Influence of soil properties on the turnover of nitric oxide and nitrous oxide by nitrification and denitrification at constant temperature and moisture

 Soils are a major source of atmospheric NO and N₂O. Since the soil properties that regulate the production and consumption of NO and N₂O are still largely unknown, we studied N trace gas turnover by nitrification and denitrification in 20 soils as a function of various soil variables. Since fertilizer treatment, temperature and moisture are already known to affect N trace gas turnover, we avoided the masking effect of these soil variables by conducting the experiments in non-fertilized soils at constant temperature and moisture. In all soils nitrification was the dominant process of NO production, and in 50% of the soils nitrification was also the dominant process of N₂O production. Factor analysis extracted three factors which together explained 71% of the variance and identified three different soil groups. Group I contained acidic soils, which showed only low rates of microbial respiration and low contents of total and inorganic nitrogen. Group II mainly contained acidic forest soils, which showed relatively high respiration rates and high contents of total N and NH₄ ⁺. Group III mainly contained neutral agricultural soils with high potential rates of nitrification. The soils of group I produced the lowest amounts of NO and N₂O. The results of linear multiple regression conducted separately for each soil group explained between 44–100% of the variance. The soil variables that regulated consumption of NO, total production of NO and N₂O, and production of NO and N₂O by either nitrification or denitrification differed among the different soil groups. The soil pH, the contents of NH₄ ⁺, NO₂ ^– and NO₃ ^–, the texture, and the rates of microbial respiration and nitrification were among the important variables. Received: 28 October 1999     

Mechanisms of N2O and NO production in the soil profile of a drained and forested peatland, as studied with acetylene, nitrapyrin and dimethyl ether

 Acetylene, dimethyl ether (DME) and 2-chloro-6-trichloromethyl pyridine (nitrapyrin) were used as inhibitors to study the contributions of nitrification and denitrification to the production of N₂O and nitric oxide (NO) in samples taken from the soil profile of a peatland drained for forestry. Acetylene and DME inhibited 60–100% of the nitrification activity in field-moist samples from the 0–5 cm and 5–10 cm peat layers, whereas nitrapyrin had no inhibitory effect. In the 0–5 cm peat layer the N₂O production could be reduced by up to 90% with inhibitors of nitrification, but in the 5–10 cm peat layer this proportion was 20–30%. All the inhibitors removed 96–100% of the nitrification potential in peat-water slurries from the 0–5 cm peat layer, but the 5–10 cm layer had a much lower nitrification activity, and here the efficiency of the inhibitors was more variable. Litter was the main net source of NO in the peat profile. NO₃ ^– production was lower in the litter layer than in the peat, whereas N₂O production was much higher in the litter than in the peat. Denitrification was the most probable source of N₂O and NO in the litter, which had a high availability of organic substrates. Received: 14 July 1997     

Influence of thiosulfate on nitrification,denitrification, and production of nitric oxide and nitrous oxide in soil

Thiosulfate and CS₂ inhibit nitrification. The effect of the addition of thiosulfate on the turnover of inorganic N compounds was tested in an Egyptian and a German arable soil under nitrifying and denitrifying conditions. For nitrification, the soils were amended with NH _inf4 ^sup+ and incubated under aerobic conditions. For denitrification, the soils were amended with NO _inf3 ^sup- and incubated under anaerobic conditions. In both cases, the thiosulfate decreased with time while tetrathionate accumulated to an intermediate extent. Both compounds disappeared completely after <25 days. Production of CS₂ was not observed. Carbonyl sulfide was produced only in the Egyptian soil, but production decreased with increasing amounts of added thiosulfate. Under nitrifying conditions, the addition of increasing amounts of thiosulfate (25, 50, and 100 g S g^-1 dry weight) resulted in decreasing rates of NH _inf4 ^sup+ oxidation to NO _inf3 ^sup- ; it also resulted in an increasing intermediate accumulation of NO _inf2 ^sup- and NO, and in an increasing production of N₂O. Under denitrifying conditions, the addition of increasing amounts of thiosulfate did not significantly affect the rate of NO _inf3 ^sup- reduction, and resulted in an increasing intermediate accumulation of NO _inf2 ^sup- and of NO only in the German soil in which the production of N₂O was slightly inhibited by thiosulfate. These results demonstrate that the nitrification of NH _inf4 ^sup+ and NO _inf2 ^sup- was inhibited by increasing concentrations of thiosulfate and/or tetrathionate without involving the formation of volatile S compounds as potential nitrification inhibitors. Denitrification was not affected by the addition of thiosulfate.     

