农田和森林土壤中氧化亚氮的产生与还原

采用土壤淤浆方法对丹麦农田和山毛榉森林土壤反硝化过程中Ｎ２Ｏ的产生与还原进行了研究。同时考察了硝酸根和铵离子对反硝化作用的影响。结果表明,森林土壤反硝化活性大于农田土壤,但农田土壤中Ｎ２Ｏ还原活性大于森林土壤,表现在农田和森林土壤中Ｎ２Ｏ／Ｎ２的产生比率分别为０．１１和３．６５。硝酸根和铵离子能促进两种土壤中的Ｎ２Ｏ产生,但可降低农田土壤中的Ｎ２Ｏ还原速率,与农田土壤相比,硝酸根可降低森林土壤Ｎ２     

Nitrous oxide production by the ectomycorrhizal fungi Paxillus involutus and Tylospora fibrillosa

Nitrous oxide (N(2)O) production by filamentous fungi has been demonstrated in pure culture and has been estimated indirectly in soils. However, it is unknown whether ectomycorrhizal fungi can also produce N(2)O. We demonstrate for the first time the ability of nitrogen (N)-tolerant ectomycorrhizal fungi (Paxillus involutus and Tylospora fibrillosa), found in forest soils under moderate to high rates of N deposition, to produce N(2)O from nitrate reduction. The N(2)O concentrations from the ectomycorrhizal fungal treatments after a 10-day pure culture experiment were 0.0117±0.00015 (P. involutus) and 0.0114±0.0003 (T. fibrillosa), and 0.0114±0.00043 μmol N(2)O L(-1) from a known fungal denitrifier (Fusarium lichenicola). No N(2)O was detected in the control treatment. Our results indicate the potential for these two N-tolerant ectomycorrhizal fungi to contribute to N(2)O production. Given that these species are abundant in many forest soils, the strength and regulation of fungal N(2)O production should now be verified in situ.     

设施菜田与棚外粮田土壤菌群和反硝化气体产生的比较分析

【目的】对比设施菜田与棚外粮田土壤菌群以及N2O产生模式的差异。【方法】采用变性梯度凝胶电泳(DGGE)和反硝化功能基因(nirS,nosZ)方法分别比较两种土壤细菌群落以及功能基因类群丰度的差异,利用自动连续在线培养监测体系(Robot系统)测定两种土壤在好氧、厌氧阶段N2O等反硝化相关气态产物产生模式,分析N2O/(N2+N2O+NO)产物比。【结果】设施菜田与棚外粮田具有不同的土壤细菌群落结构,并且土壤细菌总量得到了显著的提升,然而两种反硝化功能基因(nirS,nosZ)丰度并没有显著变化。与设施菜田相比,棚外粮田有相对低的N2O积累量以及产物比,并且在厌氧初期气体产生模式有所不同。培养后铵态氮和亚硝态氮含量上升。【结论】设施菜田长期有别于棚外粮田的管理方式造成了土壤细菌群落的显著改变,增大了活跃微生物总量,造成土壤酸化,并导致N2O在气态产物中的比例升高。设施菜田土壤微生物进行了与棚外粮田不同的硝酸盐呼吸过程,异化硝酸盐还原成铵(DNRA)过程有可能贡献了两种土壤的部分厌氧N2O产生量。     

应用DNDC模型分析施肥和翻耕方式变化对旱田土壤N_2O释放的潜在影响

以贵州省玉米 油菜轮作田和大豆 冬小麦轮作田为N2 O释放通量测量对象 ,根据DNDC模型能较好地拟合田间N2 O释放通量季节变化及施肥和翻耕对释放的影响 ,采用DNDC模型定量探讨了上述作物生长季节有机肥施用量、N肥施用量及施肥日期、N肥类型和施肥深度、翻耕深度和翻耕日期等变化对亚热带旱田生态系统N2 O释放的潜在影响 .为今后通过改变施肥和耕作方式来控制农业土壤N2 O释放量提供研究基础和参考 .     

