Nitric oxide (NO) is an important signaling molecule in animals and plants. In mammals, NO is produced from Arg by the enzyme NO synthase. In plants, NO synthesis from Arg using an NO synthase-type enzyme and from nitrite using nitrate reductase has been demonstrated previously. The data presented in this report strongly support the hypothesis that plant tissues also synthesize NO via the nonenzymatic reduction of apoplastic nitrite. As measured by mass spectrometry or an NO-reactive fluorescent probe, Hordeum vulgare (barley) aleurone layers produce NO rapidly when nitrite is added to the medium in which they are incubated. NO production requires an acid apoplast and is accompanied by a loss of nitrite from the medium. Phenolic compounds in the medium can increase the rate of NO production. The possible significance of apoplastic NO production for germinating grain and for plant roots is discussed.

Apoplastic Synthesis of Nitric Oxide by Plant Tissues

This paper reviews the use of additives to reduce odor and ammonia (NH3) emissions from livestock wastes. Reduction of NH3 volatilization has been shown to be possible, particularly with acidifying and adsorbent additives, and potential exists to develop further practical and cost-effective additives in this area. Masking, disinfecting, and oxidizing agents can provide short-term control of malodor, but as the capacity of these additives is finite, they require frequent reapplication. Microbial-based digestive additives may offer a solution to this problem as they are regenerative, but they appear to have been developed without a thorough understanding of microbiological processes occurring in livestock wastes. Currently, their use to reduce odor or NH3 emissions cannot be recommend. If the potential of these types of additives is to be realized, research needs to shift from simply evaluating these unknown products to investigating known strains of bacteria or enzymes with known modes of action. To protect the farmers' interest, standard independent test procedures are required to evaluate efficacy. Such tests should be simple and quantify the capacity of the additive to perform as claimed. The principle use of additives needs to be identified and addressed during their development. Producers may not use effective additives in one area if they further compound other problems that they perceived to be more important. There is the potential to use additives to treat other problems associated with livestock wastes, particularly to improve handling properties, reduce pollution potential to watercourses, and reduce pathogenic bacteria. Further work is required in these areas.

Additives to reduce ammonia and odor emissions from livestock wastes: a review.

1. Volatilization of Ammonia.- 2. Biological denitrification.- 3. Chemodenitrification.- 4. The measurement of denitrification.- 5. Micrometeorological methods for measuring gaseous losses of nitrogen in the field.- 6. Gaseous nitrogen losses from plants.- 7. Nitrogen loss from sewage sludges and manures applied to agricultural lands.- 8. Ammonia loss from fertilizer applied to tropical pastures.- 9. Gaseous nitrogen exchanges in grazed pastures.- 10. Fate of fertilizer nitrogen applied to wetland rice.- 11. The fate of nitrogen compounds in the atmosphere.- 12. Agronomic technological approaches to minimizing gaseous nitrogen losses from croplands.

Gaseous loss of nitrogen from plant-soil systems

Nitrite is an intermediary compound formed during nitrification as well as denitrifiication. It occasionally accumulates in soils and drainage water. The nitrite can then undergo transformations to gaseous nitrogen compounds such as NO and NO2. Soil pH controls the abiotic nitrite decomposition to a large extent. Under acidic conditions(pH <5.5), nitrous acid spontaneously decomposes preferentially to NO and NO2. Nitrite also undergoes reactions with metallic cations (especially ferrous iron) and with organic matter. As a result of these reactions gaseous compounds such as NO, NO2, N2O and CH3ONO can be formed. Through reaction of nitrite with phenolic compounds nitroand nitrosocompounds can be formed, building up organic N. With normal agricultural practices on slightly acidic soils, the nitrite instability usually does not lead to economically important N losses from soils. However, the compounds formed through its degradation or interaction with other soil constituents are linked to environmental problems such as tropospheric ozone formation, acid rain, the greenhouse effect and the destruction of the stratospheric ozone.

