Variations in soil N cycling and trace gas emissions in wet tropical forests

We used a previously described precipitation gradient in a tropical montane ecosystem of Hawai’i to evaluate how changes in mean annual precipitation (MAP) affect the processes resulting in the loss of N via trace gases. We evaluated three Hawaiian forests ranging from 2200 to 4050 mm year⁻¹ MAP with constant temperature, parent material, ecosystem age, and vegetation. In situ fluxes of N₂O and NO, soil inorganic nitrogen pools (NH₄⁺ and NO₃⁻), net nitrification, and net mineralization were quantified four times over 2 years. In addition, we performed ¹⁵N-labeling experiments to partition sources of N₂O between nitrification and denitrification, along with assays of nitrification potential and denitrification enzyme activity (DEA). Mean NO and N₂O emissions were highest at the mesic end of the gradient (8.7±4.6 and 1.1±0.3 ng N cm⁻² h⁻¹, respectively) and total oxidized N emitted decreased with increased MAP. At the wettest site, mean trace gas fluxes were at or below detection limit (≤0.2 ng N cm⁻² h⁻¹). Isotopic labeling showed that with increasing MAP, the source of N₂O changed from predominately nitrification to predominately denitrification. There was an increase in extractible NH₄⁺ and decline in NO₃⁻, while mean net mineralization and nitrification did not change from the mesic to intermediate sites but decreased dramatically at the wettest site. Nitrification potential and DEA were highest at the mesic site and lowest at the wet site. MAP exerts strong control N cycling processes and the magnitude and source of N trace gas flux from soil through soil redox conditions and the supply of electron donors and acceptors.     

Dinitrogen and nitrous oxide produced by denitrification and nitrification in soil with and without barley plants

Summary To examine the effect of barley roots on denitrification, a pot experiment was designed to compare N₂O production and denitrification in soils with and without barley plants. Denitrification, N₂O resulting from denitrification and nitrification, and respiration were estimated by incubating pots with soil with and without intact plants in plastic bags at high moisture levels. C₂H₂-inhibition of nitrous oxide reductase (partial pressure of 10 kPa C₂H₂) was used to determine total denitrification rates while incubations with ambient air and with C₂H₂ at partial pressures of 2.5–5 Pa were used to estimate the amounts of N₂O released from autotrophic nitrification and from denitrification processes. Other sources of N₂O were presumed to be negligible. Potential denitrification, nitrification and root biomass were measured in subsamples collected from four soil depths. A positive correlation was found between denitrification rates and root biomass. N₂ was the predominant denitrification product found close to roots; N₂O formed by non autotrophic nitrifiers, assumed to be denitrifiers originated in soil not affected by growing roots. Apparently, roots promote denitrification because they consumed oxygen, thereby increasing the anaerobic volume of the soil. The ratio of actual to potential denitrification rates increased over time, especially in the presence of roots.     

Nitrous oxide production, its source and distribution in urine patches on grassland on peat soil

Urine patches are considered to be important sites for nitrous oxide (N₂O) production through nitrification and denitrification due to their high concentration of nitrogen (N). The aim of the present study was to determine the microbial source and size of production of N₂O in different zones of a urine patch on grassland on peat soil. Artificial urine was applied in elongated patches of 4.5 m. Four lateral zones were distinguished and sampled for four weeks using an intact soil core incubation method. Incubation of soil cores took place without any additions to the headspace to determine total N₂O production, with acetylene addition to determine total denitrification (N₂O+N₂), and with methyl fluoride to determine the N₂O produced through denitrification.Nitrous oxide production was largest in the centre and decreased towards the edge of the patch. Maximum N₂O production was about 50 mg N m^–2 d^–1 and maximum denitrification activity was 70 mg N m^–2 d^–1. Nitrification was the main N₂O producing process. Nitrous oxide production through denitrification was only of significance when denitrification activity was high. Total N loss through nitrification and denitrification over 31 days was 4.1 g N per patch which was 2.2% of the total applied urine-N.     

