田间老化生物质炭减缓稻麦轮作系统土壤N2O排放能力降低的机理

作为一种重要的土壤调节剂,生物质炭在固碳减排,尤其在氧化亚氮(N₂O)减排方面的作用日益突出。本研究通过田间定位试验,分析稻麦轮作体系新鲜和田间不同时间老化生物质炭对N₂O排放的影响,旨在明确生物质炭对田间N₂O排放的持续效应及其作用机理。试验共设置5个处理,分别为CK（不施氮肥和生物质炭）、N（施氮肥）、NB_0y（氮肥+新鲜生物质炭）、NB_2y（氮肥+2年老化生物质炭）和NB_5y （氮肥+5年老化生物质炭）,动态监测稻麦轮作周期N₂O排放,测定水稻和小麦收获后土壤理化性质和氮循环功能基因丰度。结果表明,生物质炭显著降低土壤N2O累积排放量32.4% ~ 54.0%,且表现为NB_0y> NB_2y> NB_5y。与N处理相比,NB_0y, NB_2y 和NB_5y处理显著提高土壤pH值0.6 ~ 1.2个单位、土壤有机碳（SOC）含量21.4 % ~ 58.6%、硝态氮（NO_3^--N）含量1.7% ~ 31.3%,对土壤pH改善能力随着生物质炭老化而下降。生物质炭处理显著提高nosZ基因丰度54.9% ~ 249.4%,土壤 (nirS+nirK)/nosZ比值随着生物质炭老化而增加。相关性分析表明,土壤N₂O累积排放量与pH值呈显著负相关,与NO_3^--N含量和amoA-AOB（氨氧化细菌）丰度呈显著正相关。因此,新鲜和田间不同时间老化生物质炭均能显著改善土壤理化特性,降低土壤 N₂O排放且新鲜生物质炭的作用效果优于老化生物质炭。土壤NO_3^--N 含量及(nirS+nirK)/nosZ比值的增加,是导致老化生物质炭减排N₂O能力降低的主要原因。     

区分土壤中硝化与反硝化对N2O产生贡献的方法

土壤是产生N₂O的最主要来源之一。硝化和反硝化反应是产生N₂O的主要机理，由于硝化和反硝化微生物同时存在于土壤中，因而硝化和反硝化作用能同时产生N₂O。N₂O的来源可通过使用选择性抑制剂，杀菌剂以及加入的标记底物确定。通过对生成N₂O反应的每一步分析，主要从抑制反应发生的催化酶和细菌着手，总结了测量区分硝化、反硝化和DNRA反应对N₂O产生的贡献方法。并对¹⁵N标记底物法，乙炔抑制法和环境因子抑制法作了详细介绍。     

生物质炭对酸性菜地土壤N2O排放及相关功能基因丰度的影响

【目的】 生物质炭显著影响土壤氧化亚氮 (N₂O) 排放,但关于其相关微生物机理的研究相对匮乏,尤其是生物质炭对酸性菜地土壤N₂O排放的微生物作用机理。本文通过研究氮肥配施生物质炭对酸性菜地土壤N₂O排放以及硝化和反硝化过程相关功能基因丰度的影响,探讨酸性菜地土壤N₂O排放与功能基因丰度的关系,阐释生物质炭对酸性菜地土壤试验N₂O排放的微生物作用机理。 【方法】 在田间一次性施入生物质炭 40 t/hm2,试验连续进行了3年,共9茬蔬菜。设置4个处理:对照 (CK)、氮肥 (N)、生物质炭 (Bc) 和氮肥 + 生物质炭 (N + Bc)。在施用后第三年,采集土壤样品进行室内培养,应用荧光定量PCR技术检测硝化过程氨氧化古菌 (AOA)、氨氧化细菌 (AOB) 功能基因amoA和反硝化过程亚硝酸还原酶基因 (nirK、nirS) 以及N₂O还原酶基因 (nosZ) 等相关功能基因丰度,同时监测土壤pH值、无机氮 (铵态氮、硝态氮) 含量及N₂O排放。 【结果】 与CK相比,生物质炭 (Bc) 处理的土壤有机碳 (SOC) 提高了27.1%,总氮 (TN) 提高了8.2%,amoA-AOB基因丰度显著降低了11.0%,nosZ基因丰度增加了21.2% (P < 0.05),N ₂O排放没有显著变化 (P > 0.05)。与CK相比,施用氮肥 (N) 显著降低土壤pH ( P < 0.05),显著增加土壤无机氮含量、 nirK、nirS和nosZ功能基因丰度以及土壤N₂O累积排放量 (P < 0.05)。与N处理相比,生物质炭与氮肥联合施用 (N + Bc) 处理显著增加 amoA-AOA、amoA-AOB、nirK、nirS和nosZ基因丰度,增幅分别为68.1%、39.3%、21.1%、19.8%、48.4% (P < 0.05),但 ( nirK + nirS)/nosZ的比值降低,同时N₂O累积排放量显著降低33.3% (P < 0.05)。室内培养期间N ₂O排放峰出现在1～5 d,N和N+Bc处理排放速率分别为 N 1.70 × 103和1.76 × 103 ng/(kg·h)。相关分析结果显示,N₂O排放速率与氧化亚氮还原酶的标记基因nosZ基因拷贝数 (P < 0.05)、NH ₄+-N含量 (P < 0.01) 呈显著正相关,与pH呈显著负相关 ( P < 0.01)。 【结论】 在菜地生态系统中氮肥和生物质炭联合施用可以有效缓解菜地土壤酸化,减少菜地土壤N₂O排放,主要归因于反硝化作用nosZ基因丰度增加,(nirK + nirS)/nosZ比值降低。      