Selective inhibition of ammonium oxidation and nitrification-linked n(2)o formation by methyl fluoride and dimethyl ether

Methyl fluoride (CH(3)F) and dimethyl ether (DME) inhibited nitrification in washed-cell suspensions of Nitrosomonas europaea and in a variety of oxygenated soils and sediments. Headspace additions of CH(3)F (10% [vol/vol]) and DME (25% [vol/vol]) fully inhibited NO(2) and N(2)O production from NH(4) in incubations of N. europaea, while lower concentrations of these gases resulted in partial inhibition. Oxidation of hydroxylamine (NH(2)OH) by N. europaea and oxidation of NO(2) by a Nitrobacter sp. were unaffected by CH(3)F or DME. In nitrifying soils, CH(3)F and DME inhibited N(2)O production. In field experiments with surface flux chambers and intact cores, CH(3)F reduced the release of N(2)O from soils to the atmosphere by 20- to 30-fold. Inhibition by CH(3)F also resulted in decreased NO(3) + NO(2) levels and increased NH(4) levels in soils. CH(3)F did not affect patterns of dissimilatory nitrate reduction to ammonia in cell suspensions of a nitrate-respiring bacterium, nor did it affect N(2)O metabolism in denitrifying soils. CH(3)F and DME will be useful in discriminating N(2)O production via nitrification and denitrification when both processes occur and in decoupling these processes by blocking NO(2) and NO(3) production.     

Nitrous oxide production and methane oxidation by different ammonia-oxidizing bacteria.

Ammonia-oxidizing bacteria (AOB) are thought to contribute significantly to N2O production and methane oxidation in soils. Most of our knowledge derives from experiments with Nitrosomonas europaea, which appears to be of minor importance in most soils compared to Nitrosospira spp. We have conducted a comparative study of levels of aerobic N2O production in six phylogenetically different Nitrosospira strains newly isolated from soils and in two N. europaea and Nitrosospira multiformis type strains. The fraction of oxidized ammonium released as N2O during aerobic growth was remarkably constant (0.07 to 0.1%) for all the Nitrosospira strains, irrespective of the substrate supply (urea versus ammonium), the pH, or substrate limitation. N. europaea and Nitrosospira multiformis released similar fractions of N2O when they were supplied with ample amounts of substrates, but the fractions rose sharply (to 1 to 5%) when they were restricted by a low pH or substrate limitation. Phosphate buffer (versus HEPES) doubled the N2O release for all types of AOB. No detectable oxidation of atmospheric methane was detected. Calculations based on detection limits as well as data in the literature on CH4 oxidation by AOB bacteria prove that none of the tested strains contribute significantly to the oxidation of atmospheric CH4 in soils.     

Evidence for involvement of gut-associated denitrifying bacteria in emission of nitrous oxide (N(2)O) by earthworms obtained from garden and forest soils.

Earthworms (Aporrectodea caliginosa, Lumbricus rubellus, and Octolasion lacteum) obtained from nitrous oxide (N(2)O)-emitting garden soils emitted 0.14 to 0.87 nmol of N(2)O h(-1) g (fresh weight)(-1) under in vivo conditions. L. rubellus obtained from N(2)O-emitting forest soil also emitted N(2)O, which confirmed previous observations (G. R. Karsten and H. L. Drake, Appl. Environ. Microbiol. 63:1878-1882, 1997). In contrast, commercially obtained Lumbricus terrestris did not emit N(2)O; however, such worms emitted N(2)O when they were fed (i.e., preincubated in) garden soils. A. caliginosa, L. rubellus, and O. lacteum substantially increased the rates of N(2)O emission of garden soil columns and microcosms. Extrapolation of the data to in situ conditions indicated that N(2)O emission by earthworms accounted for approximately 33% of the N(2)O emitted by garden soils. In vivo emission of N(2)O by earthworms obtained from both garden and forest soils was greatly stimulated when worms were moistened with sterile solutions of nitrate or nitrite; in contrast, ammonium did not stimulate in vivo emission of N(2)O. In the presence of nitrate, acetylene increased the N(2)O emission rates of earthworms; in contrast, in the presence of nitrite, acetylene had little or no effect on emission of N(2)O. In vivo emission of N(2)O decreased by 80% when earthworms were preincubated in soil supplemented with streptomycin and tetracycline. On a fresh weight basis, the rates of N(2)O emission of dissected earthworm gut sections were substantially higher than the rates of N(2)O emission of dissected worms lacking gut sections, indicating that N(2)O production occurred in the gut rather than on the worm surface. In contrast to living earthworms and gut sections that produced N(2)O under oxic conditions (i.e., in the presence of air), fresh casts (feces) from N(2)O-emitting earthworms produced N(2)O only under anoxic conditions. Collectively, these results indicate that gut-associated denitrifying bacteria are responsible for the in vivo emission of N(2)O by earthworms and contribute to the N(2)O that is emitted from certain terrestrial ecosystems.     

