不同氮效率水稻生育后期根表和根际土壤硝化特征

通过田间试验研究了不同氮效率粳稻品种4007(氮高效)和Elio(氮低效)生育后期在N0(0 kgN hm-2)、N180(180 kgN hm-2)和N300(300 kgN hm-2)水平下根表、根际和土体土壤pH值、铵态氮(NH+4-N)和硝态氮(NO-3-N)含量、硝化强度和氨氧化细菌(AOB)数量.结果表明无论是齐穗期、灌浆期还是成熟期,根表土壤pH值均显著低于根际和土体土壤.土壤pH值范围在5.95至6.84之间变化.土壤NH+4-N含量随水稻生长显著下降,且随施氮量增加而显著增加.根表土壤NH+4-N有明显亏缺区,且随距水稻根表距离增加,NH+4-N含量逐渐升高.土壤NO-3-N含量随水稻生长显著增加,施氮处理均显著高于不施氮处理,但N180和N300处理差异不显著.NO-3-N含量表现为根际>土体>根表.水稻根表和根际土壤硝化强度随水稻生长显著下降,而土体土壤硝化强度随时间延长小幅增加.施氮显著提高4007水稻根表土壤在齐穗和收获期硝化强度以及Elio在齐穗期根际硝化强度,但在施氮处理N180和N300中无显著差异.在整个采样期间,土壤硝化强度均表现为根际>根表>土体.水稻根表和根际AOB数量随水稻生长而显著降低,而土体土壤AOB数量无显著变化.例如,根表土壤AOB数量在齐穗期、灌浆期和收获期分别为16.7×105、8.77×105个g-1 dry soil和8.01×105个g-1 dry soil.根表和根际土壤AOB数量无显著差异,但二者显著高于土体土壤AOB数量.就两个氮效率水稻品种而言,土壤pH值基本无差异.4007土壤NH+4-N含量均显著高于Elio.在齐穗期水稻根表、根际和土体土壤NO-3-N含量在N180水平下均表现为Elio显著高于4007.而在灌浆期和收获期,水稻根表、根际和土体土壤则表现为4007显著高于Elio.在所有采样期,两个水稻品种土体土壤硝化强度和AOB数量在3个施氮量下均无显著差异.Elio根表和根际土壤硝化强度和AOB数量在水稻灌浆期之前一直显著高于4007,而在灌浆期之后则显著低于4007,且最终产量和氮素利用率(NUE)显著低于4007,这可能是由于4007灌浆期后硝化作用强,根际产生的NO-3-N含量高,从而4007根吸收NO-3-N的量也高造成的.因此水稻灌浆期和收获期根表和根际硝化作用以及AOB与水稻高产及氮素高效利用密切相关.     

Juvenile nitrogen uptake capacities and root architecture of two open-pollinated families of Picea abies. Effects of nitrogen source and ectomycorrhizal symbiosis

This study was carried out to find early physiological differences occurring in young seedlings between two contrasting Picea abies open-pollinated families (OPF), one with high- and one with low-growth performance in the field by, determining their N uptake capacities and their root architecture. We used three potential N-sources in forest soil solution, NO3-, NH4+ and amino acids, to establish N uptake rates by the plants, whether or not associated with a fungus isolated from the field and identified as Paxillus involutus. NO3- fluxes were determined locally at the root surface using NO3(-)-selective microelectrodes whereas NH4+ and amino acid (L-glutamate and L-aspartate) uptake rates were calculated from their depletion of the incubation solution by the whole root system. Root systems were digitised in order to determine the number and the length of different root types. In non-mycorrhizal plants, the results showed that the most distinguishing parameters between OPF were NO3- uptake rates measured in the white tip of the secondary roots and the root architecture, with higher values determined in high-growth than in low-growth field performance OPF. The presence of the mycorrhizal fungus decreased NO3- uptake rates in both OPF and had an opposite effect on root architecture by increasing it in low-growth and decreasing it in high-growth field performance OPF, respectively. In non-mycorrhizal plants, NH4+ and amino-acid uptake rates were not different between OPF. Mycorrhizal symbiosis did not change NH4+ uptake rates whereas it increased that of amino acids, specifically that of L-aspartate in the low-growth field performance OPF. Taken together these results suggest that the measurement of local fluxes in roots of young plants could be a good potential tool for the early evaluating of growth capacity of Picea abies OPF.     

