Consumption In Soils Research Articles

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, δ15Nbulk 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 (βNαNO) 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.

Detecting nitrous oxide reductase (NosZ) genes in soil metagenomes: method development and implications for the nitrogen cycle.

ABSTRACTMicrobial activities in soils, such as (incomplete) denitrification, represent major sources of nitrous oxide (N2O), a potent greenhouse gas. The key enzyme for mitigating N2O emissions is NosZ, which catalyzes N2O reduction to N2. We recently described “atypical” functional NosZ proteins encoded by both denitrifiers and nondenitrifiers, which were missed in previous environmental surveys (R. A. Sanford et al., Proc. Natl. Acad. Sci. U. S. A. 109:19709–19714, 2012, doi:10.1073/pnas.1211238109). Here, we analyzed the abundance and diversity of both nosZ types in whole-genome shotgun metagenomes from sandy and silty loam agricultural soils that typify the U.S. Midwest corn belt. First, different search algorithms and parameters for detecting nosZ metagenomic reads were evaluated based on in silico-generated (mock) metagenomes. Using the derived cutoffs, 71 distinct alleles (95% amino acid identity level) encoding typical or atypical NosZ proteins were detected in both soil types. Remarkably, more than 70% of the total nosZ reads in both soils were classified as atypical, emphasizing that prior surveys underestimated nosZ abundance. Approximately 15% of the total nosZ reads were taxonomically related to Anaeromyxobacter, which was the most abundant genus encoding atypical NosZ-type proteins in both soil types. Further analyses revealed that atypical nosZ genes outnumbered typical nosZ genes in most publicly available soil metagenomes, underscoring their potential role in mediating N2O consumption in soils. Therefore, this study provides a bioinformatics strategy to reliably detect target genes in complex short-read metagenomes and suggests that the analysis of both typical and atypical nosZ sequences is required to understand and predict N2O flux in soils.

Interactions of three soil bacteria species with phyllosilicate surfaces in hybrid silica gels

To simulate iron consumption in soils, iron leaching from silicate minerals due to three heterotrophic bacterial strains and a chemical treatment was studied using hybrid silica gel (HSG) doped with two phyllosilicates, nontronite (NAu-2) or low-iron-content montmorillonite (SWy-2). HSG methodology, a novel way of separating bacteria cells from a colloidal mineral source, consisted in embedding colloidal mineral particles into an amorphous porous silica matrix using a classical sol-gel procedure. Pantoae agglomerans PA1 and Rahnella aquatilis RA1 were isolated from silicate-rich soils, that is, beech and wheat rhizospheres (Vosges, France); Burkholderia sp. G5 was selected from acidic and nutrient-poor podzol soils (Vosges, France). Fe release from clay minerals and production of bacterial metabolites, that is, low molecular weight organic acids (LMWOA) and siderophores, were monitored. Two LMWOA profiles were observed with major gluconate production (>9000μM) for Burkholderia sp. G5 and moderate production of lactate, acetate, propionate, formate, oxalate, citrate, and succinate (<300μM) for R.aquatilis RA1 and P.agglomerans PA1. HSG demonstrated its usefulness in revealing clay mineral-microorganisms interactions. The effect of bacterial exsudates was clearly separated from physical contact effect.

Diurnal dynamics of CH4, CO2 and N2O fluxes in the saline-alkaline soils of the Yellow River Delta, China