不同温度下海南禾本科类植物材料制备的生物质炭对 砖红壤性质及N2O排放影响

制备温度及原料影响生物质炭性质,相应影响土壤N₂O的排放,为筛选适宜于海南砖红壤的生物质炭类型,利用室内培养试验,研究海南4种禾本科植物材料在300℃、500℃、700℃ 三种热解温度下制备的12种生物质炭对砖红壤性质及N₂O排放的影响。结果表明：所有生物质炭都能显著增加土壤有机碳、有效磷和速效钾含量,提高土壤pH,加速土壤硝化作用进行;300℃下制备的生物质炭能促进土壤N₂O排放,500℃和700℃下制备的生物质炭则对土壤N₂O排放有明显抑制作用;所有材料中,高温热解温度下由甘蔗渣制备的生物质炭处理土壤的N₂O排放量最低,可能由于甘蔗在榨糖后剩下的甘蔗渣以较稳定的碳水化合物为主。综合分析,推荐500℃热解温度下生成的甘蔗渣生物质炭实施土壤改良,更有利于N₂O减排。     

地膜覆盖对土壤中N2O释放的影响

研究地膜覆盖下土壤中N₂O释放可为进一步探明膜下土壤中N₂O的传输、消耗和排放到大气的动力学过程提供理论依据。在2001年3月至6月和2001年10月至2002年6月连续两个冬小麦生长季，采用静态漏斗法和揭膜—封闭箱法测定了地膜覆盖下耕层5、10、20 cm土层处和地表处N₂O的释放特征及相应土壤性质。结果表明：地膜覆盖下，地表和耕层10、20 cm土层处N₂O释放通量显著增加；0～5 cm土层土壤水分和10～20 cm土层土壤硝态氮的浓度的变化分别解释了休闲地和冬小麦地土壤中N₂O释放通量85.23%和92.11%的变异，它们是膜下休闲地和冬小麦地土壤中N₂O释放通量增加的主要原因。该结论对地膜覆盖下科学地控制农田水分、养分以及地膜覆盖在中国西北地区的科学使用和推行具有实际意义。     