Effects of the chemical form of inorganic nitrogen fertilizers on the dynamics of the soil solution composition and on nutrient uptake by wheat

Adding nitrogen (N) fertilizers to soil affects not only the concentration in the soil solution of the added ions, but also those of other ions already present in the soil. This secondary effect is caused by ion exchange and electrochemical equilibrium processes. We studied how different N fertilizers affected the chemical composition of the soil solution over time, and how this related to nutrient uptake by wheat. Soil was fertilized either with (NH4)2SO4 or Ca(NO3)2, or no N was added. Each of these N treatments was either planted or not with spring wheat (Triticum aestivum L.). Soil solutions were collected repeatedly with looped hollow fiber samplers from the root zone in situ, six times during a 50-day pot experiment. Plants were harvested five times, and their nutrient contents determined. In the soil solution, NO3- was significantly less concentrated if (NH4)2SO4, rather than Ca(NO3)2 was applied, until after net nitrification had ended on day 20. In contrast, Ca2+, Mg2+ and K+ were significantly more concentrated in the former treatment. This was probably caused by the greater concentration of anions that resulted from nitrification. P was always very dilute and unaffected by the form of N fertilizer. The form of N fertilizer had no significant effect on plant growth and nutrient uptake. The likely contribution of mass flow of the soil solution in supplying Ca, Mg and N to the plants was greatest when (NH4)2SO4 was supplied. The supply of K and P was unaffected by N fertilizer. The potential for N leaching loss was lower with (NH4)2SO4 than with Ca(NO3)2, especially up to day 20. However, the potential for cations leaching loss was greater in the (NH4)2SO4 treatment. This suggests that there is only a limited advantage in fertilizing with (NH4)2SO4 to reduce the total loss of nutrients from soil.     

Plant carbon substrate supply regulated soil nitrogen dynamics in a tallgrass prairie in the Great Plains,USA: results of a clipping and shading experiment

Aims Land use management affects plant carbon (C) supply and soil environments and hence alters soil nitrogen (N) dynamics, with consequent feedbacks to terrestrial ecosystem productivity. The objective of this study was to better identify mechanisms by which land-use management (clipping and shading) regulates soil N in a tallgrass prairie, OK, USA.Methods We conducted 1-year clipping and shading experiment to investigate the effects of changes in land-use management (soil microclimates, plant C substrate supply and microbial activity) on soil inorganic N (NH 4 + ? N and NO 3 ? ? N), net N mineralization and nitrification in a tallgrass prairie.Important findings Land-use management through clipping and/or shading significantly increased annual mean inorganic N, possibly due to lowered plant N uptake and decreased microbial N immobilization into biomass growth. Shading significantly increased annual mean mineralization rates (P < 0.05). Clipping slightly decreased annual mean N nitrification rates whereas shading significantly increased annual mean N nitrification rates. Soil microclimate significantly explained 36% of the variation in NO 3 ? ? N concentrations (P = 0.004). However, soil respiration, a predictor of plant C substrate supply and microbial activity, was negatively correlated with NH 4 + ? N concentrations (P = 0.0009), net N mineralization (P = 0.0037) and nitrification rates (P = 0.0028) across treatments. Our results suggest that change in C substrate supply and microbial activity under clipping and/or shading is a critical control on NH 4 + ? N, net N mineralization and nitrification rates, whereas clipping and shading-induced soil microclimate change can be important for NO 3 ? ? N variation in the tallgrass prairie.     

