capacity moisture content. The recovery of inorganic N (NH ₄ ⁺ plus NO ₂ ^- plus NO ₃ ^- )from calcium cyanamide varied from 64% to 87% in different soils. Most of the inorganic N accumulated was in NH ₄ ⁺ form. Nitrification of the accumulated NH ₄ ⁺ in all the soils was slow.Urea (75 mg N kg^–1 soil) was completely nitrified within a week in the Samaru and Mokwa soils whereas in the Onne soil the rate of nitrification was slow. Addition of CaCN₂ at the rate of 10 mg N kg^–1 soil generally delayed ammonification of urea and nitrification was severely inhibited in all the soils. Addition of CaCN₂ at the rate of 20 mg N kg^–1 soil further reduced the ammonification of urea and completely inhibited the nitrification. High recovery of inorganic N from calcium cyanamide and its effective reduction of nitrification of urea make it suitable source of N for plants in the tropics, provided it is managed to avoid phyto-toxicity.     

Effects of type and amount of applied nitrogen fertilizer on nitrous oxide fluxes from intensively managed grassland

Five field experiments and one greenhouse experiment were carried out to assess the effects of nitrogen (N) fertilizer type and the amount of applied N fertilizer on nitrous oxide (N₂O) emission from grassland. During cold and dry conditions in early spring, emission of N₂O from both ammonium (NH ₄ ⁺ ) and nitrate (NO ₃ ^– ) containing fertilizers applied to a clay soil were relatively small, i.e. less than 0.1% of the N applied. Emission of N₂O and total denitrification losses from NO ₃ ^– containing fertilizers were large after application to a poorly drained sand soil during a wet spring. A total of 5–12% and 8–14% of the applied N was lost as N₂O and via denitrification, respectively. Emissions of N₂O and total denitrification losses from NH ₄ ⁺ fertilizers and cattle slurry were less than 2% of the N applied. Addition of the nitrification inhibitor dicyandiamide (DCD) reduced N₂O fluxes from ammonium sulphate (AS). However, the effect of DCD to reduce total N₂O emission from AS was much smaller than the effect of using NH ₄ ⁺ fertilizer instead of NO ₃ ^– fertilizer, during wet conditions. The greenhouse study showed that a high groundwater level favors production of N₂O from NO ₃ ^– fertilizers but not from NH ₄ ⁺ fertilizers. Inereasing calcium ammonium nitrate (CAN) application increased the emitted N₂O on grassland from 0.6% of the fertilizer application rate for a dressing of 50 kg N ha^–1 to 3.1% for a dressing of 300 kg N ha^–1. In another experiment, N₂O emission increased proportionally with increasing N rate. The results indicate that there is scope for reducing N₂O emission from grasslands by choosing the N fertilizer type depending on the soil moisture status. Avoiding excessive N application rates may also minimize N₂O emission from intensively managed grasslands.     

Greenhouse evaluation of the effect of topsoil moisture and simulated rainfall on the volatilization of nitrogen from surfaceapplied urea,diammonium phosphate,and potassium nitrate

The effect of topsoil moisture content at the time of nitrogen fertilization and distribution of precipitation following N fertilization on volatile loss of surfaceapplied fertilizer N was studied in two greenhouse experiments using¹⁵N-labeled fertilizers. Loss of applied NO ₃ ^- -N was small compared with loss of urea-N and diammonium phosphate (DAP)-N; this suggests that NH₃ volatilization was the major pathway of N loss for urea and DAP. Loss of applied NO ₃ ^- -N averaged less than 6% of that applied regardless of initial topsoil moisture or amount of precipitation. Increased initial topsoil moisture content increased losses of urea-N greatly but losses of DAP-N only slightly. Increasing depths of precipitation, added five days after N fertilization, greatly decreased loss of urea-N but had no effect on the loss of DAP-N. Variations in moisture and precipitation treatments caused losses of urea-N to vary from 40 to 6% of that applied in a slightly acidic silty loam and from 26 to 11% in a calcareous clay. Moisture and precipitation treatments caused volatilization of DAP-N to vary from 20 to 10% in the silty loam and from 40 to 27% in the calcareous clay. In a second experiment, moisture and precipitation conditions affected N loss from urea as in the previous experiment. Addition of phenylphosphorodiamidate (PPDA), a known urease inhibitor, to urea at 20 g kg^–1 resulted in only a small reduction of N loss in the calcareous clay soil used.It was concluded that soil moisture at the time of N fertilization and precipitation following N fertilization can greatly affect volatile loss of fertilizer N. Since the effect of moisture on N loss is not the same for all N sources, moisture parameters are expected to affect the ranking of N sources by their susceptibility to N loss and their uptake by plants in field experiments. Results obtained suggest some management practices by which fertilizer N might be conserved. The great effect of moisture and precipitation on N loss in these studies underscores the need for detailed meteorological records for field sites of N trials.     