雪被和土壤水分对典型半干旱草原土壤冻融过程中CO_2和N_2O排放的影响

土壤冻融期间的温室气体排放量会显著增加,并在全年总排放量中占有重要的份额。但目前开展的土壤冻融循环模拟实验大多是在土壤冻结之前调节土壤水分含量,而忽视了雪被在整个土壤冻融过程中的作用,因此导致室内模拟研究的结果与野外原位观测的结果差异较大。为探索开展室内模拟土壤冻融实验的优化方案,采用人工浇水和覆雪两种方式调节土壤水分含量,研究了雪被和土壤水分对内蒙古典型半干旱草原土壤冻融过程中CO2和N2O排放的影响。结果表明,浇水和覆雪两种处理对冻融循环过程中土壤CO2排放影响的差异不显著,CO2排放量在消融期都会明显增加并随着冻融循环次数的增加而逐渐减小。当土壤孔隙含水率达50%左右时,浇水处理中的N2O排放量在第1次土壤冻融循环中最高并随冻融循环次数增加而降低,但在覆雪处理中,N2O在第1次冻融循环中的排放较小,而在后两次冻融循环中的排放量更为显著。造成两种处理N2O排放规律出现显著不同的原因可能是土壤剖面水分动态变化过程和微生物性状等方面的差异。土壤冻融过程中CO2和N2O排放量随土壤含水量升高而增加,但N2O在土壤含水量较低时排放不明显,这表明可能只有当土壤含水量达到一定阈值时,冻融作用才会对N2O的排放产生显著影响。这些结果显示,雪被和土壤水分显著影响土壤冻融过程中的CO2和N2O排放,室内模拟土壤冻融实验应进一步优化。     

Processes leading to N2O and NO emissions from two different Chinese soils under different soil moisture contents

Background and Aims

Great attention has been paid to N₂O emissions from paddy soils under summer rice-winter wheat double-crop rotation, while less focus was given to the NO emissions. Besides, neither mechanism is completely understood. Therefore, this study aimed at evaluating the relative importance of nitrification and denitrification to N₂O and NO emissions from the two soils at different soil moisture contents

Methods

N₂O and NO emissions during one winter wheat season were simultaneously measured in situ in two rice-wheat based field plots at two different locations in Jiangsu Province, China. One soil was neutral in pH with silt loam texture (NSL), the other soil alkaline in pH with a clay texture (AC). A ¹⁵?N tracer incubation experiment was conducted in the laboratory to evaluate the relative importance of nitrification and denitrification for N₂O and NO emissions at soil moisture contents of 40 % water holding capacity (WHC), 65 % WHC and 90 % WHC.

Results

Higher N₂O emission rates in the AC soil than in the NSL soil were found both in the field and in the laboratory experiments; however, the differences in N₂O emissions between AC soil and NSL soil were smaller in the field than in the laboratory. In the latter experiment, nitrification was observed to be the more important source of N₂O emissions (>70 %) than denitrification, regardless of the soils and moisture treatments, with the only exception of the AC soil at 90 % WHC, at which the contributions of nitrification and denitrification to N₂O emissions were comparable. The ratios of NO/N₂O also supported the evidence that the nitrification process was the dominant source of N₂O and NO both in situ and in the laboratory. The proportion of nitrified N emitted as N₂O (P _N2O ) in NSL soil were around 0.02 % in all three moisture treatments, however, P _N2O in the AC soil (0.04 % to 0.10 %) tended to decrease with increasing soil moisture content.

Conclusions

Our results suggest that N₂O emission rates obtained from laboratory incubation experiments are not suitable for the estimation of the true amount of N₂O fluxes on a field scale. Besides, the variations of P _N2O with soil property and soil moisture content should be taken into account in model simulations of N₂O emission from soils.     

