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.     

Land use effects on gross nitrogen mineralization, nitrification, and N2O emissions in ephemeral wetlands

Stable ¹⁵N isotope dilution and tracer techniques were used in cultivated (C) and uncultivated (U) ephemeral wetlands in central Saskatchewan, Canada to: (1) quantify gross mineralization and nitrification rates and (2) estimate the relative proportion of N₂O emissions from these wetlands that could be attributed to denitrification versus nitrification-related processes. In-field incubation experiments were repeated in early May, mid-June and late July. Mean gross mineralization and nitrification rates (10.3 and 3.1 mg kg⁻¹ d⁻¹, respectively) did not differ between C and U wetlands on any given date. Despite these similarities, the mean NH₄⁺ pool size in the U wetlands (17.2 mg kg⁻¹) was two to three times that of the C wetlands (6.7 mg kg⁻¹) whereas the mean NO₃⁻ pool size in U wetlands (2.2 mg kg⁻¹) was less than half that of C wetlands (5.8 mg kg⁻¹). Mean N₂O emissions from the C wetlands decreased from 112.8 to 17.0 ng N₂O m² s⁻¹ from May to July, whereas mean U-wetland N₂O emissions ranged only from 31.8 to 51.1 ng N₂O m² s⁻¹ over the same period. This trend is correlated to water-filled pore space in C wetlands, demonstrating a soil moisture influence on emissions. Denitrification is generally considered the dominant emitter of N₂O under anaerobic conditions, but in the C wetlands, only 49% of the May emissions could be directly attributed to denitrification, decreasing to 29% in July. In contrast, more than 75% of the N₂O emissions from the U wetlands arose from denitrification of the soil NO₃⁻ pool throughout the season. These land use differences in emission sources and rates should be taken into consideration when planning management strategies for greenhouse gas mitigation.     

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.     

Isotopomer signatures of soil-emitted N2O under different moisture conditions—A microcosm study with arable loess soil

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 evaluate isotopomer analysis of N₂O (intramolecular distribution of ¹⁵N) as well as conventional nitrogen and oxygen isotope ratios (i) as a tool to identify N₂O production processes in soils and (ii) to constrain the isotopic fingerprint of soil-derived N₂O. We conducted a microcosm study with arable loess soil fertilized with 20 mg N kg⁻¹ of ¹⁵NO₃⁻-labeled or non-labeled ammonium nitrate. Soils were incubated for 16 d at varying moisture (55%, 75% and 85% water-filled pore space (WFPS)) in order to establish different levels of nitrification and denitrification. 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). Total rates and N₂O emission of denitrification and nitrification were determined by ¹⁵N analysis of headspace gases and soil extracts of the ¹⁵NO₃⁻ treatment. N₂O emission and denitrification increased with moisture whereas gross nitrification was almost constant. In the 55% WFPS treatment, more than half of the N₂O flux was derived from nitrification, whereas denitrification was the dominant N₂O source in the 75% WFPS and 85% WFPS treatments. Moisture conditions were reflected by the isotopic signatures since highly significant differences were observed for average δ¹⁵N^bulk, SP and δ¹⁸O. Experiment means of the 75% WFPS and 85% WFPS treatments gave negative δ¹⁵N^bulk (−18.0‰ and −34.8‰, respectively) and positive SP (8.6‰ and 15.3‰, respectively), which we explained by the fractionation during N₂O production and partial reduction to N₂. In the 55% WFPS treatment, mean SP was relatively low (1.9‰), which suggests that nitrification produced N₂O with low or negative SP. The observed influence of process condition on isotopomer signatures suggests that the isotopomer approach might be suitable for identifying N₂O source processes. However, more research is needed to determine the impact from process rates and microbial community structure. Isotopomer signatures were within the range reported from previous soil studies which supports the assumption that SP of soil-derived N₂O is lower than SP of tropospheric N₂O.     

Microbial processes and the site of N2O production in a temperate grassland soil

To understand nitrous oxide (N₂O) emissions from terrestrial ecosystems it is necessary to understand the processes leading to N₂O production. Here, for the first time, results are presented which identify in situ the processes of N₂O production in a temperate grassland soil. A small portion of the nitrogen (N) applied in the summer to the grassland soil was rapidly transported below the main rooting zone (>20 cm) and resulted in large N₂O productions at depths of 20-50 cm. Preferential pathways must have been responsible for this movement because the soil conditions were not conducive to leaching by piston flow. The N₂O was entirely produced by nitrate (NO₃⁻) reduction which was surprising because the bulk soil was aerobic. Therefore, reduction processes can operate during times of the year when it is least expected and cause large N₂O concentrations deep in the soil profile.     

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.     

土壤温度、水分和NH4+-N浓度对土壤硝化反应速度及N2O排放的影响

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

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.     

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.     

