Assessment of volatilization loss of ammonia from surface-applied urea on forest soil by N15 recovery

Summary Using vertically isolated micro-plots the isotopic recovery technique was tested for assessing the extent of ammonia volatilization loss from N¹⁵-labelled urea applied on the forest floor (Pinus silvestris L.). The size of the ammonia loss was obtained as a difference between the amounts of labelled urea N added and the amounts of labelled N recovered in the soil profile after 13, 31, and 39 days's exposure, respectively. Urea materials of two different pellet size were used: granulated small pellets (280 pellets per gram) and tablets (2.06 g each). The nitrogen application rate was 200 kg N per hectare. The recovery data for 13 days' exposure indicated a volatilization loss, which for the small pellet urea was 24.9 per cent and for the tabletted urea 12.1 per cent. The corresponding figures for the 31 days' exposure, during which the total amount of precipitation was 14 mm, were 15.1 and 26.9 per cent, respectively. The pattern of labelled N distribution in the soil profile examined showed that during the period of exposure in question a leaching loss of labelled N was rather unlikely. It was demonstrated, furthermore, that nitrogen from the tabletted urea had diffused to a greater depth of the soil than that from the small-pellet urea. Nitrogen from the small-pellet urea was to a large extent recovered in the litter layer. On exposure to heavy rain the tabletted urea was subjected to the highest leaching loss. An addition of 10 per cent (w/w) of metaphosphoric acid or sublimed sulphur to the tabletted urea did not result in any further reduction of the volatilization loss. The merits and limits of the isotopic recovery technique are discussed.     

Further observations on ammonia loss from urea applied to forest soil with special reference to the effect of pellet size

Summary Using a static system of ammonia sorbers, the extent of ammonia loss from surface-applied urea materials of different pellet sizes was studied under field conditions. Observations made at two different sites revealed that during the initial stage of exposure the gaseous ammonia loss was significantly retarded by increasing the pellet size. On extending the period of exposure to 20 and 30 days, respectively, this difference in the cumulative ammonia loss levelled out or even became negative. re]19750714     

The simple open soil method of measuring urea volatilisation losses

Summary A simple technique of estimating ammonia volatilisation loss from urea applied to soil is described. The soil is incubated with urea, after which hydrochloric acid is added to prevent loss of N from the hydrolysed urea during drying of the soil. The volatilisation loss is estimated by difference between the N recovered from the soil and the urea applied. Recovery studies of urea hydrolysis on 3 Malaysian soils show that the hydrolysis products, consisting of ammonium N and any unreacted urea, can be quantitatively recovered from the soil by this technique, with average recoveries of 95%–97%. A cross-check by 9 laboratories indicated good accuracy and precision in the method. When the open soil method was compared with the direct measurement of ammonia loss in simple volatilisation chambers, it gave much higher results suggesting that the closed system of trapping tended to underestimate volatilisation losses during urea hydrolysis.     

Enhancement of nitrogen release properties of urea–kaolinite fertilizer with chitosan binder

The use of controlled release fertilizer (CRF) has become a new trend to minimize environmental pollution. In this study, urea–kaolinite containing 20 wt% urea after one hour dry grinding was mixed with different concentrations of chitosan as a binder to prepare nitrogen-based CRF. Fourier transform infrared spectroscopy confirmed the hydrogen bonding between urea and kaolinite. Covalent interaction between urea–kaolinite and chitosan make the granules stronger. The nitrogen release was measured in 5 days interval using a diacetylmonoxime calorimetric method at a wavelength of 527 nm. The results illustrated that by increasing the chitosan concentration from 3 to 7.5%, nitrogen release decreased from 41.23 to 25.25% after one day and from 77.31 to 59.27% after 30 days incubation in water. Compressive stress at break tests confirmed that granules with chitosan 6% had the highest resistance and were chosen for ammonia volatilization tests. Ammonia volatilization was carried out using the forced-draft technique for a period of 10 weeks. The results showed that the total amount of ammonia loss for conventional urea fertilizer and urea–kaolinite–chitosan granules was 68.63 and 56.75%, respectively. This controlled release product could be applied in agricultural crop production purpose due to its controlled solubility in the soil, high nutrient use efficiency and potential economic benefits.     