Management Practices of <Emphasis Type="Italic">Miscanthus</Emphasis> × <Emphasis Type="Italic">giganteus</Emphasis> Strongly Influence Soil Properties and N<Subscript>2</Subscript>O Emissions Over the Long Term

Cropping practices of Miscanthus × giganteus, a promising energy crop, can influence C and N cycles and therefore potentially affect N₂O emissions. They may vary in harvesting date, either early (EH) or late harvest (LH), and the fertiliser form (NH₄ or NO₃). In this study, we combined a long-term field experiment and simulations with the STICS model to investigate the effect of these practices on soil parameters, N₂O emissions and the contribution of nitrification and denitrification. Daily N₂O fluxes and soil parameters were measured during the 4-month period following fertilisation in 2014 and 2015. Mean cumulative N₂O emissions were markedly higher in LH than EH (4.21 vs. 0.89 kg N₂O–N ha^?1 year^?1) but did not differ significantly between fertiliser forms or years. The difference was mainly attributed to the higher soil water-filled pore space (WFPS) observed in LH (80 vs. 56 % in EH) which resulted itself from the leaf mulch present in LH and not in EH. WFPS explained 67 % of the variance of N₂O emissions. The large decrease in pH observed after NH₄ fertilisation stimulated N₂O emissions probably through less-efficient reduction of N₂O to N₂ as simulated by STICS. Model outputs suggest a large contribution of nitrification in EH and a dominant contribution of denitrification in LH. Our study highlights the crucial impact of management practises on N₂O emissions in Miscanthus crops through changes in physico-chemical parameters and soil processes on the short and long term and brings knowledge required to maximise the benefits of bioenergy crops.     

Four years of experimental climate change modifies the microbial drivers of N2O fluxes in an upland grassland ecosystem

Emissions of the trace gas nitrous oxide (N₂O) play an important role for the greenhouse effect and stratospheric ozone depletion, but the impacts of climate change on N₂O fluxes and the underlying microbial drivers remain unclear. The aim of this study was to determine the effects of sustained climate change on field N₂O fluxes and associated microbial enzymatic activities, microbial population abundance and community diversity in an extensively managed, upland grassland. We recorded N₂O fluxes, nitrification and denitrification, microbial population size involved in these processes and community structure of nitrite reducers (nirK) in a grassland exposed for 4 years to elevated atmospheric CO₂ (+200 ppm), elevated temperature (+3.5 °C) and reduction of summer precipitations (?20%) as part of a long‐term, multifactor climate change experiment. Our results showed that both warming and simultaneous application of warming, summer drought and elevated CO₂ had a positive effect on N₂O fluxes, nitrification, N₂O release by denitrification and the population size of N₂O reducers and NH₄ oxidizers. In situ N₂O fluxes showed a stronger correlation with microbial population size under warmed conditions compared with the control site. Specific lineages of nirK denitrifier communities responded significantly to temperature. In addition, nirK community composition showed significant changes in response to drought. Path analysis explained more than 85% of in situ N₂O fluxes variance by soil temperature, denitrification activity and specific denitrifying lineages. Overall, our study underlines that climate‐induced changes in grassland N₂O emissions reflect climate‐induced changes in microbial community structure, which in turn modify microbial processes.     

Influence of O2 availability on NO and N2O release by nitrification and denitrification in soils

The availability of O₂ is believed to be one of the main factors regulating nitrification and denitrification and the release of NO and N₂O. The availability of O₂ in soil is controlled by the O₂ partial pressure in the gas phase and by the moisture content in the soil. Therefore, we investigated the influence of O₂ partial pressures and soil moisture contents on the NO and N₂O release in a sandy and a loamy silt and differentiated between nitrification and denitrification by selective inhibition of nitrification with 10 Pa acetylene. At 60% whc (maximum water holding capacity) NO and N₂O release by denitrification increased with decreasing O₂ partial pressure and reached a maximum under anoxic conditions. Under anoxic conditions NO and N₂O were only released by denitrification. NO and N₂O release by nitrification also increased with decreasing O₂ partial pressure, but reached a maximum at 0.1–0.5% O₂ and then decreased again. Nitrification was the main source of NO and N₂O at O₂ partial pressures higher than 0.1–0.5% O₂. At lower O₂ partial pressures denitrification was the main source of NO and N₂O. With decreasing O₂ partial pressure N₂O release increased more than NO release, indicating that the N₂O release was more sensitive against O₂ than the NO release. At ambient O₂ partial pressure (20.5% O₂) NO and N₂O release by denitrification increased with increasing soil moisture content. The maximum NO and N₂O release was observed at soil moisture contents of 65–80% whc and 100% whc, respectively. NO and N₂O release by nitrification also increased with increasing soil moisture content with a maximum at 45–55% whc and 90% whc, respectively. Nitrification was the main source of NO and N₂O at soil moisture contents lower than 90% whc and 80% whc, respectively. Higher soil moisture contents favoured NO and N₂O release by denitrification. Soil texture had also an effect on the release of NO and N₂O. The coarse-textured sandy silt released more NO than N₂O compared with the fine-textured loamy silt. At high soil moisture contents (80–100% whc) the fine-textured soil showed a higher N₂O release by denitrification than the coarse-textured soil. We assume that the fine-textured soil became anoxic at a lower soil moisture content than the coarse-textured soil. In conclusion, the effects of O₂ partial pressure, soil moisture and soil texture were consistent with the theory that denitrification increasingly contributes to the release of NO and in particular N₂O when conditions for soil microorganisms become increasingly anoxic.     

