桧叶白发藓对不同氮源胁迫的形态和生理响应

桧叶白发藓(Leucobryum juniperoideum)在我国东南部常见,被认为是一种理想的、适用于庭院栽培的苔藓植物,而氮是植物必需的矿质元素,但过量摄入会对其造成伤害,近年来氮沉降水平的提高对苔藓植物的多样性造成了严重影响。该研究为揭示氮沉降加剧对桧叶白发藓的影响,以经6个月断茎培养的桧叶白发藓配子体为材料,用Ca(NO3)2、NH4HCO3和NH4NO3代表三种氮源,设置2、4、8、16gN·m-2 4个水平,以喷洒去离子水为对照,进行不同氮源的胁迫试验。结果表明:氮处理浓度的增加引起组织氮含量的显著提高,增加幅度分别为69.1%、25.7%和43.1%;同时引起植株坏死率显著上升,增加幅度分别为16.5%、12.5%和13.9%。三种氮源处理对株高和净重的影响有显著差异,低浓度的铵态氮(4gN·m-2)引起株高和净重的显著增加,而硝态氮和混合态氮处理差异不显著;加氮浓度的进一步提高,引起株高和净重的减低,硝态氮处理的减低幅度最大,铵态氮的降低幅度最小。三种氮源处理均引起叶绿素含量先上升后下降,但同一水平铵态氮处理的叶绿素含量要高于其它两种氮处理,而且引起叶绿素含量下降的处理浓度要高于其它两种氮源;三种氮源均引起SOD活性显著增加、可溶性蛋白和脯氨酸含量先升后降,但不同氮源间生理指标的变化不同步。这说明桧叶白发藓对硝态氮胁迫的响应比铵态氮敏感,硝态氮的增加对桧叶白发藓造成严重危害,而少量的铵态氮(4gN·m-2)则能促进桧叶白发藓的生长。研究结果可作为桧叶白发藓繁殖与生产的氮源。     

Enhanced In Vitro Plantlet Regeneration from Mature Embryo-derived Primary Callus of a Basmati Rice Cultivar Through Modification of Nitrate-nitrogen and Ammonium-nitrogen Concentrations

Mature-embryo derived primary calli of the basmati rice (Oryza sativa L.) cv Karnal Local showed significant enhancement in in vitro green-plantlet regeneration efficiency through modification of nitrogen content of the callusing medium. Using KNO₃ as the source of nitrate nitrogen and (NH₄)₂SO₄ as the source of ammonium nitrogen, forty-five media combinations involving 9 levels of KNO₃ (0–40 mM) and 5 concentrations (0–6.5 mM) of (NH₄)₂SO₄ were examined. The highest frequency of plantlet regeneration (100%) and a maximum number of green-plantlets (～ 7) per embryo-derived primary callus was obtained in calli derived from the medium having 35 mM KNO₃ and 5 mM (NH₄)₂SO₄. Higher concentrations of KNO₃ and/or (NH₄)₂SO₄ showed a decline in the regeneration efficiency. It was also observed that although the nitrogen content of the callus induction medium had a profound effect on the regenerability of the callus, the nitrogen composition of the regeneration medium also affected it significantly.     

Nitrate reductase activity and nitrogen compounds in xylem exudate of Juglans nigra seedlings: relation to nitrogen source and supply

