土壤中“接力反硝化”机制的部分证据

设想土壤中存在不同类型的不完全反硝化细菌 ;这些细菌可以彼此配合 ,此菌产物作为彼菌的底物 ,共同完成完整的反硝化过程。该机制称之为“接力反硝化”机制 ,有别于传统的反硝化机制。本文为“接力反硝化”机制的存在提供部分证据。以土壤浸提液为培养基、N2 O为电子受体富集土壤微生物 ,获得了 1株仅完成NO-3 →NO-2 反应的细菌 (原始编号 2 1 6 9 2 )、1株仅完成NO-2 →N2 O反应的细菌 (原始编号 1 9 5 3)、1株仅完成NO-2 →N2 O→N2 反应的细菌 (原始编号 2 1 6 3 6 )。把菌株 2 1 6 9 2和 1 9 5 3两菌株以适当的数量比例混合于灭菌的土壤中 ,不添外来碳源 ,仅添加NO-3 ,厌气培养 1周后 ,测得土壤中剩余的NO-3 仅为原添加量的 39 4 %～ 5 3 0 % ,与此同时有 5 2 %～ 13 9%的NO-3 被还原成NO-2 ,有 2 8 6 %～ 30 8%以N2 O形态被回收 ,总回收率为 75 4 %～ 95 5 % ,说明两者可以相互配合 ,菌株 2 1 6 9 2的硝酸根还原产物可以被菌株 1 9 5 3用作底物 ,共同完成反硝化过程 ,从而支持我们设想的“接力反硝化”机制。     

尿素和KNO3对水稻土无机氮转化过程和产物的影响Ⅱ.N2O生成过程

采用15N技术标记尿素和KNO3,研究了淹水条件下黄泥土和红壤性水稻土生成N2 O的主要过程。结果表明 ,黄泥土反硝化过程产物以N2 为主 ,N2 O的生成量可以略而不计。加入KNO3促进NO- 3异化还原成铵过程 ,从而增加N2 O生成速率。红壤性水稻土主要通过反硝化或好气反硝化过程生成N2 O ,随着土壤pH的提高或NO- 3 浓度升高 ,N2 O生成速率增大。无论是黄泥土还是红壤性水稻土 ,有相当一部分样本的N2 O的15N丰度在NO- 2 、NO- 3 、NH 4的15N丰度范围外 ,由此推论 ,氮转化生成N2 O的过程应在微生物细胞内进行。     

广西沿海地区红树林根系土壤中放线菌的分离与鉴定

本研究通过分离纯培养,从广西北海及防城港红树林根系土壤中分离出放线菌并提取其总DNA,用放线菌通用引物对获得菌株的16SrDNA进行PCR扩增,对获得的扩增产物进行DNA序列测定及菌株鉴定。研究结果表明,从红树林根系土壤样品中分离出15株典型放线菌菌株。16SrDNA测序比对鉴定结果显示,15株典型放线菌菌株中有12株属于链霉菌属（Streptomyces）,是常见菌属;3株属于拟诺卡氏菌属（Nocardiopsis）,为稀有放线菌。本研究分离纯化获得15株典型放线菌,初步揭示了广西沿海地区红树林土壤中放线菌的多样性。     

西安市北郊污灌区土壤中抗铅链霉菌的多样性分析

从西安市北郊污灌区采集土样,用添加K2Cr2O7 75 μg·mL^-1和青霉素2 μg·mL^-1的高氏1号、HV和SC固体培养基分离到120株具有链霉菌特征的放线菌。采用浓度梯度法,用含Pb^2＋培养基对分离菌株进行筛选,得到50株抗铅链霉菌。在抗铅能力实验结果的基础上,选取14株抗性较强链霉菌代表菌株进行形态培养特征、生理生化和16S rDNA基因序列相似性等分析。结果表明,14株代表菌株可归为6个不同的颜色类群,其表型特征与生理生化性质和链霉菌相符合,在系统发育树上处于8个不同的进化分支。菌株HQ0031与已知链霉菌相似性差异较大,可能为链霉菌属内1个潜在新种。研究表明西安市北郊污灌区土壤中抗铅链霉菌具有丰富的多样性,可为重金属污染环境的生物修复提供有益的微生物资源。     