田间非饱和流条件下土壤硝态氮运移的模拟

运用马尔可夫过程的理论 ,建立了土壤非饱和流条件下 ,模拟硝态氮通过土层运移的随机模型 .模型把时间可变系统假设为由紧密相连的时间均质情况相接而成 ,使得运用马尔可夫过程成为可能 ,在给定土壤水流量及汇源项转移强度的土壤层次中 ,给出了硝态氮溶质的统计分布 .模型将随机过程与确定性过程相结合 ,在计算各土层间的转移概率时考虑了硝态氮的作物吸收、淋洗、硝化和反硝化等主要过程 ,并用相关函数修正 N素转化关系 .在褐土农田土壤非饱和流条件下 ,用微区试验对该模型运行效果进行了验证 ,结果显示模拟计算值与实测值之间吻合性较好 ,说明模型可以用于相似类型区 ,预测和评价土壤 -作物系统中硝态氮溶质的运移行为 .     

Ammonium and nitrate uptake by soybean during recovery from nitrogen deprivation

Soybean [Glycine max (L.) Merrill] plants that had been subjected to 15 d of nitrogen deprivation were resupplied for 10 d with 1.0 mol m-3 nitrogen provided as NO3-, NH4+, or NH4(+) + NO3- in flowing hydroponic culture. Plants in a fourth hydroponic system received 1.0 mol m-3 NO3- during both stress and resupply periods. Concentrations of soluble carbohydrates and organic acids in roots increased 210 and 370%, respectively, during stress. For the first day of resupply, however, specific uptake rates of nitrogen, determined by ion chromatography as depletion from solution, were lower for stressed than for non-stressed plants by 43% for NO3- resupply, by 32% for NH4(+) + NO3- resupply, and 86% for NH4+ resupply. When specific uptake of nitrogen for stressed plants recovered to rates for non-stressed plants at 6 to 8 d after nitrogen resupply, carbohydrates and organic acids in their roots had declined to concentrations lower than those of non-stressed plants. Recovery of nitrogen uptake capacity of roots thus does not appear to be regulated simply by the content of soluble carbon compounds within roots. Solution concentrations of NH4+ and NO3- were monitored at 62.5 min intervals during the first 3 d of resupply. Intermittent 'hourly' intervals of net influx and net efflux occurred. Rates of uptake during influx intervals were greater for the NH4(+)-resupplied than for the NO3(-)-resupplied plants. For NH4(+)-resupplied plants, however, the hourly intervals of efflux were more numerous than for NO3(-)-resupplied plants. It thus is possible that, instead of repressing NH4+ influx, increased accumulation of amino acids and NH4+ in NH4(+)-resupplied plants inhibited net uptake by stimulation of efflux on NH4+ absorbed in excess of availability of carbon skeletons for assimilation. Entry of NH4+ into root cytoplasm appeared to be less restricted than translocation of amino acids from the cytoplasm into the xylem.     

Ammonium and nitrate uptake rates and rhizosphere pH in non-mycorrhizal roots of Norway spruce [Picea abies (L.) Karst.]

Summary Relationships between root zone temperature, concentrations and uptake rates of NH ₄ ⁺ and NO ₃ ^– were studied in non-mycorrhizal roots of 4-year-old Norway spruce under controlled environmental conditions. Additionally, in a forest stand NH ₄ ⁺ and NO ₃ ^– uptake rates along the root axis and changes in the rhizosphere pH were measured. In the concentration (C_min) range of 100–150 M uptake rates of NH ₄ ⁺ were 3–4 times higher than those of NO ₃ ^– The preference for NH ₄ ⁺ uptake was also reflected in the minimum concentration (C_min) values. Supplying NH₄NO₃, the rate of NO ₃ ^– uptake was very low until the NH ₄ ⁺ concentrations had fallen below about 100 M. The shift from NH ₄ ⁺ to NO ₃ ^– uptake was correlated with a corresponding shift from net H⁺ production to net H⁺ consumption in the external solution. The uptake rates of NH ₄ ⁺ were correlated with equimolar net production of H⁺. With NO ₃ ^– nutrition net consumption of H⁺ was approximately twice as high as uptake rates of NO ₃ ^– In the forest stand the NO ₃ ^– concentration in the soil solution was more than 10 times higher than the NH ₄ ⁺ concentration (<100 M), and the rhizosphere pH of non-mycorrhizal roots considerably higher than the bulk soil pH. The rhizosphere pH increase was particularly evident in apical root zones where the rates of water and NO ₃ ^– uptake and nitrate reductase activity were also higher. The results are summarized in a model of water and nutrient transport to, and uptake by, non-mycorrhizal roots of Norway spruce in a forest stand. Model calculations indicate that delivery to the roots by mass flow may meet most of the plant demand of nitrogen and calcium, and that non-mycorrhizal root tips have the potential to take up most of the delivered nitrate and calcium.     