Nitrogen (N) limits plant productivity and its uptake and assimilation may be regulated by N source, N availability, and nitrate reductase activity (NRA). Knowledge of how these factors interact to affect N uptake and assimilation processes in woody angiosperms is limited. We fertilized 1-year-old, half-sib black walnut (Juglans nigra L.) seedlings with ammonium (NH₄ ⁺) [as (NH₄)₂SO₄], nitrate (NO₃ ⁻) (as NaNO₃), or a mixed N source (NH₄NO₃) at 0, 800, or 1,600 mg N plant⁻¹ season⁻¹. Two months following final fertilization, growth, in vivo NRA, plant N status, and xylem exudate N composition were assessed. Specific leaf NRA was higher in NO₃ ⁻-fed and NH₄NO₃-fed plants compared to observed responses in NH₄ ⁺-fed seedlings. Regardless of N source, N addition increased the proportion of amino acids (AA) in xylem exudate, inferring greater NRA in roots, which suggests higher energy cost to plants. Root total NRA was 37% higher in NO₃ ⁻-fed than in NH₄ ⁺-fed plants. Exogenous NO₃ ⁻ was assimilated in roots or stored, so no difference was observed in NO₃ ⁻ levels transported in xylem. Black walnut seedling growth and physiology were generally favored by the mixed N source over NO₃ ⁻ or NH₄ ⁺ alone, suggesting NH₄NO₃ is required to maximize productivity in black walnut. Our findings indicate that black walnut seedling responses to N source and level contrast markedly with results noted for woody gymnosperms or herbaceous angiosperms.     

Assay of molybdenum cofactor of barley

Conditions for assay of molybdenum cofactor in barley shoot extracts in the presence of molybdate (25 mM N₂MoO₄) and the sulphydryl-group protector, reduced glutathione (5 mM) were optimized. Both total Mo-cofactor (assayed after heat-treatment of cell-free extracts) and ‘free’ Mo-cofactor (assayed in untreated cell-free extracts) were assayed. Compared to control plants grown in the absence of an exogenous nitrogen source total Mo-cofactor levels increased around 70 % when plants were grown for 4 days in the presence of either 15 mM KNO₃ or 15 mM NH₄NO₃. Growth in the presence of 15 mM (NH₄)₂SO₄ did not affect the Mo-cofactor level. Very similar results were seen when plants were transferred to these nitrogen sources for 24 hr after previous growth in the absence of an exogenous nitrogen source. In contrast ‘free’ Mo-cofactor levels of both KNO₃ and NH₄NO₃-treated plants were increased 2-3-fold over untreated controls. Growth in the presence of (NH₄)₂SO₄ did not affect the ‘free’ Mo-cofactor level.     

INORGANIC NITROGEN NUTRITION OF THE SEEDLINGS OF THE ORCHID,CATTLEYA

When grown in vitro in a medium containing NH₄NO₃ as the sole source of nitrogen, seeds ro the orchid, Cattleya (C. labiata ‘Wonder’ X C. labiata ‘Treasure'), germinated readily and proceeded to form small plantlets. Development of the embryos was accompanied by an increase in their total nitrogen and a decline in the percent dry weight. Growth responses of the seedlings in other ammonium salts like (NH₄)₂SO₄, (NH₄)₂HPO₄, NH₄Cl, ammonium acetate and ammonium oxalate were similar to that in NH₄NO₃. However, when grown in a medium containing NaNO₃, development of the seedlings was drastically inhibited; KNO₃, Ca(NO₃)₂, KNO₂ and NaNO₂ also were poor nitrogen sources. Attempts to grow the seedlings in NaNO₃ by changing the pH or by addition of kinetin, molybdenum or ascorbic acid as supplements were completely unsuccessful. When seedlings growing in NH₄NO₃ for varying periods were transferred to NaNO₃, it was found that those plants allowed to grow for 60 or more days in NH₄NO₃ could resume normal growth thereafter in NaNO₃. Determination of the nitrate reductase activity in seedlings of different ages grown in NaNO₃, after NH₄NO₃, showed that the ability of the seedlings to assimilate inorganic nitrogen was paralleled by the appearance of the enzyme.     