硝态氮浓度对亚热带土壤反硝化潜力和产物组成的影响

在实验室条件下,采用密闭、淹水、充 N2 的严格厌氧培养方法研究了NO3--N?浓度对亚热带土壤反硝化潜力和产物组成的影响。研究表明,在NO3-?-N?浓度为 10 ~ 200 mg /kg 范围内,该土壤的反硝化势变化于 0.024 ~ 0.224 mg/(kg×h) 之间,随着NO3--N?浓度的增加而呈显著线性增加(R2 = 0.94,P<0.01)。N2O 始终是反硝化的主要产物,占反硝化产物的 56% ~ 92%;NO 是次要产物,占 6% ~ 40%。在野外原位状态下,土壤的还原条件难以达到供试实验室条件,由此估计,亚热带森林土壤反硝化的主要产物并非 N2,而是 N2O 和 NO,这可能是该类土壤虽反硝化作用弱,但 N2O 排放量大的主要原因。     

硝化反硝化细菌菌落与N_2O排放关系研究

虽然硝化反硝化细菌菌落组成成分与从土壤中排放出来的N2O之间的关联尚不清楚,但是,硝化反硝化细菌的菌落组成、数量与N2O的排放活动已在两个常见的耕地型湿地(CW)与非耕地型湿地(UW)上做过探讨。本研究的假设有:1)不同的硝化反硝化菌落选择不同的地形;2)反硝化是产生N2O的主要步骤;3)在硝化反硝化细菌菌群组成、数量与N2O排放之间是有某种联系的。选在圣丹尼斯国家野生动物保护区(SDNWA)的3块CW与3块UW上进行比较试验。结果表明:1)硝化作用是N2O排放的根本来源;2)耕作土壤增加了硝化细菌的产量,同时消减了硝化细菌的数量;3)反硝化细菌的数量没有因为耕作活动而增加;4)在土地利用和地形为变量的前提下,硝化细菌、反硝化细菌菌落组成和数量与N2O的排放是没有关联的。     

水稻土和菜田添加碳氮后的气态产物排放动态

【目的】动态连续监测添加碳氮底物后各气体产物—O2、 NO、 N2O、 CH4和N2的排放,对土壤碳氮转化过程和气体产生过程做更深入的理解,揭示不同土地利用方式典型红壤的温室气体产生机制。【方法】采集长江中游金井小流域不同土地利用方式稻田和菜地土壤为研究对象,利用全自动连续在线培养检测体系(Robot系统),通过两组试验分别研究土壤碳氮转化过程中各气体产物的动态变化。试验1采用菜地和稻田土壤进行好气培养,设置不施氮对照、 添加40 mg/kg铵态氮、 添加40 mg/kg铵态氮+1%硝化抑制剂、 添加40 mg/kg硝态氮、 添加40 mg/kg硝态氮+1%葡萄糖、 缺氧条件下添加40 mg/kg硝态氮+1%葡萄糖6个处理。试验2采用稻田土壤进行淹水培养,设不施氮对照、 添加40 mg/kg铵态氮、 添加40 mg/kg铵态氮+1%硝化抑制剂、 添加40 mg/kg铵态氮+1%秸秆、 缺氧条件下添加40 mg/kg铵态氮+1%的葡萄糖、 添加40 mg/kg硝态氮、 添加40 mg/kg硝态氮+1%葡萄糖、 缺氧条件下添加40 mg/kg硝态氮+1%葡萄糖8个处理。培养温度均为20℃,土壤水分含量为70% WFPS (土壤孔隙含水量),培养周期为15天。【结果】从菜地和稻田土壤不同碳氮添加处理气态产物及无机氮的动态变化可看出: 1)菜地土壤好气培养初期硝化作用产生了大量N2O; 受低碳和低含水量的限制,反硝化作用较弱。当提供充足碳源和厌氧条件,出现N2O和NO的大量排放。2)在好气稻田和淹水稻田培养过程中,反硝化作用是N2O产生的主要途径。3)稻田土壤中,提供充足碳源和厌氧条件,各气态产物出现的顺序依次是NO、 N2O和N2,与三种气体在反硝化链式反应过程中的生成顺序一致。淹水稻田加铵态氮和碳源处理N2为主要产物,添加硝态氮处理后,N2O成为主要气态产物。当土壤碳源充足时,反硝化过程进行彻底,反硝化产物以终产物(N2)为主。4)在稻田土壤出现厌氧或添加碳源条件下,均检测到大量CH4产生; 且在甲烷产生的同时,NO-3几乎消耗殆尽。【结论】金井小流域典型红壤菜地N2O主要来自于硝化作用,好气和淹水稻田N2O主要来源于反硝化作用; 当碳源充足和厌氧时,菜地及稻田反硝化作用增强; 反硝化产物组成、 产物累积量及出峰顺序与碳源和氧气浓度有关。     