Plant and microbial nitrogen use and turnover: Rapid conversion of nitrate to ammonium in soil with roots

Immobilization of ammonium (NH ₄ ⁺ ) by plants and microbes, a controlling factor of ecosystem nitrogen (N) retention, has usually been measured based on uptake of¹⁵NH ₄ ⁺ solutions injected into soil. To study the influence of roots on N dynamics without stimulating consumption of NH ₄ ⁺ , we estimated gross nitrification in the presence or absence of live roots in an agricultural soil. Tomato (Lycopersicon esculentum var. Peto76) plants were grown in microcosms containing root exclosures. When the plants were 7 weeks old,¹⁵N enriched nitrate (NO ₃ ⁻ ) was applied in the 0–150 mm soil layer. After 24 h, > 30 times more¹⁵NH ₄ ⁺ was found in the soil with roots than in the soil of the root exclosures. At least 18% of the NH ₄ ⁺ -N present at this time in the soil with roots had been converted from NO ₃ ⁻ . We estimated rates of conversion of NO ₃ ⁻ to NH ₄ ⁺ , and rates ofNH ₄ ⁺ immobilization by plants and microbes, by simulating N-flow of¹⁴⁺¹⁵N and¹⁵N in three models representing mechanisms that may be underlying the experimental data: Dissimilatory NO ₃ ⁻ reduction to NH ₄ ⁺ (DNRA), plant N efflux, and microbial biomass nitrogen (MBN) turnover. Compared to NO ₃ ⁻ uptake, plant NH ₄ ⁺ uptake was modest. Ammonium immobilization by plants and microbes was equal to at least 35% of nitrification rates. The rapid recycling of NO ₃ ⁻ to NH ₄ ⁺ via plants and/or microbes contributes to ecosystem N retention and may enable plants growing in agricultural soils to capture more NH ₄ ⁺ than generally assumed.     

硝化抑制剂DCD、DMPP对褐土氮总矿化速率和硝化速率的影响

采用¹⁵N库稀释-原位培养法研究了硝化抑制剂DCD、DMPP对华北盐碱性褐土氮总矿化速率和硝化速率的影响.试验在山西省运城市种植玉米的盐碱性土壤上进行,设单施尿素、尿素+DCD、尿素+DMPP 3个处理.结果表明：施肥后2周,DCD、DMPP分别使氮总矿化速率和氮总硝化速率减少了25.5%、7.3%和60.3%、59.1%,DCD对氮总矿化速率的影响显著高于DMPP,两者对氮总硝化速率的影响无显著差异;而在施肥后7周,不同硝化抑制剂对氮总硝化速率的影响存在差异.施肥后2周,3个处理的土壤氮总矿化速率和硝化速率分别是施肥前的7.2～10.0倍和5.5～21.5倍;NH₄⁺和NO₃^-消耗速率分别是施肥前的9.1～12.2倍和5.1～8.4倍,这是由氮肥对土壤的激发效应所致.硝化抑制剂使氮肥更多地以NH₄⁺形式保持在土壤中,减少了NO₃^-的积累.土壤氮总矿化速率和总硝化速率受硝化抑制剂的抑制是N₂O减排的主要原因.     