Carbon dioxide concentration in soil. Effects on nitrification,denitrification and associated nitrous oxide production

Experiments were conducted to study the effects of a range of CO₂ concentrations (ambient to 100%) on nitrification, denitrification and associated nitrous oxide (N₂O) production in a silt loam soil. It was found that increase in CO₂ concentration from 0.3 to 100% CO₂ increasingly retarded the rate of nitrification. No nitrification occurred at 100% CO₂. Nitrous oxide production associated with nitrification increased as CO₂ increased from 0.3 to 2.6% and tended to be greater as CO₂ concentration increased to 73%. At 100% CO₂, no N₂O was produced during 7 days at 25°C.Carbon dioxide did not affect N₂O production or reduction in a saturated NO₃^?amended soil or the rate of N₂O reduction in anaerobic environments.     

Influence of nitrification inhibitors on nitrification and nitrous oxide (N2O) emission from a clay loam soil fertilized with urea

Laboratory incubation experiments were conducted to compare the effects of the nitrification inhibitors 3,4-dimethylpyrazole phosphate (DMPP) and 2-Chloro-6-(trichloromethyl)-pyridine (N-serve) on nitrification and nitrous oxide (N₂O) emission from a Vertosol from southern Australia, under controlled moisture and temperature. Nitrification rates in the control soil were strongly influenced by the temperature and moisture, increasing by a factor of 3.6 for each 10 °C increase between 5 and 25 °C. DMPP inhibited nitrification effectively for 42 days at 5-15 °C and 40-60% water filled pore space (WFPS). DMPP also slowed nitrification appreciably at 25 °C when the soil was at 40% WFPS, but was less effective at 60% water filled pore space. N-serve inhibited nitrification effectively for 42 days under all test conditions. Emissions of N₂O from the urea treatment (no inhibitors) significantly increased with increasing temperature and moisture. The ratio of total N₂O emission to total nitrification was not constant and varied from around 0.03% at 5 °C and 40% WFPS to 0.12% at 25 °C and 60% WFPS. DMPP and N-serve reduced cumulative N₂O emission over 42 days by more than 65% under all the imposed conditions.     

Role of nitrite and nitric oxide in the processes of nitrification and denitrification in soil: Results from N tracer experiments

Recent research has proven soil nitrite to be a key element in understanding N-gas production (NO, N₂O, N₂) in soils. NO is widely accepted to be an obligatory intermediate of N₂O formation in the denitrification pathway. However, studies with native soils could not confirm NO as a N₂O precursor, and field experiments mainly revealed ammonium nitrification as the source of NO. The hypothesis was constructed, that the limited diffusion of NO in soil is the reason for this contradiction. To test this diffusion limitation hypothesis and to verify nitrite and NO as free intermediates in native soils we conducted through-flow (He/O₂ atmosphere) ¹⁵N tracer experiments using black earth soil in an experimental set up free of diffusion limitation. All of the three relevant inorganic N soil pools (ammonium, nitrite, nitrate) were ¹⁵N labelled in separate incubation experiments lasting 81 h based on the kinetic isotope method. During the experiments the partial pressure of O₂ was decreased in four steps from 20% to about 0%. The net NO emission increased up to 3.7 μg N kg⁻¹ h⁻¹ with decreasing O₂ partial pressure. Due to the special experimental set up with little to no obstructions of gas diffusion, only very low N₂O emission could be observed. As expected the content of the substrates ammonium, nitrate and nitrite remained almost constant over the incubation time. The ¹⁵N abundance of nitrite revealed high turnover rates. The contribution of nitrification of ammonium to the total nitrite production was approx. 88% under strong aerobic soil conditions but quickly decreased to zero with declining O₂ partial pressure. It is remarkable that already under the high partial pressure of 20% O₂ 12 % of nitrite is generated by nitrate denitrification, and under strict anaerobic conditions it increases to 100%. Nitrite is present in two separate endogenous pools at least, each one fed by the nitrification of ammonium or the denitrification of nitrate. The experiments clearly revealed that nitrite is almost 100% the direct precursor of NO formation under anaerobic as well as aerobic conditions. Emitted N₂O only originated to about 100% from NO under strict anaerobic conditions (0-0.2% O₂), providing evidence that NO is a free intermediate of N₂O formation by denitrification. To the best of our knowledge this is the first time that NO has been detected in a native soil as a free intermediate product of N₂O formation at denitrification. These results clearly verify the “diffusion limitation” hypothesis.     

