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  • 1
    Language: English
    In: Analytical chemistry, 04 April 2017, Vol.89(7), pp.4124-4130
    Description: N isotope tracing is an effective and direct approach to investigate sources of nitric oxide (NO) formed in soils. However, NO is highly reactive and rapidly converted to nitrogen dioxide (NO) in the presence of ozone, making it impossible to directly measure N in NO. Various wet-chemical methods for conversion of NO to nitrite (NO) and nitrate (NO) have been proposed for N analysis in high-concentration NO sources, such as combustion processes. In contrast, NO concentrations in the soil surface-near air are usually small (ppbv-range), posing major challenges to conversion efficiency and blank correction. Here, we present a modified method in which NO is oxidized quantitatively to NO by chromium trioxide (CrO), before conversion to NO and NO in an alkaline hydrogen peroxide (HO) solution. A denitrifier method was used to reduce NO and NO in the trapping solution quantitatively to nitrous oxide (NO) for subsequent N analysis. NO trapping efficiencies of 〉85% were obtained with 50 ppb NO in a 0.5 L min air stream bubbling through a solution of 1.2 M HO and 0.5 M NaOH. In a laboratory test with distinct NO abundances, the overall precision was 0.29‰ (δ-values) for natural abundance NO and 0.13 atom % for labeled NO, suggesting that our method can be used for both natural abundance studies and N labeling experiments. In a soil incubation experiment with NHNO, NHNO, or NaNO amendments, we found distinct N abundances in NO, indicating that our method is well suited to investigate NO sources in soils.
    Keywords: Nitrogen Dioxide ; Efficiency ; Soil Investigations ; Soil (Material) ; Abundance ; Conversion ; Trapping ; Combustion ; Analysis (MD) ; Chemical Analysis (Ep) ; Chemical Analysis (Ed) ; Chemical Analysis (EC);
    ISSN: 00032700
    E-ISSN: 1520-6882
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  • 2
    In: Global Change Biology, November 2016, Vol.22(11), pp.3662-3674
    Description: In forests of the humid subtropics of China, chronically elevated nitrogen (N) deposition, predominantly as ammonium (NH), causes significant nitrate (NO) leaching from well‐drained acid forest soils on hill slopes (HS), whereas significant retention of NO occurs in near‐stream environments (groundwater discharge zones, GDZ). To aid our understanding of N transformations on the catchment level, we studied spatial and temporal variabilities of concentration and natural abundance (δN and δO) of nitrate (NO) in soil pore water along a hydrological continuum in the N‐saturated Tieshanping (TSP) catchment, southwest China. Our data show that effective removal of atmogenic NH and production of NO in soils on HS were associated with a significant decrease in δN‐NO, suggesting efficient nitrification despite low soil pH. The concentration of NO declined sharply along the hydrological flow path in the GDZ. This decline was associated with a significant increase in both δN and δO of residual NO, providing evidence that the GDZ acts as an N sink due to denitrification. The observed apparent N enrichment factor () of NO of about −5‰ in the GDZ is similar to values previously reported for efficient denitrification in riparian and groundwater systems. Episode studies in the summers of 2009, 2010 and 2013 revealed that the spatial pattern of δN and δO‐NO in soil water was remarkably similar from year to year. The importance of denitrification as a major N sink was also seen at the catchment scale, as largest δN‐NO values in stream water were observed at lowest discharge, confirming the importance of the relatively small GDZ for N removal under base flow conditions. This study, explicitly recognizing hydrologically connected landscape elements, reveals an overlooked but robust N sink in N‐saturated, subtropical forests with important implications for regional N budgets.