α-酮戊二酸对氨氮胁迫下草鱼鳃Na+/K+-ATP酶活性及血液生化指标的影响

试验旨在研究饲粮添加α-酮戊二酸(α-ketoglutarate, α-KG)对氨氮胁迫下草鱼(Ctenopharyngodon idellus)鳃Na+/K+-ATP酶活性及血液生化指标的影响。选取初始体重为(24.79±0.11) g的健康草鱼, 随机分为3个处理组(对照组, 养于曝气后氨氮浓度为1.51 mg/L的自来水中并饲喂基础饲粮; 氨氮组, 养于氨氮浓度为18.37 mg/L的水中并饲喂基础饲粮; α-KG组, 养于氨氮浓度为18.37 mg/L的水中并饲喂添加0.75% α-KG的饲粮)。每处理组设3个重复, 每个重复30尾鱼, 养殖试验为期42d, 分别于第1、第14、第28和第42天采样。结果表明:在饲料中添加α-KG能够有效缓解氨氮胁迫导致的草鱼血浆氨含量(1d)、谷草转氨酶(Aspartate transaminase, AST)活性(14d)、碱性磷酸酶(Alkaline phosphatase, ALP)活性(28d)的显著升高和鳃Na+/K+-ATP酶活性(28d)、血浆谷丙转氨酶(Alanine aminotransaminase, ALT)(28d和42d)活性、血浆尿素(UREA)含量(28d)的显著降低, 显著增加氨氮胁迫下草鱼血浆球蛋白(Globulin, GLB)含量(28d)。即饲粮α-KG的适量添加能够有效缓解草鱼氨氮胁迫所致的血氨含量升高, 维持氨氮胁迫下草鱼鳃Na+/K+-ATP酶、血浆谷丙转氨酶、谷草转氨酶、碱性磷酸酶的活性和血浆球蛋白、尿素含量的稳定, 从而有利于草鱼缓解氨氮胁迫。     

不同包膜控释尿素对农田土壤氨挥发的影响

为了探索包膜控释尿素土壤氨挥发损失规律特征和提高肥料氮素利用率,采用小麦玉米轮作田间试验,通过与普通尿素进行对比,运用土壤氨挥发原位测定方法——通气法系统研究了硫包膜和树脂包膜控释尿素的施用对小麦玉米轮作农田土壤氨挥发的影响.研究结果表明:在两种施氮量水平下(210 kg/hm2和300 kg/hm2),与普通尿素相比,硫包膜和树脂包膜控释尿素在小麦基肥期、小麦追肥期和玉米施肥期的施用均减少了土壤氨挥发的累积损失量,分别达35.1％-54.3％、59.6％-75.2％、65.6％-98.1％;有效降低了土壤氨挥发通量峰值且延迟其出现时间3-8 d,并能延缓土壤氨挥发主要阶段的时间分别为4-12 d、5-12 d.在小麦玉米轮作周年中,控释尿素土壤氨挥发累积损失量为28.39-43.35 kg/hm2,土壤氨挥发损失率为4.48％-5.63％,控释尿素时段土壤氨挥发通量比普通尿素降低了51.0％-70.8％;且树脂包膜控释尿素的施用降低小麦玉米轮作农田土壤氨挥发的效果优于硫包膜控释尿素.     

Nitrogen metabolism and excretion in the aquatic chinese soft-shelled turtle, Pelodiscus sinensis, exposed to a progressive increase in ambient salinity

This study aimed to determine effects of 6-day progressive increase in salinity from 1 per thousand to 15 per thousand on nitrogen metabolism and excretion in the soft-shelled turtle, Pelodiscus sinensis. For turtles exposed to 15 per thousand water on day 6, the plasma osmolality and concentrations of Na+, Cl- and urea increased significantly, which presumably decreased the osmotic loss of water. Simultaneously, there were significant increases in contents of urea, certain free amino acids (FAAs) and water-soluble proteins that were involved in cell volume regulation in various tissues. There was an apparent increase in proteolysis, releasing FAAs as osmolytes. In addition, there might be an increase in catabolism of certain amino acids, producing more ammonia. The excess ammonia was retained as indicated by a significant decrease in the rate of ammonia excretion on day 4 in 15 per thousand water, and a major portion of it was converted to urea. The rate of urea synthesis increased 1.4-fold during the 6-day period, although the capacity of the hepatic ornithine urea cycle remained unchanged. Urea was retained for osmoregulation because there was a significant decrease in urea excretion on day 4. Increased protein degradation and urea synthesis implies greater metabolic demands, and indeed turtles exposed to 15 per thousand water had significantly higher O2 consumption rate than the freshwater (FW) control. When turtles were returned from 15 per thousand water to FW on day 7, there were significant increases in ammonia (probably released through increased amino acid catabolism) and urea excretion, confirming that FAAs and urea were retained for osmoregulatory purposes in brackish water.     