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).     

Nitrous oxide production by denitrification and nitrification in temperate forest, grassland and agricultural soils

Nitrous oxide is produced in soils by biological denitrification and nitrification. To improve the fundamental understanding of the processes leading to N₂O fluxes from soils, the production of N₂O from denitrification and nitrification in spruce forest, beech forest, riparian grassland, coastal grassland and an agricultural field were studied. Samples were taken at a high and a low position along a topographic gradient in each site in the spring and autumn when the largest N₂O fluxes were expected. They were incubated after being amended with N, and C₂H₂ was used as biological inhibitor to distinguish nitrification and denitrification. The N₂O production in the low landscape position varied between 32 and 121 ng N cm^?3 h^?1 in the riparian grassland. 9 and 26 ng N cm^?3 h^?1 in the coastal grassland, and 135 and 195 ng N cm^?3 h^?1 in the agricultural field which was 10–100 times more than in the high positions where rates ranged between 3 and 5 ng N cm^?3 h^?1, 0.3 and 0.4 ng N cm^?3 h^?1, and 7 and 10 ng N cm^?3 h^?1, respectively. These differences almost certainly arose because the soil in the low positions was wetter and contained more organic matter. In the two forests N₂O production was less than 1 ng N cm^?3 h^?1, strongly inhibited by O₂, and not influenced by landscape position. Nitrification contributed to more than 60% of total N₂O production in the riparian grassland. In the agricultural field nitrification produced 13–74% of the total N₂O in the low position, and 10–88% in the high position. Denitrification was the dominant source of N₂O in the coastal grassland except at the low position in the autumn where nitrification produced 60% of the total N₂O. In the two forests where the soil had small nitrification potentials denitrification was the only source of N₂O. In the other sites nitrification and denitrification potentials were large and of identical magnitude. The results emphasize the need to separate nitrification and denitrification at the process level and to recognize topography at the field scale when modelling N₂O effluxes from soil.     

Contribution of Heterotrophic Nitrification to Nitrous Oxide Production in a Long-Term N-Fertilized Arable Black Soil

To understand the contribution of key microbial processes to nitrous oxide (N₂O) emission in intensively cultivated black soil, laboratory incubation were conducted at 70% water-holding capacity (WHC) and 25 °C, using different gases (air, oxygen, or argon) within the headspace of the incubation chambers to evaluate gas inhibition effects. Arable black soil was sampled from an experimental field that has received urea since October 1979. Nitrification contributed to 57% of total N₂O emission, of which as much as 67% resulted from heterotrophic nitrification. These data strongly suggest that high soil organic carbon concentrations and low pH values are more favorable to N₂O production through heterotrophic, rather than autotrophic, nitrification. Nitrous oxide produced by denitrification accounted for 28% of the total N₂O emission, and the nitrifier denitrification accounted for 15% of the N₂O emitted from the tested soil. These findings indicate that heterotrophic nitrification was the primary N₂O production process in the tested soil.     