秸秆还田对灌溉玉米田土壤反硝化及N2O排放的影响

运用乙炔抑制技术研究了不同施氮水平下秸秆还田对灌溉玉米田土壤反硝化反应和氧化亚氮(N2O)排放的影响。结果表明,土壤反硝化速率及N2O的排放受氮肥施用、秸秆处理方式及其交互作用的显著影响。与秸秆燃烧相比,不施氮或低施氮水平时,秸秆还田可刺激培养初期反硝化反应速率及N2O排放,增加培养期间N2O平均排放通量;高施氮水平时,秸秆还田可降低反硝化反应速率及反硝化过程中的N2O排放。秸秆还田可降低反硝化中N2O/N2的比例。     

华北平原农田厚包气带及含水层反硝化细菌的分离鉴定及作用机制研究

华北平原农田由于长期过量施用氮肥,造成了土壤硝酸盐累积,导致地下水硝酸盐污染日趋严重。微生物的反硝化作用可将土壤中累积的硝酸盐或亚硝酸盐还原为气态产物,是消减厚包气带土壤累积的硝酸盐的重要途径。因此筛选高效反硝化微生物资源,对人工强化厚包气带土壤反硝化脱氮,阻控地下水硝酸盐污染具有重要作用。基于此,本研究采集位于华北平原的中国科学院栾城农业生态系统试验站长期施氮[施氮量为600kg(N)·hm~(-2)·a~(-1)]定位试验0～150m农田厚包气带及含水层土壤样品,从中筛选到62株细菌。16SrRNA基因序列分析表明这62株菌株与变形菌门(Proteobacteria)、放线菌门(Actinobacteria)、厚壁菌门(Firmicutes)中的9个属具有较高的同源性。根据系统发育树的结果,挑选7株亲缘关系较远的菌株进行反硝化潜势试验,结果表明,菌株L71、L13和L103具备反硝化产气能力。电镜观察结果表明,这3株菌均为无鞭毛的杆状细菌,其长度分别为1.0μm、1.5μm和1.5μm,只有L103具有运动能力。此外,菌株L103具有完全反硝化能力,且脱氮能力受到pH的影响,在本试验条件下,菌株L103的反硝化速率高达1.62～2.36g(KNO_3)·d~(-1)·L~(-1),具备实际应用潜力。本研究表明华北平原厚包气带土壤中存在完全反硝化微生物,并可为人工强化治理厚包气带土壤硝酸盐污染提供菌种资源和理论依据。     

草甸沼泽土壤硝化-反硝化作用和有机碳矿化对氮输入的响应

通过室内培养实验.研究了草甸沼泽土壤N2O排放和反硝化损失对氮输入的响应特征,结果表明,在培养期(23 d)内N2O平均排放速率为0.32(NO).0.87(N1).17.69(N2),28.07(N3)μgN2O-N/(kg±·h),反硝化平均损失速率为0.25(NO),0.81(NI),22.29(N2),30.28(N3)μgN2O--N/(kg±·h).两者都随氮输入量增高而升高.其中,N3处理N2O平均排放速率和反硝化平均损失速率与对照差异显著(p<0.05),N1和N2与对照差异不显著.N2O排放总量占氮输入的比例为0.03%(N1),1.04%(N2).1.76%(N3),反硝化损失总量占氮输入的比例为0.04%(N1),1.29%(N2),1.93%(N3).均表现为随氮输入量的增大而增高.N1处理下有机碳矿化速率低于对照,而N2和N3有机碳矿化速率高于对照,说明低氮输入对有机碳矿化有一定抑制作用,.高氮输入促进有机碳矿化.     

