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.     

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.     

Production of NO and N2O in the presence and absence of C2H2 by soil slurries and batch cultures of denitrifying bacteria

We evaluated the potential of the C₂H₂-catalyzed NO oxidation reaction to influence N₂O production during denitrification. We measured the total amount of free NO and N₂O produced by slurries of sandy loam soil and by batch cultures of denitrifying bacteria under both anaerobic and low O₂ conditions. The maximum amount of free NO released by anaerobic Nos⁺ (able to reduce N₂O to N₂) and Nos⁻ (unable to reduce N₂O to N₂) batch cultures of Cytophaga johnsonae strains catalyzing denitrification of nitrate was 17-79 nmol NO per bottle. In all cases the maximum headspace concentrations of NO-N measured in anaerobic cultures in the absence of C₂H₂ was less than 0.21% of those of N₂O-N measured in the presence of 10 kPa C₂H₂. Peak NO production was delayed when between 2.0 and 4.5% O₂ was present. Less NO accumulated in cultures in the presence of both O₂ and C₂H₂, and the maximum amount of NO-N measured in the absence of C₂H₂ was less than 0.13% of the total amount of N₂O-N measured in the presence of C₂H₂. For an agricultural sandy loam soil, the maximum concentrations of free NO released from slurries were 598-897 ng NO-N g⁻¹ of dry soil in the absence of C₂H₂ and 118-260 ng NO-N g⁻¹ of dry soil in the presence of 10 kPa C₂H₂. The maximum concentration of NO-N released in the absence of C₂H₂ was between 0.32 and 8.1% of the maximum concentration of N₂O-N accumulated in the presence of 10 kPa C₂H₂. We conclude that scavenging of NO by the C₂H₂-catalyzed NO oxidation reaction in the presence of trace amounts of O₂ does not cause a serious underestimate of long-term measurements of active denitrification in anaerobic soils containing adequate carbon and nitrate sources.     

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.     

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。     

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).     

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.     

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.     

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     

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.     

Contribution of nitrification and denitrification to N2O emissions from urine patches

Urine deposition by grazing livestock causes an immediate increase in nitrous oxide (N₂O) emissions, but the responsible mechanisms are not well understood. A nitrogen-15 (¹⁵N) labelling study was conducted in an organic grass-clover sward to examine the initial effect of urine on the rates and N₂O loss ratio of nitrification (i.e. moles of N₂O-N produced per moles of nitrate produced) and denitrification (i.e. moles of N₂O produced per moles of N₂O+N₂ produced). The effect of artificial urine (52.9 g N m⁻²) and ammonium solution (52.9 g N m⁻²) was examined in separate experiments at 45% and 35% water-filled pore space (WFPS), respectively, and in each experiment a water control was included. The N₂O loss derived from nitrification or denitrification was determined in the field immediately after application of ¹⁵N-labelled solutions. During the next 24 h, gross nitrification rates were measured in the field, whereas the denitrification rates were measured in soil cores in the laboratory. Compared with the water control, urine application increased the N₂O emission from 3.9 to 42.3 μg N₂O-N m⁻² h⁻¹, whereas application of ammonium increased the emission from 0.9 to 6.1 μg N₂O-N m⁻² h⁻¹. In the urine-affected soil, nitrification and denitrification contributed equally to the N₂O emission, and the increased N₂O loss resulted from a combination of higher rates and higher N₂O loss ratios of the processes. In the present study, an enhanced nitrification rate seemed to be the most important factor explaining the high initial N₂O emission from urine patches deposited on well-aerated soils.     

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.     

Comparison of field and laboratory measurement of denitrification and N2O production in the saturated zone of hydromorphic soils

We evaluated a new method to measure in situ denitrification under field conditions in a number of water-saturated subsoils that had a broad range of biogeochemical properties. A test solution containing ¹⁵NO₃⁻ and/or C₂H₂ was introduced to the subsoil and the subsequent production of dissolved denitrification products was measured to quantify denitrification activity. The method showed a clear production of denitrification products over time. Results were compared to laboratory-based measurements from the same soil incubated as anaerobic slurries with added ¹⁵NO₃⁻. Rates of denitrification with the in situ and the laboratory methods ranged from 1-2800 and 1-1700 μg N kg⁻¹ d⁻¹, respectively. Generally the methods gave good agreement and we consider both to be valid. However, there were some significant deviations, which we attribute to spatial heterogeneity and laboratory effects. Because the laboratory method is so much easier to perform, we suggest it should be the preferred method for large-scale studies of denitrification from the soil types we investigated. However, the two methods showed poor agreement in determining the proportion of N₂O in the total denitrification output. This was because this proportion is subject to delicate and complex control. We conclude that neither method was suitable for quantifying N₂O emission from the denitrification measurements.     

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.     