Fluxes of nitrous oxide, methane and carbon dioxide during freezing–thawing cycles in an Inner Mongolian steppe

Combined measurements of nitrification activity and N₂O emissions were performed in a lowland and a montane tropical rainforest ecosystem in NE-Australia over a 18 months period from October 2001 until May 2003. At both sites gross nitrification rates, measured by the BaPS technique, showed a strong seasonal pattern with significantly higher rates of gross nitrification during wet season conditions. Nitrification rates at the montane site (1.48?±?0.24–18.75?±?2.38 mg N kg^?1 day^?1) were found to be significantly higher than at the lowland site (1.65?±?0.21–4.54?±?0.27 mg N kg^?1 day^?1). The relationship between soil moisture and gross nitrification rates could be described best by O’Neill functions having a soil moisture optimum of nitrification at app. 65% WFPS. At the lowland site, for which continuous measurements of N₂O emissions were available, nitrification was positively correlated with N₂O emission. Nitrification contributed significantly to N₂O formation during dry season (app.85%) but less (app. 30%) during wet season conditions. In average 0.19‰ of the N metabolized by nitrification was released as N₂O. The N₂O fraction loss for nitrification was positively correlated with changes in soil moisture and varied slightly between 0.15 and 0.22‰. Our results demonstrate that combined N₂O emission and microbial N turnover studies covering prolonged observation periods are needed to clarify and quantify the role of the microbial processes nitrification and denitrification for annual N₂O emissions from soils of terrestrial ecosystems.     

Increased nitrous oxide emissions from a drained organic forest soil after exclusion of ectomycorrhizal mycelia

The aim of this study was to determine how roots and their ectomycorrhizal symbionts affect the fluxes of nitrous oxide (N₂O) from nutrient-rich drained organic forest soils. Specifically, the relative impacts of roots and mycorrhizal mycelia on N₂O fluxes were investigated using two different trenching treatments, excluding (a) roots or (b) roots and mycorrhizal mycelia, from the soil. N₂O fluxes were measured at the soil surface, for 1 year before and 2.5 years after trenching, within the two trenching treatments and on untreated controls. While the exclusion of roots alone did not affect N₂O emissions, the simultaneous exclusion of roots and mycorrhizal mycelia doubled N₂O emissions, compared to the control plots. Two probable explanations for the increased fluxes were identified: (1) a decreased uptake of nitrogen (N) from the soil, through the mycorrhizal fungi, which increased N availability for the N₂O-producing microorganisms, and (2) a decreased uptake of water from the soil, through the mycorrhiza, which increased the soil water content and thus the N₂O emissions from denitrification. If the trenching reduced any potential stimulation of N cycling, through rhizodeposition, this mechanism did not outweigh the effects of a discontinued mycorrhizal N and/or water uptake on N₂O fluxes. The results of the study emphasise the importance of ectomycorrhiza in regulating N₂O emissions from forested organic soils.     