Salt-affected soils are extensively present and constitute about 7% of total land surface. However, our knowledge about greenhouse gas (GHG) turnover between the atmosphere and the saline soils is very limited. In order to evaluate the potential of GHG consumption in saline soils, we measured gas fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) from the soil surface to the atmosphere under saline-alkaline mudflat and various community types in the Yellow River Delta in China. In general, the emissions of GHG of different ecosystems showed a unique peak diurnal pattern, with the peak at 13:00 h and the lowest value at 07:00 h. The CH4 and N2O emission of different ecosystems followed the order: saline-alkaline mudflat>T. chinensis>S. salsa>P. australis, while the CO2 emission followed the order: T. chinensis>P. australis>S. salsa > saline-alkaline mudflat. On the whole, saline-alkaline mudflat and different vegetations acted as CO2 and N2O source in spring, while saline-alkaline mudflat and P. australis communities acted as CH4 source and CH4 sink, respectively. However, T. chinensis and S. salsa communities acted as CH4 sink before 12:00 h and CH4 source after 12:00 h. Although measurements of the CO2, CH4 and N2O fluxes were taken simultaneously, CH4 and N2O fluxes were strongly correlated with soil moisture, temperature and electrical conductivity, while no significant correlation was found between CO2 flux with above environmental factors. These results probably suggest that factors other than soil temperature, moisture and salinity exerted a larger impact on fluxes than on CH4 and N2O release and/or that there were not enough samples for CO2 flux measurements because of its higher spatial and temporal variability.

Biochar's role in mitigating soil nitrous oxide emissions: A review and meta-analysis

More than two thirds of global nitrous oxide (N2O) emissions originate from soil, mainly associated with the extensive use of nitrogen (N) fertilizers in agriculture. Although the interaction of black carbon with the N cycle has been long recognized, the impact of biochar on N2O emissions has only recently been studied. Herein we reflect on proposed hypotheses to explain N2O decrease with biochar, linking them to specific mechanisms for N2O formation and consumption in soil. Moreover, to assist in elucidating key mechanisms in which biochar may act in mitigating emissions of N2O, we undertook a meta-analysis using published literature from 2007 to 2013. This quantitative analysis used 30 studies with 261 experimental treatments. Overall, we found that biochar reduced soil N2O emissions by 54% in laboratory and field studies. The biochar feedstock, pyrolysis conditions and C/N ratio were shown to be key factors influencing emissions of N2O while a direct correlation was found between the biochar application rate and N2O emission reductions. Interactions between soil texture and biochar and the chemical form of N fertilizer applied with biochar were also found to have a major influence on soil N2O emissions. While there is clear evidence that, in many cases, emissions of N2O are reduced, there is still a significant lack in understanding of the key mechanisms which result in these changed emissions. As such, we have guided readers with suggestions to address specific research gaps, which we anticipate will enhance our knowledge and understanding of biochar's N2O emission mitigation potential.

Seasonal dynamics in nitrous oxide emissions under different types of vegetation in saline-alkaline soils of the Yellow River Delta, China and implications for eco-restoring coastal wetland

Salt-affected soils are extensively present and constitute about 7% of total land surface. However, our knowledge about nitrous oxide (N2O) production through rapid nitrification and denitrification processes between the atmosphere and the saline soil is very limited. In order to evaluate the potential of N2O consumption in saline soils, this study was therefore designed to quantify the variability in N2O emissions monthly in the Yellow River Delta in China. Main issues include: different saline-alkaline soils and temporal aspects. Our aim was to quantify N2O emissions and identify the major drivers controlling its emissions for providing guidance in eco-restoring coastal wetland on large scale. By using in situ closed chambers the annual average emissions of N2O from the mudflat was determined and it was significantly higher than plant communities, especially herbage communities. In general, the emissions of N2O of different ecosystems showed a unique-peak annual pattern, with the peak in September. Saline-alkaline mudflat and different vegetations acted as N2O source in the Yellow River Delta and the N2O emission of different ecosystems followed the order: Saline-alkaline mudflat>T. chinensis >S. salsa>P. australis. Therefore restoration of saline land through revegetation was necessary to reduce the N2O emission of saline soils. The effects of air and soil temperature on N2O fluxes were significant in salt-affected soils except P. australis. Soil water content and electrical conductivity correlated positively or negatively with N2O emissions in mudflat and P. australis community. While relationships between N2O production and other soil properties (TC, TN, C:N ratio, NH4+-N and NO3−-N) were only significant in mudflat and T. chinensis community. Temporal variations of N2O emission were related to the interactions of abiotic factors (air and soil temperature, soil water content and electrical conductivity) and the variations of other soil properties, while spatial variations were mainly affected by the vegetation composition at spatial scale.