Isotopomer analysis of nitrous oxide accumulated in soil cultivated with tea (Camellia sinensis) in Shizuoka,central Japan

Nitrous oxide (N₂O) is a greenhouse gas that is destroying the stratospheric ozone to an increasing degree. Because of nitrogenous fertilizer application, agricultural soil is an important contributor of global N₂O. In Japan, tea fields are amended with the highest level of N fertilizers among agricultural soils, causing soil acidification and large N₂O flux. In soil, microbes play key roles in producing and consuming N₂O. A previous study investigated net N₂O production in tea fields using N₂O flux measurement and soil incubation, which are indirect methods to analyze relevant processes of N₂O production and consumption in soil. In the present study, to analyze N₂O concentrations and isotopomer ratios (bulk nitrogen and oxygen isotope ratios, δ¹⁵N^bulk and δ¹⁸O, and intramolecular ¹⁵N site preference, SP) and to reveal most probable microbial production processes and consumption (N₂O reduction to N₂) process of N₂O, soil gas was collected from a tea field (pH 3.1–4.5) at 10–50 cm depths using a silicone tube. The combination of fertilization, precipitation, and temperature rise produced significantly high N₂O concentrations. During the period of high N₂O concentration (above 5.7 ppmv), SP, the difference in ¹⁵N/¹⁴N ratio between central (α) and terminal (β) nitrogen position in the linear N₂O molecule (^βN^αNO) showed low values of 1.4‰–9.8‰, suggesting that the contribution of bacterial denitrification (nitrifier-denitrification and bacterial denitrifier-denitrification) was greater than that of bacterial nitrification or fungal denitrification. High SP values of 15.0‰–20.1‰ obtained at 10, 35, and 50 cm depths on 31 May 2011 (after one of fertilizations) during which soil temperatures were 15.8 °C–17.9 °C and water-filled pore space (WFPS) was 0.73–0.89 suggest that fungal denitrification and bacterial nitrification contributed to N₂O production to a degree equivalent to that of bacterial denitrification.     

Nitrogen oxides emission from soils bearing a potato crop as influenced by fertilization with treated pig slurries and composts

Nitrous oxide, nitric oxide and denitrification losses from an irrigated soil amended with organic fertilizers with different soluble organic carbon fractions and ammonium contents were studied in a field study covering the growing season of potato (Solanum tuberosum). Untreated pig slurry (IPS) with and without the nitrification inhibitor dicyandiamide (DCD), digested thin fraction of pig slurry (DTP), composted solid fraction of pig slurry (CP) and composted municipal solid waste (MSW) mixed with urea were applied at a rate of 175 kg available N ha⁻¹, and emissions were compared with those from urea (U) and a control treatment without any added N fertilizer (Control). The cumulative denitrification losses correlated significantly with the soluble carbohydrates, dissolved N and total C added. Added dissolved organic C (DOC) and dissolved N affected the N₂O/N₂ ratio, and a lower ratio was observed for organic fertilizers than from urea or unfertilized controls. The proportion of N₂O produced from nitrification was higher from urea than from organic fertilizers. Accumulated N₂O losses during the crop season ranged from 3.69 to 7.31 kg N₂O-N ha⁻¹ for control and urea, respectively, whereas NO losses ranged from 0.005 to 0.24 kg NO-N ha⁻¹, respectively. Digested thin fraction of pig slurry compared to IPS mitigated the total N₂O emission by 48% and the denitrification rate by 33%, but did not influence NO emissions. Composted pig slurry compared to untreated pig slurry increased the N₂O emission by 40% and NO emission by 55%, but reduced the denitrification losses (34%). DCD partially inhibited nitrification rates and reduced N₂O and NO emissions from pig slurry by at least 83% and 77%, respectively. MSW+U, with a C:N ratio higher than that of the composted pig slurry, produced the largest denitrification losses (33.3 kg N ha⁻¹), although N₂O and NO emissions were lower than for the U and CP treatments.This work has shown that for an irrigated clay loam soil additions of treated organic fertilizers can mitigate the emissions of the atmospheric pollutants NO and N₂O in comparison with urea.     

