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.     

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 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.     

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.     

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.     

农田土壤N2O生成与排放影响因素及N2O总量估算的研究

综述了国内外农田土壤N2 O生成与排放及其影响因素、N2 O排放测定技术及总量估算等方面的研究进展 ,指出硝化与反硝化过程均可产生N2 O ,而影响硝化、反硝化过程的土壤水分含量、温度、pH、有机碳含量和土壤质地等是影响农田土壤N2 O生成与排放的重要因素。根据我国各地农田土壤N2 O排放通量测定结果及相应模型分析 ,初步估算全国农田土壤N2 O年排放总量为N 398Gg ,约占全球农田土壤排放总量的 1 0 % ,其中旱田N2 O年排放总量为N 31 0Gg ,水田为N 88Gg。     

Dinitrogen and N2O emissions in arable soils: Effect of tillage, N source and soil moisture

A laboratory investigation was performed to compare the fluxes of dinitrogen (N₂), N₂O and carbon dioxide (CO₂) from no-till (NT) and conventional till (CT) soils under the same water, mineral nitrogen and temperature status. Intact soil cores (0-10 cm) were incubated for 2 weeks at 25 °C at either 75% or 60% water-filled pore space (WFPS) with ¹⁵N-labeled fertilizers (100 mg N kg⁻¹ soil). Gas and soil samples were collected at 1-4 day intervals during the incubation period. The N₂O and CO₂ fluxes were measured by a gas chromatography (GC) system while total N₂ and N₂O losses and their ¹⁵N mole fractions in the soil mineral N pool were determined by a mass spectrometer. The daily accumulative fluxes of N₂ and N₂O were significantly affected by tillage, N source and soil moisture. We observed higher (P<0.05) fluxes of N₂+N₂O, N₂O and CO₂ from the NT soils than from the CT soils. Compared with the addition of nitrate (NO₃⁻), the addition of ammonium (NH₄⁺) enhanced the emissions of these N and C gases in the CT and NT soils, but the effect of NH₄⁺ on the N₂ and/or N₂O fluxes was evident only at 60% WFPS, indicating that nitrification and subsequent denitrification contributed largely to the gaseous N losses and N₂O emission under the lower moisture condition. Total and fertilizer-induced emissions of N₂ and/or N₂O were higher (P<0.05) at 75% WFPS than with 60% WFPS, while CO₂ fluxes were not influenced by the two moisture levels. These laboratory results indicate that there is greater potential for N₂O loss from NT soils than CT soils. Avoiding wet soil conditions (>60% WFPS) and applying a NO₃⁻ form of N fertilizer would reduce potential N₂O emissions from arable soils.     

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.     

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.     

Elevated CO2 stimulates N2O emissions in permanent grassland

To evaluate climate forcing under increasing atmospheric CO₂ concentrations, feedback effects on greenhouse gases such as nitrous oxide (N₂O) with a high global warming potential should be taken into account. This requires long-term N₂O flux measurements because responses to elevated CO₂ may vary throughout annual courses. Here, we present an almost 9 year long continuous N₂O flux data set from a free air carbon dioxide enrichment (FACE) study on an old, N-limited temperate grassland. Prior to the FACE start, N₂O emissions were not different between plots that were later under ambient (A) and elevated (E) CO₂ treatments, respectively. However, over the entire experimental period (May 1998–December 2006), N₂O emissions more than doubled under elevated CO₂ (0.90 vs. 2.07 kg N₂O-N ha⁻¹ y⁻¹ under A and E, respectively). The strongest stimulation occurred during vegetative growth periods in the summer when soil mineral N concentrations were low. This was surprising because based on literature we had expected the highest stimulation of N₂O emissions due to elevated CO₂ when mineral N concentrations were above background values (e.g. shortly after N application in spring). N₂O emissions under elevated CO₂ were moderately stimulated during late autumn–winter, including freeze–thaw cycles which occurred in the 8th winter of the experiment. Averaged over the entire experiment, the additional N₂O emissions caused by elevated CO₂ equaled 4738 kg CO₂-equivalents ha⁻¹, corresponding to more than half a ton (546 kg) of CO₂ ha⁻¹ which has to be sequestered annually to balance the CO₂-induced N₂O emissions. Without a concomitant increase in C sequestration under rising atmospheric CO₂ concentrations, temperate grasslands may be converted into greenhouse gas sources by a positive feedback on N₂O emissions. Our results underline the need to include continuous N₂O flux measurements in ecosystem-scale CO₂ enrichment experiments.     