Litter and seed burying alter food availability and foraging efficiency of granivorous birds in the Monte desert

We assessed experimentally if the main granivorous bird species that feed on the ground in the central Monte desert are able to detect and consume seeds buried in the soil or trapped within litter. Understanding seed vulnerability to birds allows 1) a better understanding of how seed abundance translates into seed availability, a necessary step to assess seed limitation scenarios, and 2) whether birds alter the distribution of soil seeds through their consumption. Rufous‐collared sparrows found and consumed high proportions of buried seeds, though less seeds were eaten at increasing depths. In contrast, many‐colored chaco‐finches, common diuca‐finches and cinnamon warbling‐finches did not find buried seeds. All bird species fed on every substrate offered but, as a whole, birds reduced by 50% their seed consumption in Prosopis litter, and by 30% in Larrea litter, compared to consumption in bare soil. This effect was less notable for rufous‐collared sparrows, whose ‘double scratch’ foraging method would contribute to its great diet breath and abundance in the Monte desert. As birds do not reach a fraction of seeds buried and trapped by litter, seeds readily available for them may be scarcer than previously estimated through soil seed bank studies. Furthermore, since the four bird species detect and consume seeds from littered microhabitats, seed consumption by them surely affects the seasonal dynamics of the soil seed bank in all microhabitat types of the Monte desert.

Fraction of Cd in oasis soil and its bioavailability to commonly grown crops in Northwest China

Heavy metals in the soil–plant system resulting from mining and wastewater irrigation have greatly threatened human health and sustainable development in Northwest China. This research used pot experiments to study the bioavailability of Cd in irrigated desert soil from the oasis regions and conducted a human health risk assessment of the Cd content in vegetables. The results show that the content of Cd associated with exchangeable and carbonate metal fractionations is not uniform in arid oasis soils cultivated with the commonly grown vegetables. These common growth vegetables are: Cole (Brassica campestris L.), Celery (Apium Graveolens L.) and carrots (Daucus carota L.). The results show that the content of Cd in the edible part of cole was significantly higher than celery and carrots in almost all of the treatments. However, cole can grow normally and keep increasing its biomass at a sustainable rate under the highest concentration of Cd added in the experiment. Cole is not a suitable vegetable to be planted for consumption in arid soil contaminated with Cd, celery planted under lower concentration of Cd will not threaten human health, and carrots are suitable to be planted in arid soil contaminated with Cd even in the highest concentration used in the experiment. According to this conclusion, stricter soil management systems should be implemented to protect arid land soil resources and to protect human health from the toxicity impacts.

N2O consumption by low-nitrogen soil and its regulation by water and oxygen

Soils can be a source and sink for atmospheric nitrous oxide (N2O). Consumption of N2O has been reported for anoxic soils and sediments rich in organic matter and depleted in nitrates (NO3−), and also for some dry, oxic soils. However, the mechanisms and controls of N2O consumption in dry soil are not clear.Here, we report on a field study in China (Taihang mountain region, Shijiazhuang), in which N2O uptake by a sandy loam soil was measured for the greater part of the season (from April to October in 2011), and on four incubation experiments, in which we tried to reveal the roles of water content and oxygen (O2) concentrations on N2O consumption. Flux measurements in the field were made bi-weekly on unfertilized cropped land with static flux chambers (5 replicates) for 6 months. The results show that N2O–N fluxes ranged from −26.0 to −726.6 μg m−2 h−1. Consumption of N2O was largest when the soil was dry (5–20% soil water filled pore space). In the incubation experiments, N2O consumption and N2 production were measured in (an)aerobic soil with soil moisture content ranging from 1% to 50% (wt/wt) and with N2O addition, using a thermostatic, robotized incubation system. Under anaerobic conditions, N2O was rapidly consumed at water content of >10% (wt/wt). However, a significant consumption also occurred at 1% soil moisture. Under aerobic conditions, N2O consumption increased with increasing soil moisture content, but significant consumption was still measured at 2% moisture. Sterilization of oxic soil completely blocked N2O consumption, suggesting that the consumption had a biological nature. In conclusion, the steady N2O consumption measured in the field was confirmed by the laboratory experiments, but the relationship with soil moisture content was reversed. Further studies are required to understand this apparent anomaly.