CO2 and N2O emissions from gleyic soils in the Russian tundra and a German forest during freeze-thaw periods—a microcosm study

Knowledge is scarce on mineralization of soil organic carbon (SOC) in and N₂O emissions from tundra soils in periods of alternate freezing and thawing. Our objectives were to study the CO₂ and N₂O emissions from two silty gleyic soils formed in different climate zones (a gleyic Cryosol located in the Russian tundra, and a stagnic Gleysol located in an oak stand in central Germany) during freeze-thaw events. Soils were adjusted to a matric potential of −0.2 kPa and emissions were measured in 3-h intervals during an incubation period of 50 days including three freeze-thaw cycles. CO₂ emissions from the German oak forest soil were twofold higher than those of the tundra soil. The ratios of the mean CO₂ production rate before the freezing to the mean CO₂ production rate after thawing ranged from 0.63 to 0.73 for the forest soil and from 0.85 to 0.89 for the tundra soil. The specific CO₂-C production rate (CO₂-C/SOC) was 0.16 for the tundra soil and 0.57 for the forest soil. The results indicate that bioavailability of SOC was markedly smaller in the tundra soil than in the forest soil. Large N₂O emissions were found for the German forest soil, but no N₂O emissions were observed for the tundra soil. The main reason for the absence of N₂O emissions was most likely the negligible availability of nitrate for denitrification. There was some indication that the initial increase in mineralization of SOC induced by freezing and thawing differs between soils from various climatic regions, probably mainly due to a differing bioavailability of the SOC and differing releases of nutrients after thawing.     

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。     

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 emissions and product ratios of nitrification and denitrification as affected by freezing and thawing

Agricultural soils contribute significantly to atmospheric nitrous oxide (N₂O). A considerable part of the annual N₂O emission may occur during the cold season, possibly supported by high product ratios in denitrification (N₂O/(N₂+N₂O)) and nitrification (N₂O-N/(NO₃⁻-N+NO₂⁻-N)) at low temperatures and/or in response to freeze-thaw perturbation. Water-soluble organic materials released from frost-sensitive catch crops and green manure may further increase winter emissions. We conducted short-term laboratory incubations under standardized moisture and oxygen (O₂) conditions, using nitrogen (N) tracers (¹⁵N) to determine process rates and sources of emitted N₂O after freeze-thaw treatment of soil or after addition of freeze-thaw extract from clover. Soil respiration and N₂O production was stimulated by freeze-thaw or addition of plant extract. The N₂O emission response was inversely related to O₂ concentration, indicating denitrification as the quantitatively prevailing process. Denitrification product ratios in the two studied soils (pH 4.5 and 7.0) remained largely unaltered by freeze-thaw or freeze-thaw-released plant material, refuting the hypothesis that high winter emissions are due to frost damage of N₂O reductase activity. Nitrification rates estimated by nitrate (NO₃⁻) pool enrichment were 1.5-1.8 μg NO₃-N g⁻¹ dw soil d⁻¹ in freeze-thaw-treated soil when incubated at O₂ concentrations above 2.3 vol% and one order of magnitude lower at 0.8 vol% O₂. Thus, the experiments captured a situation with severely O₂-limited nitrification. As expected, the O₂ stress at 0.8 vol% resulted in a high nitrification product ratio (0.3 g g⁻¹). Despite this high product ratio, only 4.4% of the measured N₂O accumulation originated from nitrification, reaffirming that denitrification was the main N₂O source at the various tested O₂ concentrations in freeze-thaw-affected soil. N₂O emission response to both freeze-thaw and plant extract addition appeared strongly linked to stimulation of carbon (C) respiration, suggesting that freeze-thaw-induced release of decomposable organic C was the major driving force for N₂O emissions in our soils, both by fuelling denitrifiers and by depleting O₂. The soluble C (applied as plant extract) necessary to induce a CO₂ and N₂O production rate comparable with that of freeze-thaw was 20-30 μg C g⁻¹ soil dw. This is in the range of estimates for over-winter soluble C loss from catch crops and green manure plots reported in the literature. Thus, freeze-thaw-released organic C from plants may play a significant role in freeze-thaw-related N₂O emissions.     

The production of N2O in Douglas fir litter as affected by anoxic conditions within litter particles and pores

The development of anoxic conditions in forest litter and the relation with nitrous oxide (N₂O) production and emission rates are not completely understood. Water content is an important factor in the regulation of N₂O production due to its effect on the development of anoxic conditions. A combination of simulation modeling and incubation experiments was used to study (1) O₂ concentrations in water and organic matter at various water saturation fractions of inter-particle pores in Douglas fir litter (F2 horizon), (2) the relationship between N₂O production and moisture content of litter and (3) to test whether diffusion constraints of nitrate (NO₃⁻) could have explained measured N₂O production rates within litter fragments. Model simulations showed that the occurrence of high N₂O production rates in samples with extremely high water contents coincided with the development of anoxic conditions in water-filled inter-particle pores. Measured N₂O production rates started to increase exponentially after 1-2 days in glucose-amended samples, during which substantial microbial growth was established. For these latter samples model simulations showed that the increase in O₂ consumption due to microbial growth lead to anoxic conditions in water-filled pores at locations which were far from the O₂ saturated air-filled pores. It was concluded that anoxic conditions in water-filled pores was the crucial factor for the development of high N₂O production rates. Diffusion limitation of NO₃⁻ and glucose were estimated to be negligible in the highly fragmented litter material used. The occurrence of diffusion limitation depended on litter particle size, the NO₃⁻ reduction potential and the NO₃⁻ concentration. Therefore, diffusion limitation together with N₂O production in litter cannot be neglected under field conditions with a low NO₃⁻ concentration or a high NO₃⁻ reduction potential.     

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.