高氮和NO2-对中亚热带森林土壤N2O和NO产生的影响

利用15N同位素标记方法,研究在两种水分条件即60％和90％ WHC下,添加硝酸盐(NH4NO3,N 300 mg kg-1)和亚硝酸盐(NaNO2,N 1 mg kg-1)对中亚热带天然森林土壤N2O和NO产生过程及途径的影响.结果表明,在含水量为60％ WHC的情况下,高氮输入显著抑制了N2O和NO的产生(p＜0.01);但当含水量增为90％ WHC后,实验9h内抑制N2O产生,之后转为促进.所有未灭菌处理在添加NO2-后高氮抑制均立即解除并大量产生N2O和NO,与对照成显著差异(p＜0.01),在60％ WHC条件下,这种情况维持时间较短(21 h),但如果含水量高(90％ WHC)这种情况会持续很长时间(2周以上),说明水分有效性的提高和外源NO2-在高氮抑制解除中起到重要作用.本实验中N2O主要来源于土壤反硝化过程,而且加入未标记NO2-后导致杂合的N2O(14N15NO)分子在实验21 h内迅速增加,表明这种森林土壤的反硝化过程可能主要是通过真菌的“共脱氮”来实现,其贡献率可多达80％以上.Spearman秩相关分析表明未灭菌土壤NO的产生速率与N2O产生速率成显著正相关性(p＜0.05),土壤含水量越低二者相关性越高.灭菌土壤添加NO2-能较未灭菌土壤产生更多的NO,但却几乎不产生N2O,表明酸性土壤的化学反硝化对NO的贡献要大于N2O.     

Dinitrogen emissions and the N2:N2O emission ratio of a Rendzic Leptosol as influenced by pH and forest thinning

Reduction of nitrous oxide (N₂O) to dinitrogen (N₂) by denitrification in soils is of outstanding ecological significance since it is the prevailing natural process converting reactive nitrogen back into inert molecular dinitrogen. Furthermore, the extent to which N₂O is reduced to N₂ via denitrification is a major regulating factor affecting the magnitude of N₂O emission from soils. However, due to methodological problems in the past, extremely little information is available on N₂ emission and the N₂:N₂O emission ratio for soils of terrestrial ecosystems. In this study, we simultaneously determined N₂ and N₂O emissions from intact soil cores taken from a mountainous beech forest ecosystem. The soil cores were taken from plots with distinct differences in microclimate (warm-dry versus cool-moist) and silvicultural treatment (untreated control versus heavy thinning). Due to different microclimates, the plots showed pronounced differences in pH values (range: 6.3–7.3). N₂O emission from the soil cores was generally very low (2.0 ± 0.5–6.3 ± 3.8 μg N m⁻² h⁻¹ at the warm-dry site and 7.1 ± 3.1–57.4 ± 28.5 μg N m⁻² h⁻¹ at the cool-moist site), thus confirming results from field measurements. However, N₂ emission exceeded N₂O emission by a factor of 21 ± 6–220 ± 122 at the investigated plots. This illustrates that the dominant end product of denitrification at our plots and under the given environmental conditions is N₂ rather than N₂O. N₂ emission showed a huge variability (range: 161 ± 64–1070 ± 499 μg N m⁻² h⁻¹), so that potential effects of microclimate or silvicultural treatment on N₂ emission could not be identified with certainty. However, there was a significant effect of microclimate on the magnitude of N₂O emission as well as on the mean N₂:N₂O emission ratio. N₂:N₂O emission ratios were higher and N₂O emissions were lower for soil cores taken from the plots with warm-dry microclimate as compared to soil cores taken from the cool-moist microclimate plots. We hypothesize that the increase in the N₂:N₂O emission ratio at the warm-dry site was due to higher N₂O reductase activity provoked by the higher soil pH value of this site. Overall, the results of this study show that the N₂:N₂O emission ratio is crucial for understanding the regulation of N₂O fluxes of the investigated soil and that reliable estimates of N₂ emissions are an indispensable prerequisite for accurately calculating total N gas budgets for the investigated ecosystem and very likely for many other terrestrial upland ecosystems as well.     