    Keywords: Denitrification ; Hydrological Continuum ; Nitrification ; Δ 15 N ; Δ 18 O
    ISSN: 1354-1013
    E-ISSN: 1365-2486
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  • 3
    Language: English
    In: Soil Biology and Biochemistry, February 2013, Vol.57, pp.606-614
    Description: Nitric oxide (NO) is known to induce denitrification in model organisms, but the quantitative effect of NO and its concentration dependency has not been assessed for denitrification in soils. NO is chemically unstable in the presence of oxygen due to autoxidation, and the oxidation of NO is accelerated by acetylene (C H ) which is commonly used as an inhibitor of N O reductase in denitrification studies. As a first step to experimentally explore the role of NO in soil denitrification, we investigated NO oxidation kinetics for a closed “two phase” system (i.e. liquid phase + headspace) typically used for denitrification experiments with soil slurries, both with and without acetylene present. Models were developed to adequately predict autoxidation and acetylene-accelerated oxidation. The minimum oxygen concentration in the headspace ([O ] , mL L ) for acetylene-accelerated NO oxidation was found to increase linearly with the NO concentration ([NO], mL L ); [O ]  = 0.192 + [NO] * 0.1 (  = 0.978). The models for NO oxidation were then used to assess NO oxidation rates in denitrification experiments with batches of bacterial cells extracted from soil. The batches were exposed to low initial oxygen concentrations in gas tight serum flasks (with and without C H ), and monitored for O , NO, N O and N production while depleting the oxygen and switching to anoxic respiration. Acetylene effectively scavenged NO from the cultures until oxygen concentration reached below ∼0.19 mL L , and the estimated rate of acetylene-accelerated NO oxidation was more than sufficient to explain an observed reduction of the N O production induced by acetylene. When [O ] reached below 0.19 mL L , the NO concentrations increased and stabilized at the same level as in the treatments without acetylene, but the rate of denitrification was much lower than without acetylene. The results indicate that the early accumulation of 10–20 nM NO during oxygen depletion has a significant effect on the expression of denitrification in soil communities. This warrants a greater interest in NO as a regulator of denitrification in soils and shows that the acetylene inhibition method may be problematic even for intentionally anoxic incubations, unless precautions are taken to secure initial O concentrations below 0.19 mL O L . ► The kinetics of NO oxidation in typical denitrification experiments can be simulated by simple models. ► Acetylene accelerates NO oxidation only if oxygen concentration exceed 0.19 μL L in the headspace. ► Acetylene-accelerated NO oxidation was utilized to elucidate the role of NO in inducing denitrification in soil bacteria. ► Early accumulation of NO during oxygen depletion exerts a strong positive feedback on the induction of denitrification.
    Keywords: Denitrification ; Nitric Oxide ; Acetylene ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 4
    Language: English
    In: Soil biology & biochemistry, 2013, Vol.57, pp.606-614
    Description: Nitric oxide (NO) is known to induce denitrification in model organisms, but the quantitative effect of NO and its concentration dependency has not been assessed for denitrification in soils. NO is chemically unstable in the presence of oxygen due to autoxidation, and the oxidation of NO is accelerated by acetylene (C₂H₂) which is commonly used as an inhibitor of N₂O reductase in denitrification studies. As a first step to experimentally explore the role of NO in soil denitrification, we investigated NO oxidation kinetics for a closed “two phase” system (i.e. liquid phase + headspace) typically used for denitrification experiments with soil slurries, both with and without acetylene present. Models were developed to adequately predict autoxidation and acetylene-accelerated oxidation. The minimum oxygen concentration in the headspace ([O₂]ₘᵢₙ, mL L⁻¹) for acetylene-accelerated NO oxidation was found to increase linearly with the NO concentration ([NO], mL L⁻¹); [O₂]ₘᵢₙ = 0.192 + [NO] * 0.1 (r² = 0.978). The models for NO oxidation were then used to assess NO oxidation rates in denitrification experiments with batches of bacterial cells extracted from soil. The batches were exposed to low initial oxygen concentrations in gas tight serum flasks (with and without C₂H₂), and monitored for O₂, NO, N₂O and N₂ production while depleting the oxygen and switching to anoxic respiration. Acetylene effectively scavenged NO from the cultures until oxygen concentration reached below ∼0.19 mL L⁻¹, and the estimated rate of acetylene-accelerated NO oxidation was more than sufficient to explain an observed reduction of the N₂O production induced by acetylene. When [O₂] reached below 0.19 mL L⁻¹, the NO concentrations increased and stabilized at the same level as in the treatments without acetylene, but the rate of denitrification was much lower than without acetylene. The results indicate that the early accumulation of 10–20 nM NO during oxygen depletion has a significant effect on the expression of denitrification in soil communities. This warrants a greater interest in NO as a regulator of denitrification in soils and shows that the acetylene inhibition method may be problematic even for intentionally anoxic incubations, unless precautions are taken to secure initial O₂ concentrations below 0.19 mL O₂ L⁻¹. ; p. 606-614.