Nitrite in soils: accumulation and role in the formation of gaseous N compounds

Publisher Summary Loss of nitrogen (N) as NH 3 from applied fertilizers, organic amendments, and crop residues is only one of several possible fates of soil and applied N. Leaching losses of N, largely as nitrate (NO 3 –N), may be severe on sandy, gravelly, or lateritic soils under conditions of heavy rainfall or excessive irrigation. This chapter reviews the research results on NH 3 volatile losses from applied N fertilizers and examines the impact of greater use of urea N. The chapter discusses the losses of NH 3 from crop residues, applied manures, and other organic amendments. Techniques for measuring N volatilization losses directly as NH 3 and indirectly by measuring crop uptake of applied N. NH 3 absorption by soils and vegetation are also discussed in the chapter. Research results have shown that NH 3 –N losses from fertilizers surface-applied to moist soils increase with intensity of drying conditions and with decrease in soil sorption capacity for NH 4 –N. Results from laboratory, greenhouse pot, and field plot experiments agree, in general, concerning the various factors contributing to NH 3 –N losses.

Volatilization Losses of Nitrogen as Ammonia from Surface-Applied Fertilizers, Organic Amendments, and Crop Residues

Summary

Plant damage and reduced yields were associated with delayed ammonification or nitrification of added urea-nitrogen In a neutral sandy loam, the adverse effects of urea application were associated with the accumulation of toxic levels of first ammonia and later nitrite Addition of hydrogen-ion as phosphoric or sulphuric acid alleviated the damage thereby supporting the conclusion that ammonia and nitrite accumulation were the causes of phytotoxicity In an acid sand, toxicity was associated first with the presence of large quantities of urea remaining in the soil though the evidence was insufficient to show in what way the urea was toxic There was then a second phase of toxicity associated with the presence of considerable quantities of ammonium-nitrogen under conditions where toxic levels of ammonia were likely to have occurred The later phase (nitrite toxicity) did not occur Phytotoxicity arising from urea addition to the soil could be due apparently to the persistence of urea as such in the soil and also to the accumulation of significant quantities of free ammonia and nitrite The occurrence of one or more of these phases of urea toxicity depended on differences in soil conditions

Toxicity as a cause of the inefficiency of urea as a fertilizer.

Toxic Effect of Urea on Plants: Nitrite Toxicity arising from the Use of Urea as a Fertilizer

Summary

The causes and mechanisms of phytotoxicity that can bring about poor or adverse crop responses to applications of solid urea to soil are reviewed Compounds which may cause phytotoxicity are urea, transformation products of urea formed during manufacture (biuret), and transformation products produced in the soil or in plants (cyanate, carbamate, ammonia, and nitrite) Of these, ammonia and nitrite are considered to be the most likely causes of phytotoxicity when urea of low biuret content is used

Toxicity as a cause of the inefficiency of urea as a fertilizer. i. review

Summary

A greenhouse experiment compared the effects of ammonium nitrate and urea (1 per cent biuret), applied to forty-two soils, on the growth and yield of maize foliage.



Dry-matter yields of maize in most soils were similar when both sources supplied 0·15 g or 0·30 g N per pot. At 0·45 and 0·60 g N per pot plants in soils with urea yielded less than with ammonium nitrate in sixteen and nineteen soils respectively. Urea adversely affected plant development to a greater extent in light soils than in heavy soils and peats. A partial correlation analysis indicated that responses to urea at the two higher rates of application were positively correlated with ammonia absorption capacity and a correlation with clay content was indicated. No significant partial correlations were obtained with organic matter content, pH, exchangeable-hydrogen, cation-exchange capacity, and moisture content.

J. S. Waid

Papers

Toxicity as a cause of the inefficiency of urea as a fertilizer.

Toxic Effect of Urea on Plants: Nitrite Toxicity arising from the Use of Urea as a Fertilizer

Toxicity as a cause of the inefficiency of urea as a fertilizer. i. review

The influence of soil type on the response of maize to urea in glasshouse experiments