不同耕作措施下土壤N2O排放及其农学效率

为评价不同耕作措施下华北平原农田土壤N2O排放及其农学效率,通过设置常规耕作秸秆还田(CT+)、常规耕作无秸秆还田(CT?)、免耕秸秆还田(NT+)、免耕无秸秆还田(NT?)4个处理田间定位试验,采用静态箱?气相色谱法测定分析了连续3个小麦生长期的表层土壤N2O排放及其主要相关因子,同时测定了小麦产量与氮吸收量等相关指标。结果表明:在4个处理下,小麦生长期内表层土壤N2O排放动态基本一致,而土壤N2O累积排放量却存在显著差异,而且耕作方式与秸秆还田存在显著的互作效应。在常规耕作和免耕措施下,单位面积土壤N2O累积排放量均表现为秸秆还田土壤显著高于无秸秆还田土壤,CT+和NT+分别比CT?和NT?高26.2%和74.6%;在无秸秆还田条件下,土壤N2O排放量表现为常规耕作比免耕高42.4%。相关分析表明,土壤N2O排放通量与地下5 cm土壤温度、土壤孔隙充水率(WFPS)之间呈显著正相关关系,与土壤溶解性有机氮(DON)含量之间呈显著负相关关系。利用农学效率指标度量N2O排放量时可知,虽然小麦籽粒产量和氮肥偏生产力在各处理间没有达到显著性差异,但每生产1 kg小麦籽粒表层土壤N2O排放量为0.18~0.73 g N2O-N,每投入1 kg氮素表层土壤N2O排放量为5.1~18.0 g N2O-N,处理间存在显著差异;与单位面积土壤N2O排放量表现一致,单位籽粒产量N2O排放量和单位氮素投入N2O排放量均表现为无论是常规耕作还是免耕措施,秸秆还田土壤均显著高于秸秆不还田土壤,在秸秆不还田条件下,常规耕作土壤均显著高于免耕土壤。总之,免耕是有效减少土壤N2O排放的一种耕作措施。     

生物质炭和腐殖质对稻田土壤CH4和N2O排放的影响

为探讨生物质炭与腐殖质单独施用与配合施用对稻田土壤CH₄和N₂O气体排放以及水稻产量的影响。以浙江临安潜育性水稻土的稻田系统为研究对象,设置2个水稻秸秆生物质炭添加水平(0,20 t/hm²)和3个腐殖质水平(0,0.6,1.2 t/hm²),共6个处理,分别为:(1)B0F0(对照,不添加生物质炭和腐殖质);(2)B0F1(腐殖质用量为0.6 t/hm²);(3)B0F2(腐殖质用量为1.2 t/hm²);(4)B1F0(生物质炭用量为20 t/hm²);(5)B1F1(生物质炭和腐殖质用量分别为20,0.6 t/hm²);(6)B1F2(生物质炭和腐殖质用量分别为20,1.2 t/hm²),研究生物质炭和腐殖质输入对水稻产量、稻田CH₄和N₂O气体排放的影响。结果表明:(1)与B0F0相比,单独施用生物质炭和腐殖质或生物质炭与腐殖质配施均降低了土壤CH₄累积排放量,但增加了土壤N₂O累积排放量;(2)生物质炭处理对GWP(global warming potential)和GHGI(greenhouse gas intensity)没有显著影响(P>0.05),腐殖质处理显著降低了GWP和GHGI(P<0.05),生物质炭和腐殖质对GWP和GHGI存在显著交互作用(P<0.05);(3)与B0F0相比,单独施用生物质炭和腐殖质或者生物质炭与腐殖质配施均能在一定程度上减少单位水稻产量的温室气体排放强度(GHGI),B0F2处理的GHGI最低,表明单施腐殖质处理(腐殖质用量为1.2 t/hm²)稻田土壤的减排效果和环境效应最好。研究结果为进一步探讨稻田土壤固碳减排提供数据支撑和理论依据。     