Nitrogenous fertilizer transformations in the sudan Gezira soil

Summary and Conclusions Direct measurements were made of losses of ammonia during the transformation of urea and ammonium sulphate, surface-applied to alkaline Gezira soil in containers incubated in the field, under different rates of nitrogen application and moisture conditions.The highest rate of ammonia loss occurred during the first week after application with both fertilizers, thereafter decreasing to lower values. The cumulative ammonia loss was higher with higher application of nitrogen. Ammonium sulphate gave consistently higher ammonia losses than urea and losses from open soil system were generally less than from soil in polythene bags.With lowest irrigation level used, ammonia loss attained a sizeable value throughout the incubation period with both fertilizers. With the higher moisture levels, the magnitude of ammonia loss decreased appreciably, much more so with urea than with ammonium sulphate. Induced drying and rewetting prolonged the duration of loss and increased the magnitude of cumulative loss. An appreciable loss of ammonia may take place from fertillzed Gezira Soil under warm conditions, low moisture levels and high fertilizer concentration; this may be the case with patchy fertilizer distribution and frequent light showers during early summer. It is advisable to apply the urea or ammonium sulphate when conditions are most favourable for nitrification.     

Salmonella reduction in manure by the addition of urea and ammonia

Animal manure may contain pathogenic microorganisms and disinfection is suggested to avoid reinfection of animal herds and introduction of zoonotic disease into the food chain. Ammonia and urea were tested for disinfection of bovine manure and Salmonella was found to be rapidly eliminated by the addition of 0.5% aqueous ammonia or 2% w/w urea (s). Treatments (2% urea and 0.5% ammonia), temperature (4 degrees C or 14 degrees C) and combinations of these factors significantly affected the inactivation rate. Decimal values (T(90)) were reduced from 8.3 days in the control to 2.0 days and 0.4 days at 14 degrees C after the addition of urea and ammonia, respectively. At 4 degrees C, the decimal values were reduced from 34 to 4.8 and 1.1 days, respectively. Recommended treatments of bovine manure based on Monte Carlo simulations are 0.5% ammonia followed by storage for one week or 2% urea followed by storage for two weeks at 14 degrees C, one month at 4 degrees C. Storage without additives should include at least one summer in temperate regions. Enterococci were evaluated as indicators for Salmonella but significantly slower decay rate and different behaviour in the material made them unsuitable as indicators for Salmonella in manure disinfected by ammonia or urea. Free ammonia treatment of Salmonella-contaminated manure disinfects the material and raises its fertilizer value.     

Rate of hydrolysis of urea as influenced by thiourea and pellet size

Summary Two incubation experiments and a number of field experiments were conducted to determine the effect of soil moisture tension, pellet size and addition of thiourea to urea on the rate of urea hydrolysis. In the incubation experiments at 20°C, the rate of hydrolysis of urea increased from 15 bar to 1/3 bar soil moisture tension, with the largest change (doubling) occurring from 15 bar to 7 bar moisture tension. Increasing pellet size reduced the rate of urea hydrolysis by about 12% with urea pellets weighing 0.21 g as compared to 0.01 g urea pellets after 114h. When thiourea (a metabolic inhibitor) was pelleted with urea in a ratio of two parts urea and one part thiourea, the rate of hydrolysis was halved.In a field experiment, the addition of thiourea to urea and increasing pellet size suppressed the rate of urea hydrolysis considerably for 8 days. The amount of urea hydrolyzed with urea+thiourea (21) pellets weighing 2.51 g was one-fourth of the amount of urea hydrolyzed with 0.01 g pellets of urea alone. In the other six field experiments which were set out in October, only 22% to 39% of urea +thiourea (21) was hydrolyzed at two weeks after application, while almost all of the urea was hydrolyzed when it was mixed into the soil without an inhibitor.Unter our field conditions, we would estimate that the hydrolysis of urea can be inhibited for at least one week. The inhibition of urea hydrolysis appears to be great enough that the problems encountered from the rapid hydrolysis of urea, wherever these occur, may be reduced by combined use of thiourea and either increased pellet size or band placement.     