Potential nitrification in alum-treated soil slurries amended with poultry manure

Alum is used to reduce environmental pollutants in poultry production. Alum decreases NH3 volatilization and increases total N and NH4+-N compared to untreated poultry manure. Nitrification in poultry wastes could therefore be stimulated due to higher NH4+ concentrations or could be inhibited because the soil environment is acidified. A 10-day laboratory study was conducted to study potential nitrification rates in soil slurries (20 g soil in 150 ml water) amended with 2.0 g alum-treated poultry manure. Fecal bacteria, NH4+, NO2-, NO3-, orthophosphate, pH, and NH3 were measured at 2-day intervals. Alum significantly reduced fecal bacteria concentrations through day 6. Water-soluble P was reduced 82% by day 10. Alum-treated manure had significantly increased NH4+ concentrations by day 8 and 10, and significantly decreased NO2- and NO3- concentrations by days 6-10. Alum's effect on potential nitrification was inhibitory in the soil environment. Slurries with alum-treated poultry manure had reduced nitrification rates, fecal bacteria, and soluble P. Therefore, in addition to reducing P loss, alum could temporarily reduce the risk for environmental pollution from land-applied manures in terms of both NO3- and fecal bacteria loss.     

三江平原典型小叶章湿地土壤氮素净矿化与硝化作用

2004年6月—2005年7月,利用PVC顶盖原位培育法研究了三江平原典型草甸小叶章湿地和沼泽化草甸小叶章湿地土壤(0~15cm)无机氮库、净矿化/硝化速率动态、影响因素及年净矿化/硝化量.结果表明:两种湿地土壤的无机氮均呈明显的动态变化特征,其NH4 -N、NO3-N含量均表现为典型草甸小叶章湿地>沼泽化草甸小叶章湿地.两种湿地土壤的净矿化/硝化速率均呈明显的波动变化,生物固持作用、反硝化作用以及雨季较多降水是导致净矿化/硝化速率出现负值的主要原因.温度、降水、土壤有机质含量、C/N和pH是引起二者土壤无机氮库、净矿化/硝化速率存在明显差异的重要原因.典型草甸小叶章湿地的年净矿化量(19.41kg·hm-2)、年净硝化量(4.27kg·hm-2)以及净硝化量占净矿化量的百分比(22.00%)明显高于沼泽化草甸小叶章湿地(5.51kg·hm-2、0.28kg·hm-2和5.08%),说明前者的氮有效性以及维持可利用氮的能力明显高于后者.     

元素硫和双氰胺对蔬菜地土壤硝态氮淋失的影响

采用温室盆栽淋洗试验,以NH₄HCO₃为氮肥源,研究了元素硫(S₀)和双氰胺（DCD）对种葱和不种作物土壤NO₃^--N淋失量和NO₃^--N、NH₄⁺-N浓度的影响.结果表明,在12周试验期间,与对照相比,S₀+DCD和S₀处理NO₃^--N淋失量分别低83%～86%和83%;NH₄⁺-N淋失量分别高16.8～21.0 mg·盆^-1和20.4～25.0 mg·盆^-1;而同期无机氮（NO₃^--N、NH₄⁺-N）淋失量则低60%.试验结束后,,S₀+DCD和S₀处理土壤无机氮含量分别比对照高79.9%～85.4%和74.9%～82.6%,以NH₄⁺-N为主.S₀+DCD处理无机氮淋失量比S₀和DCD处理分别低4.6%～14.4%和15.4%～30.1%;试验结束后土壤无机氮分别高6.1%和16.8～36.0%.在Na₂S₂O₃+DCD、Na₂S₂O₃和DCD处理中也发现类似结果.可见S₀施入土壤具有与DCD同样的氨稳定和硝化抑制作用.S₀与DCD配合施用可使DCD的硝化抑制性增强,其作用机理是S₀氧化中间体S₂O₃^2-、S₄O₆^2-,具有抑制硝化和DCD降解作用,延缓DCD硝化抑制效果.S0与DCD配合施用可用于延缓太湖流域蔬菜地土壤NH₄⁺-N向NO₃^--N转化,减少氮向水体迁移风险.     