竹叶及其生物质炭输入对板栗林土壤N2O通量的影响

【目的】氧化亚氮(N2O)是温室气体的主要组成部分,其增温效应极强,陆地生态系统是N2O的主要排放源之一。人工林生态系统是陆地生态系统的重要组成部分,但目前关于经营措施对人工林生态系统土壤N2O通量的影响研究较少。本文研究了竹叶及其生物质炭输入对板栗林土壤N2O排放通量的影响,为调控亚热带人工林土壤N2O排放通量提供理论基础与科学依据。【方法】定位试验于2012年7月~2013年7月在浙江省临安市三口镇典型板栗林区进行,设对照、输入竹叶、输入生物质炭3个处理,利用静态箱-气相色谱法测定板栗林土壤N2O通量的动态变化以及土壤温度、土壤含水量、水溶性有机碳(WSOC)、水溶性有机氮(WSON)、微生物量碳(MBC)、微生物量氮(MBN)、NH+4-N和NO-3-N含量。【结果】不同处理条件下,板栗林土壤N2O排放通量均呈显著的季节性变化特征,最高值出现在7月,最低值出现在1月。与对照相比,竹叶处理的土壤N2O年平均通量和年累积排放量分别增加了17.2%和12.8%,而生物质炭处理的土壤N2O年平均通量和年累积排放量分别降低了27.4%和20.5%。竹叶处理的土壤WSON、MBN、NH+4-N及NO-3-N含量增加12.4%、19.1%、8.3%和13%,而生物质炭处理的NH+4-N和NO-3-N含量分别降低了14.1%和18%。在对照、竹叶以及生物质炭处理条件下,板栗林土壤N2O排放通量与土壤温度(表层5 cm处)和WSOC含量均有显著相关性(P 0.05),与土壤MBC含量均无显著相关性。竹叶处理土壤N2O通量与NH+4-N、NO-3-N及WSON含量均有显著相关性(P0.05)。【结论】在不同处理条件下,板栗林土壤N2O排放通量均呈现明显的季节性变化特征,表现为夏季高、 冬季低。输入竹叶可显著增加板栗林土壤N2O排放通量,而输入生物质炭N2O排放通量显著降低;输入竹叶和生物质炭可能是通过影响土壤碳库与氮库特征而影响土壤N2O的排放通量。     

生物质炭添加对不同pH紫色土硝化作用及N2O排放的影响

生物质炭施用可能对土壤中氮素硝化过程和N₂O排放产生影响。本研究通过室内培养试验,研究铵态氮肥与玉米秸秆生物质炭施用量(0、1%、2%、5%、10%,m/m)对酸性(pH 5.10)和石灰性(pH 8.15)紫色土氮素硝化率、净硝化速率及N₂O排放特征的影响。结果表明:①酸性和石灰性紫色土生物质炭处理平均净硝化速率相比对照分别降低了33.7%～93.7%和7.5%～40.9%,生物质炭添加抑制了酸性和石灰性紫色土硝化作用,在酸性紫色土中生物质炭对硝化作用的抑制作用随施用量的增加而增强,在石灰性紫色土中无明显规律。②与对照相比,酸性紫色土N₂O累计排放量在1%生物质炭和2%生物质炭处理下降幅分别为15.9%和27.7%,在5%和10%生物质炭处理下增幅分别为60.1%和93.2%;石灰性紫色土生物质炭各处理N₂O累积排放量均显著高于对照。③综合考虑,在酸性紫色土中1%、2%生物质炭施用对硝化作用抑制和N₂O减排综合效果最好,但在石灰性紫色土中无明显抑制和减排效果。     

土壤温度、水分和NH4+-N浓度对土壤硝化反应速度及N2O排放的影响

硝化反应是土壤、特别是干旱半干旱地区农业土壤N2O产生的重要途径之一。但是,目前环境条件对硝化反应中N2O排放的影响研究较少,而在国内外通用的几个模型中均用固定比例估算硝化反应过程中N2O的排放。本文通过砂壤土培养试验,研究了土壤温度、水分和NH4+-N浓度对硝化反应速度及硝化反应中N2O排放的影响,并用数学模型定量表示了各因素对硝化反应的作用,用最小二乘法最优拟合求得该土壤的最大硝化反应速度及N2O最大排放比例。结果表明,随着温度升高,硝化反应速度呈指数增长;水分含量由20%充水孔隙度(WFPS)增加到40%WFPS时,反应速度增加,水分含量增加到60%WFPS时反应速度略有降低;NH4+-N浓度增加对硝化反应速度起抑制作用。用米氏方程描述该土壤的硝化反应过程,其最大硝化反应速度为6.67mg·kg?1·d?1。硝化反应中N2O排放比例随温度升高而降低;随NH4+-N浓度增加而略有增加;20%和40%WFPS水分含量时,硝化反应中N2O排放比例为0.43%～1.50%,最小二乘法求得的最大比例为3.03%,60%WFPS时可能由于反硝化作用,N2O排放比例急剧增加,还需进一步研究水分对硝化反应中N2O排放的影响。     

Nitrous oxide production and consumption in serially diluted soil suspensions as related to in situ N2O emission in submerged soils