Bi-directional soil/atmosphere N2O exchange over two mown grassland systems with contrasting management practices

Nitrous oxide (N₂O) fluxes from soil under mown grassland were monitored using static chambers over three growing seasons in intensively and extensively managed systems in Central Switzerland. Emissions were largest following the application of mineral (NH₄NO₃) fertilizer, but there were also substantial emissions following cattle slurry application, after grass cuts and during the thawing of frozen soil. Continuous flux sampling, using automatic chambers, showed marked diurnal patterns in N₂O fluxes during emission peaks, with highest values in the afternoon. Net uptake fluxes of N₂O and subambient N₂O concentrations in soil open pore space were frequently measured on both fields. Flux integration over 2.5 years yields a cumulated emission of +4.7 kgN₂O‐N ha^?1 for the intensively managed field, equivalent to an average emission factor of 1.1%, and a small net sink activity of ?0.4 kg N₂O‐N ha^?1 for the unfertilized system. The data suggest the existence of a consumption mechanism for N₂O in dry, areated soil conditions, which cannot be explained by conventional anaerobic denitrification. The effect of fertilization on greenhouse gas budgets of grassland at the ecosystem level is discussed.     

Denitrification and N2O emission from urine-affected grassland soil

Denitrification and N₂O emission rates were measured following two applications of artificial urine (40 g urine-N m^–2) to a perennial rye-grass sward on sandy soil. To distinguish between N₂O emission from denitrification or nitrification, urine was also applied with a nitrification inhibitor (dicyandiamide, DCD). During a 14 day period following each application, the soil was frequently sampled, and incubated with and without acetylene to measure denitrification and N₂O emission rates, respectively.Urine application significantly increased denitrification and N₂O emission rates up to 14 days after application, with rates amounting to 0.9 and 0.6 g N m^–2 day^–1 (9 and 6 kg N ha^–1 day^–1), respectively. When DCD was added to the urine, N₂O emission rates were significantly lower from 3 to 7 days after urine application onwards. Denitrification was the main source of N₂O immediately following each urine application. 14 days after the first application, when soil water contents dropped to 15% (v/v) N₂O mainly derived from nitrification.Total denitrification losses during the 14 day periods were 7 g N m^–2, or 18% of the urine-N applied. Total N₂O emission losses were 6.5 and 3 g N m^–2, or 16% and 8% of the urine-N applied for the two periods. The minimum estimations of denitrification and N₂O emission losses from urine-affected soil were 45 to 55 kg N ha^–1 year^–1, and 20 to 50 kg N ha^–1 year^–1, respectively.     

The legacy effect of biochar application on soil nitrous oxide emissions

Existing studies suggest that biochar application can reduce soil nitrous oxide (N₂O) emissions, mainly based on short-term results. However, it remains unclear what the effects (i.e., legacy effects) and underlying mechanisms are on N₂O emissions after many years of a single application of biochar. Here, we collected intact soil columns from plots without and with biochar application in a subtropical tea plantation 7 years ago for an incubation experiment. We used the N₂O isotopocule analysis combined with ammonia oxidizer-specific inhibitors and molecular biology approaches to investigate how the legacy effect of biochar affected soil N₂O emissions. Results showed that the soil in the presence of biochar had lower N₂O emissions than the control albeit statistically insignificant. The legacy effect of biochar in decreasing N₂O emissions may be attributed to the reduced effectiveness of the soil substrate, nitrification and denitrification activities, and the promotion of the further reduction of N₂O. The legacy effect of biochar reduced the relative contribution of nitrifier denitrification/bacterial denitrification, nitrification-related N₂O production, and the relative abundance of several microorganisms involved in the nitrogen cycle. Our global meta-analysis also showed that the reduction of N₂O by biochar increased with increasing application rate but diminished and possibly even reversed with increasing experimental time. In conclusion, our findings suggest that the abatement capacity of biochar on soil N₂O emissions may weaken over time after biochar application, but this remains under further investigation.     