A GAS FLOW-THROUGH SYSTEM FOR STUDYING DENITRIFICATION IN SOI

A continuous gas ‘now-through’ system, incorporating a soil incubation cell and a gas sampling device, was developed to measure the nitrogen and carbon losses as gaseous N₂, N₂O, NH₃ and CO₂ from soil during denitrification under ‘steady state’ anaerobic conditions. Nitrogen was added as KNO₃ to the soil in amounts equivalent to 0, 30,100 and 300 kg N ha^-1. At 25 °C and one quarter of the water-holding capacity, N₂ and N₂O evolution accounted for most of the nitrogen lost. In the effluent gas stream, N₂O appeared about six hours after anaerobic conditions were established, and increased steadily. Initial N, production was small and reached a maximum between 90 and 100 h with a simultaneous decrease in N₂O evolution. Gaseous N losses were highest with the 100 kg N ha^?1 treatment and accounted for 59 per cent of the nitrate lost. A weight ratio, CO₂ -C/(N₂ O + N₂)-N of 1.0–1.3 was observed in the effluent gas once a ‘steady state’ was achieved, which conforms with that expected from nitrate respiration when N, and N₂O are produced simultaneously. When denitrification ceased, CO₂ production decreased to a steady rate. An increase in the duration, rather than the rate, of denitrification was observed when more N was added. A time lag before the onset of a maximum rate of denitrification was also observed, which increased with the amount of added N.     

Effects of transient anaerobic conditions in the presence of acetylene on subsequent aerobic respiration and N2O emission by soil aggregates

Our objective was to assess the effect of anaerobic conditioning in the presence of acetylene on subsequent aerobic respiration and N₂O emission at the scale of soil aggregates. Nitrous oxide production was measured in intact soil aggregates Δ (compacted aggregates without visible porosity) and Γ (aggregates with visible porosity) incubated under oxic conditions, with or without anaerobic conditioning for 6 d. N₂O emissions were much higher in aggregates that had been submitted to anaerobic conditioning than in aggregates that did not experience this conditioning, although very little NO₃⁻ remained in soil after the anaerobic period. ¹⁵N isotope tracing technique was used to check whether N₂O came from nitrification or denitrification. The results showed that denitrification was the major process responsible for N₂O emissions. The aerobic CO₂ production rate was also measured in intact soil aggregates. It was greater in aggregates submitted to anaerobic conditioning than in those that were not, suggesting that the anaerobic conditioning lead to an accumulation of small compounds including fatty acids that are readily available for microbial decomposition in aerobic conditions. This process increases the aerobic CO₂ production and favours the N₂O emissions through denitrification.     

