湖泊微生物硝化过程研究进展

湖泊中微生物介导的硝化作用在生境内部氮周转和温室气体N_2O释放方面扮演着关键的角色。因此,研究湖泊微生物硝化过程及速率有助于我们整体评估湖泊生境内部的氮循环状态,全面认识湖泊响应区域乃至全球气候变化的规律。本文综述了湖泊生境中硝化过程及其驱动微生物和影响因素,包括氨氧化过程、亚硝酸盐氧化过程和完全氨氧化过程,同时聚焦前沿,归纳了氨氧化古菌、氨氧化细菌和完全氨氧化菌产生N_2O的机制和相对贡献。最后对湖泊硝化过程研究现状和未来发展方向提出总结和展望。     

真菌反硝化过程及其驱动的N2O产生机制研究进展

真菌反硝化过程的发现打破了反硝化过程只发生在原核生物中的传统认识,是对全球微生物氮循环过程的重要补充。真菌参与的反硝化过程由于缺乏N_2O还原酶,其终产物为具有强辐射效应的温室气体N_2O。真菌在环境中分布广泛,生物量巨大,故真菌反硝化作用对全球N_2O释放通量的贡献是不容忽视的。近年来许多研究表明,真菌反硝化过程是自然环境中N_2O产生的重要途径。本文对反硝化真菌的发现、多样性及分布、产生N_2O的机制和活性测定方法等几个方面进行综述,并对未来的研究提出展望。     

分离于河口区芦苇湿地1株好氧反硝化菌的鉴定及其反硝化特性

【目的】筛选高效脱氮且N_2O释放量少的好氧反硝化细菌,并对菌株的反硝化特性进行研究,可为河口湿地富营养化水体的生物修复提供技术支撑。【方法】经BTB培养基初筛和反硝化能力测定,从辽河河口区芦苇湿地土壤中分离得到1株具有较高反硝化能力的好氧反硝化菌C3。经形态观察、生理生化鉴定和16S rRNA序列分析,对菌株进行鉴定。研究温度、碳源、pH及C/N对其生长量、反硝化能力及N2O释放的影响。【结果】筛选得到的高效好氧反硝化细菌C3,经鉴定属于假单胞菌属(Pseudomonas sp.)。反硝化特性研究结果表明,该菌最适碳源为柠檬酸三钠,在温度为30°C、pH为7.0、C/N为10时生长速率和脱氮效率最高且N_2O释放量较少。在此条件下,该菌在36 h内使NO_3~–由179.55 mg/L降至5.08 mg/L,脱氮率高达97.17%。该菌株在整个反硝化过程中中间产物N_2O的最大累积量较低,为0.22 mg/L。【结论】从湿地土壤中分离所得好氧反硝化菌C3为假单胞菌属的1个种(Pseudomonas sp.),该菌株在高效除氮和低N_2O累积方面均具有明显优势,对后续河口湿地富营养化水体治理具有重要意义。     

浅表层水稻土N2O消耗能力及其与N2O还原微生物的耦合关系

土壤不仅能够产生、排放温室气体N_2O,还具有截留、吸收、转化N_2O的能力。土壤消耗N_2O已经成为很重要的一种降低大气N_2O浓度的途径,但目前关于土壤N_2O消耗过程及其微生物调控机制的系统研究较为缺乏。试验以浅表层水稻土柱(0—5 cm)为研究对象,通过外源添加N_2O气体研究N_2O迁移通过淹水土柱的动态过程,以及N_2O消耗能力与氧化亚氮还原酶基因丰度变化和其他土壤养分含量变化的联系,揭示浅表层水稻土N_2O消纳量与N_2O还原微生物之间的耦合关系。结果显示,淹水厌氧条件下5 cm土壤深度外源添加的N_2O迁移通过浅表层土柱后,仅有7.17—9.80%部分逸散出土表,表明0—5 cm淹水水稻土层具有极强的N_2O截留能力(90%以上)而减少N_2O净排放量。排放出土表的N_2O也可被淹水土柱继续吸收消耗,且吸收转化速率随N_2O浓度增加而大幅提高,最高可达到3896.75μg N m~(-2) h~(-1)。与此同时,土壤DOC含量大量消耗,含nosZⅠ基因的反硝化微生物数量显著增长(P0.01),而nosZⅡ基因丰度的无显著变化。说明高浓度N_2O添加能够促进淹水土壤N_2O吸收消耗能力,此刺激作用可能主要由含nosZⅠ基因的N_2O还原微生物进行调控。浅表层土壤强大的N_2O吸收消耗功能可进一步深入系统研究,为实践温室气体减排提供理论基础。     