In situ 15N-N2O site preference and O2 concentration dynamics disclose the complexity of N2O production processes in agricultural soil

Arable soil continues to be the dominant anthropogenic source of nitrous oxide (N₂O) emissions owing to application of nitrogen (N) fertilizers and manures across the world. Using laboratory and in situ studies to elucidate the key factors controlling soil N₂O emissions remains challenging due to the potential importance of multiple complex processes. We examined soil surface N₂O fluxes in an arable soil, combined with in situ high-frequency measurements of soil matrix oxygen (O₂) and N₂O concentrations, in situ ¹⁵N labeling, and N₂O ¹⁵N site preference (SP). The in situ O₂ concentration and further microcosm visualized spatiotemporal distribution of O₂ both suggested that O₂ dynamics were the proximal determining factor to matrix N₂O concentration and fluxes due to quick O₂ depletion after N fertilization. Further SP analysis and in situ ¹⁵N labeling experiment revealed that the main source for N₂O emissions was bacterial denitrification during the hot-wet summer with lower soil O₂ concentration, while nitrification or fungal denitrification contributed about 50.0% to total emissions during the cold-dry winter with higher soil O₂ concentration. The robust positive correlation between O₂ concentration and SP values underpinned that the O₂ dynamics were the key factor to differentiate the composite processes of N₂O production in in situ structured soil. Our findings deciphered the complexity of N₂O production processes in real field conditions, and suggest that O₂ dynamics rather than stimulation of functional gene abundances play a key role in controlling soil N₂O production processes in undisturbed structure soils. Our results help to develop targeted N₂O mitigation measures and to improve process models for constraining global N₂O budget.     

Short term effect of ploughing a permanent pasture on N2O production from nitrification and denitrification

Soils are an important source of N₂O, which can be produced both in the nitrification and the denitrification processes. Grassland soils in particular have a high potential for mineralization and subsequent nitrification and denitrification. When ploughing long term grassland soils, the resulting high supply of mineral N may provide a high potential for N₂O losses. In this work, the short-term effect of ploughing a permanent grassland soil on gaseous N production was studied at different soil depths. Fertiliser and irrigation were applied in order to observe the effect of ploughing under a range of conditions. The relative proportions of N₂O produced from nitrification and denitrification and the proportion of N₂ gas produced from denitrification were determined using the methyl fluoride and acetylene specific inhibitors. Irrespectively to ploughing, fertiliser application increased the rates of N₂O production, N₂O production from nitrification, N₂O production from denitrification and total denitrification (N₂O + N₂). Application of fertiliser also increased the denitrification N₂O/N₂ ratio both in the denitrification potential and in the gaseous N productions by denitrification. Ploughing promoted soil organic N mineralization which led to an increase in the rates of N₂O production, N₂O production from nitrification, N₂O production from denitrification and total denitrification (N₂O + N₂). In both the ploughed and unploughed treatments the 0–10 cm soil layer was the major contributing layer to gaseous N production by all the above processes. However, the contribution of this layer decreased by ploughing, gaseous N productions from the 10 to 30 cm layer being significantly increased with respect to the unploughed treatment. Ploughing promoted both nitrification and denitrification derived N₂O production, although a higher proportion of N₂O lost by denitrification was observed as WFPS increased. Recently ploughed plots showed lower denitrification derived N₂O percentages than those ploughed before as a result of the lower soil water content in the former plots. Similarly, a lower mean nitrification derived N₂O percentage was found in the 10–30 cm layer compared with the 0–10 cm.     

Nitrogen loss from grassland on peat soils through nitrous oxide production

Nitrous oxide (N₂O) in soils is produced through nitrification and denitrification. The N₂O produced is considered as a nitrogen (N) loss because it will most likely escape from the soil to the atmosphere as N₂O or N₂. Aim of the study was to quantify N₂O production in grassland on peat soils in relation to N input and to determine the relative contribution of nitrification and denitrification to N₂O production. Measurements were carried out on a weekly basis in 2 grasslands on peat soil (Peat I and Peat II) for 2 years (1993 and 1994) using intact soil core incubations. In additional experiments distinction between N₂O from nitrification and denitrification was made by use of the gaseous nitrification inhibitor methyl fluoride (CH₃F).Nitrous oxide production over the 2 year period was on average 34 kg N ha^-1 yr^-1 for mown treatments that received no N fertiliser and 44 kg N ha^-1 yr^-1 for mown and N fertilised treatments. Grazing by dairy cattle on Peat I caused additional N₂O production to reach 81 kg N ha^-1 yr^-1. The sub soil (20–40 cm) contributed 25 to 40% of the total N₂O production in the 0–40 cm layer. The N₂O production:denitrification ratio was on average about 1 in the top soil and 2 in the sub soil indicating that N₂O production through nitrification was important. Experiments showed that when ratios were larger than l, nitrification was the major source of N₂O. In conclusion, N₂O production is a significant N loss mechanism in grassland on peat soil with nitrification as an important N₂O producing process.     