    Keywords: Blood Serum ; Acetylene ; Oxygen ; Nitric Oxide ; Nitrous Oxide ; Soil Ecology ; Nitrogen ; Models ; Headspace Analysis ; Denitrification ; Slurries ; Autoxidation ; Soil
    ISSN: 0038-0717
    Source: AGRIS (Food and Agriculture Organization of the United Nations)
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  • 5
    In: PLoS ONE, 2015, Vol.10(9)
    Description: Biochar (BC) application to soil suppresses emission of nitrous- (N 2 O) and nitric oxide (NO), but the mechanisms are unclear. One of the most prominent features of BC is its alkalizing effect in soils, which may affect denitrification and its product stoichiometry directly or indirectly. We conducted laboratory experiments with anoxic slurries of acid Acrisols from Indonesia and Zambia and two contrasting BCs produced locally from rice husk and cacao shell. Dose-dependent responses of denitrification and gaseous products (NO, N 2 O and N 2 ) were assessed by high-resolution gas kinetics and related to the alkalizing effect of the BCs. To delineate the pH effect from other BC effects, we removed part of the alkalinity by leaching the BCs with water and acid prior to incubation. Uncharred cacao shell and sodium hydroxide (NaOH) were also included in the study. The untreated BCs suppressed N 2 O and NO and increased N 2 production during denitrification, irrespective of the effect on denitrification rate. The extent of N 2 O and NO suppression was dose-dependent and increased with the alkalizing effect of the two BC types, which was strongest for cacao shell BC. Acid leaching of BC, which decreased its alkalizing effect, reduced or eliminated the ability of BC to suppress N 2 O and NO net production. Just like untreated BCs, NaOH reduced net production of N 2 O and NO while increasing that of N 2 . This confirms the importance of altered soil pH for denitrification product stoichiometry. Addition of uncharred cacao shell stimulated denitrification strongly due to availability of labile carbon but only minor effects on the product stoichiometry of denitrification were found, in accordance with its modest effect on soil pH. Our study indicates that stimulation of denitrification was mainly due to increases in labile carbon whereas change in product stoichiometry was mainly due to a change in soil pH.
    Keywords: Research Article
    E-ISSN: 1932-6203
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  • 6
    Language: English
    In: Soil Biology and Biochemistry, December 2017, Vol.115, pp.337-345
    Description: Numerous studies have shown that soil moisture controls NO flux from soils. Less is known, however, to what extent microbial N-cycling mediates this control. Does soil moisture control NO release primarily by affecting the physical gas exchange between soil and atmosphere, by modulating microbial activities involved in biotic NO turnover, or by both? Using a novel dynamic chamber system for high-resolution measurement of NO release, we found one or several soil-specific maxima of NO release during dry-out experiments in different soils. A mid-latitude arable soil displayed a single maximum at 0.10 water holding capacity (whc), whereas a dryland farming and a rice paddy soil showed two maxima at 0.65/0.10 and 0.90/0.10 whc, respectively. Transcription of genes in the dryland soil at 0.65 whc was low, but larger than at 0.10 whc, while transcriptional activity of archaeal ammonia oxidizers showed the opposite pattern with higher activity at 0.10 whc, suggesting biogenic NO production at low soil moisture. Our study is a first attempt to link NO emission to soil moisture responses of different microbial groups.