水肥一体化条件下设施菜地的N2O排放

在保证作物产量的前提下,研究减少农田土壤N_2O排放的水肥统筹管理措施对全球温室气体减排具有重要意义。以京郊典型设施菜地为例,设置了农民习惯(FP)、水肥一体化(FPD)、优化水肥一体化(OPTD)和对照(CK)4个处理,采用静态箱-气相色谱法,对果菜-叶菜(黄瓜-芹菜)轮作周期内土壤N_2O排放进行了观测,并分析了氮肥施用量、灌溉方式、土壤温度和湿度等因素对土壤N_2O排放的影响。结果表明:在黄瓜-芹菜种植模式中,各施氮处理除基肥施用后N_2O排放峰持续10—15d外,一般施肥、施肥+灌溉事件后土壤N_2O排放峰均呈现3—5d短而急促的情形。黄瓜生长季N_2O排放通量与土壤湿度(WFPS)之间呈现显著相关的关系;芹菜生长季N_2O排放通量与土壤温度之间呈现显著相关的关系。观测期内FP处理N_2O排放量为(31.00±2.15)kg N/hm~2,FPD处理与之相比N_2O排放量减少了4.2%,而OPTD处理在减少40%化肥氮量的情况下,N_2O累积排放量比FP处理减少了42.7%,且达到显著水平。说明在水肥一体化条件下,合理改变施肥体系是减少N_2O排放的前提,在此基础上进行水肥优化是设施菜地保持产量、减少N_2O排放的重要技术措施。     

控释肥施用对土壤N2O排放的影响——以华北平原冬小麦/夏玉米轮作系统为例

控释肥作为一种能够提高肥料利用率、保障作物产量和节约劳动力的新型肥料已经在作物生产中得到广泛应用,而控释肥对土壤N_2O排放影响结果的差异使其成为当前科学评估控释肥施用环境效应的焦点问题之一。因此,旨在探讨不同种类控释肥及氮素水平施用对华北平原冬小麦/夏玉米轮作系统土壤N_2O排放的影响,为科学评价控释肥施用的环境效应及其推广应用提供科学依据。本研究监测采用静态暗箱—气相色谱法对不同控释肥施用下土壤N_2O排放、环境因素以及产量进行了周年监测,探讨了不同处理(对照处理(CK)、控释肥处理1(CRF1)、优化控释肥处理1(80%CRF1)、优化控释肥处理2(80%CRF2)和控释肥处理3(CRF3+尿素))下土壤N_2O排放特征及土壤温湿度对其的影响。结果表明:控释肥施用下冬小麦/夏玉米轮作系统中土壤N_2O排放峰高值主要出现在基肥施用并伴随灌溉(或降雨)后,一般持续时间约为7—10 d,小麦返青期灌溉以及玉米后期降雨会引起微弱的N_2O排放峰。不同处理土壤N_2O排放通量变化范围为~(-2)35.61—2625.01μg N_2O m~(-2)h~(-1),平均排放通量为23.88—51.39μg N_2O m~(-2)h~(-1),与CRFI相比,80%CRF1和80%CRF2处理能够减小施肥期的N_2O排放峰值,但不改变轮作周期土壤N_2O排放季节变化规律。CK处理和CRF3+尿素处理土壤N_2O排放通量与5 cm深度土壤温度之间表现出显著的正相关性(r~2=0.38,P0.01;r~2=0.30,P0.05);CRF1处理和80%CRF1处理在冬小麦生长季及整个轮作周期内与土壤孔隙含水率(WFPS)表现为显著的正相关关系(冬小麦生长季分别为r~2=0.50,P0.01;r~2=0.39,P0.05;整个轮作周期分别为r~2=0.39,P0.05;r~2=0.43,P0.05)。80%CRF2处理N_2O年排放总量最高,为(2.89±0.24)kg N/hm~2。相同控释肥种类条件下,80%CRF1处理比CRF1处理减少了14.23%,但并未达到显著水平;相同施氮量水平下,CRF1处理与(CRF3+尿素)处理之间N_2O年排放总量差异不显著,而80%CRF1处理比80%CRF2处理N_2O年排放总量减少16.16%,并达到显著水平(P0.05)。本研究不同处理之间N_2O直接排放系数在0.29%—0.42%之间,均明显低于IPCC 1.0%的默认值。各控释肥处理产量与当地农民常规施肥量条件下产量没有显著性差异。因此在华北地区冬小麦/夏玉米轮作系统中应用控释肥技术可以在保证产量的前提下有效减少土壤N_2O排放,并且仍存在一定的减排空间。     