Nitrogen dynamics in paddy field as influenced by free-air CO2 enrichment (FACE) at three levels of nitrogen fertilization

As part of a FACE (free-air CO₂ enrichment) experiment in a rice paddy field in Shizukuishi (Iwate Prefecture, Japan), studies were conducted to determine the effects of elevated CO₂ on N dynamics at three levels of N application. Rice plants were grown under ambient CO₂ or ambient + 200 ppmV CO₂ conditions throughout the growing season in an Andosol soil with each treatment having 4 replicated plots. Three levels of N fertilizer (high, standard and low; 15, 9 and 4 gN m^–2, respectively) were applied to examine different N availability under both CO₂ conditions. Soil samples were collected at 4 different times from upper and lower soil layers (0–1 cm and 1–10 cm soil depths, respectively) and analyzed for microbial biomass N (B_N), mineralizable N (Min. N) and NH₄ ⁺-N in soil. Plant sampling was also done at 3 different times during the growing season to determine the N uptake by plant. Elevated CO₂ significantly increased B_N and Min. N in the upper soil layer at harvest by 25–42% and 18–24%, respectively, compared to ambient CO₂, regardless of N application rate. In low N soil, these significant increases were also observed at the ripening stage. In addition, elevated CO₂ only significantly increased the NH₄ ⁺-N in the upper soil layer at harvest in low N soil compared to ambient CO₂. The N uptake was not significantly affected by CO₂ treatment. These results indicate that elevated CO₂ had significant positive influence on B_N and Min. N in the upper soil layer in paddy soil at the later period of the cropping season at all levels of N application rates, but only at low levels of application rate on NH₄ ⁺-N.     

Nitrogen mineralization,nitrate leaching and crop growth following cultivation of a temporary leguminous pasture in autumn and winter

A field experiment was conducted to investigate the effect of timing and method of cultivation of a 3-year old ryegrass/white clover pasture on subsequent N mineralization, NO ₃ ^- -N leaching, and growth and N uptake of a wheat crop in the following season. The size of various N pools and decomposition of¹⁴C-labelled ryegrass material were also investigated. Cultivation method (mouldboard or chisel ploughing) generally had no significant effect on the accumulation of mineral N in the profile in the autumn or on the amount of NO ₃ ^- -N leached over winter.¹⁴C measurements suggested that initial decomposition rate of plant material was faster from May than March cultivation treatments. Despite this, overall net mineralization of organic N (of soil plus plant origin) increased with increasing fallow period between cultivation and leaching. The total amounts of mineral N accumulated in the soil profile before the start of leaching were 139, 119 and 22 kg N ha^–1 for the March, May and July cultivated soils respectively. Cumulative leaching losses over the trial calculated from soil solution samples were 78, 40 and 5 kg N ha^–1 for the March, May and July cultivated soils respectively. Differences in N mineralization over the season were generally not reflected by changes in amounts of potentially-mineralizable soil N (as measured by extraction or laboratory incubation) or levels of microbial biomass during the season. The amount of mineral N in the profile in spring increased with decreasing fallow period. This was reflected in an approximately 15% and 25% greater grain yield and N uptake respectively by the following wheat crop in plots cultivated in July rather than in March.     