The effect of some ammonium salts on nitrate reductase level,onin vivo nitrate reduction and on nitrate content in excisedpisum sativum roots

The effect of some ammonium salts on nitrate reductase (NR) level, onin vivo nitrate reduction and on nitrate content was followed in the presence of nitrate in the medium, under changing experimental conditions, in excisedPisum sativum roots, and their effect was compared with that of KNO₃, Ca(NO₃)₂ and NaNO₃ at 15 mM NO₃ ^- concentration, i.e. at a concentration which considerably exceeded the level of saturation with nitrate with respect to nitrate reductase. The effect of ammonium salts on NR level is indirect and changes from a positive one to a strongly negative one which is dependent on the time of action of the salt, on the presence of other cations, on pH of the solution of the ammonium salt and on the nature of the anion of the ammonium salt. A positive effect on the enzyme level can be observed in the presence of other cations than NH₄ ⁺ at suitable concentrations of those ammonium salts, the solutions of which have their pH values in the acid region (i.e. NH₄H₂PO₄, (NH₄)₂SO₄ and NH₄NO₃). However their positive effect is independent of the presence of NH₄ ⁺ ions, and it is obviously the result of an increased concentration of H⁺ ions. A clear-cut negative effect on NR level can be observed after 24 h in one-salt NH₄NO₃ solution where NH₄ ⁺ is not balanced with other cations and thus certainly can adversely influence many metabolic processes, and in the solutions containing neutral (pH 6.2) and dibasic ammonium phosphates in which dissolved undissociated ammonia [(NH₃). (H₂O) which can also affect many metabolic processes incl. proteosynthesis] probably has a toxic influence. Thein vivo nitrate reduction is always depressed in excised pea roots in the presence of ammonium salts in the medium, regardless of the level of nitrate reductase. Under the described conditions, no relationship could be established between the enzyme level and the so-called metabolic NO₃ ^- pool (i.e. NO₂ ^- production under anaerobic conditions), nor between NR level and the total nitrate content in the roots. One-salt solutions of NaNO₃, Ca(NO₃)₂ and KNO₃ exert different effects on the level of nitrate reductase and on the content of NO₃ ^- in the roots, but the in vivo NO₃ ^- reduction shows the same trend as NR level in the roots influenced by these salts. Cl^- ions, supplied in NH₄C1, depress both NR level and NO₃ ^- content in the roots at higher concentrations, but they do not significantly affect the in vivo nitrate reduction in comparison with other ammonium salts. These results indicate that NR level,in vivo nitrate reduction, and nitrate uptake can be regulated in pea roots independently of each other.     

Effect of nitrogen forms on growth and chemical changes in the rhizosphere of strawberry plants

The experiment was set up to examine the influence of different nitrogen forms: (NH₄)₂SO₄, Ca(NO₃)₂ or NH₄NO₃ on growth response, root induced pH changes in the rhizosphere, root-borne acid phosphatase activity in strawberry plants cv. Senga Sengana. The plants grown on sandy mineral soil were fertilized with 3 forms of nitrogen, in concentrations of 46 mg N·kg⁻¹ soil. The plants were grown in rhizoboxes with removable plexiglass lids. To ensure the root growth along the plexiglass lids, the rhizoboxes were placed at an angle of about 50° with the lid on the lower side. In case of ammonium supply, the nitrification inhibitor DIDIN was added (10 mg·kg⁻¹ of moist soil) to prevent conversion of ammonium into nitrate. The growth response (roots and shoots) of strawberry plants were determined after 11 weeks of treatment with different N forms. The best development of the root system and shoots (root and shoot dry weight and root length) was obtained, when ammonium nitrate was supplied. It is suggested therefore, that NH₄NO₃ stimulates vegetative growth of strawberry plants cv. Senga Sengana. However, there were no statistical differences in a leaf and flower number of the plants grown under different forms of N-fertilization. Determination of rhizosphere pH, and acid phosphatase activity were executed using non-destructive techniques, which enabled weekly measurement of chemical changes in the rhizosphere. The results revealed that the form of nitrogen supplied had a predominant effect on chemical changes in the rhizosphere of strawberry plants. The highest pH values (average pH 6.8) were measured in the rhizosphere of individual plants supplied with Ca(NO₃)₂. Whereas the lowest pH values (average pH 5.8) were detected in the presence of (NH₄)₂SO₄. The curve of rhizosphere pH measured along individual roots of the plants treated with Ca(NO₃)₂ represents the highest pH values whereas the curve of rhizosphere pH under (NH₄)₂SO₄ treatment had the lowest pH values. The highest activity of acid phosphatase were observed in the rhizosphere of strawberry plants grown in the presence of (NH₄)₂SO₄, at pH 5.8.     