Responses of nitrous oxide, dinitrogen and carbon dioxide production and oxygen consumption to temperature in forest and agricultural light-textured soils determined by model experiment

 The experiment, carried out on a forest and arable light-textured soil, was designed to study the temperature response of autotrophic and heterotrophic N₂O production and investigate how the N₂O flux relates to soil respiration and O₂ consumption. Although N₂O production seemed to be stimulated by a temperature increase in both soils, the relationship between production rate and temperature was different in the two soils. This seemed to depend on the different contribution of nitrification and denitrification to the overall N₂O flux. In the forest soil, almost all N₂O was derived from nitrification, and its production rate rose linearly from 2  °C to 40  °C. A stronger effect of temperature on N₂O production was observed in the arable soil, apparently as a result of an incremental contribution of denitrification to the overall N₂O flux with rising temperature. The soil respiration rate increased exponentially with temperature and was significantly correlated with N₂O production. O₂ consumption stimulated denitrification in both soils. In the arable soil, N₂O and N₂ production increased exponentially with decreasing O₂ concentration, though N₂O was the main gas produced at any temperature. In the forest soil, only the N₂ flux was related exponentially to O₂ consumption and it outweighed the rate of N₂O production only at >34  °C. Thus, it appears that in the forest soil, where nitrification was the main source of N₂O, temperature affected the N₂O flux less dramatically than in the arable soil, where a temperature increase strongly stimulated N₂O production by enhancing favourable conditions for denitrification. Received: 26 August 1998     

Immediate and adaptational temperature effects on nitric oxide production and nitrous oxide release from nitrification and denitrification in two soils

 Nitrification and denitrification are, like all biological processes, influenced by temperature. We investigated temperature effects on N trace gas turnover by nitrification and denitrification in two soils under two experimental conditions. In the first approach ("temperature shift experiment") soil samples were preincubated at 25  °C and then exposed to gradually increasing temperatures (starting at 4  °C and finishing at 40–45  °C). Under these conditions the immediate effect of temperature change was assessed. In the second approach ("discrete temperature experiment") the soil samples were preincubated at different temperatures (4–35  °C) for 5 days and then tested at the same temperatures. The different experimental conditions affected the results of the study. In the temperature shift experiment the NO release increased steadily with increasing temperature in both soils. In the discrete temperature experiment, however, the production rates of NO and N₂O showed a minimum at intermediate temperatures (13–25  °C). In one of the soils (soil B9), the percent contribution of nitrification to NO production in the discrete temperature experiment reached a maximum (>95% contribution) at 25  °C. In the temperature shift experiment nitrification was always the dominant process for NO release and showed no systematic temperature dependency. In the second soil (soil B14), the percent contribution of nitrification to NO release decreased from 50 to 10% as the temperature was increased from 4  °C to 45  °C, but no differences were evident in the discrete temperature experiment. The N₂O production rates were measured in the discrete temperature experiment only. The contribution of nitrification to N₂O production in soil B9 was considerably higher at 25–35  °C (60–80% contribution) than at 4–13  °C (15–20% contribution). In soil B14 the contribution of nitrification to N₂O production was lowest at 4  °C. The effects of temperature on N trace gas turnover differed between the two soils and incubation conditions. The experimental set-up allowed us to distinguish between immediate effects of short-term changes in temperature on the process rates, and longer-term effects by which preincubation at a particular temperature presumably resulted in the adaptation of the soil microorganisms to this temperature. Both types of effects were important in regulating the release of NO and N₂O from soil. Received: 20 October 1998     

区分土壤中硝化与反硝化对N2O产生贡献的方法

土壤是产生N₂O的最主要来源之一。硝化和反硝化反应是产生N₂O的主要机理，由于硝化和反硝化微生物同时存在于土壤中，因而硝化和反硝化作用能同时产生N₂O。N₂O的来源可通过使用选择性抑制剂，杀菌剂以及加入的标记底物确定。通过对生成N₂O反应的每一步分析，主要从抑制反应发生的催化酶和细菌着手，总结了测量区分硝化、反硝化和DNRA反应对N₂O产生的贡献方法。并对¹⁵N标记底物法，乙炔抑制法和环境因子抑制法作了详细介绍。     