Emissions and spatial variability of N2O, N2 and nitrous oxide mole fraction at the field scale, revealed with N isotopic techniques

The accurate measurement of nitrous oxide (N₂O) and dinitrogen (N₂) during the denitrification process in soils is a challenge which will help to estimate the contribution of soil N₂O emissions to global warming. Oxygen concentration, nitrate concentration and carbon availability are generally the main factors that control soil denitrification rate and the amount of N₂O or N₂ emitted. The aim of this paper is to present a database of the N₂O mole fraction measured at the field scale, and to test hypotheses concerning its regulation. A ¹⁵N-nitrate tracer solution was added to 36 undisturbed soil cores on a 20 m×20 m cultivated field plot. Fluxes of CO₂, N₂O and N₂ from the soil surface were monitored for 24 h. Soil moisture, bulk density, carbon, nitrogen and mineral nitrogen concentration were also measured to investigate possible spatial relationships between their variations and those of N₂O, N₂ and nitrous oxide mole fraction. Under high water content, nitrous oxide and N₂ emissions were highly variable with variation coefficients of 70-140%. N₂O emission rates were about twice as high as those of N₂, with a total denitrification rate ranging from 269 to 3843 g N ha⁻¹ d⁻¹. After 24 h of incubation, the values of nitrous oxide mole fraction ranged from 0.15 to 0.94 and no significant decline during incubation time was observed. Spatial variability of N₂O, N₂ and nitrous oxide mole fraction was high and no spatial dependence was observed at the scale of the experimental plot. Only tenuous relationships between gaseous nitrogen emissions and soil properties (mainly nitrate concentration and moisture content) were found. Meanwhile, a positive correlation was observed between N₂ and CO₂ emissions. This result supports the hypothesis that an increase in soil available organic carbon leads to N₂ emissions as the end product of denitrification.     

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.     

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

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

Interactions between residue placement and earthworm ecological strategy affect aggregate turnover and N2O dynamics in agricultural soil

Previous laboratory studies using epigeic and anecic earthworms have shown that earthworm activity can considerably increase nitrous oxide (N₂O) emissions from crop residues in soils. However, the universality of this effect across earthworm functional groups and its underlying mechanisms remain unclear. The aims of this study were (i) to determine whether earthworms with an endogeic strategy also affect N₂O emissions; (ii) to quantify possible interactions with epigeic earthworms; and (iii) to link these effects to earthworm-induced differences in selected soil properties. We initiated a 90-day ¹⁵N-tracer mesocosm study with the endogeic earthworm species Aporrectodea caliginosa (Savigny) and the epigeic species Lumbricus rubellus (Hoffmeister). ¹⁵N-labeled radish (Raphanus sativus cv. Adagio L.) residue was placed on top or incorporated into the loamy (Fluvaquent) soil. When residue was incorporated, only A. caliginosa significantly (p < 0.01) increased cumulative N₂O emissions from 1350 to 2223 μg N₂O-N kg⁻¹ soil, with a corresponding increase in the turnover rate of macroaggregates. When residue was applied on top, L. rubellus significantly (p < 0.001) increased emissions from 524 to 929 μg N₂O-N kg⁻¹, and a significant (p < 0.05) interaction between the two earthworm species increased emissions to 1397 μg N₂O-N kg⁻¹. These effects coincided with an 84% increase in incorporation of residue ¹⁵N into the microaggregate fraction by A. caliginosa (p = 0.003) and an 85% increase in incorporation into the macroaggregate fraction by L. rubellus (p = 0.018). Cumulative CO₂ fluxes were only significantly increased by earthworm activity (from 473.9 to 593.6 mg CO₂-C kg⁻¹ soil; p = 0.037) in the presence of L. rubellus when residue was applied on top. We conclude that earthworm-induced N₂O emissions reflect earthworm feeding strategies: epigeic earthworms can increase N₂O emissions when residue is applied on top; endogeic earthworms when residue is incorporated into the soil by humans (tillage) or by other earthworm species. The effects of residue placement and earthworm addition are accompanied by changes in aggregate and SOM turnover, possibly controlling carbon, nitrogen and oxygen availability and therefore denitrification. Our results contribute to understanding the important but intricate relations between (functional) soil biodiversity and the soil greenhouse gas balance. Further research should focus on elucidating the links between the observed changes in soil aggregation and controls on denitrification, including the microbial community.     

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.     

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.     