Distribution and recovery of15N after fertilization of Douglas-fir saplings with different nitrogen sources

Summary Four-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) saplings planted in pots with a sand and peat mix (11) were fertilized at the rate of 200 kg N/ha of (¹⁵NH₂)₂CO (U-15),¹⁵NH₄NO₃ (A-15) and NH₄ ¹⁵NO₃(An-15). They were placed in a shadehouse and watered regularly to maintain soil moisture at field capacity over periods of one and two years. Quantity of¹⁵N in foliage generally increased from old to current growth, irrespective of the nitrogen source. Utilization of¹⁵N fertilizers by saplings after the first and second growing seasons following fertilization was greatest with nitrate labelled ammonium nitrate AN-15, and nearly equal for urea U-15 and ammonium labelled ammonium nitrate A-15. The soil immobilized more fertilizer nitrogen-15 from U-15 and A-15 than from AN-15. Data from the present study, in which leaching losses of fertilizer were minimized, demonstrated that in terms of nitrogen uptake by the saplings the nitrate fertilizer was superior to ammonium fertilizer.     

The effect of ammonium and nitrate nitrogen sources on copper uptake and amino acid status of cereals

Summary The effects of different nitrogen sources (NH₄, NO₃, and NH₄ NO₃) on the uptake of copper by wheat and barley growing in solution culture were compared in three experiments. Both the copper concentration and weight gain of shoots and roots were found to decrease in the order NO₃>NH₄ NO₃>NH₄ irrespective of the solution copper concentration. Ammonium nitrogen was also found to decrease the copper concentration of wheat grown on a copper deficient soil compared with a nitrate source of nitrogen. Increasing concentrations of ammonium ions in solution culture caused ammonium toxicity and reduced both plant copper concentrations and vegetative yield. Biochemical investigations using paper chromatography revealed that the amino acid asparagine was the major detoxification product of ammonia in wheat. Copper deficient plants were found to have elevated levels of amino acids compared with controls, irrespective of the nitrogen source.     

Effect of nitrogen source on the antimicrobial activity of the bacilli air flora

The antimicrobial activity of strainsBacillus megaterium NB-3,Bacillus cereus NB-4,Bacillus cereus NB-5,Bacillus subtilis NB-6 andBacillus circulans NB-7, previously isolated from the air flora, now in the Jerash Culture Collection (Jordan), was investigated in media containing different nitrogen sources. Maximum antimicrobial activity of strains NB-4, NB-5 and NB-6 was observed using Ca(NO₃)₂ as nitrogen source, (NH₄)₂SO₄ and KNO₂ strongly enhanced the antimicrobial activity of strains NB-3 and NB-7, respectively. The lowest level of the antimicrobial activity of strains NB-4 and NB-5 was observed using NaNO₃. In case of strains NB-3, NB-6 and NB-7, the lowest antimicrobial activity was observed using NH₄NO₃, KNO₃ and (NH₄)₂SO₄ as nitrogen source, respectively.     

Effects of nitrogen fertilisers on the Javanese root-knot nematode Meloidogyne javanica and its interaction with cucumber

The influence of different nitrogen salts at five electrical conductivity levels (EC 2, 4, 6 and 8 mmhos/cm) on the Javanese root-knot nematode (RKN) (Meloidogyne javanica) and its interaction with cucumber was evaluated under in vitro, growth chamber and greenhouse conditions. Percentages of egg-hatching and second-stage juvenile viability of M. javanica were greatly reduced when NH₄Cl, (NH4)₂SO₄ and NH₄NO₃ were used especially at the higher levels of EC and accompanied with reduction in cucumber root galling. The lower root galling (less than 2.5) was accompanied with NH₄Cl, (NH₄)₂SO₄ and (NH₄)₂HPO₄, while KNO₃ and NH₄NO₃ resulted in moderate root galling. In contrast to the nitrogen salts, NaCl caused a reduction in both nematode infection and root growth especially at higher EC levels and this could be due to salinity effect. Diammonium phosphate was superior over the tested salts in increasing plant and root fresh and dry weights and cucumber phosphorus content, while KNO₃ was superior in increasing in plant content of potassium.     

