Trace amounts of O2 affect NO and N2O production during denitrifying enzyme activity (DEA) assays

We evaluated the potential of trace amounts of O₂ inadvertently introduced into anaerobic incubations to initiate the C₂H₂-catalyzed NO oxidation reaction and affect NO and N₂O production rates during denitrifying enzyme activity (DEA) assays. We measured the rate of NO production in the presence and absence of 10 kPa C₂H₂ and N₂O production in the presence of 10 kPa C₂H₂ by short-term incubations of slurries of humisol and sandy loam soil. NO production, in the absence of C₂H₂, was similar for both soils (0.73-1.32 ng NO-N g dry soil⁻¹ min⁻¹) and replicate measurements of NO production rates were linear and exhibited small standard deviations. It was not possible to consistently measure NO production in the presence of C₂H₂. Replicate measurements of NO production were always lower and exhibited a wide range of variability when C₂H₂ was present. The rate of NO production in the absence of C₂H₂ was only 2.3-3.0% of the rate of N₂O production in the presence of C₂H₂ in humisol soil but was much larger (31.8-35.0%) in the sandy loam soil. Rates of NO production from sandy loam soil were reduced by 58% when trace amounts (30 μl) of O₂ were added to slurry incubations containing C₂H₂. We conclude that trace amounts of O₂ inadvertently introduced into the slurries during sampling could initiate scavenging of NO by the C₂H₂-catalyzed NO oxidation reaction and cause an underestimate of N₂O production during DEA assays in some soils.     

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.     

Diffusion of N-labelled N2O into soil columns: a promising method to examine the fate of N2O in subsoils

Nitrous oxide research has generally focused directly on measuring fluxes of N₂O from the soil surface. The fate of N₂O in the subsoil has often been placed in the ‘too hard’ basket. However, determining the production, fate and movement of N₂O in the subsoil is vital in fully understanding the sources of surface fluxes and in compiling accurate inventories for N₂O emissions. The aim of this study was to generate and introduce into soil columns ¹⁵N labelled N₂O, and to try and determine the consumption of the ¹⁵N₂O and production of ambient N₂O. Columns, 100 cm long by 15 cm diameter, were repacked with sieved soil (sampled from 0 to 5 cm depth) and instrumented with silicone rubber gas sampling ports. Nitrous oxide enriched with ¹⁵N was generated using a thermal decomposition process at 300 °C and then transferred to 2 l flasks. After equilibrating with SF₆ tracer gas the ¹⁵N₂O was introduced into the soil columns via passive diffusion. Gas samples from the soil profile and headspace flux were taken over a 12-day period. A watering event was simulated to perturb the ¹⁵N₂O gas composition in the soil profile. Using the measured ¹⁵N enriched fluxes and the rate of decline in ¹⁵N in the N₂O reservoir, from which the N₂O diffused into the soil, we calculated an N₂O sink (consumption plus absorption by water) equal to 0.48 ng N₂O g⁻¹ soil h⁻¹. The decrease in the ¹⁵N enrichment between successive soil depths indicated N₂O production in the soil profile and we calculated a net N₂O production rate of 0.88 ng N₂O g⁻¹ soil h⁻¹. This pilot study demonstrated the potential for simultaneously measuring both N₂O consumption and production rates, using the ¹⁵N enrichment of the N₂O measured. Further potential refinements of the methodology are discussed.     

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.     

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).     