Modelling oxygen transport in soil with plant root and microbial oxygen consumption: depth of oxygen penetration

A set of equations governing oxygen diffusion and consumption in soils has been developed to include microbial and plant-root sinks. The dependent variable is the transformed oxygen concentration, which is the difference between the gaseous concentration and a scaled value of the aqueous oxygen concentration at the root–soil interface. The results show how, as the air-filled porosity decreases, the reduced oxygen flux causes the depth of extinction to decrease. The results also show how the depth of extinction at a particular value of soil water content decreases with increasing temperature, due to increased microbial respiration. The critical value of water content at which the oxygen concentration goes to extinction at a finite depth was compared with alternative calculations with only a microbial sink. By ignoring the feedback of oxygen concentration on root uptake, the alternative calculations yielded substantially higher critical values of water content at all temperatures. Two soil oxygen diffusion coefficient functions from the literature were compared and shown to give significantly different critical values of water content for fine-textured soils, one more realistic than the other. A single relationship between the extinction depth and the ratio of the water content to the critical value was shown to apply for all temperatures and soil textures. The oxygen profiles were used along with a function relating redox potential to oxygen concentration to generate redox potential profiles. This application of the model could be useful in explaining soil biochemical processes in soils. For one such process, denitrification, the depth at which a critical oxygen concentration is reached was calculated as a function of the air-filled porosity and temperature of the soil. The implications of the critical value of soil water content in terms of water-filled pore space and matric potential are discussed in relation to the diffusion coefficient functions and recent literature.

Strong spatial variability in trace gasdynamics following experimental drought in a humid tropical forest

Soil moisture is a key driver of biogeochemical processes in terrestrial ecosystems, strongly affecting carbon (C) and nutrient availability as well as trace gas production and consumption in soils. Models predict increasing drought frequency in tropical forest ecosystems, which could feed back on future climate change directly via effects on trace gasdynamics and indirectly through changes in nutrient availability. We used throughfall exclusion shelters to determine effects of short‐term (3 month) drought on trace gas fluxes and nutrient availability in humid tropical forests in Puerto Rico. Exclusion and control plots were replicated within and across three topographic zones (ridge, slope, valley) to account for spatial heterogeneity typical of these ecosystems. Throughfall exclusion reduced soil moisture in all sites and lowered exchangeable phosphorus (P) on ridges and slopes. Drought decreased soil carbon dioxide (CO2) emissions by 30% in ridge sites and 28% in slope sites, and increased net methane (CH4) consumption by 480% in valley sites. Both valley and ridge sites became net nitrous oxide (N2O) sinks in response to soil drying. Emissions of CO2 and N2O, as well as CH4 consumption were positively related to exchangeable P and the nitrate:ammonium ratio. These findings suggest that drought has the potential to decrease net trace gas emissions from humid tropical forest soils. The differential response of trace gas emissions and nutrients from different topographic zones to drought underscores the complexity of biogeochemical cycling in these ecosystems and the importance of considering spatial heterogeneity when estimating whole system responses.