A simple method for characterizing soil microbial community composition relevant to N₂O production and consumption was proposed. Ten-fold series soil dilution was prepared. Nitrate or N₂O was provided as the sole electron acceptor. Nitrous oxide concentration in the headspace gas across the serially diluted soil suspensions was measured against controls. Results showed that the patterns of N₂O production and consumption across the soil suspensions provided useful information on the microbial community composition relevant to N₂O production and consumption in these soils. An independent method, to that proposed here, was also employed to characterize denitrifier community compositions of the same soils. Data indicated that information on the soil microbial community composition characterized by both methods were compatible or mutually supporting and apparently related to in situ N₂O emissions. Soil samples from manure (applied with animal manure plus chemical fertilizer) plots had higher denitrification rates than the samples from normal fertilizer (applied with chemical fertilizer only) plots. It was concluded that functional characteristics of soil microbial communities relevant to N₂O production and consumption could be characterized at ecological levels and may potentially affect N₂O emissions.     

No-till only increases N2O emissions in poorly-aerated soils

Denitrification rates are often greater in no-till than in tilled soils and net soil-surface greenhouse gas emissions could be increased by enhanced soil N₂O emissions following adoption of no-till. The objective of this study was to summarize published experimental results to assess whether the response of soil N₂O fluxes to the adoption of no-till is influenced by soil aeration. A total of 25 field studies presenting direct comparisons between conventional tillage and no-till (approximately 45 site-years of data) were reviewed and grouped according to soil aeration status estimated using drainage class and precipitation during the growing season. The summary showed that no-till generally increased N₂O emissions in poorly-aerated soils but was neutral in soils with good and medium aeration. On average, soil N₂O emissions under no-till were 0.06 kg N ha⁻¹ lower, 0.12 kg N ha⁻¹ higher and 2.00 kg N ha⁻¹ higher than under tilled soils with good, medium and poor aeration, respectively. Our results therefore suggest that the impact of no-till on N₂O emissions is small in well-aerated soils but most often positive in soils where aeration is reduced by conditions or properties restricting drainage. Considering typical soil C gains following adoption of no-till, we conclude that increased N₂O losses may result in a negative greenhouse gas balance for many poorly-drained fine-textured agricultural soils under no-till located in regions with a humid climate.     

Emission of N2O, N2 and CO2 from soil fertilized with nitrate: effect of compaction, soil moisture and rewetting

Soil compaction and soil moisture are important factors influencing denitrification and N₂O emission from fertilized soils. We analyzed the combined effects of these factors on the emission of N₂O, N₂ and CO₂ from undisturbed soil cores fertilized with (150 kg N ha⁻¹) in a laboratory experiment. The soil cores were collected from differently compacted areas in a potato field, i.e. the ridges (ρ_D=1.03 g cm⁻³), the interrow area (ρ_D=1.24 g cm⁻³), and the tractor compacted interrow area (ρ_D=1.64 g cm⁻³), and adjusted to constant soil moisture levels between 40 and 98% water-filled pore space (WFPS).High N₂O emissions were a result of denitrification and occurred at a WFPS≥70% in all compaction treatments. N₂ production occurred only at the highest soil moisture level (≥90% WFPS) but it was considerably smaller than the N₂O-N emission in most cases. There was no soil moisture effect on CO₂ emission from the differently compacted soils with the exception of the highest soil moisture level (98% WFPS) of the tractor-compacted soil in which soil respiration was significantly reduced. The maximum N₂O emission rates from all treatments occurred after rewetting of dry soil. This rewetting effect increased with the amount of water added. The results show the importance of increased carbon availability and associated respiratory O₂ consumption induced by soil drying and rewetting for the emissions of N₂O.     