Carbon and oxygen controls on N2O and N2 production during nitrate reduction

Here we provide evidence that the form of carbon compound and O₂ concentration exert an inter-related regulation on the production and reduction of N₂O in soil. 6.7 mM d-glucose, 6.7 mM D-mannitol, 8 mM L-glutamic acid or 10 mM butyrate (all equivalent to 0.48 g C l⁻¹) were applied to slurries of a sandy loam soil. At the start of the experiment headspace O₂ concentrations were established at ∼2%, 10% and 21% O₂ v/v for each C treatment, and 2 mM K¹⁵NO₃ (25 atom % excess ¹⁵N) was applied, enabling quantification of ¹⁵N-N₂ production, ¹⁵N-(N₂O-to-N₂) ratios and DNRA. The form of C compound was most important in the initially oxic (21% O₂ v/v) soils, where addition of butyrate and glutamic acid resulted in greater N₂O production (0.61 and 0.3 μg N₂O-N g⁻¹ soil for butyrate and glutamic acid, respectively) than the addition of carbohydrates (glucose and mannitol). Although, there was no significant effect of C compound at low initial O₂ concentrations (∼2% O₂ v/v), production of ¹⁵N-N₂ was greatest where headspace O₂ concentrations were initially, or fallen to, ∼2% O₂ v/v, with greatest reduction of N₂O and lowering ¹⁵N-(N₂O-to-N₂) ratios (∼0-0.27). This may reflect that the effect of C is indirect through stimulation of heterotrophic respiration, lowering O₂ concentrations, providing sub-oxic conditions for dissimilatory nitrate reduction pathways. Addition of carbohydrates (glucose and mannitol) also resulted in greatest recovery of ¹⁵N in NH₄⁺ from applied ¹⁵N-NO₃⁻, indicative of the occurrence of DNRA, even in the slurries with initial 10% and 21% O₂ v/v concentrations. Our ¹⁵N approach has provided the first direct evidence for enhancement of N₂O reduction in the presence of carbohydrates and the dual regulation of C compound and O₂ concentration on N₂O production and reduction, which has implications for management of N₂O emissions through changing C inputs (exudates, rhizodeposition, residues) with plant species of differing C traits, or through plant breeding.     

植物排放N2O研究进展

氧化亚氮(N_2O)是主要的温室气体之一,对大气环境质量与全球气候变化具有重要的影响。N_2O排放不仅增加温室效应,同时也会导致陆地生态系统氮损失与平流层臭氧消耗。长期以来土壤被认为是陆地生态系统N_2O的主要排放源,但近年来越来越多的证据表明,植物可能是陆地生态系统N_2O排放的另一重要来源。近年来有关植物排放N_2O的报道逐年增多,但对植物排放N_2O的途径及其调控机制方面还缺乏文献综述。本文首先在总结长期以来人们普遍认为的N_2O源与汇的基础上,提出陆地植物可能是另一个尚未被广泛认可的重要的N_2O的排放源。植物排放N_2O可能有两种潜在途径:1)植物作为土壤中通过微生物产生的N_2O的运输通道,2)植物通过自身代谢或内生菌的作用产生N_2O并排放到大气中。然后分析了关键因素(养分、光照、温度和植物器官及生长阶段)对植物排放N_2O的影响机制。最后指出未来需进一步探明植物体内产生N_2O的具体途径及其对全球N_2O排放的贡献,重点是探明植物自身的生理生化过程以及与其伴生、共生的微生物在N_2O产生中的作用。     

Effect of management, climate and soil conditions on N2O and NO emissions from an arable crop rotation using high temporal resolution measurements

Quantifying the nitrous oxide (N₂O) and nitric oxide (NO) fluxes emitted from croplands remains a major challenge. Field measurements in different climates, soil and agricultural conditions are still scarce and emissions are generally assessed from a small number of measurements. In this study, we report continuously measured N₂O and NO fluxes with a high temporal resolution over a 2-year crop sequence of barley and maize in northern France. Measurements were carried out using 6 automatic chambers at a rate of 16 mean flux measurements per day. Additional laboratory measurements on soil cores were conducted to study the response of NO and N₂O emissions to environmental conditions.The detection limit of the chamber setup was found to be 3 ng N m⁻² s⁻¹ for N₂O and 0.1 ng N m⁻² s⁻¹ for NO. Nitrous oxide fluxes were higher than the threshold 37% of the time, while they were 72% of the time for NO fluxes.The cumulated annual NO and N₂O emissions were 1.7 kg N₂O-N ha⁻¹ and 0.5 kg NO-N ha⁻¹ in 2007, but 2.9 kg N₂O-N ha⁻¹ and 0.7 kg NO-N ha⁻¹ in 2008. These inter-annual differences were largely related to crop types and to their respective management practices. The forms, amounts and timing of nitrogen applications and the mineralization of organic matter by incorporation of crop residues were found to be the main factor controlling the emissions peaks. The inter-annual variability was also due to different weather conditions encountered in 2007 and 2008. In 2007, the fractioned N inputs applied on barley (54 kg ha⁻¹ in March and in April) did not generate N₂O emissions peaks because of the low rainfall during the spring. However, the significant rainfall observed in the summer and fall of 2007, promoted rapid decomposition of barley residues which caused high levels of N₂O emissions. In 2008, the application of dairy cattle slurry and mineral fertilizer before the emergence of maize (107 kg N_min ha⁻¹ or 130 kg N_tot ha⁻¹ in all) coincided with large rainfalls promoting both NO and N₂O emissions, which remained high until early summer.Laboratory measurements corroborated the field observations: NO fluxes were maximum at a water-filled pore space (WFPS) of around 27% while N₂O fluxes were optimal at 68% WFPS, with a maximum potentially 14 times larger than for NO.     