Ammonium and nitrate uptake by the floating plant Landoltia punctata

BACKGROUND AND AIMS: Plants from the family Lemnaceae are widely used in ecological engineering projects to purify wastewater and eutrophic water bodies. However, the biology of nutrient uptake mechanisms in plants of this family is still poorly understood. There is controversy over whether Lemnaceae roots are involved in nutrient uptake. No information is available on nitrogen (N) preferences and capacity of Landoltia punctata (dotted duckweed), one of the best prospective species in Lemnaceae for phytomelioration and biomass production. The aim of this study was to assess L. punctata plants for their ability to take up NH4+ and NO3- by both roots and fronds. METHODS: NO3- and NH4+ fluxes were estimated by a non-invasive ion-selective microelectrode technique. This technique allows direct measurements of ion fluxes across the root or frond surface of an intact plant. KEY RESULTS: Landoltia punctata plants took up NH4+ and NO3- by both fronds and roots. Spatial distribution of NH4+ and NO3- fluxes demonstrated that, although ion fluxes at the most distal parts of the root were uneven, the mature part of the root was involved in N uptake. Despite the absolute flux values for NH4+ and NO3- being lower in roots than at the frond surface, the overall capacity of roots to take up ions was similar to that of fronds because the surface area of roots was larger. L. punctata plants preferred to take up NH4+ over NO3- when both N sources were available. CONCLUSIONS: Landoltia punctata plants take up nitrogen by both roots and fronds. When both sources of N are available, plants prefer to take up NH4+, but will take up NO3- when it is the only N source.     

A comparison of ammonium, nitrate and proton net fluxes along seedling roots of Douglas-fir and lodgepole pine grown and measured with different inorganic nitrogen sources

Significant spatial variability in NH4+, NO3- and H+ net fluxes was measured in roots of young seedlings of Douglas-fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta) with ion-selective microelectrodes. Seedlings were grown with NH4+, NO3-, NH4NO3 or no nitrogen (N), and were measured in solutions containing one or both N ions, or no N in a full factorial design. Net NO3- and NH4+ uptake and H+ efflux were greater in Douglas-fir than lodgepole pine and in roots not exposed to N in pretreatment. In general, the rates of net NH4+ uptake were the same in the presence or absence of NO3-, and vice versa. The highest NO3- influx occurred 0-30 mm from the root apex in Douglas-fir and 0-10 mm from the apex in lodgepole pine. Net NH4+ flux was zero or negative (efflux) at Douglas-fir root tips, and the highest NH4+ influx occurred 5-20 mm from the root tip. Lodgepole pine had some NH4+ influx at the root tips, and the maximum net uptake 5 mm from the root tip. Net H+ efflux was greatest in the first 10 mm of roots of both species. This study demonstrates that nutrient uptake by conifer roots can vary significantly across different regions of the root, and indicates that ion flux profiles along the roots may be influenced by rates of root growth and maturation.     

Metabolism of nitric oxide and nitrous oxide during nitrification and denitrification in soil at different incubation conditions

Abstract NO production and consumption rates as well as N₂O accumulation rates were measured in a loamy cambisol which was incubated under different conditions (i.e. soil moisture content, addition of nitrogen fertilizer and/or glucose, aerobic or anaerobic gas phase). Inhibition of nitrification with acetylene allowed us to distinguish between nitrification and denitrification as sources of NO and N₂O. Under aerobic conditions untreated soil showed very low release of NO and N₂O but high consumption of NO. Fertilization with NH₄⁺ or urea stimulated both NO and N₂O production by nitrification. Addition of glucose at high soil moisture contents led to increased N₂ and N₂O production by denitrification, but not to increased NO production rates. Anaerobic conditions, however, stimulated both NO and N₂O production by denitrification. The production of NO and N₂O was further stimulated at low moisture contents and after addition of glucose or NO₃⁻. Anaerobic consumption of NO by denitrification followed Michaelis-Menten kinetics and was stimulated by addition of glucose and NO₃⁻. Aerobic consumption of NO followed first-order kinetics up to mixing ratios of at least 14 ppmv NO, was inhibited by autoclaving but not by acetylene, and decreased with increasing soil moisture content. The high NO-consumption activity and the effects of soil moisture on the apparent rates of anaerobic and aerobic production and consumption of NO suggest that diffusional constraints have an important influence on the release of NO, and may be a reason for the different behaviour of NO release vs N₂O release.     

