土壤N2O和NO产生机制研究进展

N2O和NO是大气中两种重要的活性氮气体,强烈影响着全球变化和生态环境。土壤是N2O和NO的重要排放源,生物和非生物途径均可产生N2O和NO。本文详细论述了自养硝化、异养硝化、生物反硝化、化学反硝化、硝化细菌反硝化和硝态氮异化还原成铵作用产生N2O和（或）NO的机制,并对研究中存在的一些问题进行了探讨。     

农田土壤N2O排放的影响因素

氧化亚氮是大气湿室效应气体之一。本文概括论述影响农田土壤Ｎ２Ｏ排放的氧气、温度、土壤湿度和水分、有机质、土壤ｐＨ、微生物、土壤质地以及施肥等因素。     

免耕对农田土壤N_2O排放影响及作用机制研究进展

免耕在促进农业可持续发展和有效分馏大气碳的同时还可影响土壤N2O排放,但迄今为止关于免耕对农田土壤N2O排放影响的研究结果却不尽一致,正效应间或负效应都存在。本文综述了免耕条件下土壤理化性状和生物性状的变化及其对N2O排放的影响,并指出实施免耕后土壤反硝化强度变化程度的不同是导致免耕对N2O排放影响效应不同的主要原因,最后提出了一些有待研究的问题。     

农田土壤N2O排放和减排措施的研究进展

氧化亚氮(N2O)是一种受人类活动影响的重要温室气体。农业土壤是其主要的排放源之一,土壤中硝化和反硝化作用是N2O产生的主要过程。N2O的排放受多种因素的影响,农业活动尤其是施用化学氮肥是农田N2O排放量增加的主要因素。提高氮肥利用率,使用硝化抑制剂等措施将有助于减少N2O的排放量,更有效的减排措施还有待进一步的研究与应用。     

东北农田黑土N2O排放研究进展

农田是温室气体氧化亚氮（N2O）的重要排放源,位于东北地区的黑土地是我国重要的粮食生产基地。目前我国农田N2O排放增速正在放缓,但是东北黑土区仍在加快。针对我国东北黑土区的自身特点和N2O排放研究现状,本文综合分析了黑土N2O排放特征、产生过程与影响因素。结果表明,东北农田黑土N2O-N背景排放量平均为0.56±0.29 kg·hm?2,施用化肥黑土N2O-N平均排放量为1.49±1.09 kg·hm?2,化肥氮诱导的N2O排放系数（EF）为0.45%±0.42%。与中国旱地和世界其他黑土区相比,东北农田黑土的背景排放量和EF均处于较低水平。这是因为在正常降雨条件下,东北黑土N2O主要是由硝化作用产生,反硝化作用受到活性碳缺乏的限制。冻融过程则可能促进反硝化作用进行,诱导春融期N2O出现爆发式排放。与我国其他农田相比,东北黑土N2O排放研究明显不足,今后应加强对不同区域黑土N2O排放的原位观测,阐明冻融过程N2O的产生机制,评估黑土N2O排放对气候变化的响应;同时应加强研究秸秆还田、有机肥施用等措施对N2O排放的影响效应,从而制定出黑土地质量提升和N2O减排的双赢措施。     

外加可溶性碳源对华北典型农田土壤N2O、CO2排放的影响

以华北平原典型农田土壤为对象,运用静态培养系统研究方法,设置室内培养试验,研究添加不同浓度葡萄糖对土壤N2O、CO2排放的影响.结果表明:碳氮配施的外源添加方式明显促进N2O和CO2排放,其排放通量均高于对照组和只添加氮源的处理.在配施碳源葡萄糖浓度为0.5 g/kg时N2O排放通量最高(NH4+组2 500 μg/(kg·d),单位以N计,下同,NO3-组1 500 μg/(kg·d)),4.0 g/kg时N2O排放通量最低(NH4+组500 μg/(kg·d),NO3-组800 μg/(kg·d));葡萄糖浓度为2.0 g/kg时CO2排放通量最高(NH+组500mg/(kg· d)),0.5 g/kg时CO2排放通量最低(NH+组100 mg/(kg,d)).从培养开始到结束,只添加氮源的土壤NH+含量变化不明显,NO3-含量增至29.21 mg/kg(NH4+组)和62.25 mg/kg(NO3-组);而配施葡萄糖的土壤NH+含量降为不足1 mg/kg(NH4+组),NO3-含量明显减少.N2O累积排放通量与葡萄糖浓度呈负相关(NH4+组),CO2累积排放通量与葡萄糖浓度呈正相关.分析结果表明,外加可溶性碳源明显减少土壤中NH4+和NO3-含量,并且促进土壤N2O、CO2排放,其排放通量大小与C/N比有关.     