Nitrification controls N2O production rates in a frozen boreal forest soil

The winter season has been identified as a significant contributor to N₂O emissions from boreal soils, but our understanding of the processes regulating these emissions is fragmentary. We investigated potential N-sources and pathways involved in N₂O formation in a frozen boreal forest soil by labeling soil samples with ¹⁵N-containing substrates, and measured rates of ¹⁵N₂O/¹⁵N₂ formation under both oxic and anoxic conditions. Our results showed that all N₂O produced in the frozen samples originate from denitrification, but the rate-limiting factor is NO₃⁻ availability, which is largely governed by nitrification. This suggests that N₂O formation in frozen boreal soils may be sustained for a prolonged period of time, but is governed by a delicate balance of the O₂ regime.     

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.     

CH4 oxidation and N2O emissions at varied soil water-filled pore spaces and headspace CH4 concentrations

Emission of N₂O and CH₄ oxidation rates were measured from soils of contrasting (30-75%) water-filled pore space (WFPS). Oxidation rates of ¹³C-CH₄ were determined after application of 10 μl ¹³C-CH₄ l⁻¹ (10 at. % excess ¹³C) to soil headspace and comparisons made with estimates from changes in net CH₄ emission in these treatments and under ambient CH₄ where no ¹³C-CH₄ had been applied. We found a significant effect of soil WFPS on ¹³C-CH₄ oxidation rates and evidence for oxidation of 2.2 μg ¹³C-CH₄ d⁻¹ occurring in the 75% WFPS soil, which may have been either aerobic oxidation occurring in aerobic microsites in this soil or anaerobic CH₄ oxidation. The lowest ¹³C-CH₄ oxidation rate was measured in the 30% WFPS soil and was attributed to inhibition of methanotroph activity in this dry soil. However, oxidation was lowest in the wetter soils when estimated from changes in concentration of ¹²⁺¹³C-CH₄. Thus, both methanogenesis and CH₄ oxidation may have been occurring simultaneously in these wet soils, indicating the advantage of using a stable isotope approach to determine oxidation rates. Application of ¹³C-CH₄ at 10 μl ¹³C-CH₄ l⁻¹ resulted in more rapid oxidation than under ambient CH₄ conditions, suggesting CH₄ oxidation in this soil was substrate limited, particularly in the wetter soils. Application of and (80 mg N kg soil⁻¹; 9.9 at.% excess ¹⁵N) to different replicates enabled determination of the respective contributions of nitrification and denitrification to N₂O emissions. The highest N₂O emission (119 μg ¹⁴⁺¹⁵N-N₂O kg soil⁻¹ over 72 h) was measured from the 75% WFPS soil and was mostly produced during denitrification (18.1 μg ¹⁵N-N₂O kg soil⁻¹; 90% of ¹⁵N-N₂O from this treatment). Strong negative correlations between ¹⁴⁺¹⁵N-N₂O emissions, denitrified ¹⁵N-N₂O emissions and ¹³C-CH₄ concentrations (r=−0.93 to −0.95, N₂O; r=−0.87 to −0.95, denitrified ¹⁵N-N₂O; P<0.05) suggest a close relationship between CH₄ oxidation and denitrification in our soil, the nature of which requires further investigation.     

Emissions of N2O and CO2, denitrification measurements and soil properties in red clover and ryegrass stands

The relationships between the fluxes of nitrous oxide (N₂O) and carbon dioxide (CO₂), and their concentrations in the soil air, three different measures of potential denitrification, soil moisture, soil temperature and precipitation were investigated in soils from beneath ryegrass (Lolium multiflorum Lam.), red clover (Trifolium pratense L.) and mixture of ryegrass-red clover stands on a gleic cambisol. Investigations were carried out in order to test the hypothesis that the measure(s) of potential denitrification are good predictor(s) of N₂O fluxes and thus may be used in empirical models of N₂O emission. Potential denitrification characteristics used in this study involved (i) short-term denitrifying enzyme activity (DEA), (ii) long-term denitrification potential (DP), both determined in soils amended with nitrate and glucose, and (iii) denitrification rate (DR) measured using intact soil cores. Flux measurements were made using cylindrical chambers (internal diameter 31 cm, volume 0.015 m³). The fluxes of N₂O and CO₂ and many other characteristics showed large spatial and temporal variability. Emissions of N₂O from the grass plots were closely related to N₂O concentrations in the soil atmosphere at 22.5 cm depth. Most soil properties did not correlate with N₂O fluxes. It was concluded that DP was not a good predictor for N₂O flux. DEA did not show significant relationship with N₂O flux, but it is suggested that if determined in representative, large soil samples, DEA could be a predictor of N₂O fluxes; this assumption needs, however, verification. The only potential denitrification characteristic which was significantly related to N₂O emission both in grass and clover treatments was DR, which was determined in soil cores.     