Isotopologue data reveal bacterial denitrification as the primary source of N2O during a high flux event following cultivation of a native temperate grassland

The source of N₂O in terrestrial ecosystems has long been debated. Both nitrification and denitrification produce N₂O but their relative importance remains uncertain. Here we apply site preference, SP (the difference in δ¹⁵N between the central and outer N atom in N₂O), to estimate the relative importance of bacterial denitrification (including nitrifier denitrification) to total N₂O production from soil. We measured SP over a diurnal cycle following the third year of tillage of a previously uncultivated grassland soil at the Kellogg Biological Station (KBS) in southwestern Michigan. Fluxes of N₂O in our study ranged between 7.8 and 12.1 g N₂O-N ha⁻¹ d⁻¹ and were approximately 3 and 10 times greater than fluxes observed in managed agricultural and successional fields, respectively, at KBS. Consequently, our study captured a period of high flux resulting from the cultivation of a historically never-tilled soil. Concentration weighted SP values decreased from 12.9‰ in the morning to a minimum value of −0.1‰ in the afternoon.Based on SP values reported for bacterial denitrification (−5 to 0‰; Toyoda et al., 2005; Sutka et al., 2006), hydroxylamine oxidation (nitrification) and fungal denitrification (33-37‰; Sutka et al., 2006) we found that production attributable to bacterial denitrification increased from between 52.9 and 60.9% in the morning to between 87.5 and 100% in the afternoon. Further, we observed diurnal variation in flux and SP that is consistent with increased production from bacterial denitrification associated with temperature-driven increases in respiration.     

三氯生和三氯卡班对水稻土好氧氮转化及N_2O排放的影响

三氯生(Triclosan, TCS)和三氯卡班(Triclocarban, TCC)是典型的药品与个人护理用品,在土壤生态系统中被广泛检出,且存在增加土壤微生物抗药性及抑制土壤呼吸的潜在风险,但目前有关TCS和TCC对土壤氮转化过程及氧化亚氮(N_2O)排放的影响尚不清楚。基于此,采用室内培养实验和15N稀释-富集法,结合氮转化数值模型,研究了不同浓度梯度下TCS(2和5mg·kg~(-1))和TCC(1和2 mg·kg~(-1))的单独及联合存在对水稻土氮初级转化速率以及N_2O排放的影响。结果表明,1mg·kg~(-1)TCC及5mg·kg~(-1)TCS+2mg·kg~(-1)TCC处理对水稻土氮素的矿化-同化无显著影响,其余TCS和TCC处理均显著促进了氮的矿化-同化循环。此外,TCS和TCC处理显著降低了自养硝化速率、硝态氮的微生物固定速率以及硝酸盐异化还原成铵(Dissimilatory nitrate reduction to ammonium, DNRA)速率(2 mg·kg~(-1)TCS处理及5mg·kg~(-1)TCS+2mg·kg~(-1)TCC对DNRA速率无显著影响)。值得关注的是,TCS和TCC单一和联合处理均显著增加了N_2O的累积排放量,其累积排放量为对照的1.13倍～1.44倍。本研究表明,TCS和TCC改变了水稻土好氧氮转化过程,可能对稻田生态系统氮循环产生不利影响;TCC和TCS对水稻土N_2O排放的促进作用也增加了稻田生态系统对温室效应和臭氧层破坏的潜在贡献,因此,未来评价TCS和TCC土壤生态风险时,应考虑其对氮转化过程和N_2O排放的潜在影响。     

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.     