    Keywords: Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 7
    Language: English
    In: Soil biology & biochemistry, 2013, Vol.57, pp.577-586
    Description: Subtropical forests in SW China, receiving high nitrogen (N) deposition, have been reported to show high N retention. N removal by denitrification may be one important process favored by warm and wet soils during monsoonal summer when most of the atmogenic N input occurs. Due to low pH soils which are common in this region, N removal by denitrification may entail substantial N₂O emissions. In this study, we explored intrinsic denitrification characteristics of soils from different landscape elements of a forested headwater catchment in SW China (Tieshanping, Chongqing). Laboratory incubations were used to measure instantaneous denitrification rates, apparent specific denitrifier growth rates, denitrification product stoichiometries (N₂O/(N₂O + N₂)) and response to carbon-addition as a function of soil depth and landscape position. The results revealed that potential denitrification and denitrifier growth rates were highest in top soils along a hill slope, and decreased strongly with soil depth due to C-limitation. A hydrologically connected, colluvial groundwater discharge zone showed equally high instantaneous denitrification rates which were more equally distributed with depth. Denitrifying communities in the top horizons of the hill slope were less efficient in expressing N₂O reductase in response to anoxia resulting in higher N₂O/(N₂ + N₂O) product ratios than found in soils of the groundwater discharge zone, suggesting that a significant share of the deposited N can be lost as N₂O from the hill slopes. Differences in denitrification traits appeared to be linked to eco-hydrological conditions in the two landscape elements and to substrate availability along the hydrological flowpath. Our study supports the notion that denitrification plays an important role for observed N removal in SW Chinese forest ecosystems and illustrates how habitat functions constrain the amount of N₂O emitted during N removal. ; p. 577-586.
    Keywords: Forests ; Hills ; Habitats ; Emissions ; Nitrous Oxide ; Forested Watersheds ; Hypoxia ; Nitrogen ; Groundwater ; Soil Depth ; Landscape Position ; Topographic Slope ; Denitrification ; Summer ; Forest Ecosystems ; Landscapes ; Ph
    ISSN: 0038-0717
    Source: AGRIS (Food and Agriculture Organization of the United Nations)
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  • 8
    Language: English
    In: Soil Biology and Biochemistry, December 2012, Vol.55, pp.104-112
    Description: Agricultural soils emit significant amounts of N O to the atmosphere, and annual emissions are in some proportion to the input of reactive nitrogen to the system. Hence the ongoing intensification of cropping systems in South Asia will result in increased emissions of N O. The prospects are potentially worse than those predicted by the increasing doses of N-fertilizers, however. The reason for this is that intensive cropping systems may acidify the soils, which could increase the N O/(N  + N O) product ratio of denitrification due to interference with the expression of the different enzymatic steps in this process ( FEMS Microbiol Ecol 72 407–417). We investigated this phenomenon for agricultural soils in the central mid-hills of Nepal. We sampled soils from fields that had been under intensified cultivation for ≥20 years, and adjacent fields with more traditional cultivation, in areas with permanently drained soils as well as areas with frequent flooding. The characteristic kinetics of NO, N O and N production by denitrification in these soils was measured by anoxic incubations after flooding and drainage of the soils with 2 mM NO , to secure similar NO -concentrations for all soils. The results demonstrate that intensification invariably lowered the soil pH and increased the N O/(N  + N O) product ratios of denitrification. This effect of intensification was observed both for incubations with and without C-substrates (glutamic acid) added. The transient accumulation of NO varied grossly between sites, but was not affected by intensification. The results demonstrate convincingly that the intensification has resulted in higher intrinsic propensity of the soils to emit N O to the atmosphere, and the correlation with pH suggests that acidification is responsible. This causal relationship is underpinned by emerging evidence that low pH interferes with the assembly of the enzyme N O-reductase. We conclude that the ongoing intensification of agriculture in South Asia may result in severely increasing N O emissions unless acidification of the soil is counteracted. ► Intensification of plant production may gradually acidify the soil. ► Acidification leads to high N O/(N  + N O) product ratio of denitrification. ► Intensification of plant production may lead to escalating N O emission. ► Soil pH management can moderate the effect of intensification on N O emission.