Nitrous oxide production by nitrification and denitrification in soil aggregates as affected by O2 concentration

Nitrous oxide emitted by soils can be produced either by denitrification in anoxic conditions or by nitrification in presence of O₂. The relative importance of the two processes, particularly under varied partial pressures of O₂, is not always known. This paper focuses on the influence of O₂ concentration on N₂O production by nitrification and denitrification in an arable Orthic Luvisol. Soil aggregates (2-3 mm size), water unsaturated, received 116 mg N kg⁻¹ as ammonium sulphate labelled with ¹⁵N and were incubated during 14 days at different O₂ partial pressures: 0, 0.35, 0.76, 1.5, 4.3 and 20.4 kPa. A ¹⁵N tracing technique was used to quantify nitrification and denitrification rates. ¹⁵N₂O and ¹⁵N₂ were measured. Oxygen pressure appeared to strongly influence both nitrification and denitrification rates and also N₂O emissions. Nitrification rates were reduced by a factor of 6-9 when O₂ decreased from 20.4 to 0.35 kPa. They were highly correlated with O₂ consumption rates. Denitrification mainly occurred in complete anoxic conditions. The proportion of N₂O emitted by denitrification was estimated by two independent methods: one based on ¹⁵N tracing using isotope composition of NH₄, NO₃ and N₂O, the other based on the measurement of the ¹⁵N₂O:¹⁵N₂ ratio. The two methods gave close results. The highest N₂O emissions were obtained under complete anoxic conditions and were due to denitrification. However, N₂O emissions almost as important were obtained at day 14 with 1.5 kPa O₂ pressure, and they were due to nitrification. Nitrification was the main source of N₂O at O₂ concentrations greater than 0.35 kPa. The amounts of N₂O-N emitted by nitrification were linearly related to the amounts of N nitrified, but the slope of the regression was highly dependent on O₂ concentration: it varied from 0.16 to 1.48% when O₂ concentration was reduced from 20.4 to 0.76 kPa. Emissions of N₂O by nitrification may then be quite significant if nitrification occurs at a reduced O₂ concentration.     

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。     

The N2O product ratio of nitrification and its dependence on long-term changes in soil pH

The contribution of nitrification to the emission of nitrous oxide (N₂O) from soils may be large, but its regulation is not well understood. The soil pH appears to play a central role for controlling N₂O emissions from soil, partly by affecting the N₂O product ratios of both denitrification (N₂O/(N₂+N₂O)) and nitrification (N₂O/(NO₂⁻+NO₃⁻). Mechanisms responsible for apparently high N₂O product ratios of nitrification in acid soils are uncertain. We have investigated the pH regulation of the N₂O product ratio of nitrification in a series of experiments with slurries of soils from long-term liming experiments, spanning a pH range from 4.1 to 7.8. ¹⁵N labelled nitrate (NO₃⁻) was added to assess nitrification rates by pool dilution and to distinguish between N₂O from NO₃⁻ reduction and NH₃ oxidation. Sterilized soil slurries were used to determine the rates of chemodenitrification (i.e. the production of nitric oxide (NO) and N₂O from the chemical decomposition of nitrite (NO₂⁻)) as a function of NO₂⁻ concentrations. Additions of NO₂⁻ to aerobic soil slurries (with ¹⁵N labelled NO₃⁻ added) were used to assess its potential for inducing denitrification at aerobic conditions. For soils with pH?5, we found that the N₂O product ratios for nitrification were low (0.2-0.9‰) and comparable to values found in pure cultures of ammonia-oxidizing bacteria. In mineral soils we found only a minor increase in the N₂O product ratio with increasing soil pH, but the effect was so weak that it justifies a constant N₂O product ratio of nitrification for N₂O emission models. For the soils with pH 4.1 and 4.2, the apparent N₂O product ratio of nitrification was 2 orders of magnitude higher than above pH 5 (76‰ and 14‰). This could partly be accounted for by the rates of chemodenitrification of NO₂⁻. We further found convincing evidence for NO₂⁻-induction of aerobic denitrification in acid soils. The study underlines the role of NO₂⁻, both for regulating denitrification and for the apparent nitrifier-derived N₂O emission.     

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。     

Fractionation of N2O isotopomers during production by denitrifier

Isotopomer ratios of N₂O, which include intramolecular ¹⁵N-site preference in addition to conventional isotope ratios for N and O in NNO (we designate N^α and N^β for the center and end N atom, respectively, in the asymmetric molecule), reflect production and consumption processes of this greenhouse gas. Therefore, they are useful parameters for deducing global N₂O budget. This paper reports the first precise measurement of ¹⁵N-site preference in N₂O produced by two species of denitrifying bacteria, Pseudomonas fluorescens (ATCC 13525) and Paracoccus denitrificans (ATCC 17741).Cultures were incubated in a batch mode with a liquid medium that contains KNO₃ as unique nitrogen supply under acetylene/helium (10% v/v) atmosphere at 27 °C. Enrichment factors for ¹⁵N in bulk nitrogen in N₂O (average for N^α and N^β) fluctuated in a few tens permil showing a slight difference between the species. In contrast, ¹⁵N-site preference (difference in isotope ratios between N^α and N^β) showed nearly constant and distinct value for the two species (23.3±4.2 and −5.1±1.8‰ for P. fluorescens and P. denitrificans, respectively). The site preference was also measured for N₂O produced by inorganic reactions (nitrite reduction and hydroxylamine oxidation); a unique value (about 30‰ for the both reactions) was obtained. These results and those recently reported for nitrifying bacteria suggest that ¹⁵N-site preference in N₂O can be used to identify the production processes of N₂O on the level of bacterial species or enzymes involved.