Denitrification losses from puddled rice soils in the tropics

Summary Although denitrification has long been considered a major loss mechanism for N fertilizer applied to lowland rice (Oryza sativa L.) soils, direct field measurements of denitrification losses from puddled rice soils in the tropics have only been made recently. This paper summarizes the results of direct measurement and indirect estimation of denitrification losses from puddled rice fields and reviews the status of research methodology for measurement of denitrification in rice fields. The direct recovery of (N₂+N₂O)-¹⁵N from ¹⁵N-enriched urea has recently been measured at sites in the Philippines, Thailand, and Indonesia. In all 12 studies, recoveries of (N₂+N₂O)-¹⁵N ranged from less than 0.1 to 2.2% of the applied N. Total gaseous N losses, estimated by the ¹⁵N-balance technique, were much greater, ranging from 10 to 56% of the applied urea-N. Denitrification was limited by the nitrate supply rather than by available C, as indicated by the values for water-soluble soil organic C, floodwater (nitrate+nitrite)-N, and evolved (N₂+N₂O)-¹⁵N from added nitrate. In the absence of runoff and leaching losses, the amount of (N₂+N₂O)-¹⁵N evolved from ¹⁵N-labeled nitrate was consistently less than the unrecovered ¹⁵N in ¹⁵N balances with labeled nitrate, which presumably represented total denitrification losses. This finding indicates that the measured recoveries of (N₂+N₂O)-¹⁵N had underestimated the denitrification losses from urea. Even with a probable two-or threefold underestimation, direct measurements of (N₂+N₂O)-¹⁵N failed to confirm the appreciable denitrification losses often estimated by the indirect difference method. This method, which determines denitrification losses by the difference between total ¹⁵N loss and determined ammonia loss, is prone to high variability. Measurements of nitrate disappearance and ¹⁵N-balance studies suggest that nitrification-denitrification occurs under alternate soil drying and wetting conditions both during the rice cropping period and between rice crops. Research is needed to determine the magnitude of denitrification losses when soils are flooded and puddled for production of rice.     

Role of nitrite and nitric oxide in the processes of nitrification and denitrification in soil: Results from N tracer experiments

Recent research has proven soil nitrite to be a key element in understanding N-gas production (NO, N₂O, N₂) in soils. NO is widely accepted to be an obligatory intermediate of N₂O formation in the denitrification pathway. However, studies with native soils could not confirm NO as a N₂O precursor, and field experiments mainly revealed ammonium nitrification as the source of NO. The hypothesis was constructed, that the limited diffusion of NO in soil is the reason for this contradiction. To test this diffusion limitation hypothesis and to verify nitrite and NO as free intermediates in native soils we conducted through-flow (He/O₂ atmosphere) ¹⁵N tracer experiments using black earth soil in an experimental set up free of diffusion limitation. All of the three relevant inorganic N soil pools (ammonium, nitrite, nitrate) were ¹⁵N labelled in separate incubation experiments lasting 81 h based on the kinetic isotope method. During the experiments the partial pressure of O₂ was decreased in four steps from 20% to about 0%. The net NO emission increased up to 3.7 μg N kg⁻¹ h⁻¹ with decreasing O₂ partial pressure. Due to the special experimental set up with little to no obstructions of gas diffusion, only very low N₂O emission could be observed. As expected the content of the substrates ammonium, nitrate and nitrite remained almost constant over the incubation time. The ¹⁵N abundance of nitrite revealed high turnover rates. The contribution of nitrification of ammonium to the total nitrite production was approx. 88% under strong aerobic soil conditions but quickly decreased to zero with declining O₂ partial pressure. It is remarkable that already under the high partial pressure of 20% O₂ 12 % of nitrite is generated by nitrate denitrification, and under strict anaerobic conditions it increases to 100%. Nitrite is present in two separate endogenous pools at least, each one fed by the nitrification of ammonium or the denitrification of nitrate. The experiments clearly revealed that nitrite is almost 100% the direct precursor of NO formation under anaerobic as well as aerobic conditions. Emitted N₂O only originated to about 100% from NO under strict anaerobic conditions (0-0.2% O₂), providing evidence that NO is a free intermediate of N₂O formation by denitrification. To the best of our knowledge this is the first time that NO has been detected in a native soil as a free intermediate product of N₂O formation at denitrification. These results clearly verify the “diffusion limitation” hypothesis.     

Gaseous nitrogen losses from fertilized soil during nitrification and denitrification using the acetylene blockage technique

Summary A sandy soil amended with different forms and amounts of fertilizer nitrogen (urea, ammonium sulphate and potassium nitrate) was investigated in model experiments for N₂O emission, which may be evolved during both oxidation of ammonia to nitrate and anaerobic respiration of nitrate. Since C₂H₂ inhibits both nitrification and the reduction of N₂O to N₂ during denitrification, the amount of N₂O evolved in the presence and absence of C₂H₂ represents the nitrogen released through nitrification and denitrification.Results show that amounts of N₂O-N lost from soils incubated anaerobically with 0.1% C₂H₂ and treated with potassium nitrate (23.1 µg N-NO ₃ ^– /g dry soil) exceeded those from soils incubated in the presence of 20% oxygen and treated with even larger amounts of nitrogen as urea and ammonium sulphate. This indicates that nitrogen losses by denitrification may potentially be higher than those occurring through nitrification.     