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.     

Effect of elevated CO2 on soil N dynamics in a temperate grassland soil

The response of terrestrial ecosystems to elevated atmospheric CO₂ is related to the availability of other nutrients and in particular to nitrogen (N). Here we present results on soil N transformation dynamics from a N-limited temperate grassland that had been under Free Air CO₂ Enrichment (FACE) for six years. A ¹⁵N labelling laboratory study (i.e. in absence of plant N uptake) was carried out to identify the effect of elevated CO₂ on gross soil N transformations. The simultaneous gross N transformation rates in the soil were analyzed with a ¹⁵N tracing model which considered mineralization of two soil organic matter (SOM) pools, included nitrification from NH₄⁺ and from organic-N to NO₃⁻ and analysed the rate of dissimilatory NO₃⁻ reduction to NH₄⁺ (DNRA). Results indicate that the mineralization of labile organic-N became more important under elevated CO₂. At the same time the gross rate of NH₄⁺ immobilization increased by 20%, while NH₄⁺ oxidation to NO₃⁻ was reduced by 25% under elevated CO₂. The NO₃⁻ dynamics under elevated CO₂ were characterized by a 52% increase in NO₃⁻ immobilization and a 141% increase in the DNRA rate, while NO₃⁻ production via heterotrophic nitrification was reduced to almost zero. The increased turnover of the NH₄⁺ pool, combined with the increased DNRA rate provided an indication that the available N in the grassland soil may gradually shift towards NH₄⁺ under elevated CO₂. The advantage of such a shift is that NH₄⁺ is less prone to N losses, which may increase the N retention and N use efficiency in the grassland ecosystem under elevated CO₂.     

生物炭施用下中国农田土壤N2O排放的Meta分析

为明确施加生物炭对中国农田土壤N_2O排放的影响和主要控制因素,以公开发表的试验数据为研究对象,采用Meta-analysis法定量分析了施加生物炭条件下,气候、土壤性质、田间管理方式、生物炭性质与施加量对土壤N_2O排放的影响,并对各影响因素进行通径分析。结果表明,当年降雨量≥600 mm时,生物炭显著降低土壤N_2O排放量(P0.05),且随年降雨量的增加而增强;当年日照时数大于1 000 h时,生物炭对土壤N_2O的减排效果随年日照时数的增加而减弱。当土壤p H≥6.5时,生物炭对土壤N_2O的减排效果随土壤p H的增加呈先增后减趋势;在壤土中施加生物炭对N_2O的减排效果显著(P0.05),而砂土和黏土不显著(P0.05)。生物炭对覆膜土壤N_2O的减排效果优于不覆膜土壤;生物炭对土壤N_2O的减排效果随施氮肥量增加而减弱,而随生物炭比表面积的增加而增强。当生物炭C/N处于30~500时,生物炭施用下土壤N_2O排放量显著降低(P0.05);当生物炭施加量处于20~160 t×hm-2时,生物炭对土壤N_2O的减排效果随施加量增加而增强。生物炭对土壤N_2O减排的影响存在显著的区域性特征,对华南、华东、华中和东北地区影响显著(P0.05),而对西北地区不显著(P0.05);施氮肥量、生物炭施加量、年均温和年降雨量是影响生物炭减排效果的最主要因素,这些因素的相互作用共同影响生物炭对土壤N_2O的减排效果。该研究可为生物炭在我国农区的推广应用和农田N_2O减排提供参考。     

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。