Sources of ammonium in oat leaves treated with tabtoxin or methionine sulfoximine

Excised 7-day-old oat (Avena sativa L. cv. Jaycee) leaves were incubated in media containing 7.1 millimolar KNO₃ and 0.15 millimolar tabtoxin or 1 millimolar methionine sulfoximine (MSO) to investigate the sources of the observed ammonium accumulated. Tabtoxin and MSO are known inhibitors of glutamine synthetase, the first enzyme in the primary pathway of ammonium assimilation. During a 4- to 6-hour incubation, there was little net change in protein or total amino acid concentration. Alanine, aspartate/asparagine, and glutamate/glutamine decreased markedly under these treatments, whereas several other amino acids increased. Exogenous ¹⁵N from K¹⁵NO₃ was taken up and incorporated into the nitrate and ammonium fractions of leaves treated with tabtoxin or MSO. This result and the high in vitro activities of nitrate reductase indicated that reduction of nitrate was one source of the accumulated ammonium. Leaves incubated under 2% O₂ to reduce photorespiration accumulated only about 13% as much ammonium as did those under normal atmospheres. We conclude that most of the tabtoxin- or MSO-induced ammonium came from photo-respiration, and the remainder was from nitrate reduction.     

Effect of some nitrogenous salts on nitrogen transfer and protease activity in germinatingZea mays L. seeds

Maize seeds were allowed to germinate in the presence of different nitrogenous salts for 72 h. Changes in the ethanol soluble and insoluble nitrogen were studied in the embryo and in the endosperm. Supply of Ca(NC₃)₂ enhanced germination and protease activity in the endosperm resulting in greater solubilisation of protein to soluble nitrogen in the seeds. NH₄NO₃ and (NH₄)₂SO₄ were less effective as compared to Ca(NO₃)₂. Cycloheximide inhibited germination and protease activity. Pretreatment also resulted in increase in growth, soluble and insoluble nitrogen, and nitrate reductase activity in the primary leaves. Ca(NO₃)₂ was more effective than NH₄NO₃ and (NH₄)₂SO₄.     

Biological role of mycorrhizal fungi on the assimilation and transportation of carbon and nitrogen to Anacamptis palustris and Anacamptis laxiflor

Fungal is a physiological trail and its understanding in the assimilation with the transfer of carbon (C) cum nitrogen (N) or (C/N) to orchid-seedlings have not been determined. Labelled stable isotopes ¹³C and ¹⁵N were used to plan the flow of C and N between orchid plants and mycorrhizal connotations in-terms of bulk transfer for C/N. This study attends to comprehend the mechanism, supporting mycorrhizal fungi which influences on orchid-seedling growth. Determined integration and transfer of C/N from amino acids (AA), ammonium nitrate (NH₄NO₃) and sugar for orchid-plant may lead to understand these mechanisms. This current study tries to estimate the importance of organic compounds as a source for C/N over the inorganic-NH₄NO₃. Generally, after begging of germination and when it is found to be associated to the nutrient resource, organic compound enhance the biomass accumulation of two orchid species. AA significantly increase the mass of ¹³C assimilated by two species. With amino acids the concentration of ¹³C in two species was greater than with NH₄NO₃ and sugar. At another phase, amount of ¹⁵N content shoots was a higher value in Anacamptis laxiflora shoots assimilated substantially additional of ¹⁵N with NH₄NO₃ plus sugar compared with ammonium nitrate only. This study showed that two terrestrial orchids species are reliant on organic compounds as a source of carbon and nitrogen more than inorganic compounds.     