    Keywords: Nitrous Oxide ; Soil ; Agricultural Intensification ; Fertilizers ; Denitrification ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 9
    Language: English
    In: Soil Biology and Biochemistry, August 2016, Vol.99, pp.36-46
    Description: Experiments with soils have provided ample evidence that soil pH controls the N O/(N O + N ) ratio of denitrification, which increases with decreasing pH, most probably because low pH interferes with the expression of N O reductase in denitrifying bacteria. In contrast, the N O/NO product ratio of nitrification appears to be unaffected by soil pH within the range relevant for agricultural soils (pH 5.5–7.0). We hypothesized that local pH variations in cultivated soil may control N O emissions during periods of active denitrification. To test this hypothesis, we identified three plots with slightly different soil pH (5.4–5.9) within an agricultural field under spring ploughed cereal cropping, and placed four frames within each plot for measuring N O emissions throughout autumn and spring. Soil samples were taken from each frame after the experiment to characterize the kinetics of NO, N O and N production by anoxic incubation. The data were used to calculate an N O index, , which is an inverse measure of the capacity of the denitrifying community to effectively express N O reductase under anoxia and hence a proxy for the soil’s propensity to emit N O under denitrifying conditions. N O emissions were greatest during spring thaw, intermediate in autumn and low in late spring. Emissions during autumn and spring thaw were inversely related to soil pH, supporting the hypothesis that soil pH influences N O emissions when denitrification is the main source of N O. During these periods, emissions were positively correlated with , further substantiating the idea that soil pH affects denitrification product ratios . Total organic carbon and nitrate content were negatively correlated with soil pH, thus co-varying with N O emissions. However, the relationship of N O emission to TOC and nitrate appeared weaker than to pH. Off-season emissions dominate N O budgets in many regions. If the pH relationship holds at greater scales, careful soil pH management by precision liming could be a viable tool to reduce N O emissions.
    Keywords: N2o Emissions ; Soil Ph ; Denitrification Kinetics ; N2o Product Ratio ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 10
    Language: English
    In: Soil Biology and Biochemistry, February 2013, Vol.57, pp.577-586
    Description: Subtropical forests in SW China, receiving high nitrogen (N) deposition, have been reported to show high N retention. N removal by denitrification may be one important process favored by warm and wet soils during monsoonal summer when most of the atmogenic N input occurs. Due to low pH soils which are common in this region, N removal by denitrification may entail substantial N O emissions. In this study, we explored intrinsic denitrification characteristics of soils from different landscape elements of a forested headwater catchment in SW China (Tieshanping, Chongqing). Laboratory incubations were used to measure instantaneous denitrification rates, apparent specific denitrifier growth rates, denitrification product stoichiometries (N O/(N O + N )) and response to carbon-addition as a function of soil depth and landscape position. The results revealed that potential denitrification and denitrifier growth rates were highest in top soils along a hill slope, and decreased strongly with soil depth due to C-limitation. A hydrologically connected, colluvial groundwater discharge zone showed equally high instantaneous denitrification rates which were more equally distributed with depth. Denitrifying communities in the top horizons of the hill slope were less efficient in expressing N O reductase in response to anoxia resulting in higher N O/(N  + N O) product ratios than found in soils of the groundwater discharge zone, suggesting that a significant share of the deposited N can be lost as N O from the hill slopes. Differences in denitrification traits appeared to be linked to eco-hydrological conditions in the two landscape elements and to substrate availability along the hydrological flowpath. Our study supports the notion that denitrification plays an important role for observed N removal in SW Chinese forest ecosystems and illustrates how habitat functions constrain the amount of N O emitted during N removal. ► Forests with acid soils in subtropical China have high denitrification potentials. ► Potential denitrification and product stoichiometry differ with landscape position. ► Denitrification in mesic top soils on hillslopes has inherently high N O/N ratios. ► Colluvium with high groundwater levels have lower ratios. ► Contrasts in denitrifier functioning explain spatial distribution of N O emissions.
    Keywords: Denitrification ; Nitrous Oxide ; Product Stoichiometry ; Denitrifier Growth ; Subtropical Forest ; N Saturation ; Landscape ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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