Einfluß chemischer Bodeneigenschaften auf Ausmaß und Zusammensetzung der Denitrifikationsverluste drei verschiedener Bakterien

Effect of soil properties on the quantity and quality of denitrification with different bacteria The influence of 4 different soils on the intensity and quality of gaseous denitrification losses from 3 bacteria (Aeromonas “denitrificans” S224, Azospirillum lipoferum DSM 1843 and Bacillus licheniformis ATCC 14580) was examined in model experiments at complete anaerobic conditions at the expense of a relatively high nitrate concentration (300 μg NO₃^? N/g dry soil) at standard conditions (30°C, 80% WHC). The soils (Ah-material) were obtained from gleyo-eutric Fluvisol (A), orthic Luvisol (L), calcaric Fluvisol (AR) and eutric Cambisol (KB) and represented different chemical properties. Gas production (CO₂, NO, N₂O, N₂ and CH₄) was analyzed by gaschromatography in regular intervals, whereas changes in N_t, C_t, water extractable organic carbon (C), nitrate, nitrite, ammonium, pH (H₂O) were determined at the end of each experiment. The intensity and composition of denitrification (NO, N₂O, N₂) differed considerably from organism to organism and from soil to soil. With A. “denitrificans” NO was released from the calcaric Fluvisol and orthic Luvisol, whereas B. licheniformis produced this gas only from the Cambisol. A. lipoferum did not produce NO in any of the soils tested. N₂O was liberated by A. “denitrificans” in all soils, but A. lipoferum produced it only in the Fluvisol and B. licheniformis exclusively in the Cambisol. Apparently, the production of NO and N₂O as products of incomplete denitrification at relatively high nitrate concentration is determined primarily by the organism in question and in the second place by the chemical properties of the soil. The main properties that govern the quality of denitrification in soils are discussed.     

Nitrogen isotope discrimination in denitrification of nitrate in soils

Nitrogen isotope discrimination during denitrification in soils of nitrate containing natural concentrations of ¹⁴N and ¹⁵N was studied by determining the amount and the ¹⁵N content of nitrate-N and (nitrate + nitrite)-N in nitrate-treated soils incubated under anaerobic conditions (He atmosphere) for various times after treatment with glucose to promote denitrification. Analyses performed showed that the nitrate-N lost on incubation of these soils could largely be accounted for as products of denitrification (nitrite, NO. N₂O and N₂).The studies reported show that marked discrimination between ¹⁴N and ¹⁵N occurs during denitrification of nitrate in soils and that significant N isotope effects occur both in reduction of nitrate to nitrite and in reduction of nitrite to gaseous forms of N. They also indicate that the overall N isotope effect during denitrification of nitrate in soil will depend upon the tendency of the soil to accumulate nitrite under conditions that induce denitrification.It is concluded that discrimination between ¹⁴N and ¹⁵N during denitrification in soils of nitrate containing natural concentrations of these isotopes is of sufficient magnitude to invalidate the use of N isotope-ratio analyses for assessment of the contributions of soil and fertilizer N to nitrate in surface or ground waters or to nitrous oxide in the atmosphere.     

Long-term animal impact modifies potential production of N<Subscript>2</Subscript>O from pasture soil