Marine (Taeniura lymma) and freshwater (Himantura signifer) elasmobranchs synthesize urea for osmotic water retention

The objective of this study was to elucidate whether the marine blue-spotted fantail ray, Taeniura lymma, and the freshwater white-edge whip ray, Himantura signifer, injected with NH(4)Cl intraperitoneally would excrete the majority of the excess ammonia as ammonia per se to ameliorate ammonia toxicity despite being ureogenic. To examine the roles of urea and the ornithine-urea cycle, experimental fishes were exposed to salinity changes after being injected with NH(4)Cl. The ammonia excretion rates of the marine ray, T. lymma, injected with NH(4)Cl followed by exposure to seawater (30 per thousand) or diluted seawater (25 per thousand) increased 13-fold and 10-fold, respectively, within the first 3 h. Consequently, the respective percentage of nitrogenous wastes excreted as ammonia were 55% and 65% compared with 21% of the saline-injected control, indicating that T. lymma became apparently ammonotelic after injection with NH(4)Cl. By hour 6, large portions (70%-85%) of the ammonia injected into T. lymma exposed to seawater or diluted seawater had been excreted, and T. lymma excreted much more nitrogenous wastes (135%-180%), in excess of the ammonia injected into the fish, during the 24-h period. For T. lymma exposed to seawater, a small portion (30%) of the ammonia injected into the fish was detoxified to urea during the first 6 h, but there was an apparent suppression of urea synthesis thereafter, contributing partially to the large decrease (19%) in urea contents in its muscle at hour 24. A major contributing factor to the decrease in urea content was a reduction in ammonia production, as indicated by a large deficit between urea loss in the muscle and excess ammonia accumulated plus excess nitrogen excreted in the experimental fish. The freshwater ray, H. signifer, injected with NH(4)Cl followed by exposure to freshwater (0.7 per thousand) or brackish water (10 per thousand) was capable of excreting all the ammonia injected into the body, mainly as ammonia, within 12 h. Like T. lymma, it also excreted the injected ammonia mainly as ammonia during the first 3 h postinjection. During this period, the percentage of the injected ammonia excreted in fish exposed to brackish water (28.4%+/-4.6%) was significantly lower than those exposed to freshwater (56.1%+/-8.26%). In contrast, the percentage of nitrogenous wastes being excreted as urea in the former (38.4%) was significantly greater than that in the latter (14.1%). These results suggest that a portion of the ammonia injected into the fish was turned into urea, and urea synthesis was increased transiently in fish exposed to brackish water during the initial postinjection period. However, urea was not retained effectively by H. signifer. Taken together, these results suggest that the primary function of the ornithine-urea cycle in ureogenic marine and freshwater elasmobranchs is to synthesize urea for osmotic water retention and not for ammonia detoxification.     

Nitrification and ammonia volatilisation losses from urea and dicyandiamide-treated urea in a sandy loam soil

Summary Nitrification and ammonia volatilisation losses from urea and dicyandiamide (DCD)-treated urea were studied in a sandy loam soil. Laboratory experiments indicated that 20 ppm (of soil) DCD effectively inhibited nitrification of urea over sixty days. If the urea was treated with DCD (20 ppm), ammonia emission from the soil was extended over 105 days; with urea alone, it was negligible after 15 days. A field study indicated that DCD treatment increased volatilisation losses of ammonia tremondously if urea was applied to the soil surface; these losses were minimised if the urea was placed at 5 cm depth. It would seem that nitrification inhibitors must be combined with a placement technique.     

Ammonia volatilization losses from prilled urea,urea supergranules (USG) and coated USG in rice fields

Summary About 8.4 per cent of applied nitrogen was lost as ammonia during a week after application when prilled urea was broadcast or banded and incorporated in soil 20 days after sowing of rice. Ammonia volatilization was reduced to 3.3 per cent when urea supergranules (USG) were used. Coating of USG with DCD or neem cake showed no advantage. Ammonia volatilization was only 0.7 to 1.6 per cent when fertilizer was applied at panicle initiation stage of rice; highest values were again obtained with prilled urea. The experiments were carried out in closed cages.     