生物炭施用下中国农田土壤N2O排放的Meta分析

为明确施加生物炭对中国农田土壤N_2O排放的影响和主要控制因素,以公开发表的试验数据为研究对象,采用Meta-analysis法定量分析了施加生物炭条件下,气候、土壤性质、田间管理方式、生物炭性质与施加量对土壤N_2O排放的影响,并对各影响因素进行通径分析。结果表明,当年降雨量≥600 mm时,生物炭显著降低土壤N_2O排放量(P0.05),且随年降雨量的增加而增强;当年日照时数大于1 000 h时,生物炭对土壤N_2O的减排效果随年日照时数的增加而减弱。当土壤p H≥6.5时,生物炭对土壤N_2O的减排效果随土壤p H的增加呈先增后减趋势;在壤土中施加生物炭对N_2O的减排效果显著(P0.05),而砂土和黏土不显著(P0.05)。生物炭对覆膜土壤N_2O的减排效果优于不覆膜土壤;生物炭对土壤N_2O的减排效果随施氮肥量增加而减弱,而随生物炭比表面积的增加而增强。当生物炭C/N处于30~500时,生物炭施用下土壤N_2O排放量显著降低(P0.05);当生物炭施加量处于20~160 t×hm-2时,生物炭对土壤N_2O的减排效果随施加量增加而增强。生物炭对土壤N_2O减排的影响存在显著的区域性特征,对华南、华东、华中和东北地区影响显著(P0.05),而对西北地区不显著(P0.05);施氮肥量、生物炭施加量、年均温和年降雨量是影响生物炭减排效果的最主要因素,这些因素的相互作用共同影响生物炭对土壤N_2O的减排效果。该研究可为生物炭在我国农区的推广应用和农田N_2O减排提供参考。     

施肥对夏玉米季紫色土N2O排放及反硝化作用的影响

采用原状土柱-乙炔抑制培养法研究了施肥对紫色土玉米生长季土壤N2O排放通量和反硝化作用的影响.结果表明:玉米季施肥显著增加土壤N2O排放和反硝化损失,同时,各施肥处理间N2O排放与反硝化损失量差异显著.猪厩肥、猪厩肥配施氮磷钾肥、氮肥、氮磷钾肥和秸秆配施氮磷钾肥等处理的土壤N,O排放量分别为3.01、2.86、2.51、2.19和1.88 kg hm-2,分别占当季氮肥施用量的1.63％、1.53％、1.30％、1.09％和0.88％,反硝化损失量分别为6.74、6.11、5.23、4.69和4.12 kg hm-2,分别占当季氮肥施用量的3.97％、3.55％、2.97％、2.61％和2.23％,不施肥土壤的N2O排放量和反硝化损失量仅为0.56和0.78 kg hm-2.施肥是紫色土玉米生长前期(2周内)土壤N2O排放和反硝化速率出现高峰的主要驱动因子,土壤铵态氮和硝态氮含量是影响土壤N2O排放、土壤硝化和反硝化作用的限制因子,土壤含水量是重要影响因子,降雨是主要促发因素.土壤N2O排放量与反硝化损失量的比值介于0.45 ～0.72之间,土壤反硝化损失量极显著高于土壤N2O排放量,说明土壤反硝化作用是紫色土玉米生长季氮肥损失的重要途径.     

液面上部气体的置换对土壤厌氧培养下的N2O和CO2排放的影响

To study effect of C2H2 and change of headspace gas on N2O emission,denitrification,as well as CO2 emission,slurries of an agricultural soil were anaerobically incubated for 7 days at 25℃.Both N2O reduction and CO2 emissions were inhibited by the addition of 100 mL L^-1 of C2H2.However,the inhibition to CO2 emission was alleviated by the replacement of headspace gas,and the N2O emission was enhanced by the replacement.Acetylene disappeared evidently from the soil slurries during the incubation.Consequently results obtained from the traditional C2H2 blocking technique for determination of denitrifcation rate,especially in a long-time incubation,should be explained with care because of its side effect exsting in the incubation environments without change of headspace gas.To reduce the possible side effect on the processes other than denitrification ,it is suggested that headspace gas should be replaced several times during a long-time incubation.     