土壤质地对小麦和棉花田N2O排放的影响

１９９４年－１９９５年在中国科学院封丘生态试验站通过小区试验研究了土壤质地对小麦和棉花田Ｎ２Ｏ排放量的影响。结果表明,土壤质地明显影响小麦和棉花田Ｎ２Ｏ排放量,壤质土壤排放的Ｎ２Ｏ高于砂质和粘质土壤,小麦和棉花生长期壤质,砂质及粘质土壤的平均Ｎ２Ｏ排放通量分别为３７．９３,２３．８１,１２．９０及７０．３９,４５．８７,２７．８５μｇＮ２Ｏ－Ｎ／（ｍ＾２．ｈ）。     

不同施肥方式对农田土壤CO2和N2O排放的影响

采用静态箱/气相色谱法研究不同施肥方式以及环境因子对农田土壤CO2和N2O排放通量的影响,结果表明,不同施肥方式对农田土壤CO2排放的季节模式无明显影响,但是影响了N2O排放的季节模式。不同施肥方式对土壤CO2排放通量影响不明显,主要影响土壤N2O排放,整个小麦、玉米生长季,分两次施肥的F2与分四次施肥的F1相比,土壤N2O排放量增加,化肥配合有机肥施用(MF)的土壤N2O通量大于单纯的化肥处理,秸秆还田降低了土壤N2O的排放。相关分析结果表明,土壤CO2排放与大气温度、地表温度、土壤温度和土壤水分均呈显著正相关关系(P<0.01)。由于肥料施用的影响,土壤N2O排放和土壤温度、水分的相关分析并不显著。土壤N2O排放受土壤硝态氮和铵态氮变化的影响。     

秸秆还田对农田棕壤氧化亚氮排放动态的影响

通过田间试验,采用静态箱法对不同量秸秆还田(单施氮肥、半量秸秆还田、全量秸秆还田)下不同时期土壤氧化亚氮排放动态进行了研究。结果表明,和单施氮肥相比,秸秆覆盖还田提高了土壤氮素积累,也同时影响土壤N_2O排放动态。在玉米生长期间,半收获量秸秆还田增加了7.8%的N_2O排放,全量秸秆还田降低了2.2%的N_2O排放,与土壤铵态氮变化一致;玉米收获后,秸秆还田显著增加了N_2O排放,增幅分别为6.7%(半量秸秆还田)和22.6%(秸秆全量还田)。试验期间单施氮肥、半量秸秆还田和全量秸秆还田三个处理N_2O排放的年累计量分别为1183 g hm~(-2)、1269 g hm~(-2)和1294 g hm~(-2)。尽管秸秆还田增加了总的N_2O排放,但是,由于秸秆增加了土壤氮素的投入,当以N_2O损失的氮素占总氮素投入的比例进行估算时,全量秸秆还田损失率最低为0.52%、半量秸秆还田为0.57%、单施氮肥为0.59%。由此可见,秸秆还田在增加土壤肥力的同时,降低了单位氮素的氧化亚氮损失,有利于氮素在土壤中的保留。     

中国农田秸秆还田土壤N_2O排放及其影响因素的Meta分析

农田N2O排放是全球人为温室气体主要的来源之一,了解农作措施对其排放的影响对中国农田减排具有重要的意义。该研究采用Meta分析方法,定量分析了秸秆还田对中国农田土壤N2O排放的影响,并对其影响因素进行解析。研究结果表明,在中国不同区域秸秆还田对土壤N2O排放有一定的差异,其中华东地区显著减排18.61%(P0.05),而华中和华北地区则分别显著增加排放62.3%和27.73%(P0.05)。同时,施氮量介于0~240 kg/hm2(以N计,下同)时,随着施氮量的增加,秸秆还田对土壤N2O影响的效应值逐渐由负值增加为正值;当施氮量介于241~300 kg/hm2时,秸秆还田有显著降低土壤N2O排放的趋势。当土壤p H值介于6.5~7.5时,秸秆还田对N2O排放影响的效应值为正值;当黏粒质量分数为15%~25%时,秸秆还田对N2O排放影响的效应值为正值,当黏粒质量分数15%时,秸秆还田显著降低土壤N2O排放。秸秆的碳氮比与秸秆还田量对N2O的排放也有不同程度的影响,另外,秸秆还田下不同的种植制度间N2O的排放也有差异。因此,秸秆还田下实施农田N2O减排措施应综合考虑区域农业资源特点、种植制度、土壤类型和水肥管理因素。研究可为科学管理秸秆与减少农田N2O排放提出理论支撑。     