Metabolic and immune responses in Chinese mitten-handed crab (Eriocheir sinensis) juveniles exposed to elevated ambient ammonia

The toxicity of ammonia to Eriocheir sinensis juveniles was determined. The 24 h-, 48 h-, 72 h-, 96 h-LC(50) values of total ammonia (TAN) were 251.68, 217.61, 156.05, and 119.67 mg L(-1), respectively. Following these results, crabs were then exposed for a 2-day period to 20, 40, 60 and 80 mg L(-1) TAN and sampled at 3, 6, 24 and 48 h for changes in metabolic parameters (including haemolymph ammonia concentration, glucose, lactate, urea, triacylglycerol, glutamine, and glutamate levels) and immunity indicators (the total of haemocyte count and superoxide dismutase activity). Results showed a distinct linear relationship between ambient ammonia and haemolymph ammonia and a notable increase in haemolymph ammonia content after ammonia exposure. Compared with the control group, lower concentration of triglycerides and significantly higher glucose, urea, and lactate level in haemolymph were observed when ambient ammonia increased. This suggested a reduced use of carbohydrates through anaerobic metabolism and an increase in the use of lipids to satisfy the metabolic demand. A significant surge of the ammonia metabolic product, glutamate, was observed after 3 h ammonia exposure, and the compensatory response to reduced glutamate was manifested by increased glutamine synthesis. During the same period, total haemocyte count decreased while ambient ammonia increased. Superoxide dismutase (SOD) activity in haemolymph was stimulated by lower ambient ammonia concentration after short time exposure and depressed by higher ammonia concentration. Therefore, haemolymph ammonia accumulation resulted in an increase in energy demand and a depression in immune capacity. The mechanism to detoxification of ammonia may be to transform ammonia to urea and glutamine.     

The African lungfish, Protopterus dolloi, detoxifies ammonia to urea during environmental ammonia exposure

The African lungfish, Protopterus dolloi, was able to maintain a low level of blood plasma ammonia during exposure to high concentrations of environmental ammonia. After 6 d of exposure to 30 or 100 mM NH(4)Cl, the total ammonia concentrations in the blood plasma were 0.288 and 0.289 mM, respectively, which were only 1.7-fold greater than the control value of 0.163 mM. In addition, accumulation of ammonia occurred only in the muscle, but not in the liver. This was achieved in part through urea synthesis, as reflected by significant increases in urea contents in the muscle, liver, and plasma of the experimental animals. In contrast with plasma ammonia, the plasma urea concentrations of specimens exposed to 30 or 100 mM NH(4)Cl for 6 d increased 15.4-fold and 18.8-fold, respectively. Taken together, these results suggest that P. dolloi upregulated the rate of urea synthesis to detoxify ammonia during environmental ammonia exposure and that the increased rate of urea synthesis was fast enough to compensate for the rate of endogenous ammonia production plus the net influx of exogenous ammonia in these experimental animals. Simultaneously, there were increases in the rates of urea excretion in the experimental animals between day 2 and day 6 of environmental ammonia exposure. Interestingly, the rates of urea excretion in specimens exposed to 100 mM NH(4)Cl were lower than those exposed to 30 mM NH(4)Cl, despite the presumably greater load of ammonia to be detoxified to urea in the former situation. It would appear that P. dolloi was regulating the rate of urea excretion during ammonia exposure to retain urea, which might have some physiological functions under environmental stresses yet to be determined. There were decreases in the contents of glutamate, glutamine, and total free amino acids in the liver of the experimental animals, which indirectly suggest that a reduction in the rate of proteolysis and/or amino acid catabolism would have occurred that might lead to a decrease in ammonia production. Our results suggest that, unlike marine elasmobranchs and coelacanths, which synthesize and retain urea for osmoregulatory purposes, the ureogenic P. dolloi was adapted to synthesizing and excreting urea for the purpose of ammonia detoxification.     