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     

Measurement of nitrous oxide and di-nitrogen emissions from agricultural soils

Nitrous oxide can be produced during nitrification, denitrification, dissimilatory reduction of NO ₃ ^- to NH ₄ ⁺ and chemo-denitrification. Since soils are a mosaic of aerobic and anaerobic zones, it is likely that multiple processes are contributing simultaneously to N₂O production in a soil profile. The N₂O produced by all processes may mix to form one pool before being reduced to N₂ by denitrification. Reliable methods are needed for measuring the fluxes of N₂O and N₂ simultaneously from agricultural soils. The C₂H₂ inhibition and ¹⁵N gas-flux methods are suitable for use in undisturbed soils in the field. The main disadvantage of C₂H₂ is that as well as blocking N₂O reductase, it also blocks nitrification and dissimilatory reduction of NO ₃ ^- to NH ₄ ⁺ . Potentially the ¹⁵ N gas-flux method can give reliable measurements of the fluxes of N₂O and N₂ when all N transformation processes proceed naturally. The analysis of ¹⁵N in N₂ and N₂O is now fully automated by continuous-flow isotope-ratio mass spectrometry for 12-ml gas samples contained in septum-capped vials. Depending on the methodology, the limit of detection ranges from 4 to 11 g N ha^-1day^-1 for N₂ and 4 to 15 g N ha^-1day^-1 for N₂O. By measuring the ¹⁵N content and distribution of ¹⁵N atoms in the N₂O molecules, information can also be obtained to help diagnose the sources of N₂O and the processes producing it. Only a limited number of field studies have been done using the ¹⁵N gas-flux method on agricultural soils. The measured flux rates and mole fractions of N₂O have been highly variable. In rain-fed agricultural soils, soil temperature and water-filled pore space change with the weather and so are difficult to modify. Soil organic C, NO ₃ ^- and pH should be amenable to more control. The effect of organic C depends on the degree of anaerobiosis generated as a result of its metabolism. If conditions for denitrification are not limiting, split applications of organic C will produce more N₂O than a single application because of the time lag in the synthesis of N₂O reductase. Increasing the NO ₃ ^- concentration above the K _m value for NO ₃ ^- reductase, or decreasing soil pH from 7 to 5, will have little effect on denitrification rate but will increase the mole fraction of N₂O. The effect of NO ₃ ^- concentration on the mole fraction of N₂O is enhanced at low pH. Manipulating the interaction between NO ₃ ^- supply and soil pH offers the best hope for minimising N₂O and N₂ fluxes.     

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.     

一株异养硝化菌的分离鉴定及其最佳亚硝化条件

采用传统微生物分离纯化方法,从焦化废水活性污泥中筛选到一株高效去除氨氮并显著积累亚硝酸盐氮的异养硝化细菌C16.该菌株为G-,短杆状;菌落为白色、半透明.经形态、生理生化特性以及16S rRNA基因序列分析,初步鉴定该菌属于产碱杆菌属(命名为Alcaligenes sp.C16).对该菌的异养硝化功能进行了研究,结果表...     

The fate of algal nitrogen in a flooded soil system

Algal N labelled with ¹⁵N added to a flooded soil in laboratory columns without plants was studied to determine the changes over time in the fate of N assimilated by algae and to study how its fate is affected by (a) exclusion of light simulating complete closure of the rice canopy, and (b) addition of fertilizer-NH₄ ^*. In the light but with no added fertilizer-N there was little net mineralization of the added algal N during the first 4 weeks, but after 8 weeks 42% had been mineralized, of which 95% was denitrified. Exclusion of light caused net mineralization to proceed more rapidly in the first 4 weeks due to the death of algal cells and lowered reassimilation. After 8 weeks 51% had been mineralized, of which 54% was denitrified, 16% volatilized and 30% was present as KCl exchangeable NH₄ ⁺-N. Application of fertilizer-NH₄ ⁺ apparently caused mineralization of 25% of the algal N within one week but the results were probably affected by pool substitution in which labelled N mineralized to NH₄ ⁺-N was diluted with fertilizer – NH⁺ ₄ and then immobilized leaving more labelled NH₄–N in the mineral pool. After 8 weeks, 42% of algal N had been mineralized, of which 69% was estimated to have been denitrified, 19% lost through NH₃ volatilization and 12% remained as extracted NH₄ ⁺+NO^- ₃. Uptake of N by a rice crop would reduce the gaseous losses. Algal N was mineralized quickly enough to be available during the growing season of a rice crop and, depending on field conditions, algae may have a role in assimilating N and protecting it from loss as well as being a major driving force for NH₃ volatilization through diurnal increases in pH.     

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.     

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.