Effect of different biochar and fertilizer types on N2O and NO emissions

The use of biochar as soil improver and climate change mitigation strategy has gained much attention, although at present the effects of biochar on soil properties and greenhouse gas emissions are not completely understood. The objective of our incubation study was to investigate biochar's effect on N₂O and NO emissions from an agricultural Luvisol upon fertilizer (urea, NH₄Cl or KNO₃) application. Seven biochar types were used, which were produced from four different feedstocks pyrolyzed at various temperatures. At the end of the experiment, after 14 days of incubation, soil nitrate concentrations were decreased upon biochar addition in all fertilizer treatments by 6–16%. Biochar application decreased both cumulative N₂O (52–84%) and NO (47–67%) emissions compared to a corresponding treatment without biochar after urea and nitrate fertilizer application, and only NO emissions after ammonium application. N₂O emissions were more decreased at high compared to low pyrolysis temperature.Several hypotheses for our observations exist, which were assessed against current literature and discussed thoroughly. In our study, the decreased N₂O and NO emissions are expected to be mediated by multiple interacting phenomena such as stimulated NH₃ volatilization, microbial N immobilization, non-electrostatic sorption of NH₄⁺ and NO₃⁻, and biochar pH effects.     

N2O and NO production in various Chinese agricultural soils by nitrification

Eleven types of agricultural soils were collected from Chinese uplands and paddy fields to compare their N₂O and NO production by nitrification under identical laboratory conditions. Before starting the assays, all air-dried soils were preincubated for 4 weeks at 25 °C and 40% WFPS (water-filled pore space). The nitrification activities of soils were determined by adding (NH₄)₂SO₄ (200 mg N kg⁻¹ soil) and incubating for 3 weeks at 25 °C and 60% WFPS. The net nitrification rates obtained fitted one of two types of models, depending on the soil pH: a zero-order reaction model for acidic soils and one neutral soil (Group 0); or a first-order reaction model for one neutral soil and alkaline soils (Group 1). The results suggest that pH is the most important factor in determining the kinetics of soil nitrification from ammonium. In the Group 1 soils, initial emissions (i.e. during the first week) of N₂O and NO were 82.6 and 83.6%, respectively, of the total emissions during 3 weeks of incubation; in the Group 0 soils, initial emissions of N₂O and NO were 54.7 and 59.9%, respectively, of the total emissions. The net nitrification rate in the first week and second-third weeks were highly correlated with the initial and subsequent emissions (i.e. during the second and third weeks), respectively, of N₂O and NO. The average percentages of emitted (N₂O+NO)-N relative to net nitrification N in initial and subsequent periods were 2.76 and 0.59 for Group 0, and 1.47 and 0.44 for the Group 1, respectively. The initial and subsequent emission ratios of NO/N₂O from Group 0 (acidic) soils were 3.77 and 2.52 times, respectively, higher than those from Group 1 soils (P<0.05).     

Recovery of nitrification and production of NO and N2O after exposure of soil to acetylene

Incubation of soil under low partial pressures of acetylene (10 Pa) is a widely used method to specifically inhibit nitrification due to the suicide inhibition of ammonium monooxygenase (AMO), the first enzyme in NH₄ ⁺ oxidation by nitrifying bacteria. Although the inhibition of AMO is irreversible, recovery of activity is possible if new enzyme is synthesized. In experiments with three different soils, NH₄ ⁺ concentrations decreased and NO₃ ^– concentrations increased soon after acetylene was removed from the atmosphere. Recovery of NO production started immediately after the removal of acetylene. The release rates of NO and N₂O were higher in soil samples which were only preincubated with 10 Pa acetylene than in those which were kept in the presence of 10 Pa acetylene. In the permanent presence of 10 Pa acetylene, NH₄ ⁺ and NO₃ ^– concentrations stayed constant, and the release rates of NO and N₂O were low. These low release rates were apparently due to processes other than nitrification. Our experiments showed that the blockage of nitrification by low (10 Pa) acetylene partial pressures is only reliable when the soil is kept in permanent contact with acetylene. Received: 17 July 1996     

Temperature responses of NO and N2O emissions from boreal organic soil

Both NO and N₂O are produced in soil microbial processes and have importance in atmospheric physics and chemistry. In recent years several studies have shown that N₂O emissions from organic soils can be high at low temperatures. However, the effects of low temperature on NO emissions from soil are unknown. We studied in laboratory conditions, using undisturbed soil cores, the emissions of NO and N₂O from organic soils at various temperatures, with an emphasis on processes and emissions during soil freezing and thawing periods. We found no soil freezing- or thawing-related emission maxima for NO, while the N₂O emissions were higher both during soil freezing and thawing periods. The results suggest that different factors are involved in the regulation of NO and N₂O emissions at low temperatures.     