Physiological and metabolic changes associated with weaning in the tammar wallaby,Macropus eugenii

Summary The tammar wallaby (Macropus eugenii) is a small macropodid marsupial in which the major part of weaning occupies the period between 28 and 36 weeks of pouch life. Before weaning the diet of the tammar is high in carbohydrate and low in lipid/volatile fatty acid whereas the reverse applies after weaning. The adult tammar is a forestomach fermenter. The aim of this study was to elucidate some of the physiological and metabolic changes associated with this major change in the diet.Hepatic glycogen content increased gradually early in development to a maximum of 7% of liver weight at 28–30 weeks of pouch life. It then fell precipitously to less than 1% of liver weight at 36 weeks before recovering to the adult level of about 3% liver weight. Plasma glucose levels were maintained at about 10 mM until 36 weeks, after which they fell gradually to adult values of about 4 mM. Hepatic hexokinase activity increased several-fold between 18 and 30 weeks of pouch life, remained high until 42 weeks, and then fell to the adult level. The hepatic activities of fructose-bisphosphatase and particulate phosphoenolpyruvate carboxykinase (PEPCK) were unchanged during development but soluble hepatic PEPCK activity, which was low until 28 weeks of pouch life, increased 3–4 fold between 30 and 36 weeks and then fell slightly to the adult level. Hepatic pyruvate kinase increased in activity up to 28 weeks and then fell to about half peak values at 36 weeks and 20% of peak activity in the adult. There was a greater than ten-fold increase in the ratio of soluble PEPCK activity to pyruvate kinase activity between 28 and 36 weeks of development. It has previously been reported that hepatic gluconeogenesis is inducible in pouch young but constitutive in adults. We conclude that the change in regulation of hepatic gluconeogenesis at the PEPCK/pyruvate kinase level is part of the weaning process.The urea content of the plasma changed little during development but plasma ammonia increased consistently through pouch life. Urine urea content was low until about 28 weeks of age and then increased rapidly to adult levels. Urine ammonia increased from about 20 mM early in pouch life to a maximum of more than 100 mM at 28 weeks. Thereafter, urine ammonia content fell rapidly to the adult value of about 20 mM. For the first 27 weeks of pouch life, urine pH was consistently between 4.4 and 5.7, but subsequently it rose and became more variable. Urine pH in adults was 8.1±0.3. The activities of the five enzymes of the ornithine-urea cycle increased 3–5 fold in activity between 28 and 36 weeks of pouch life.These findings indicate that there are major changes in metabolic regulation associated with weaning in the tammar. During weaning, glucose becomes essentially unavailable to the young animal and there is an increase in the rate of hepatic gluconeogenesis which is attributable primarily to increased activity of soluble PEPCK. Metabolism, which is acidotic before weaning, becomes alkalotic and there is a decrease in urinary ammonia content as proton excretion decreases. As ammonia excretion falls, the activity of the urea cycle increases and the concentration of urea in the urine rises. Weaning in the tammar is therefore a complex and well-orchestrated process which may be associated with the change in diet.     

The effect of intermittent flooding on the growth and yield of wetland rice and nitrogen-loss mechanism with surface applied and deep placed urea

Summary Wetland rice was cultivated in pots of puddled soil under continuous and intermittent flooding conditions. The soil was either fertilized with the surface application of prilled urea in three split doses or once with urea supergranules applied at different soil levels.The grain yield, fertilizer efficiency and percent nitrogen recovery by the grains were increased by deep placement of urea supergranules independent of the water regime. Grain yield was always lower with intermittent flooding, particularly when the plants were fertilized with the surface application of prilled urea.Nitrogen loss by ammonia volatilization, measured in a closed cuvette system, was reduced from 24% with the surface application of urea prills and 20% with surface application of urea supergranules to approximately 2% with deep placement of urea supergranules. Intermittent flooding created conditions which promoted additional nitrogen loss by nitrification and denitrification processes. The total nitrogen loss, measured in an open cuvette system, was about 38% with the surface application of urea supergranules, whereas this loss was reduced to 10% with deep placement of urea supergranules. Furthermore, deep placement of urea fertilizer reduced the nitrogen loss irrespective of water regime.     