At cattle overwintering areas, inputs of nutrients in animal excrements create conditions favourable for intensive microbial activity in soil. During nitrogen transformations, significant amounts of N₂O are released, which makes overwintering areas important sources of N₂O emission. In previous studies, however, increasing intensity of long-term cattle impact did not always increase emissions of N₂O from the soil: in some cases, N₂O emissions from the soil were lower at the most impacted area than at the moderately impacted one. Thus, the relationships between the level of long-term animal impact and potential production of N₂O from soil by denitrification were investigated in field and laboratory experiments. Field measurements indicated that the production of N₂O after glucose and nitrate amendments was greater in severely and moderately impacted locations than in an unimpacted location, while differences between the severely and moderately impacted locations were not significant. In laboratory experiments, the potential production of N₂O (measured as anaerobic production of N₂O after addition of glucose and nitrate) was highest in the moderately impacted soil. Surprisingly, potential N₂O production was lower in the most impacted than in the moderately impacted soil, and the net N₂O production in the highly impacted soil was further decreased by a significant reduction of N₂O to N₂. The expected stimulating effect of an increasing ratio of glucose C to nitrate N on the reduction of N₂O to N₂ during denitrification was not confirmed. The results show that cattle increase the denitrification potential of the soil but suggest that the denitrification potential does not increase indefinitely with increasing cattle impact.     

Nitrous oxide fluxes and denitrification sensitivity to temperature in Irish pasture soils

Nitrous oxide (N₂O) emissions from grazed pastures constitute approximately 28% of total global anthropogenic N₂O emissions. The aims of this study were to investigate the effect of inorganic N fertilizer application on fluxes of N₂O, quantify the emission factors (EFs) for a sandy loam soil which is typical of large areas in Ireland and to investigate denitrification sensitivity to temperature. Nitrous oxide flux measurements from a cut and grazed pasture field for 1 year and denitrification laboratory incubation were carried out. The soil pH was 7.3 and had a mean organic C and N content at 0–20 cm of 44.1 and 4.4 g/kg dry weight, respectively. The highest observed peaks of N₂O fluxes of 67 and 38.7 g N₂O‐N per hectare per day were associated with times of application of inorganic N fertilizer. Annual fluxes of N₂O from control and fertilized treatments were 1 and 2.4 kg N₂O‐N per hectare, respectively. Approximately 63% of the annual flux was associated with N fertilizer application. Multiple regression analysis revealed that soil nitrate and the interaction between soil nitrate and soil water content were the main factors controlling N₂O flux from the soil. The derived EF of 0.83% was approximately 66% of the IPCC default EF value of 1.25% as used by the Irish EPA to estimate greenhouse gases (GHGs) in Ireland. The IPCC‐revised EF value is 0.9%. A highly significant exponential regression (r² = 0.98) was found between denitrification and incubation temperature. The calculated Q₁₀ ranged from 4.4 to 6.2 for a temperature range of 10–25 °C and the activation energy was 47 kJ/mol. Our results show that denitrification is very sensitive to increasing temperature, suggesting that future global warming could lead to a significant increase in soil denitrification and consequently N₂O fluxes from soils.     

Litter decomposition reduces either N2O or NO production in strongly acidic coniferous and broad-leaved forest soils under anaerobic conditions

Purpose

The beneficial effect to the environment of nitrate (NO₃ ^?) removal by denitrification depends on the partitioning of its end products into nitrous oxide (N₂O), nitric oxide (NO), and dinitrogen (N₂). However, in subtropical China, acidic forest mineral soils are characterized by negligible denitrification capacity and thus reactive forms of N could not be effectively converted to inert N₂, resulting in a negative environmental consequence. In this study, the influences of C input from litter decomposition on denitrification rate and its gaseous products under anoxic conditions in the acidic coniferous and broad-leaved forest soils in subtropical China were investigated using the acetylene (C₂H₂) blockage technique in the laboratory.

Materials and methods

The coniferous and broad-leaved forest soils with and without litter addition were incubated under anaerobic conditions for 244 h. There were three treatments for each forest soil including addition of 0.5 and 1% corresponding litter (gram of litter per gram of soil) and the control without addition of litter.

Results and discussion

The results showed that litter addition into the broad-leaved forest soil had no effect on average rates of denitrification (calculated as the sum of NO, N₂O, and N₂), whereas in the coniferous forest soil, the addition resulted in a significant increase in average denitrification rate. In the broad-leaved forest soil, both rates of litter addition decreased the production of NO but increased the production of N₂, and high rates of litter addition into the coniferous forest soil promoted the reduction of N₂O to N₂.

Conclusions

Increased decomposition of litter in the forest soils could effectively reduce N₂O and NO production through denitrification under anaerobic conditions.