森林土壤氮素的转化与循环

森林土壤氮素转化与循环在森林生态系统功能中占有极其重要的意义。本文综述了森林土壤氮素转化与循环的研究历程和现状 ;介绍了凋落物的归还、施肥、大气沉降、自生固氮、氨化、硝化、反硝化、植物吸收、NH3 的挥发、NO3 -淋溶等土壤氮素输入、转化和输出的途径和过程 ;最后从研究目标、研究方法、研究对象和研究内容 4个方面归纳了森林土壤氮素转化与循环的发展趋势。     

Metabolism of nitric oxide and nitrous oxide during nitrification and denitrification in soil at different incubation conditions

Abstract NO production and consumption rates as well as N₂O accumulation rates were measured in a loamy cambisol which was incubated under different conditions (i.e. soil moisture content, addition of nitrogen fertilizer and/or glucose, aerobic or anaerobic gas phase). Inhibition of nitrification with acetylene allowed us to distinguish between nitrification and denitrification as sources of NO and N₂O. Under aerobic conditions untreated soil showed very low release of NO and N₂O but high consumption of NO. Fertilization with NH₄⁺ or urea stimulated both NO and N₂O production by nitrification. Addition of glucose at high soil moisture contents led to increased N₂ and N₂O production by denitrification, but not to increased NO production rates. Anaerobic conditions, however, stimulated both NO and N₂O production by denitrification. The production of NO and N₂O was further stimulated at low moisture contents and after addition of glucose or NO₃⁻. Anaerobic consumption of NO by denitrification followed Michaelis-Menten kinetics and was stimulated by addition of glucose and NO₃⁻. Aerobic consumption of NO followed first-order kinetics up to mixing ratios of at least 14 ppmv NO, was inhibited by autoclaving but not by acetylene, and decreased with increasing soil moisture content. The high NO-consumption activity and the effects of soil moisture on the apparent rates of anaerobic and aerobic production and consumption of NO suggest that diffusional constraints have an important influence on the release of NO, and may be a reason for the different behaviour of NO release vs N₂O release.     

Root-zone acidity and nitrogen source affects Typha latifolia L. growth and uptake kinetics of ammonium and nitrate

The NH(4)(+) and NO(3)(-) uptake kinetics by Typha latifolia L. were studied after prolonged hydroponics growth at constant pH 3.5, 5.0, 6.5 or 7.0 and with NH(4)(+) or NO(3)(-) as the sole N-source. In addition, the effects of pH and N source on H(+) extrusion and adenine nucleotide content were examined. Typha latifolia was able to grow with both N sources at near neutral pH levels, but the plants had higher relative growth rates, higher tissue concentrations of the major nutrients, higher contents of adenine nucleotides, and higher affinity for uptake of inorganic nitrogen when grown on NH(4)(+). Growth almost completely stopped at pH 3.5, irrespective of N source, probably as a consequence of pH effects on plasma membrane integrity and H(+) influx into the root cells. Tissue concentrations of the major nutrients and adenine nucleotides were severely reduced at low pH, and the uptake capacity for inorganic nitrogen was low, and more so for NO(3)(-)-fed than for NH(4)(+)-fed plants. The maximum uptake rate, V(max), was highest for NH(4)(+) at pH 6.5 (30.9 micro mol h(-1) g(-1) root dry weight) and for NO(3)(-) at pH 5.0 (31.7 micro mol h(-1) g(-1) root dry weight), and less than 10% of these values at pH 3.5. The affinity for uptake as estimated by the half saturation constant, K((1/2)), was lowest at low pH for NH(4)(+) and at high pH for NO(3)(-). The changes in V(max) and K((1/2)) were thus consistent with the theory of increasing competition between cations and H(+) at low pH and between anions and OH(-) at high pH. C(min) was independent of pH, but slightly higher for NO(3)(-) than for NH(4)(+) (C(min)(NH(4)(+)) approximately 0.8 mmol m(-3); C(min)(NO(3)(-)) approximately 2.8 mmol m(-3)). The growth inhibition at low pH was probably due to a reduced nutrient uptake and a consequential limitation of growth by nutrient stress. Typha latifolia seems to be well adapted to growth in wetland soils where NH(4)(+) is the prevailing nitrogen compound, but very low pH levels around the roots are very stressful for the plant. The common occurrence of T. latifolia in very acidic areas is probably only possible because of the plant's ability to modify pH-conditions in the rhizosphere.     