NH4 + and NO3 − requirement for wheat somatic embryogenesis

The influence of three nitrogen salts: NH₄NO₃, KNO₃ and NH₄Cl on wheat in vitro cultures was investigated. Both NO ₃ ⁻ and NH ₄ ⁺ ions were indispensable for proliferation of embryogenic calli and development of wheat somatic embryos. It is possible to obtain wheat somatic embryos when the medium is enriched with NH₄NO₃ only as a source of inorganic nitrogen. The results of the statistical analysis showed that the level of NH₄NO₃ and KNO₃ in the medium had a great influence on the efficiency of somatic embryogenesis. We observed tendency that calli on media containing 50 mM NH₄NO₃ and 0 to 20 mM KNO₃ turned out to be more embryogenic than on control MS medium. High concentrations of KNO₃- 100 mM inhibited somatic embryogenesis, while 100 mM NH₄NO₃ did not. The level of total N did not have significant influence on wheat somatic embryogenesis. Ratio NO ₃ ⁻ :NH ₄ ⁺ also turned out to be not substantial. We observed that mutual connection of concentration levels between NH₄NO₃ and KNO₃ and between NH₄Cl and KNO₃ was more important. The efficiency of somatic embriogenesis obtained in the experiment with NH₄Cl and KNO₃ was significantly lower than in experiment with NH₄NO₃ and KNO₃.     

闽江河口湿地土壤CH4产生与氧化速率对外源氮、硫添加的响应

通过室内培养实验,研究了外源氮、硫添加对闽江河口湿地土壤CH_4产生/氧化速率以及土壤理化性质的短期影响。NH_4Cl(N1)和NH_4NO_3(N3)处理在各培养阶段均显著促进土壤CH_4产生速率(P0.05),较对照分别提高136.70%和136.55%;NH_4Cl+K_2SO_4(NS1)和NH_4NO_3+K_2SO_4(NS3)处理在培养第3、6、12、15和18天均显著促进了CH_4产生速率(P0.05)。KNO_3(N2)、K_2SO_4(S)处理在不同培养时间对CH_4产生速率影响均不显著(P0.05);KNO_3+K_2SO_4(NS2)处理除在第21天外(P0.05),其他时间影响均不显著(P0.05)。N2、N3、NS2和NS3处理均显著促进了土壤CH_4氧化速率(P0.05),平均CH4氧化速率较CK分别提高了145.30%、142.93%、139.48%和112.68%。整体而言,不同添加处理并没有显著改变湿地土壤CH_4产生/氧化速率的时间变化规律,各处理均表现为随培养时间先增加而后逐渐降低。短期培养结束后,土壤可溶性有机碳(DOC)、电导率、p H值在不同处理间均不存在显著差异(P0.05);土壤NH+4-N含量在N1、N3、NS1和NS3处理下,NO_3~--N含量在N2、N3、NS2和NS3处理下,SO_4~(2-)含量在S、NS1、NS2和NS3处理下均显著高于对照处理(P0.05)。相关分析显示,DOC、铵态氮(NH+4-N)和硝态氮(NO_3~--N)是氮、硫添加处理下影响闽江河口湿地土壤CH_4产生/氧化速率短期变化的主要控制因素。     

High nitrogen supply affects the metabolism of Matricaria chamomilla leaves

N-status of the two Matricaria chamomilla cultivars grown in the presence of high potassium nitrate concentration was evaluated and compared with ammonium nitrate supply. After 5 days of potassium nitrate treatment the visible increase of dry mass together with total chlorophyll accumulation were observed. In both cultivars, ammonium nitrate application led to increased accumulation of N-containing compounds in chamomile leaves. NH₄NO₃ nitrogen supply influenced activity of nitrate reductase positively. In vivo nitrate reductase activity reached maximum in lower nitrate supply and decreased in higher nitrate availability significantly. Among the most abundant leaf secondary metabolites, the high nitrate availability both KNO₃ and NH₄NO₃ significantly increased umbelliferone level. The highest potassium nitrate dose (60 mmol per plant) caused an osmotic stress accompanied with lower tissue water content and turgor loss. In such condition the decrease in (Z)- and (E)-2-β-d-glucopyranosyloxy-4-methoxycinnamic acid, herniarin and dicycloethers, as well as PAL activity was observed. On the other hand, strong increase of umbelliferone is likely a stress response and is related to its antioxidant activity.     