Isotopologue fractionation during microbial reduction of N2O within soil mesocosms as a function of water-filled pore space

Isotopologue analyses of N₂O within soil mesocosm experiments were used to evaluate the influence of N₂O reduction on isotope fractionation. We investigated fractionation during N₂O reduction at 60%, 80% and 100% water-filled pore space (WFPS) and found net isotope effects (NIE) for δ¹⁵N of 4.2–7.8‰, δ¹⁸O of 12.5–19.1‰, δ¹⁵N^α of 6.4–9.7‰ and δ¹⁵N^β of 2.0–5.9‰. Consequently, N₂O reduction has a marked affect on isotopologue values and the importance of this process in flux chamber studies should not be ignored. With the exception of SP (the difference between the δ¹⁵N of the central, α, and terminal, β, atoms) inverse relationships between the NIE, reaction rate and reaction rate constant and WFPS were observed. Isotopic discrimination in SP during N₂O reduction was small and the average NIE for the treatments varied between 2.9‰ and 4.5‰. A strong correlation was evident between δ¹⁸O vs. δ¹⁵N and δ¹⁸O vs. δ¹⁵N^α during reduction with slopes of 2.6 and 1.9, respectively, which contrasts from a slope of <1 commonly observed for mixing between soil-derived and atmospheric N₂O in flux chambers.     

土壤pH对硝酸根还原过程中N2O产生的影响

采用氢氧化钠和盐酸将中性和碱性土壤分别分步调节成具有不同pH的系列土壤 ,加入等量硝态氮后 ,在添加易有效碳源葡萄糖和不添加葡萄糖的厌气条件下进行培养 ,测定不同处理条件下的N2 O和N2 产生速率。结果表明 ,不加碳源培养 2 4h后 ,原中性土壤系列中N2 O的最大产生速率位于pH 5 2 5左右 ,碱性土壤系列的该值位于 5 90左右 ;加入葡萄糖后 ,中性土壤系列中最大N2 O产生速率的pH值不变 ,但产生N2 最大速率的pH已提高至 6 50。而碱性土壤系列中N2 O产生最大速率时的pH值已移至 6 90处 ,即碳源的加入对产生N2 O所需的最佳pH有所提高。试验还显示 ,酸性条件可提高总还原气体中N2 O所占的比例 ,但就N2 O产生速率的绝对值来说 ,近中性条件仍然是最为有利的     

Effect of antecedent soil moisture conditions on emissions and isotopologue distribution of N2O during denitrification

The present study determined the influence of initial moisture conditions on the production and consumption of nitrous oxide (N₂O) during denitrification and on the isotopic fingerprint of soil-emitted N₂O. Sieved arable soil was pre-incubated at two different moisture contents: pre-wet at 75% and pre-dry at 20% water-filled pore space. After wetting to 90% water-filled pore space the soils were amended with glucose (400 kg C ha⁻¹) and KNO₃ (80 kg N ha⁻¹) and incubated for 10 days under a He/O₂-atmosphere. Antecedent moisture conditions affected denitrification. N₂ + N₂O fluxes and the N₂O-to-N₂ ratio were higher in soils which were pre-incubated under dry conditions, probably because mobilization of organic C during the pre-treatment enhanced denitrification. Gaseous N fluxes showed similar time patterns of production and reduction of N₂O in both treatments, where N₂O fluxes were initially increasing and maximised 3-4 days after fertilizer application, and N₂ fluxes were delayed by 1-2 days. Time courses of δ¹⁵N^bulk-N₂O and δ¹⁸O-N₂O exhibited in both treatments increasing trends until maximum N₂ fluxes occurred, reflecting isotope fractionation during intense NO₃⁻ reduction. Later this trend slowed down in the pre-dry treatment, while δ¹⁸O-N₂O was constant and δ¹⁵N^bulk-N₂O decreased in the pre-wet treatment. We explain these time patterns by non-homogenous distribution of NO₃⁻ and denitrification activity, resulting from application of NO₃⁻ and glucose to the surface of the soil. We assume that several process zones were thus created, which affected differently the isotopic signature of N₂O and the N₂O and N₂ fluxes during the different stages of the process. We modelled the δ¹⁵N^bulk-N₂O using process rates and associated fractionation factors for the pre-treated soils, which confirmed our hypothesis. The site preference (SP) initially decreased while N₂O reduction was absent, which we could not explain by the N-flux pattern. During the subsequent increase in N₂ flux, SP and δ¹⁸O-N₂O increased concurrently, confirming that this isotope pattern is indicative for N₂O reduction to N₂. The possible effect of the antecedent moisture conditions of the soil on N₂O emissions was shown to be important.     