Deciphering the oxygen isotope composition of nitrous oxide produced by nitrification

The ability to use δ¹⁸O values of nitrous oxide (N₂O) to apportion environmental emissions is currently hindered by a poor understanding of the controls on δ¹⁸O–N₂O from nitrification (hydroxylamine oxidation to N₂O and nitrite reduction to N₂O). In this study fertilized agricultural soils and unfertilized temperate forest soils were aerobically incubated with different ¹⁸O/¹⁶O waters, and conceptual and mathematical models were developed to systematically explain the δ¹⁸O–N₂O formed by nitrification. Modeling exercises used a set of defined input parameters to emulate the measured soil δ¹⁸O–N₂O data (Monte Carlo approach). The Monte Carlo simulations implied that abiotic oxygen (O) exchange between nitrite (NO₂^?) and H₂O is important in all soils, but that biological, enzyme‐controlled O‐exchange does not occur during the reduction of NO₂^? to N₂O (nitrifier‐denitrification). Similarly, the results of the model simulations indicated that N₂O consumption is not characteristic of aerobic N₂O formation. The results of this study and a synthesis of the published literature data indicate that δ¹⁸O–N₂O formed in aerobic environments is constrained between +13‰ and +35‰ relative to Vienna Standard Mean Ocean Water (VSMOW). N₂O formed via hydroxylamine oxidation and nitrifier‐denitrification cannot be separated using δ¹⁸O unless ¹⁸O tracers are employed. The natural range of nitrifier δ¹⁸O–N₂O is discussed and explained in terms of our conceptual model, and the major and minor controls that define aerobically produced δ¹⁸O–N₂O are identified. Despite the highly complex nature of δ¹⁸O–N₂O produced by nitrification this δ¹⁸O range is narrow. As a result, in many situations δ¹⁸O values may be used in conjunction with δ¹⁵N–N₂O data to apportion nitrifier‐ and denitrifier‐derived N₂O. However, when biological O‐exchange during denitrification is high and N₂O consumption is low, there may be too much overlap in δ¹⁸O values to distinguish N₂O formed by these pathways.     

Nitrous oxide fluxes from savanna (miombo) woodlands in Zimbabwe

Aim We test the hypothesis that land use and climate are important controls of nitrous oxide (N₂O) emissions from savanna ecosystems, and that these emissions can be represented by a mechanistic model of carbon (C) and nitrogen (N) transformations. Location Miombo woodlands in Zimbabwe are part of widespread woody savanna formations in southern and central Africa that cover more than 2.7 million km². The rainfall in this region is around 800 mm and is concentrated in the period between November and March. Methods Losses of N₂O were measured along transects in two field areas using static chambers over a period of 1 year. The vegetation in both areas was dominated by Julbernardia globiflora and Brachystegia spiciformis, but had differing management systems (burned and unburned), soil properties and site characteristics (slope and drainage). The effects of simulated rainfall and fertilizer additions were studied in laboratory incubations. Results Patterns of N₂O emissions were strongly linked to rainfall. The highest fluxes at both sites were measured within 18 days of the onset of the first rains in November, with fluxes of up to 42 μg N m^?2 h^?1. During the dry season, fluxes were lower, but a large proportion (R² values between 0.8 and 0.95, P < 0.001) of the N₂O flux could be predicted by variations in soil moisture. Soil columns were set up in the laboratory to which simulated rainwater was added, and the amounts and timing of rainwater addition were varied. Losses of N₂O were highest within the first week of the laboratory study. Altering the amount of rainwater addition did not significantly affect N₂O loss; however, a continuous addition of water resulted in higher losses of N₂O (up to 79 μg N m^?2 h^?1) than periodic addition of the same amount. A model (denitrification–decomposition) was used to simulate N₂O release over a 12 month period, using meteorological data recorded in the vicinity of the field site. The simulations and field data suggest that nitrification was the main process responsible for N₂O release during the dry season but that denitrification was more important during the wet season. Main conclusions The release of N₂O from dryland savannas was shown to constitute an important nutrient flux, and emissions were strongly linked to patterns of rainfall; however, there was evidence to suggest that the magnitude of fluxes is also influenced locally by differences in soil organic matter concentration and drainage.     