Decreased N2O reduction by low soil pH causes high N2O emissions in a riparian ecosystem

Quantification of harmful nitrous oxide (N(2)O) emissions from soils is essential for mitigation measures. An important N(2)O producing and reducing process in soils is denitrification, which shows deceased rates at low pH. No clear relationship between N(2)O emissions and soil pH has yet been established because also the relative contribution of N(2)O as the denitrification end product decreases with pH. Our aim was to show the net effect of soil pH on N(2)O production and emission. Therefore, experiments were designed to investigate the effects of pH on NO(3)(-) reduction, N(2)O production and reduction and N(2) production in incubations with pH values set between 4 and 7. Furthermore, field measurements of soil pH and N(2)O emissions were carried out. In incubations, NO(3)(-) reduction and N(2) production rates increased with pH and net N(2)O production rate was highest at pH 5. N(2)O reduction to N(2) was halted until NO(3)(-) was depleted at low pH values, resulting in a built up of N(2)O. As a consequence, N(2)O:N(2) production ratio decreased exponentially with pH. N(2)O reduction appeared therefore more important than N(2)O production in explaining net N(2)O production rates. In the field, a negative exponential relationship for soil pH against N(2)O emissions was observed. Soil pH could therefore be used as a predictive tool for average N(2)O emissions in the studied ecosystem. The occurrence of low pH spots may explain N(2)O emission hotspot occurrence. Future studies should focus on the mechanism behind small scale soil pH variability and the effect of manipulating the pH of soils.     

Regulation of denitrification at the cellular level: a clue to the understanding of N2O emissions from soils

Denitrifying prokaryotes use NO(x) as terminal electron acceptors in response to oxygen depletion. The process emits a mixture of NO, N(2)O and N(2), depending on the relative activity of the enzymes catalysing the stepwise reduction of NO(3)(-) to N(2)O and finally to N(2). Cultured denitrifying prokaryotes show characteristic transient accumulation of NO(2)(-), NO and N(2)O during transition from oxic to anoxic respiration, when tested under standardized conditions, but this character appears unrelated to phylogeny. Thus, although the denitrifying community of soils may differ in their propensity to emit N(2)O, it may be difficult to predict such characteristics by analysis of the community composition. A common feature of strains tested in our laboratory is that the relative amounts of N(2)O produced (N(2)O/(N(2)+N(2)O) product ratio) is correlated with acidity, apparently owing to interference with the assembly of the enzyme N(2)O reductase. The same phenomenon was demonstrated for soils and microbial communities extracted from soils. Liming could be a way to reduce N(2)O emissions, but needs verification by field experiments. More sophisticated ways to reduce emissions may emerge in the future as we learn more about the regulation of denitrification at the cellular level.     

Genetic characterization of denitrifier communities with contrasting intrinsic functional traits

Microorganisms capable of denitrification are polyphyletic and exhibit distinct denitrification regulatory phenotypes (DRP), and thus, denitrification in soils could be controlled by community composition. In a companion study (D?rsch et al., 2012) and preceding work, ex situ denitrification assays of three organic soils demonstrated profoundly different functional traits including N(2) O/N(2) ratios. Here, we explored the composition of the underlying denitrifier communities by analyzing the abundance and structure of denitrification genes (nirK, nirS, and nosZ). The relative abundance of nosZ (vs. nirK + nirS) was similar for all communities, and hence, the low N(2) O reductase activity in one of the soils was not because of the lack of organisms with this gene. Similarity in community composition between the soils was generally low for nirK and nirS, but not for nosZ. The community with the most robust denitrification (consistently low N(2) O/N(2) ) had the highest diversity/richness of nosZ and nirK, but not of nirS. Contrary results found for a second soil agreed with impaired denitrification (low overall denitrification activity, high N(2) O/N(2) ). In conclusion, differences in community composition and in the absolute abundance of denitrification genes clearly reflected the functional differences observed in laboratory studies and may shed light on differences in in situ N(2) O emission of the soils.     

Soil microorganisms as controllers of atmospheric trace gases (H2, CO, CH4, OCS, N2O, and NO).