氮素类型和剂量对寒温带针叶林土壤N2O排放的影响

大气氮沉降输入会增加森林生态系统氮素有效性,进而改变土壤N_2O产生与排放,然而有关不同氮素离子(氧化态NO_3~--N与还原态NH_4~+-N)沉降对土壤N_2O排放的影响知之甚少。以大兴安岭寒温带针叶林为研究对象,构建了3种类型(NH_4Cl、KNO_3、NH_4NO_3)和4个施氮水平(0、10、20、40 kg N hm~(-2)a~(-1))的增氮控制试验,利用流动化学分析仪和静态箱-气相色谱法4次/月测定凋落物层和矿质层土壤无机氮含量、土壤-大气界面N_2O净交换通量以及相关环境因子,分析施氮类型和剂量对土壤氮素有效性、土壤N_2O通量的影响探讨氮素富集条件下土壤N_2O通量的环境驱动机制。结果表明:施氮类型和剂量均显著影响土壤无机氮含量,土壤NH_4~+-N的积累效应显著高于NO_3~--N。施氮一致增加寒温带针叶林土壤N_2O排放,NH_4NO_3促进效应最为明显,增幅为442%-677%,高于全球平均水平(134%)。土壤N_2O通量与土壤温度、凋落物层NH_4~+-N含量正相关,且随着施氮水平增加而增加。结果表明大气氮沉降短期内不会导致寒温带针叶林土壤NO_3~--N大量流失,但会显著促进土壤N_2O的排放。此外,外源性NH_4~+和NO_3~-输入对土壤N_2O排放的促进作用具有协同效应,在未来森林生态系统氮循环和氮平衡研究中应该区分对待。     

微生物驱动的滨海湿地N2O产生及其机制研究进展

滨海湿地位于海陆交界,具有初级生产力高、生物多样性丰富以及微生物驱动的营养元素循环活跃等特点,同时也是大气中一氧化二氮(N_2O)的重要排放源。N_2O是仅次于二氧化碳(CO2)和甲烷(CH4)的第三大温室气体,而全球90%以上的N_2O排放由微生物主导,并与滨海湿地氮循环的微生物群落多样性及功能密切相关。因此,滨海湿地系统中N_2O的产生与转化逐渐受到关注。本文综述了滨海湿地生态系统中微生物驱动下N_2O的产生过程,以及氮元素及其与碳、硫和金属元素耦合过程中产生N_2O的代谢途径,N_2O排放的时空变化与微生物调控,并对未来相关研究方向进行了展望,旨在揭示微生物驱动的N_2O产生及环境调控机制,为减缓全球变暖提供科学依据。     

一株兼性氧化亚氮还原菌的还原N2O能力

【目的】从水稻土中分离筛选出一株兼性氧化亚氮还原菌,并探索其在不同条件下还原N_2O的能力,为减少温室气体N_2O的排放提供重要依据。【方法】通过微生物富集培养分离技术从水稻土中分离得到纯菌;利用nosZ基因和16S rRNA的测序分析鉴定菌株;通过测定菌株在不同条件下N_2O的还原量,分析该菌株还原N_2O的能力及调控因子。【结果】经鉴定,该菌株含有nos Z基因,属于假单胞菌属,在温度30°C、厌氧条件下还原N_2O速率高达0.0219μmol/min以上,改变不同温度和氧气浓度后其能力相对减弱,但仍具备较强的还原N_2O作用。【结论】从水稻土中分离筛选得到的兼性氧化亚氮还原菌为假单胞菌,它在不同环境条件下都具备较强的还原N_2O能力,该菌株可能为减少土壤N_2O排放提供新途径,对保障生态环境安全具有重要的应用价值。     