How important is N2O production in removing atmospherically deposited nitrogen from UK moorland catchments?

Nitrate (NO₃⁻) leaching due to anthropogenic nitrogen (N) deposition is an environmental problem in many parts of the UK uplands, associated with surface water acidification and affecting lake nutrient balances. It is often assumed that gaseous return of deposited N to the atmosphere as N₂O through denitrification may provide an important sink for N. This assumption was tested for four moorland catchments (Allt a’Mharcaidh in the Cairngorms, Afon Gwy in mid-Wales, Scoat Tarn in the English Lake District and River Etherow in the southern Pennines), covering gradients of atmospheric N deposition and surface water NO₃⁻ leaching, through a combination of field and laboratory experiments. Field measurements of N₂O fluxes from static chambers with and without additions of NH₄NO₃ solution were carried out every 4 weeks over 1 yr. Wetted soil cores from the same field plots were used in experimental laboratory incubations at 5 and 15 °C with and without additions of NH₄NO₃ solution, followed by measurement of N₂O fluxes. Field measurements showed that significant N₂O fluxes occurred in only a very small number of plots with most showing zero values for much of the year. The maximum fluxes were 0.24 kg-N/ha/yr from unamended plots at the River Etherow and 0.49 kg-N/ha/yr from plots with NH₄NO₃ additions at the Allt a’Mharcaidh. Laboratory incubation experiments demonstrated that large N₂O fluxes could be induced by warming and NH₄NO₃ additions, with the top 5 cm of soil cores responsible for the largest fluxes, reaching 11.8 kg-N/ha/yr from a podsol at Scoat Tarn. Acetylene block experiments showed that while N₂ was not likely to be a significant denitrification product in these soils, reduced N₂O fluxes indicated that nitrification was an important source of N₂O in many cases. A simple model of denitrification suggesting that 10-80% of net N inputs may be denitrified from non-agricultural soils was found to greatly over-estimate fluxes in the UK uplands. The proportion of deposition denitrified was found to be much closer to the IPCC suggested value of 1% with an upper limit of 10%. Interception of N deposition by vegetation may greatly reduce the net supply of N from this source, while soil acidification or other factors limiting carbon supply to soil microbes may prevent large denitrification fluxes even where NO₃⁻ supply is not limiting.     

冻融对土壤氮素转化和N2O排放的影响研究进展

在中、高纬度及高海拔地区,土壤冻融现象常有发生。冻融作用通过影响土壤理化性质和生物学性状进而影响土壤氮素转化过程及N₂O的产生和释放,但迄今关于冻融对土壤氮素转化过程影响的研究结果还不尽一致,正效应或负效应均存在,土壤冻融期间N₂O排放对全年N₂O排放总量的贡献程度也存在着较大差异。本文重点论述了土壤冻结或冻融循环过程对土壤氮矿化、固持、硝化和反硝化等主要氮素转化过程的影响机制,同时分析了可引起冻融期间N₂O排放强度变化的四种可能机理（禁锢-释放、环境-底物诱导、N₂O还原酶抑制和化学反硝化增强）。指出在全球变暖背景下研究土壤冻融格局改变影响土壤氮素转化过程及N₂O排放的必要性,并简要提出了若干理论问题及研究方向。     

Isotopologue ratios of N2O emitted from microcosms with NH4 fertilized arable soils under conditions favoring nitrification