Production and consumption processes in soils contribute to the global cycles of many trace gases (CH4, CO, OCS, H2, N2O, and NO) that are relevant for atmospheric chemistry and climate. Soil microbial processes contribute substantially to the budgets of atmospheric trace gases. The flux of trace gases between soil and atmosphere is usually the result of simultaneously operating production and consumption processes in soil: The relevant processes are not yet proven with absolute certainty, but the following are likely for trace gas consumption: H2 oxidation by abiontic soil enzymes; CO cooxidation by the ammonium monooxygenase of nitrifying bacteria; CH4 oxidation by unknown methanotrophic bacteria that utilize CH4 for growth; OCS hydrolysis by bacteria containing carbonic anhydrase; N2O reduction to N2 by denitrifying bacteria; NO consumption by either reduction to N2O in denitrifiers or oxidation to nitrate in heterotrophic bacteria. Wetland soils, in contrast to upland soils are generally anoxic and thus support the production of trace gases (H2, CO, CH4, N2O, and NO) by anaerobic bacteria such as fermenters, methanogens, acetogens, sulfate reducers, and denitrifiers. Methane is the dominant gaseous product of anaerobic degradation of organic matter and is released into the atmosphere, whereas the other trace gases are only intermediates, which are mostly cycled within the anoxic habitat. A significant percentage of the produced methane is oxidized by methanotrophic bacteria at anoxic-oxic interfaces such as the soil surface and the root surface of aquatic plants that serve as conduits for O2 transport into and CH4 transport out of the wetland soils. The dominant production processes in upland soils are different from those in wetland soils and include H2 production by biological N2 fixation, CO production by chemical decomposition of soil organic matter, and NO and N2O production by nitrification and denitrification. The processes responsible for CH4 production in upland soils are completely unclear, as are the OCS production processes in general. A problem for future research is the attribution of trace gas metabolic processes not only to functional groups of microorganisms but also to particular taxa. Thus, it is completely unclear how important microbial diversity is for the control of trace gas flux at the ecosystem level. However, different microbial communities may be part of the reason for differences in trace gas metabolism, e.g., effects of nitrogen fertilizers on CH4 uptake by soil; decrease of CH4 production with decreasing temperature; or different rates and modes of NO and N2O production in different soils and under different conditions.     

Plant uptake of nitrogen and phosphorus among grassland species affected by drought along a soil available phosphorus gradient

Plant and Soil - Environmental factors controlling nitrous oxide (N2O) uptake in forest soils are poorly known, and the atmospheric impact of the forest N2O sink is not well constrained compared to...     

Nitrous oxide emissions from drained organic forest soils––an up-scaling based on C:N ratios

Total emissions of N₂O from drained organic forest soils in Sweden were estimated using an equation linking the C:N ratio of the soil to N₂O emissions. Information on soil C:N ratios was derived from a national database. It was estimated that the emissions from Histosols amount to 2,820 tonnes N₂O a⁻¹. This is almost five times the value calculated for the same soils using the method suggested by the Intergovernmental Panel on Climate Change: 580 tonnes N₂O a⁻¹. The higher value in the present study can mainly be explained by improved accuracy of estimates of N₂O emissions from nutrient-rich soils, including former agricultural soils. In Sweden, in addition to 0.94 Mha of drained Histosols, there are 0.55 Mha of other types of drained organic soils. The annual emissions from these soils were estimated to amount to 1,890 tonnes of N₂O. The total emission value calculated for drained organic forest soils was thus 4,700 tonnes N₂O a⁻¹, which, if added, would increase the current estimate of the Swedish anthropogenic N₂O source strength by 18%. Of these emissions, 88% occur from sites with C:N ratios lower than 25. The exponential relationship between C:N ratio and N₂O emissions, in combination with a scarcity of data, resulted in large confidence intervals around the estimates. However, by using the C:N ratio-based method, N₂O emission estimates can be calculated from a variable that is readily available in databases. Also, the recent findings that there are exceptionally large emissions of N₂O from the most nitrogen-rich drained organic forest soils are taken into account.     

Linking soil N2O emissions with soil microbial community abundance and structure related to nitrogen cycle in two acid forest soils

Plant and Soil - Tree species and seasonal change influence N2O flux and microbial communities, but the mechanisms are unclear. We studied N2O flux in soils planted with slash pine and oil-seed...     

A comparison of NO and N2O production by the autotrophic nitrifier Nitrosomonas europaea and the heterotrophic nitrifier Alcaligenes faecalis.