脲酶抑制剂和硝化抑制剂的协同作用对尿素氮转化和N2O排放的影响

根据培养试验,论述了脲酶抑制剂氢醌和硝化抑制剂双氰胺和碳化钙的不同组合在土壤正常水分和渍水的条件下对于土中尿素的水解及其释出的氨的吸附、氧化和挥发以及Ｎ２Ｏ生成的影响．文章指出,配合使用氢醌和双氰胺既能延缓土中尿素的水解并使水解后释出的氨在土中得以更多和更长时间的保持,还能减少土中硝酸盐的累积、氨挥发的损失及Ｎ２Ｏ的生成．这表明在脲酶抑制剂和硝化抑制剂间可能存在一定的协同作用．很好利用这一作用,将有益于提高尿素肥效和减少其Ｎ损失与环境污染．     

几种控释氮肥减少氨挥发的效果及影响因素研究

采用“静态吸收法”和“土柱淋溶法”、室内模拟试验,研究几种控释氮肥施入土壤后的氨挥发损失情况、N溶出速率、土壤脲酶活性及pH值变化的关系．结果表明,施氮450mg·kg^-1土时,3种控释氮肥氨挥发损失氮总量分别比普通尿素减少49．7％、28．0％和71．2％;施氮600mg·kg^-1土时,3种控释氮肥氨挥发损失氮总量分别比普通尿素减少34．6％、12．3％和69．9％．控释氮肥能显著降低土壤氨挥发量,减少因施肥而引起的大气环境污染．控释氮肥氨挥发量与不同氮肥引起的土壤脲酶活性、pH值、土壤中氮溶出速率密切相关．土壤的氨挥发总量与肥料在土壤中溶出总量的相关系数达到0．9533,在肥料施入的前期土壤氨挥发量同土壤脲酶活性、pH值的相关系数达到0．9533和0．9908。     

Strategies for surviving high concentrations of environmental ammonia in the swamp eel Monopterus albus

The swamp eel Monopterus albus lives in muddy ponds, swamps, canals, and rice fields in the tropics. It encounters high concentrations of environmental ammonia (HEA) during dry seasons or during agricultural fertilization in rice fields. This study aimed at determining the tolerance of M. albus to environmental ammonia and at elucidating the strategies that it adopts to defend against ammonia toxicity in HEA. In the laboratory, M. albus exhibited very high environmental ammonia tolerance; the 48-, 72-, and 96-h median lethal concentrations of total ammonia at pH 7.0 and 28 degrees C were 209.9, 198.7, and 193.2 mM, respectively. It was apparently incapable of actively excreting ammonia against a concentration gradient. In addition, it did not detoxify ammonia to urea, the excretion of which would lead to a loss of nitrogen and carbon, during ammonia loading. The high tolerance of M. albus to HEA was attributable partially to its exceptionally high tolerance to ammonia at the cellular and subcellular levels. During the 144 h of exposure to 75 mM NH(4)Cl at pH 7.0, the ammonia contents in the muscle, liver, brain, and gut of M. albus reached 11.49, 15.18, 6.48, and 7.51 mu mol g(-1), respectively. Such a capability allowed the accumulation of high concentrations of ammonia in the plasma (3.54 mu mol mL(-1)) of M. albus exposed to HEA, which would reduce the net influx of exogenous ammonia. Subsequent to the buildup of internal ammonia levels, M. albus detoxified ammonia produced endogenously to glutamine. The glutamine contents in the muscle and liver reached 10.84 and 17.06 mu mol g(-1), respectively, after 144 h of exposure to HEA, which happened to be the highest known for fish. Unlike urea, the storage of glutamine in the muscle during ammonia loading allowed its usage for anabolic purposes when the adverse environmental condition subsides. Glutamine synthetase activity increased significantly in the liver and gut (2.8- and 1.5-fold, respectively) of specimens exposed to HEA for 144 h. These results suggest that the liver was the main site of ammonia detoxification and the gut was more than a digestive/absorptive organ in M. albus. Monopterus albus did not undergo a reduction in amino acid catabolism during the first 24 h of ammonia exposure. However, assuming a total inhibition of excretion of endogenous ammonia, there was a deficit of -312 mu mol N between the reduction in nitrogenous excretion (3,360 mu mol N) and the retention of nitrogen (3,048 mu mol N) after 72 h of aerial exposure. The deficit became much greater after 144 h, reaching a value of -3,243 mu mol N. These results suggest that endogenous ammonia production in M. albus was suppressed in order to prevent the newly established internal steady state concentration of ammonia from rising to an intolerable level after an extended period of exposure to HEA.