施用生物炭和秸秆还田对华北农田CO2、N2O排放的影响

以华北农田冬小麦-夏玉米轮作体系连续6a施用生物炭和秸秆还田的土壤为研究对象,于2013年10月—2014年9月,采用静态暗箱-气相色谱法,对CO_2、N_2O通量进行了整个轮作周期的连续观测,探究施用生物炭与秸秆还田对其排放通量的影响。试验共设4个处理:CK(对照)、C1(低量生物炭4.5 t hm~(-2)a~(-1))、C2(高量生物炭9.0 t hm~(-2)a~(-1))和SR(秸秆还田straw return)。结果表明:在整个轮作周期内,各处理CO_2、N_2O通量随时间的变化趋势基本一致。随着生物炭施用量的增加,CO_2排放通量分别增加了0.3%—90.3%(C1)、1.0%—334.2%(C2)和0.4%—156.3%(SR)。其中,C2处理对CO_2累积排放量影响最大,增幅为42.9%。对N_2O而言,C2处理显著降低了N_2O累积排放量,但增加了CO_2和N_2O排放的综合增温潜势,C1和SR处理对N_2O累积排放量及综合增温潜势均没有显著影响。相关分析表明,土壤温度和土壤含水量是影响CO_2通量最主要的因素,两者之间呈极显著的正相关关系;N_2O通量与土壤温度、土壤含水量、NO_3~--N和NH_4~+-N均表现出极显著的正相关关系,而与土壤p H值表现出极显著的负相关关系。由此可见,添加生物炭对于减少氮素的气体损失具有较大的潜力。     

Short term effect of ploughing a permanent pasture on N2O production from nitrification and denitrification

Soils are an important source of N₂O, which can be produced both in the nitrification and the denitrification processes. Grassland soils in particular have a high potential for mineralization and subsequent nitrification and denitrification. When ploughing long term grassland soils, the resulting high supply of mineral N may provide a high potential for N₂O losses. In this work, the short-term effect of ploughing a permanent grassland soil on gaseous N production was studied at different soil depths. Fertiliser and irrigation were applied in order to observe the effect of ploughing under a range of conditions. The relative proportions of N₂O produced from nitrification and denitrification and the proportion of N₂ gas produced from denitrification were determined using the methyl fluoride and acetylene specific inhibitors. Irrespectively to ploughing, fertiliser application increased the rates of N₂O production, N₂O production from nitrification, N₂O production from denitrification and total denitrification (N₂O + N₂). Application of fertiliser also increased the denitrification N₂O/N₂ ratio both in the denitrification potential and in the gaseous N productions by denitrification. Ploughing promoted soil organic N mineralization which led to an increase in the rates of N₂O production, N₂O production from nitrification, N₂O production from denitrification and total denitrification (N₂O + N₂). In both the ploughed and unploughed treatments the 0–10 cm soil layer was the major contributing layer to gaseous N production by all the above processes. However, the contribution of this layer decreased by ploughing, gaseous N productions from the 10 to 30 cm layer being significantly increased with respect to the unploughed treatment. Ploughing promoted both nitrification and denitrification derived N₂O production, although a higher proportion of N₂O lost by denitrification was observed as WFPS increased. Recently ploughed plots showed lower denitrification derived N₂O percentages than those ploughed before as a result of the lower soil water content in the former plots. Similarly, a lower mean nitrification derived N₂O percentage was found in the 10–30 cm layer compared with the 0–10 cm.     

Occurrence of effective nitrogen-scavenging bacteria in the rhizosphere of kallar grass

Bacteria occurring in high numbers on the rhizoplane of kallar grass grown at a natural site in Pakistan were effective scavengers of traces of combined nitrogen from the atmosphere. Bacteria grew under appropriate conditions in nitrogen-free semi-solid malate medium in the form of a typical subsurface pellicle which resulted in a significant nitrogen gain in the medium within 3 to 4 days of incubation; this could be also measured by¹⁵N-dilution. Bacteria grew and incorporated nitrogen under an atmosphere containing NH₃ and N₂O. A rapid and strong binding of strain W1 to roots of kallar grass grown in hydroponic culture was found by using a³²P-tracer technique. We obtained no evidence for diazotrophy of our strains because they failed to grow on nitrogen-free media when gases of high purity were used. No¹⁵N₂ was incorporated when bacteria were grown on¹⁵N₂ although a nitrogen gain was found, no acetylene reduction was observed and no homology with DNA containing sequences ofnifHDK structural genes for the nitrogenase components fromKlebsiella pneumoniae were detected. Owing to close contact of these bacteria with roots of kallar grass, utilization of scavenged nitrogen by the plant may have to be taken into account.     