Soils represent the major source of the atmospheric greenhouse gas nitrous oxide (N₂O) and there is a need to better constrain the total global flux and the relative contribution of the microbial source processes. The aim of our study was to determine variability and control of the isotopic fingerprint of N₂O fluxes following NH₄⁺-fertilization and dominated by nitrification. We conducted a microcosm study with three arable soils fertilized with 0–140 mg NH₄⁺–N kg⁻¹. Fractions of N₂O derived from nitrification and denitrification were determined in parallel experiments using the ¹⁵N tracer and acetylene inhibition techniques or by comparison with unfertilized treatments. Soils were incubated for 3–10 days at low moisture (30–55% water-filled pore space) in order to establish conditions favoring nitrification. Dual isotope and isotopomer ratios of emitted N₂O were determined by mass spectrometric analysis of δ¹⁸O, average δ¹⁵N (δ¹⁵N^bulk) and ¹⁵N site preference (SP = difference in δ¹⁵N between the central and peripheral N positions of the asymmetric N₂O molecule). N₂O originated mainly from nitrification (>80%) in all treatments and the proportion of NH₄⁺ nitrified that was lost as N₂O ranged between 0.07 and 0.45%. δ¹⁸O and SP of N₂O fluxes ranged from 15 to 28.4‰ and from 13.9 to 29.8‰, respectively. These ranges overlapped with isotopic signatures of N₂O from denitrification reported previously. There was a negative correlation between SP and δ¹⁸O which is opposite to reported trends in N₂O from denitrification. Variation of average ¹⁵N signatures of N₂O (δ¹⁵N^bulk) did not supply process information, apparently because a strong shift in precursor signatures masked process-specific effects on δ¹⁵N^bulk. Maximum SP of total N₂O fluxes and of nitrification fluxes was close to reported SP of N₂O from NH₄⁺ or NH₂OH conversion by autotrophic nitrifiers, suggesting that SP close to 30‰ is typical for autotrophic nitrification in soils following NH₄⁺-fertilization. The results suggest that the δ¹⁸O/SP fingerprint of N₂O might be used as a new indicator of the dominant source process of N₂O fluxes in soils.     

硝化抑制剂影响小麦产量、N2O与NH3排放的研究

通过田间小区试验研究不同施氮水平下,施用硝化抑制剂CP对小麦产量、氮素利用率、氧化亚氮(N_2O)排放与氨(NH_3)挥发的综合影响规律。结果表明:在施氮水平为140 kg/hm2与180 kg/hm2时,施用CP促使小麦产量分别显著增加17.8%和15.4%,在同一施氮水平下,施用CP促进小麦氮素利用率提高11.3%~25.2%。施用硝化抑制剂CP可以降低麦季(特别是基肥与穗肥施用时期)土壤N_2O的排放速率,并显著减少39.3%~53.7%的累积N_2O排放量。但是在两个施氮水平下,施用CP导致麦季NH_3挥发量增加1.46~1.75倍,而且此效应主要发生于基肥与穗肥观测期。本研究说明:在麦季施用硝化抑制剂CP可以提高氮素利用率,从而提高小麦产量,并且能减少N_2O排放,但同时会导致一定程度的NH_3挥发增加,需加以控制。     

矿山废水灌溉区农田土壤N2O的产生及释放机制研究

农田系统是温室气体N₂O的主要排放源,目前对酸性矿山废水（acid mine drainage,AMD）灌溉影响下,农田土壤剖面N₂O的来源识别、转换机制及其控制因子缺乏深入研究。本文选择广东省大宝山矿区下游沿岸水稻田和甘蔗田两种典型农田,针对酸性矿山废水灌溉区（上坝村）和天然来水灌溉区（连心村）,对土壤理化性质、重金属含量及包气带N₂O浓度、同位素特征值进行了测定,定量计算了硝化和反硝化作用对土壤中N₂O的贡献比和N₂O转化为N₂的还原比,评价了其相关影响因素。结果表明：在AMD影响下,灌区农田土壤剖面N₂O浓度均高于同种作物类型天然来水区土壤,同种灌溉处理下甘蔗田土壤N₂O浓度高于水稻田。甘蔗田表层土壤（0~30 cm）反硝化作用对N₂O产生量的贡献比高于硝化作用,约71.29% N₂O由反硝化作用产生。AMD灌区甘蔗田土壤剖面中N₂O还原成N₂的比例随深度增加逐渐减小,在N₂O浓度峰值处仅有15.54% N₂O被还原成为N₂,而天然来水区N₂O还原成N₂的平均比率高达49.80%。这表明较弱的土壤N₂O还原能力导致较高浓度的N₂O残留在土壤中。相关性分析表明,AMD灌溉通过改变上坝村土壤的pH、重金属含量、含水率从而改变了土壤N₂O的来源途径及还原能力。组合同位素特征值溯源法有效地揭示了农田土壤N₂O的来源和AMD灌区土壤的潜在生态风险,为日后的治理修复工作提供了科学依据。