Soil microorganisms are important sources of the nitrogen trace gases NO and N2O for the atmosphere. Present evidence suggests that autotrophic nitrifiers such as Nitrosomonas europaea are the primary producers of NO and N2O in aerobic soils, whereas denitrifiers such as Pseudomonas spp. or Alcaligenes spp. are responsible for most of the NO and N2O emissions from anaerobic soils. It has been shown that Alcaligenes faecalis, a bacterium common in both soil and water, is capable of concomitant heterotrophic nitrification and denitrification. This study was undertaken to determine whether heterotrophic nitrification might be as important a source of NO and N2O as autotrophic nitrification. We compared the responses of N. europaea and A. faecalis to changes in partial O2 pressure (pO2) and to the presence of typical nitrification inhibitors. Maximal production of NO and N2O occurred at low pO2 values in cultures of both N. europaea (pO2, 0.3 kPa) and A. faecalis (pO2, 2 to 4 kPa). With N. europaea most of the NH4+ oxidized was converted to NO2-, with NO and N2O accounting for 2.6 and 1% of the end product, respectively. With A. faecalis maximal production of NO occurred at a pO2 of 2 kPa, and maximal production of N2O occurred at a pO2 of 4 kPa. At these low pO2 values there was net nitrite consumption. Aerobically, A. faecalis produced approximately the same amount of NO but 10-fold more N2O per cell than N. europaea did. Typical nitrification inhibitors were far less effective for reducing emissions of NO and N2O by A. faecalis than for reducing emissions of NO and N2O by N. europaea.(ABSTRACT TRUNCATED AT 250 WORDS)     

The biogeochemical controls of N2O production and emission in landfill cover soils: the role of methanotrophs in the nitrogen cycle

Emissions of N2O from cover soils of both abandoned (> 30 years) and active landfills greatly exceed the maximum fluxes previously reported for tropical soils, suggesting high microbial activities for N2O production. Low soil matrix potentials (<-0.7 MPa) indicate that nitrification was the most likely mechanism of N2O formation during most of the time of sampling. Soil moisture had a strong influence on N2O emissions. The production of N2O was stimulated by as much as 20 times during laboratory incubations, when moisture was increased from -2.0 MPa to -0.6 MPa. Additional evidence from incubation experiments and delta13C analyses of fatty acids (18:1) diagnostic of methanotrophs suggests that N2O is formed in these soils by nitrification via methanotrophic bacteria. In a NH3(g)-amended landfill soil, the rate of N2O production was significantly increased when incubated with 100 ppmv methane compared with 1.8 ppmv (atmospheric) methane. Preincubation of a landfill soil with 1% CH4 for 2 weeks resulted in higher rates of N2O production when subsequently amended with NH3(g) relative to a control soil preincubated without CH4. At one location, at the soil depth (9-16 cm) of maximum methane consumption and N2O production, we observe elevated concentrations of organic carbon and nitrogen and distinct minima in delta15N (+1.0%) and delta13C (-33.8%) values for organic nitrogen and organic carbon respectively. A delta13C value of -39.3% was measured for 18:1 carbon fatty acids in this soil, diagnostic of type II methanotrophs. The low delta15N value for organic nitrogen is consistent with N2 fixation by type II methanotrophs. These observations all point to a methanotrophic origin for the organic matter at this depth. The results of this study corroborate previous reports of methanotrophic nitrification and N2O formation in aqueous and soil environments and suggest a predominance of type II rather than type I or type X methanotrophs in this landfill soil.     

Simulation of Effects of Soils,Climate and Management on N<Subscript>2</Subscript>O Emission from Grasslands

Nitrous oxide (N₂O) is a potent greenhouse gas with a high contribution from agricultural soils and emissions that depend on soil type, climate, crops and management practices. The N₂O emissions therefore need to be included as an integral part of environmental assessments of agricultural production systems. An algorithm for N₂O production and emission from agricultural soils was developed and included in the FASSET whole-farm model. The model simulated carbon and nitrogen (N) turnover on a daily basis. Both nitrification and denitrification was included in the model as sources for N₂O production, and the N₂O emissions depended on soil microbial and physical conditions. The model was tested on experimental data of N₂O emissions from grasslands in UK, Finland and Denmark, differing in climatic conditions, soil properties and management. The model simulated the general time course of N₂O emissions and captured the observed effects of fertiliser and manure management on emissions. Scenario analyses for grazed and cut grasslands were conducted to evaluate the effects of soil texture, climatic conditions, grassland management and N fertilisation on N₂O emissions. The soils varied from coarse sand to sandy loam and the climatic variation was taken to represent the climatic variation within Denmark. N fertiliser rates were varied from 0 to 500 kg N ha⁻¹. The simulated N₂O emissions showed a non-linear response to increasing N rates with increasing emission factors at higher N rates. The simulated emissions increased with increasing soil clay contents. N₂O emissions were slightly increased at higher temperatures, whereas increasing annual rainfall generally lead to decreasing emissions. Emissions were slightly higher from grazed grasslands compared with cut grasslands at similar rates of total N input (fertiliser and animal excreta). The results indicate higher emission factors and thus higher potentials for reducing N₂O emissions for intensively grazed grasslands on fine textured soils than for extensive cut-based grasslands on sandy soils.     