Total denitrification and the ratio between N2O and N2 during the growth of spring barley

Summary Total denitrification (N₂O+N₂) and nitrous oxide emission were measured on intact soil cores using the acetylene inhibition technique.Total denitrification from the depth 0–8 cm during the growth period from April to August was 7 kg N/ha from plots supplied with 30 kg N/ha and 19 kg N/ha from plots supplied with 120 kg N/ha. The amounts of precipitation, plant growth, and N application were found to affect the denitrification rate. These factors also affected the ratio (N₂O+N₂)/N₂O, which varied from 1.0 to 7.2. Plant growth and precipitation increased the proportion of N₂ produced, whereas a high nitrate content increased the proportion of N₂O.     

Production of N 2 O by Ammonia Oxidizing Bacteria at Subfreezing Temperatures as a Model for Assessing the N 2 O Anomalies in the Vostok Ice Core

It was suggested that the abnormally high N ₂ O values found in 130,000–160,000 year-old Vostok ice core samples, characterized by high δ ¹⁵ N and low δ ¹⁸ O values, resulted from in situ microbial N ₂ O production. To substantiate these observations we obtained new geochemical data from the last glacial period and showed the existence of additional small N ₂ O anomalies. To test the hypothesis that microbial metabolism could contribute to these anomalies, we developed protocols for examining the ability of Nitrosomonas cryotolerans cells to produce N ₂ O at subfreezing temperatures. Our results show that these model, frozen cultures produce N ₂ O at temperatures as low as ?32°C.     

Contribution of plants to N 2 O emissions in soil-winter wheat ecosystem: pot and field experiments

Outdoor pot and field experiments were conducted to assess the role of growing plants in agricultural ecosystem N₂O emissions. N₂O emissions from plants were quantified as the difference in soil-crop system N₂O emissions before and immediately after cutting plants during the main growth stages in 2001–02 and 2002–03 winter wheat seasons. Emissions of N₂O from plants depended on biomass within the same plant developmental status. Field results indicated that the seasonal contribution of N₂O emissions from plants to ecosystem fluxes averaged 25%, ranging from 10% at wheat tillering to 62% at the heading stage. The fluxes of N₂O emissions from plants varied between 0.3 and 3.9 mg N₂O-N m⁻² day⁻¹ and its seasonal amount was equivalent to 0.23% of plant N released as N₂O. A N₂O emission coefficient (N₂O_E, mg N₂O-N g⁻¹ C day⁻¹), defined as N₂O-N emission in milligrams from per gram carbon of plant dry matter within a day, was represented by a 5-fold variation ranging from 0.021 to 0.004 mg N₂O-N g C⁻¹ day⁻¹. A linear relationship (y=0.4611x+0.0015, r ²=0.9352, p < 0.001) between N₂O_E (y) and plant dark respiration rate (x, mg CO₂-C g C⁻¹ day⁻¹) suggested that in the absence of photosynthesis, some N₂O production in plant N assimilation was associated with plant respiration. Although this study could not show whether N₂O was produced or transferred by winter wheat plants, these results indicated an important role for higher plant in N₂O exchange. Identifying its potential contribution is critical for understanding agricultural ecosystem N₂O sources.     

Molecular Dynamics Study of High Temperature Phase-Separation in a H2O/N2 Mixture with Exp-6 Interactions

Abstract

Equimolar H₂O/N₂ fluid mixture was studied by molecular dynamics simulations for NVT ensemble. Calculations were performed with the modified Buckingham (exp-6) potentials at T = 2000 K. Particular attention was given to the phase separation at very high pressures relevant to a detonation environment. Calculations of pair correlation functions and local mole fractions clearly indicated the occurrence of the fluid separation into N₂-rich and H₂O-rich phase. The density at the phase boundary between homogeneous and inhomogeneous phase-separated state was determined to be p = 1.35 g/cm³ on the basis of the static cross correlation factor which is defined by the sum of the local mole fractions. The ratio of the self-diffusion coefficients of N₂ and H₂O at p < 1.35 g/cm³ was found to be approximately equal to the value predicted by the kinetic theory of the ideal gas, whereas the ratio was close to unity at the phase-separated state (p > 1.35 g/cm³). In addition, two distinctive behaviors of the system could be observed for the relaxation from the initial uniform mixture to the phase-separated fluid: at lower densities (1.35 < p < 2.0 g/cm³) the fluid mixture began to relax into the phase-separated system without obvious incubation time, while clear incubation period was associated for the separation at higher densities. During this incubation period, discontinuous jumps in the mean square displacements were found.     