Small-scale heterogeneity in carbon dioxide, nitrous oxide and methane production from aggregates of a cultivated sandy-loam soil

Spatial variability in carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) emissions from soil is related to the distribution of microsites where these gases are produced. Porous soil aggregates may possess aerobic and anaerobic microsites, depending on the water content of pores. The purpose of this study was to determine how production of CO₂, N₂O and CH₄ was affected by aggregate size and soil water content. An air-dry sandy loam soil was sieved to generate three aggregate fractions (<0.25 mm, 0.25–2 mm and 2–6 mm) and bulk soil (<2 mm). Aggregate fractions and bulk soil were moistened (60% water-filled pore space, WFPS) and pre-incubated to restore microbial activity, then gradually dried or moistened to 20%, 40%, 60% or 80% WFPS and incubated at 25 °C for 48 h. Soil respiration peaked at 40% WFPS, presumably because this was the optimum level for heterotrophic microorganisms, and at 80% WFPS, which corresponded to the peak N₂O production. More CO₂ was produced by microaggregates (<0.25 mm) than macroaggregate (>0.25 mm) fractions. Incubation of aggregate fractions and soil at 80% WFPS with acetylene (10 Pa and 10 kPa) and without acetylene showed that denitrification was responsible for 95% of N₂O production from microaggregates, while nitrification accounted for 97–99% of the N₂O produced by macroaggregates and bulk soil. This suggests that oxygen (O₂) diffusion into and around microaggregates was constrained, whereas macroaggregates remained aerobic at 80% WFPS. Methane consumption and production were measured in aggregates, reaching 1.1–6.4 ng CH₄–C kg⁻¹ soil h⁻¹ as aggregate fractions and soil became wetter. For the sandy-loam soil studied, we conclude that nitrification in aerobic microsites contributed importantly to total N₂O production, even when the soil water content permitted denitrification and CH₄ production in anaerobic microsites. The relevance of these findings to microbial processes controlling N₂O production at the field scale remains to be confirmed.     

Nitrous oxide production of heavy metal contaminated soil

Arsenic (As), lead (Pb), copper (Cu) and zinc (Zn) can be found in large concentrations in mine spills of central and northern Mexico. Interest in these heavy metals has increased recently as they contaminate drinking water and aquifers in large parts of the world and severely affect human health, but little is known about how they affect biological functioning of soil. Soils were sampled in seven locations along a gradient of heavy metal contamination with distance from a mine in San Luis Potosí (Mexico), active since about 1800 AD. C mineralization and N₂O production were monitored in an aerobic incubation experiment. Concentrations of As in the top 0-10 cm soil layer ranged from 8 to 22,992 mg kg⁻¹, from 31 to 1845 mg kg⁻¹ for Pb, from 27 to 1620 mg kg⁻¹ for Cu and from 81 to 4218 mg kg⁻¹ for Zn. There was a significant negative correlation between production rates of CO₂ and concentrations of As, Pb, Cu and Zn, and there was a significant positive correlation with pH, water holding capacity (WHC), total N and soil organic C. There was a significant negative correlation (P<0.05) between production rate of nitrous oxide (N₂O) attributed to nitrification by the inhibition method in soil incubated at 50% WHC and total concentrations of Pb and Zn, and there was a significant positive correlation (P<0.05) with pH and total N content. There was a significant negative correlation (P<0.05) between the production rate of N₂O attributed to denitrification by the inhibition method in soil incubated at 100% WHC and total concentrations of Pb, Cu and Zn, and a significant positive correlation (P<0.01) with pH; there was a significant positive correlation (P<0.05) between the production of N₂O attributed to other processes by the inhibition method and WHC, inorganic C and clay content. A negative value for production rate of N₂O attributed to nitrifier denitrification by the inhibition method was obtained at 100% WHC. The large concentrations of heavy metals in soil inhibited microbial activity and the production rate of N₂O attributed to nitrification by the inhibition method when soil was incubated at 50% WHC and denitrification when soil was incubated at 100% WHC. The inhibitor/suppression technique used appeared to be flawed, as negative values for nitrifier denitrification were obtained and as the production rate of N₂O through denitrification increased when soil was incubated with C₂H₂.     