Elevated CO2 stimulates N2O emissions in permanent grassland

To evaluate climate forcing under increasing atmospheric CO₂ concentrations, feedback effects on greenhouse gases such as nitrous oxide (N₂O) with a high global warming potential should be taken into account. This requires long-term N₂O flux measurements because responses to elevated CO₂ may vary throughout annual courses. Here, we present an almost 9 year long continuous N₂O flux data set from a free air carbon dioxide enrichment (FACE) study on an old, N-limited temperate grassland. Prior to the FACE start, N₂O emissions were not different between plots that were later under ambient (A) and elevated (E) CO₂ treatments, respectively. However, over the entire experimental period (May 1998–December 2006), N₂O emissions more than doubled under elevated CO₂ (0.90 vs. 2.07 kg N₂O-N ha⁻¹ y⁻¹ under A and E, respectively). The strongest stimulation occurred during vegetative growth periods in the summer when soil mineral N concentrations were low. This was surprising because based on literature we had expected the highest stimulation of N₂O emissions due to elevated CO₂ when mineral N concentrations were above background values (e.g. shortly after N application in spring). N₂O emissions under elevated CO₂ were moderately stimulated during late autumn–winter, including freeze–thaw cycles which occurred in the 8th winter of the experiment. Averaged over the entire experiment, the additional N₂O emissions caused by elevated CO₂ equaled 4738 kg CO₂-equivalents ha⁻¹, corresponding to more than half a ton (546 kg) of CO₂ ha⁻¹ which has to be sequestered annually to balance the CO₂-induced N₂O emissions. Without a concomitant increase in C sequestration under rising atmospheric CO₂ concentrations, temperate grasslands may be converted into greenhouse gas sources by a positive feedback on N₂O emissions. Our results underline the need to include continuous N₂O flux measurements in ecosystem-scale CO₂ enrichment experiments.     

The effect of growing soybean (Glycine max. L.) on N2O emission from soil

A pot experiment was conducted to investigate the effect of growing soybean on N₂O emission from soil. When soybean was growing in pots, the cumulative N₂O emission during the growing season was 2.26 mg N pot⁻¹, which was 5.9 times greater than that from the identical but unplanted pots (CK). However, the difference in N₂O fluxes between the two treatments was not significant until the grain-filling stage. Of the total N₂O emission, 94% took place during the period from grain-filling to ripening. Premature harvesting of the aerial parts of the plants at various growth stages substantially stimulated N₂O emission from the soil. These results implied that the process of symbiotic N fixation per se does not stimulate N₂O production or emission, but rather senescence and decomposition of the roots and nodules in the late growth stage. Therefore, additional N₂O would be emitted from the soil after harvesting of soybean with roots, litter, and residues left in situ.     

Dinitrogen emissions and the N2:N2O emission ratio of a Rendzic Leptosol as influenced by pH and forest thinning

Reduction of nitrous oxide (N₂O) to dinitrogen (N₂) by denitrification in soils is of outstanding ecological significance since it is the prevailing natural process converting reactive nitrogen back into inert molecular dinitrogen. Furthermore, the extent to which N₂O is reduced to N₂ via denitrification is a major regulating factor affecting the magnitude of N₂O emission from soils. However, due to methodological problems in the past, extremely little information is available on N₂ emission and the N₂:N₂O emission ratio for soils of terrestrial ecosystems. In this study, we simultaneously determined N₂ and N₂O emissions from intact soil cores taken from a mountainous beech forest ecosystem. The soil cores were taken from plots with distinct differences in microclimate (warm-dry versus cool-moist) and silvicultural treatment (untreated control versus heavy thinning). Due to different microclimates, the plots showed pronounced differences in pH values (range: 6.3–7.3). N₂O emission from the soil cores was generally very low (2.0 ± 0.5–6.3 ± 3.8 μg N m⁻² h⁻¹ at the warm-dry site and 7.1 ± 3.1–57.4 ± 28.5 μg N m⁻² h⁻¹ at the cool-moist site), thus confirming results from field measurements. However, N₂ emission exceeded N₂O emission by a factor of 21 ± 6–220 ± 122 at the investigated plots. This illustrates that the dominant end product of denitrification at our plots and under the given environmental conditions is N₂ rather than N₂O. N₂ emission showed a huge variability (range: 161 ± 64–1070 ± 499 μg N m⁻² h⁻¹), so that potential effects of microclimate or silvicultural treatment on N₂ emission could not be identified with certainty. However, there was a significant effect of microclimate on the magnitude of N₂O emission as well as on the mean N₂:N₂O emission ratio. N₂:N₂O emission ratios were higher and N₂O emissions were lower for soil cores taken from the plots with warm-dry microclimate as compared to soil cores taken from the cool-moist microclimate plots. We hypothesize that the increase in the N₂:N₂O emission ratio at the warm-dry site was due to higher N₂O reductase activity provoked by the higher soil pH value of this site. Overall, the results of this study show that the N₂:N₂O emission ratio is crucial for understanding the regulation of N₂O fluxes of the investigated soil and that reliable estimates of N₂ emissions are an indispensable prerequisite for accurately calculating total N gas budgets for the investigated ecosystem and very likely for many other terrestrial upland ecosystems as well.