多年冻土退化地区湿地土壤温室气体排放及其影响因子

采用野外原位实验静态箱-气相色谱法,研究了兴安岭多年冻土不同程度退化地区生长季湿地土壤温室气体CH4、CO2和N2O的排放通量特征,同时分析了环境因子对土壤温室气体排放的影响。结果表明:1)3种类型冻土区(季节性冻土区、岛状多年冻土区、连续多年冻土区,分别用D1、D2、D3表示)土壤在生长季时期表现为CO2和N2O的源;D1和D3为CH4的源,D2为CH4的汇。D1、D2、D3土壤在生长季中平均CH4排放通量分别为(0.127±0.021)、(-0.020±0.006)、(0.082±0.019)mg·m^-2·h^-1;CO2排放通量分别为(371.50±66.73)、(318.43±55.67)、(213.19±37.05)mg·m^-2·h^-1;N2O排放通量分别为(24.05±2.62)、(8.07±2.42)、(2.17±0.25)μg·m-2·h-1。土壤CO2和N2O排放通量随多年冻土退化程度的加剧呈现出升高的趋势。2)细根生物量、凋落物生物量、全碳、全氮、可溶性有机碳、总可溶性氮、土壤容重、土壤温度、土壤含水量等均影响温室气体排放,3种不同类型冻土区土壤CH4、CO2和N2O的排放差异是诸多影响因子综合作用的结果。     

Biological sources and sinks of nitrous oxide and strategies to mitigate emissions

Nitrous oxide (N(2)O) is a powerful atmospheric greenhouse gas and cause of ozone layer depletion. Global emissions continue to rise. More than two-thirds of these emissions arise from bacterial and fungal denitrification and nitrification processes in soils, largely as a result of the application of nitrogenous fertilizers. This article summarizes the outcomes of an interdisciplinary meeting, 'Nitrous oxide (N(2)O) the forgotten greenhouse gas', held at the Kavli Royal Society International Centre, from 23 to 24 May 2011. It provides an introduction and background to the nature of the problem, and summarizes the conclusions reached regarding the biological sources and sinks of N(2)O in oceans, soils and wastewaters, and discusses the genetic regulation and molecular details of the enzymes responsible. Techniques for providing global and local N(2)O budgets are discussed. The findings of the meeting are drawn together in a review of strategies for mitigating N(2)O emissions, under three headings, namely: (i) managing soil chemistry and microbiology, (ii) engineering crop plants to fix nitrogen, and (iii) sustainable agricultural intensification.     

Characterization of N<Subscript>2</Subscript>O emissions and associated microbial communities from the ant mounds in soils of a humid tropical rainforest

Tropical rainforest soils harbor a considerable diversity of soil fauna that contributes to emissions of N₂O. Despite their ecological dominance, there is limited information available about the contribution of epigeal ant mounds to N₂O emissions in these tropical soils. This study aimed to determine whether ant mounds contribute to local soil N emissions in the tropical humid rainforest. N₂O emission was determined in vitro from individual live ants, ant-processed mound soils, and surrounding reference soils for two trophically distinct and abundant ant species: the leaf-cutting Atta mexicana and omnivorous Solenopsis geminata. The abundance of total bacteria, nitrifiers (AOA and AOB), and denitrifiers (nirK, nirS, and nosZ) was estimated in these soils using quantitative PCR, and their respective mineral N contents determined. There was negligible N₂O emission detected from live ant individuals. However, the mound soils of both species emitted significantly greater (3-fold) amount of N₂O than their respective surrounding reference soils. This emission increased significantly up to 6-fold in the presence of acetylene, indicating that, in addition to N₂O, dinitrogen (N₂) is also produced from these mound soils at an equivalent rate (N₂O/N₂?=?0.57). Functional gene abundance (nitrifiers and denitrifiers) and mineral N pools (ammonium and nitrate) were significantly greater in mound soils than in their respective reference soils. Furthermore, in the light of the measured parameters and their correlation trends, nitrification and denitrification appeared to represent the major N₂O-producing microbial processes in ant mound soils. The ant mounds were estimated to contribute from 0.1 to 3.7% of the total N₂O emissions of tropical rainforest soils.