A method of selective inhibition to distinguish between nitrification and denitrification as sources of nitrous oxide in soil

Summary Nitrapyrin and C₂H₂ were evaluated as nitrification inhibitors in soil to determine the relative contributions of denitrification and nitrification to total N₂O production. In laboratory experiments nitrapyrin, or its solvent xylene, stimulated denitrification directly or indirectly and was therefore considered unsuitable. Low partial pressures of C₂H₂ (2.5–5.0 Pa) inhibited nitrification and had only a small effect on denitrification, which made it possible to estimate the contribution of denitrification. The contribution of nitrification was estimated by subtracting the denitrification value from total N₂O production (samples without C₂H₂). The critical C₂H₂ concentrations needed to achieve inhibition of nitrification, without affecting the N₂O reductase in denitrifiers, must be individually determined for each set of experimental conditions.     

The influence of soluble carbon and fertilizer nitrogen on nitric oxide and nitrous oxide emissions from two contrasting agricultural soils

Contradictory effects of simultaneous available organic C and N sources on nitrous oxide (N₂O), carbon dioxide (CO₂) and nitric oxide (NO) fluxes are reported in the literature. In order to clarify this controversy, laboratory experiments were conduced on two different soils, a semiarid arable soil from Spain (soil I, pH=7.5, 0.8%C) and a grassland soil from Scotland (soil II, pH=5.5, 4.1%C). Soils were incubated at two different moisture contents, at a water filled pore space (WFPS) of 90% and 40%. Ammonium sulphate, added at rates equivalent to 200 and 50 kg N ha^?1, stimulated N₂O and NO emissions in both soils. Under wet conditions (90% WFPS), at high and low rates of N additions, cumulative N₂O emissions increased by 250.7 and 8.1 ng N₂O–N g^?1 in comparison to the control, respectively, in soil I and by 472.2 and 2.1 ng N₂O–N g^?1, respectively, in soil II. NO emissions only significantly increased in soil I at the high N application rate with and without glucose addition and at both 40% and 90% WFPS. In both soils additions of glucose together with the high N application rate (200 kg N ha^?1) reduced cumulative N₂O and NO emissions by 94% and 55% in soil I, and by 46% and 66% in soil II, respectively. These differences can be explained by differences in soil properties, including pH, soil mineral N and total and dissolved organic carbon content. It is speculated that nitrifier denitrification was the main source of NO and N₂O in the C-poor Spanish soil, and coupled nitrification–denitrification in the C-rich Scottish soil.     

Processes responsible for the nitrous oxide emission from a Costa Rican Andosol under a coffee agroforestry plantation

We used the inhibitor acetylene (C₂H₂) at partial pressures of 10 Pa and 10 kPa to inhibit autotrophic nitrification and the reduction of nitrous oxide (N₂O) to N₂, respectively. Soils (Andosol) from a Coffea arabica plantation shaded by Inga densiflora in Costa Rica were adjusted to 39, 58, 76 and 87% water-filled pore space (WFPS) and incubated for 6 days in the absence or presence of C₂H₂. Soil respiration, nitrification rates and N₂O emissions by both processes were measured in relation to soil moisture conditions. At all WFPS studied, rates of N₂O and N₂ productions were small (4.8; 14.7; 23 and 239.6 ng N–N₂O g⁻¹ d.w. d⁻¹ at 39, 58, 76 and 87% WFPS, respectively), and despite a low soil pH (4.7), N₂O was mainly produced by nitrification, which was responsible for 85, 91, 84 and 87% of the total N₂O emissions at 39, 58, 76 and 87% WFPS, respectively. At the three smaller values of WFPS, a linear relationship was established between WFPS, soil respiration, nitrification and N₂O released by nitrification; no N₂ was produced by denitrification. At more anaerobic conditions achieved by a WFPS of 87%, a large rate of N₂O production was measured during nitrification, and N₂ production accounted for 84% of the gaseous N fluxes caused by denitrification.