The global nitrogen cycle in the twenty-first century

Global nitrogen fixation contributes 413 Tg of reactive nitrogen (N_r) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic N_r are on land (240 Tg N yr⁻¹) within soils and vegetation where reduced N_r contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer N_r contribute to nitrate (NO₃⁻) in drainage waters from agricultural land and emissions of trace N_r compounds to the atmosphere. Emissions, mainly of ammonia (NH₃) from land together with combustion related emissions of nitrogen oxides (NO_x), contribute 100 Tg N yr⁻¹ to the atmosphere, which are transported between countries and processed within the atmosphere, generating secondary pollutants, including ozone and other photochemical oxidants and aerosols, especially ammonium nitrate (NH₄NO₃) and ammonium sulfate (NH₄)₂SO₄. Leaching and riverine transport of NO₃ contribute 40–70 Tg N yr⁻¹ to coastal waters and the open ocean, which together with the 30 Tg input to oceans from atmospheric deposition combine with marine biological nitrogen fixation (140 Tg N yr⁻¹) to double the ocean processing of N_r. Some of the marine N_r is buried in sediments, the remainder being denitrified back to the atmosphere as N₂ or N₂O. The marine processing is of a similar magnitude to that in terrestrial soils and vegetation, but has a larger fraction of natural origin. The lifetime of N_r in the atmosphere, with the exception of N₂O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 10²–10³ years), the lifetime is a few decades. In the ocean, the lifetime of N_r is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N₂O that will respond very slowly to control measures on the sources of N_r from which it is produced.     

Effect of chromium and nitrogen form on photosynthesis and anti-oxidative system in barley

The effect of nitrogen forms on photosynthesis and anti-oxidative systems of barley plants under chromium stress was studied in a hydroponic experiment. The treatments comprised three chromium concentrations (0, 75, and 100 μM) and three N forms (NH₄)₂SO₄, urea, and Ca(NO₃)₂. In comparison with the urea or (NH₄)₂SO₄ fed plants, the Ca(NO₃)₂ fed plants had higher net photosynthetic rate, intercellular CO₂ concentration, stomatal conductance, transpiration rate, photosynthetically active radiation utilization efficiency, variable to maximum chlorophyll fluorescence ratio, and the content of chlorophylls and carotenoids. Cr toxicity caused oxidative stress in all plants but the Ca(NO₃)₂ fed plants had the least oxidative stress. Moreover, the Ca(NO₃)₂ fed plants had higher activities of anti-oxidative enzymes and content of non-enzymatic antioxidants than the urea or (NH₄)₂SO₄ fed plants. In addition, the Ca(NO₃)₂ fed plants had higher N and lower Cr content in all plant tissues than the urea or (NH₄)₂SO₄ fed plants. The current results indicate that the reasonable choice of N fertilizer is important for barley production on the Cr-contaminated soils.     

Effect of pH on nitrogen immobilization in two California soils

Summary The effect of pH on nitrogen immobilization from tagged NH₄Cl and KNO₃ was investigated in two soils. Although the ammonium source was utilized by the soil population preferentially at all pH levels in both soils, there was a marked influence of pH on the quantity immobilized. Nitrogen immobilized from the ammonium source increased, as pH increased, whereas the reverse was true of the nitrate source. These results appear to be related to the physiological acidity or alkalinity of the nitrogen source.Most of the nitrogen incorporated into the organic fraction was present in the amino form.Contribution from the Department of Soils and Plant Nutrition University of California, Davis, and the Department of Vegetable Crops, University of California, Riverside. This work was supported in part by a grant from the National Science Foundation.