Contributions of nitrification and denitrification to N2O emissions from soils at different water-filled pore space

A combination of stable isotope and acetylene (0.01% v/v) inhibition techniques were used for the first time to determine N₂O production during denitrification, autotrophic nitrification and heterotrophic nitrification in a fertilised (200 kg N ha^–1) silt loam soil at contrasting (20–70%) water-filled pore space (WFPS). ¹⁵N-N₂O emissions from ¹⁴NH₄¹⁵NO₃ replicates were attributed to denitrification and ¹⁵N-N₂O from ¹⁵NH₄¹⁵NO₃ minus that from ¹⁴NH₄¹⁵NO₃ replicates was attributed to nitrification and heterotrophic nitrification in the presence of acetylene, as there was no dissimilatory nitrate reduction to ammonium or immobilisation and remineralisation of ¹⁵N-NO₃^–. All of the N₂O emitted at 70% WFPS (31.6 mg N₂O-N m^–2 over 24 days; 1.12 g N₂O-N g dry soil^–1; 0.16% of N applied) was produced during denitrification, but at 35–60% WFPS nitrification was the main process producing N₂O, accounting for 81% of ¹⁵N-N₂O emitted at 60% WFPS, and 7.9 g ¹⁵N-N₂O m^–2 (0.28 ng ¹⁵N-N₂O g dry soil^–1) was estimated to be emitted over 7 days during heterotrophic nitrification in the 50% WFPS treatment and accounted for 20% of ¹⁵N-N₂O from this treatment. Denitrification was the predominant N₂O-producing process at 20% WFPS (2.6 g ¹⁵N-N₂O m^–2 over 7 days; 0.09 ng ¹⁵N-N₂O g dry soil^–1; 85% of ¹⁵N-N₂O from this treatment) and may have been due to the occurrence of aerobic denitrification at this WFPS. Our results demonstrate the usefulness of a combined stable isotope and acetylene approach to quantify N₂O emissions from different processes and to show that several processes may contribute to N₂O emission from agricultural soils depending on soil WFPS.     

Influence of pH on the release of NO and N2O from fertilized and unfertilized soil

Summary NO and N₂O release rates were measured in an acidic forest soil (pH 4.0) and a slightly alkaline agricultural soil (pH 7.8) after the pH was adjusted to values ranging from pH 4.0 to 7.8. The total release of NO and N₂O during 20 h of incubation was determined together with the net changes in the concentrations of NH ₄ ⁺ , NO ₂ ^– and NO ₃ ^– in the soil. The release of NO and N₂O increased after fertilization with NH ₄ ⁺ and/or NO ₃ ^– ; it strongly decreased with increasing pH in the acidic forest soil; and it increased when the pH of the alkaline agricultural soil was decreased to pH 6.5. However, there was no simple correlation between NO and N₂O release or between these compounds and activities such as the NO ₂ ^– accumulation, NO ₃ ^– reduction, or NH ₄ ⁺ oxidation. We suggest that soil pH exerts complex controls, e.g., on microbial populations or enzyme activities involved in nitrification and denitrification.     

Compaction influences N2O and N2 emissions from 15N-labeled synthetic urine in wet soils during successive saturation/drainage cycles