Effect of stand age on greenhouse gas fluxes from a Sitka spruce [Picea sitchensis (Bong.) Carr.] chronosequence on a peaty gley soil

The influence of forest stand age in a Picea sitchensis plantation on (1) soil fluxes of three greenhouse gases (GHGs – CO₂, CH₄ and N₂O) and (2) overall net ecosystem global warming potential (GWP), was investigated in a 2‐year study. The objective was to isolate the effect of forest stand age on soil edaphic characteristics (temperature, water table and volumetric moisture) and the consequent influence of these characteristics on the GHG fluxes. Fluxes were measured in a chronosequence in Harwood, England, with sites comprising 30‐ and 20‐year‐old second rotation forest and a site clearfelled (CF) some 18 months before measurement. Adjoining unforested grassland (UN) acted as a control. Comparisons were made between flux data, soil temperature and moisture data and, at the 30‐year‐old and CF sites, eddy covariance data for net ecosystem carbon (C) exchange (NEE). The main findings were: firstly, integrated CO₂ efflux was the dominant influence on the GHG budget, contributing 93–94% of the total GHG flux across the chronosequence compared with 6–7% from CH₄ and N₂O combined. Secondly, there were clear links between the trends in edaphic factors as the forest matured, or after clearfelling, and the emission of GHGs. In the chronosequence sites, annual fluxes of CO₂ were lower at the 20‐year‐old (20y) site than at the 30‐year‐old (30y) and CF sites, with soil temperature the dominant control. CH₄ efflux was highest at the CF site, with peak flux 491±54.5 μg m⁻² h⁻¹ and maximum annual flux 18.0±1.1 kg CH₄ ha⁻¹ yr⁻¹. No consistent uptake of CH₄ was noted at any site. A linear relationship was found between log CH₄ flux and the closeness of the water table to the soil surface across all sites. N₂O efflux was highest in the 30y site, reaching 108±38.3 μg N₂O‐N m⁻² h⁻¹ (171 μg N₂O m⁻² h⁻¹) in midsummer and a maximum annual flux of 4.7±1.2 kg N₂O ha⁻¹ yr⁻¹ in 2001. Automatic chamber data showed a positive exponential relationship between N₂O flux and soil temperature at this site. The relationship between N₂O emission and soil volumetric moisture indicated an optimum moisture content for N₂O flux of 40–50% by volume. The relationship between C : N ratio data and integrated N₂O flux was consistent with a pattern previously noted across temperate and boreal forest soils.     

Mesocosms Mimic Natural Meadows as regards Greenhouse Gas Fluxes and Potential Activities of Nitrifying and Denitrifying Bacteria

The objective of this study was to determine whether a planted mesocosm mimics a natural habitat in terms of N₂O and CH₄ fluxes, soil characteristics and potential nitrification and denitrification activities. We compared mesocosms in unchambered open-field plots and in open-top chambers with nonfiltered ambient air with three natural meadows that had similar soil characteristics and species composition. The N₂O fluxes in the mesocosms were very similar to the fluxes in the three natural meadows. There were no marked differences in potential nitrification and denitrification activities between the mesocosms and the natural meadows, either. Only the CH₄ fluxes differed slightly between the mesocosms and some of the natural meadows. Therefore, it seems that the mesocosms compared rather well to natural habitats. The open-top chambers modified only the soil water content, the values being higher in the unchambered plots than in the chambered plots. These results thus suggest that the open-top chamber experiment enables estimates of greenhouse gas and potential activities of nitrifying and denitrifying bacteria in unfertilized Finnish meadows, in spite of the chamber effects on the soil water content.