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Urea

About: Urea is a research topic. Over the lifetime, 21394 publications have been published within this topic receiving 382444 citations. The topic is also known as: carbamide & carbonic acid diamide.


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Journal ArticleDOI
TL;DR: Although the use of ammonium nitrate fertiliser would significantly reduce NH3 volatilisation losses in summer, this fertiliser costs 45% more per unit N than urea, so there is no economic justification for recommending its use over urea for the other seasons, however, theUse of urea during the cooler, wetter months may result in significantly less denitrification loss.
Abstract: The use of nitrogen (N) fertiliser on dairy pastures in south-eastern Australia has increased exponentially over the past 15 years. Concerns have been raised about the economic and environmental impact of N loss through volatilisation and denitrification. Emissions of NH3, N2, and N2O were measured for 3 years in the 4 different seasons from a grazed grass/clover pasture, with or without 200 kg N fertiliser/ha, applied as ammonium nitrate and urea. Nitrogen-fertilised treatments lost significantly more N than the control treatments in all cases. More NH3 was lost from urea-fertilised treatments than from either the control or ammonium nitrate treatments, whereas ammonium nitrate treatments lost significantly more N through denitrification than the control or urea treatments in all seasons, except for summer. More NH3 was lost in summer than in the other seasons, whereas denitrification and N2O losses were highest in winter and lowest in summer. The total annual NH3 loss from the control, ammonium nitrate, and urea treatments averaged 17, 32, and 57 kg N/ha.year, respectively. Annual denitrification losses were estimated at around 6, 15, and 13 kg N/ha.year for the control, ammonium nitrate, and urea treatments, respectively. Total gaseous N losses were estimated to be 23, 47, and 70 kg N/ha.year from the control, ammonium nitrate, and urea treatments respectively. Although the use of ammonium nitrate fertiliser would significantly reduce NH3 volatilisation losses in summer, this fertiliser costs 45% more per unit N than urea, so there is no economic justification for recommending its use over urea for the other seasons. However, the use of urea during the cooler, wetter months may result in significantly less denitrification loss. The results are discussed in terms of potential management strategies to improve fertiliser efficiency and reduce adverse effects on the environment.

94 citations

Journal ArticleDOI
TL;DR: In this paper, the synthesis of dimethyl carbonate (DMC) from urea and methanol was investigated, to explore the catalysts and reaction conditions, and ZnO showed the highest catalytic activity toward DMC synthesis.
Abstract: The synthesis of dimethyl carbonate (DMC) from urea and methanol was investigated, to explore the catalysts and reaction conditions. Among the catalysts, ZnO showed the highest catalytic activity toward DMC synthesis. Based on the experimental investigation, the reaction conditions over ZnO for a batch process were optimized, and the highest DMC yield attained (based on urea) was ∼30%, because of the presence of side reactions. In addition, the reusability test indicated that ZnO had high stability.

94 citations

Journal ArticleDOI
TL;DR: The ammonia produced by H. pylori urease activity facilitates the organism's nitrogen metabolism at neutral pH as well as protecting it from acid damage at low pH, and utilises exogenous urea as a nitrogen source for amino acid synthesis.
Abstract: Helicobacter pylori has one of the highest urease activities of all known bacteria. Its enzymatic production of ammonia protects the organism from acid damage by gastric juice. The possibility that the urease activity allows the bacterium to utilise urea as a nitrogen source for the synthesis of amino acids was investigated. H. pylori (NCTC 11638) was incubated with 50 mM urea, enriched to 5 atom% excess 15N, that is the excess enrichment of 15N above the normal background, in the presence of either NaCl pH 6.0, or 0.2M citrate pH 6.0. E. coli (NCTC 9001) was used as a urease-negative control. 15N enrichment was detected by isotope ratio mass spectrometry. H. pylori showed intracellular incorporation of 15N in the presence of citrate buffer pH 6.0 but there was no significant incorporation of 15N in unbuffered saline or by E. coli in either pH 6.0 citrate buffer or unbuffered saline. The intracellular fate of the urea-nitrogen was determined by means of gas chromatography/mass spectrometry following incubation with 15N enriched 5 mM urea in the presence of either 0.2 M citrate buffer pH 6.0 or 0.2 M acetate buffer pH 6.0. After 5 min incubation in either buffer the 15N label appeared in glutamate, glutamine, phenylalanine, aspartate and alanine. It appears, therefore, that at pH and urea concentrations typical of the gastric mucosal surface, H. pylori utilises exogenous urea as a nitrogen source for amino acid synthesis. The ammonia produced by H. pylori urease activity thus facilitates the organism's nitrogen metabolism at neutral pH as well as protecting it from acid damage at low pH.

94 citations

Journal ArticleDOI
Jeff M. Sands1
TL;DR: The physiologic, biophysical, and molecular evidence for facilitated and active urea transporters, and integrative studies of their acute and long-term regulation in rats with reduced urine-concentrating ability are focused on.
Abstract: Urea is important for the conservation of body water due to its role in the production of concentrated urine in the renal inner medulla. Physiologic data demonstrate that urea is transported by facilitated and by active urea transporter proteins. The facilitated urea transporter (UT-A) in the terminal inner medullary collecting duct (IMCD) permits very high rates of transepithelial urea transport and results in the delivery of large amounts of urea into the deepest portions of the inner medulla where it is needed to maintain a high interstitial osmolality for concentrating the urine maximally. Four isoforms of the UT-A urea transporter family have been cloned to date. The facilitated urea transporter (UT-B) in erythrocytes permits these cells to lose urea rapidly as they traverse the ascending vasa recta, thereby preventing loss of urea from the medulla and decreasing urine-concentrating ability by decreasing the efficiency of countercurrent exchange, as occurs in Jk null individuals (who lack Kidd antigen). In addition to these facilitated urea transporters, three sodium-dependent, secondary active urea transport mechanisms have been characterized functionally in IMCD subsegments: (1) active urea reabsorption in the apical membrane of initial IMCD from low-protein fed or hypercalcemic rats; (2) active urea reabsorption in the basolateral membrane of initial IMCD from furosemide-treated rats; and (3) active urea secretion in the apical membrane of terminal IMCD from untreated rats. This review focuses on the physiologic, biophysical, and molecular evidence for facilitated and active urea transporters, and integrative studies of their acute and long-term regulation in rats with reduced urine-concentrating ability.

93 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
20231,000
20221,982
2021433
2020502
2019589
2018557