Nitrous oxide emitted from urine patches is a key source of agricultural greenhouse gas emissions. A better understanding of the complex soil environmental and biochemical regulation of urine-N transformations in wet soils is needed to predict N₂O emissions from grazing and also to develop targeted mitigation technologies. Soil aeration, gas diffusion and drainage are key factors regulating N transformations and are affected by compaction during grazing. To understand how soil compaction from animal treading influences N transformations of urine in wet soils, we applied pressures of 0, 220 and 400 kPa to repacked soil cores, followed by ¹⁵N-labeled synthetic urine, and then subjected the cores to three successive saturation–drainage cycles on tension tables from 0 to 10 kPa.Compaction had a relatively small effect on soil bulk density (increasing from 0.81 to 0.88 Mg m⁻³), but strongly affected the pore size distribution. Compaction reduced both total soil porosity and macroporosity. It also affected the pore size distribution, principally by decreasing the proportion of 30–60 μm and 60–100 μm pores and increasing the proportion of micropores (<30 μm).Rates of urine-N transformations, emissions of N₂ and N₂O, and the N₂O to N₂ ratio were affected by the saturation/drainage cycles and degree of compaction. During the first saturation–drainage cycle, production of both N₂O and N₂ was low (<0.4 mg N m⁻² h⁻¹), probably because of anaerobic conditions inhibiting nitrification. In the second saturation/drainage cycle, the predominant product was N₂ at all compaction rates. By the third cycle, with increasing availability of mineral-N substrates, N₂O was the dominant product in the uncompacted (max = 4.70 mg N m⁻² h⁻¹) and 220 kPa compacted soils (max = 7.65 mg N m⁻² h⁻¹) with lower amounts of N₂ produced, while N₂ was produced in similar quantities to N₂O (max = 3.11 mg N m⁻² h⁻¹) in the 400 kPa compacted soil. Reduced macroporosity in the most compacted soil contributed to more sustained N₂ and N₂O production as the soils drained. In addition, compaction affected the rate of change of soil pH and DOC, both of which affected the N₂O to N₂ ratio.Denitrification during drainage and re-saturation may make a large contribution to soil N₂O emissions. Improving soil drainage and adopting grazing management practices that avoid soil compaction while increasing macroporosity will reduce total N₂O and N₂ emissions.     

N2O and NO emissions from an Andisol field as influenced by pelleted poultry manure

Animal manures from intensive livestock operations can be pelleted to improve handlings and recyclings of embodied nutrients. The aim of this study was to evaluate the influence of pelleted poultry manure on N₂O and NO fluxes from an Andisol field. In autumn 2006 and summer 2007, poultry manure (PM), pelleted poultry manure (PP), and chemical fertilizer (CF) were applied at a rate of 120 kg N ha⁻¹ in each cultivation period to Komatsuna (Brassica rapa var. peruviridis). Nitrous oxide and NO fluxes were measured using an automated monitoring system. A soil incubation experiment was also conducted to determine the influence of intact and ground pelleted manure on N₂O, NO, and CO₂ production with a water-filled pore space (WFPS) of 30 or 50%. In the field measurements, N₂O emission rates from the organic fertilizer treatments were larger than that from the CF treatment, possibly because organic C stimulated denitrification. The highest N₂O flux was observed from the PP treatment after a rainfall following fertilization, and the cumulative emission rate (2.72 ± 0.22 kg N ha⁻¹ y⁻¹) was 3.9 and 7.1 times that from the PM and CF treatments, respectively. In contrast, NO emission rates were highest from the CF treatment. The NO/N₂O flux ratio indicated that nitrification was the dominant process for NO and N₂O production from the CF treatment. Cumulative N₂O emission rates from all treatments were generally higher during the wetter cultivation period (autumn 2006) than during the drier cultivation period (summer 2007). In contrast, NO emission rates were higher in the drier than in the wetter cultivation period. The incubation experiment results showed a synergistic effect of soil moisture and the pelleted manure form on N₂O emission rates. The intact pelleted manure with the 50% WFPS treatment produced the highest N₂O and CO₂ fluxes and resulted in the lowest soil NO₃⁻ content after the incubation. These results indicate that anaerobic conditions inside the pellets, caused by rainfall and heterotrophic microbial activities, led to denitrification, resulting in high N₂O fluxes. Controlling the timing of N application by avoiding wet conditions might be one mitigation option to reduce N₂O emission rates from the PP treatment in this study field.     

The influence of winter soil cover on spring nitrous oxide emissions from an agricultural soil

In temperate regions, a majority of N₂O is emitted during spring soil thawing. We examined the influence of two winter field covers, snow and winter rye, on soil temperature and subsequent spring N₂O emissions from a New York corn field over two years. The first season (2006-07) was a cold winter (2309 h below 0 °C at 8 cm soil depth), historically typical for the region. The snow removal treatment resulted in colder soils and higher N₂O fluxes (73.3 vs. 57.9 ng N₂O-N cm⁻² h⁻¹). The rye cover had no effect on N₂O emissions. The second season (2007-08) was a much milder winter (1271 h below freezing at 8 cm soil depth), with lower N₂O fluxes overall. The winter rye cover resulted in lower N₂O fluxes (5.9 vs. 33.7 ng N₂O-N cm⁻² h⁻¹), but snow removal had no effect. Climate scenarios predict warmer temperature and less snow cover in the region. Under these conditions, spring N₂O emissions can be expected to decrease and could be further reduced by winter rye crops.