Nitrous oxide emissions from agricultural fields: Assessment, measurement and mitigation

In this paper we discuss three topics concerning N₂O emissions from agricultural systems. First, we present an appraisal of N₂O emissions from agricultural soils (Assessment). Secondly, we discuss some recent efforts to improve N₂O flux estimates in agricultural fields (Measurement), and finally, we relate recent studies which use nitrification inhibitors to decrease N₂O emissions from N-fertilized fields (Mitigation).To assess the global emission of N₂O from agricultural soils, the total flux should represent N₂O from all possible sources; native soil N, N from recent atmospheric deposition, past years fertilization, N from crop residues, N₂O from subsurface aquifers below the study area, and current N fertilization. Of these N sources only synthetic fertilizer and animal manures and the area of fields cropped with legumes have sufficient global data to estimate their input for N₂O production. The assessment of direct and indirect N₂O emissions we present was made by multiplying the amount of fertilizer N applied to agricultural lands by 2% and the area of land cropped to legumes by 4 kg N₂O-N ha^-1. No regard to method of N application, type of N, crop, climate or soil was given in these calculations, because the data are not available to include these variables in large scale assessments. Improved assessments should include these variables and should be used to drive process models for field, area, region and global scales.Several N₂O flux measurement techniques have been used in recent field studies which utilize small and ultralarge chambers and micrometeorological along with new analytical techniques to measure N₂O fluxes. These studies reveal that it is not the measurement technique that is providing much of the uncertainty in N₂O flux values found in the literature but rather the diverse combinations of physical and biological factors which control gas fluxes. A careful comparison of published literature narrows the range of observed fluxes as noted in the section on assessment. An array of careful field studies which compare a series of crops, fertilizer sources, and management techniques in controlled parallel experiments throughout the calendar year are needed to improve flux estimates and decrease uncertainty in prediction capability.There are a variety of management techniques which should conserve N and decrease the amount of N application needed to grow crops and to limit N₂O emissions. Using nitrification inhibitors is an option for decreasing fertilizer N use and additionally directly mitigating N₂O emissions. Case studies are presented which demonstrate the potential for using nitrification inhibitors to limit N₂O emissions from agricultural soils. Inhibitors may be selected for climatic conditions and type of cropping system as well as the type of nitrogen (solid mineral N, mineral N in solution, or organic waste materials) and applied with the fertilizers.     

Do increasingly depleted δ<Superscript>15</Superscript>N values of atmospheric N<Subscript>2</Subscript>O indicate a decline in soil N<Subscript>2</Subscript>O reduction?

Growing concentrations of N₂O within the atmosphere have been accompanied by decreasing δ¹⁵N values, provoking the hypothesis of a global decline in the rate of N₂O reduction relative to its production in soil. We estimate that the ratio of N₂O produced to N₂O reduced within the soil profile has declined by about 10–25% relative to its pre-industrial value. To a smaller extent, a reduction in the uptake of atmospheric N₂O at the soil surface relative to its emission could also have contributed to the reported isotopic signal. This calls for a greater consideration of the process of N₂O reduction in soil and its role in the global turnover of N₂O.     

Soil core method for direct simultaneous determination of N2 and N2O emissions from forest soils

In order to quantify N₂-emissions from a spruce and a beech site at the Höglwald Forest, a new measuring system was developed, that allowed simultaneous, direct determination of N₂- and N₂O-emission with high accuracy (detection limit approx. 10 g N m^–2 h^–1 for N₂ and <1 g for N₂O) using a gas-flow core method. This method requires exchange of the soil atmosphere with an artificial atmosphere, that differs only in that N₂ is substituted by He. The measuring system, the methodology of measurements and validation experiments are described in detail. Due to the huge heterogeneity of denitrification activity in different soil cores taken from our forest sites, no general trends of N₂ and N₂O production in relation to soil moisture and temperature could be demonstrated. Based on reasonable number of measurements, this work gives for the first time an estimate of the magnitude of N₂-losses from temperate forest soils. Both the magnitude of N₂-emissions (spruce: 7.2±0.7 kg N₂-N ha^–1 yr^–1; beech: 12.4±3.1 kg N₂-N ha^–1 yr^–1), as well as the N₂O–N₂ ratio (spruce: 0.136±0.04; beech: 0.52±0.19) were significantly higher for soils from the beech sites as compared to soils from the spruce site. The results suggests that N₂-emissions from N-saturated forest soils, still receiving high loads of atmospheric N-deposition, are approx. 30% of atmospheric N-input at the spruce site, and approx. 50% at the beech site. Our results demonstrate that losses of nitrogen in the form of N₂ cannot be neglected in the context of calculating N-balances for given forest sites.