Linkages between organic matter mineralization and denitrification in eight riparian wetlands

Denitrification (N₂ production) and oxygen consumption rates were measured at ambient field nitrate concentrations during summer in sediments from eight wetlands (mixed hardwood swamps, cedar swamps, heath dominated shrub wetland, herbaceous peatland, and a wetland lacking live vegetation) and two streams. The study sites included wetlands in undisturbed watersheds and in watersheds with considerable agricultural and/or sewage treatment effluent input. Denitrification rates measured in intact cores of water-saturated sediment ranged from 20 to 260 mol N m^-2 h^-1 among the three undisturbed wetlands and were less variable (180 to 260 mol N M^-2 h^-1) among the four disturbed wetlands. Denitrification rates increased when nitrate concentrations in the overlying water were increased experimentally (1 up to 770 M), indicating that nitrate was an important factor controlling denitrification rates. However, rates of nitrate uptake from the overlying water were not a good predictor of denitrification rates because nitrification in the sediments also supplied nitrate for denitrification. Regardless of the dominant vegetation, pH, or degree of disturbance, denitrification rates were best correlated with sediment oxygen consumption rates (r ² = 0.912) indicating a relationship between denitrification and organic matter mineralization and/or sediment nitrification rates. Rates of denitrification in the wetland sediments were similar to those in adjacent stream sediments. Rates of denitrification in these wetlands were within the range of rates previously reported for water-saturated wetland sediments and flooded soils using whole core¹⁵N techniques that quantify coupled nitrification/denitrification, and were higher than rates reported from aerobic (non-saturated) wetland sediments using acetylene block methods.     

红壤区不同产量籼稻品种苗期根表和根际土壤硝化特征

以不同产量籼稻品种中旱22(ZH,高产品种)和禾盛10号(HS,低产品种)为材料,采用根际培养箱(三室)--速冻切片技术研究了红壤水稻土种植条件下,水稻苗期生长、氮素积累和氮素利用率(NUE)、根孔隙度(POR)、根际土壤矿质态氮含量和硝化强度.结果表明,ZH苗期生长、氮素吸收及NUE均显著优于HS,且ZH单株不定根数量及根系通气组织发育程度(用POR表示)均显著高于HS.ZH根际和土体土壤中铵(NH+4)含量始终低于HS,而硝(NO-3)含量则始终高于HS,但二者根表土壤NH+4和NO-3含量均无显著差异.ZH和HS硝化强度最大发生部位均是在距根表2 mm的根际土壤,分别为:0.48 μmol kg-1h-1和0.31 μmol kg-1h-1.随着距根表越远,硝化强度就越弱,直至距根表10～20 mm处土壤硝化强度就接近于土体土壤.ZH根际土壤硝化强度始终显著高于HS,但二者根表和土体土壤硝化强度均无显著差异.与不种水稻的CK相比,根际土壤硝化强度提高了约2～3.5倍.尽管红壤水稻土硝化作用很弱,但红壤区水稻根际硝化作用与水稻苗期生长和氮素营养密切相关.