黄河三角洲不同类型盐碱地CO2、CH4和N2O通量特征

盐碱地作为陆地的重要组成部分，在温室气体排放中的作用不可忽视。为探究滨海盐碱地温室气体排放通量，本研究于2011年10月运用静态暗箱–气相色谱法对黄河三角洲不同类型盐碱地CO2、CH4和N2O日通量进行了原位观测。结果表明，不同植被覆盖与大气界面的CO2、CH4和N2O通量存在明显差异。CO2日排放通量：柽柳群落 > 芦苇群落 > 翅碱蓬群落，分别为108.2999，84.5160，41.0333 mg·m–2·h–1，而光滩则表现为CO2吸收汇。整体来看，不同植被覆盖CH4日排放通量小，分别是0.0080，–0.0299，0.0212，–0.0068 mg·m–2·h–1，但不同类型盐碱地仍存在差异，光滩和翅碱蓬群落为排放源，柽柳和芦苇群落是吸收汇。不同群落N2O日排放通量的差异性较弱。本试验同时研究了环境因子对不同类型盐碱地上三种温室气体排放通量的影响，相关分析表明，环境因子对光滩温室气体通量没有显著影响，其余三种植被覆盖的温室气体排放通量与不同深度土层温度呈现相关性，说明环境因素对盐碱地温室气体排放作用机制的复杂性。 Salt-affected soils are extensively present and constitute about 7% of total land surface. However, our knowledge about greenhouse gas (GHG) turnover between the atmosphere and the saline soils is very limited. In order to evaluate the potential of GHG consumption in saline soils, we measured gas fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from the soil surface to the atmosphere under saline-alkaline mudflat and various com- munity types in the Yellow River Delta in China. The CO2 emission followed the order: T. chinensis > P. australis > S. salsa > Saline-alkaline mudflat. On the whole, saline-alkaline mudflat and different vegetations acted as CH4 sink be- fore 12:00 and CH4 source after 12:00. However, the four ecosystems acted as N2O source in the whole day. Although measurements of the CO2, CH4, and N2O fluxes were taken simultaneously, CH4 and N2O fluxes were strongly corre- lated with soil temperature at different depths in T. chinensis community. The significantly negative correlations were also found between air and soil temperature and CO2 or CH4 fluxes in S. salsa, N2O fluxes in P. australis. While no sig- nificant correlation was found between greenhouse gases with soil moisture and electrical conductivity. These results probably suggest that factors other than soil temperature, moisture and salinity exerted a larger impact on CO2, CH4 and N2O flux because of its higher spatial and temporal variability.

Microbial CH(4) and N(2)O Consumption in Acidic Wetlands.

Acidic wetlands are global sources of the atmospheric greenhouse gases methane (CH4), and nitrous oxide (N2O). Consumption of both atmospheric gases has been observed in various acidic wetlands, but information on the microbial mechanisms underlying these phenomena is scarce. A substantial amount of CH4 is consumed in sub soil by aerobic methanotrophs at anoxic–oxic interfaces (e.g., tissues of Sphagnum mosses, rhizosphere of vascular plant roots). Methylocystis-related species are likely candidates that are involved in the consumption of atmospheric CH4 in acidic wetlands. Oxygen availability regulates the activity of methanotrophs of acidic wetlands. Other parameters impacting on the methanotroph-mediated CH4 consumption have not been systematically evaluated. N2O is produced and consumed by microbial denitrification, thus rendering acidic wetlands as temporary sources or sinks for N2O. Denitrifier communities in such ecosystems are diverse, and largely uncultured and/or new, and environmental factors that control their consumption activity are unresolved. Analyses of the composition of N2O reductase genes in acidic wetlands suggest that acid-tolerant Proteobacteria have the potential to mediate N2O consumption in such soils. Thus, the fragmented current state of knowledge raises open questions concerning methanotrophs and denitrifiers that consume atmospheric CH4 and N2O in acidic wetlands.

Methane production and consumption in loess soil at different slope position.

Methane (CH4) production and consumption and soil respiration in loess soils collected from summit (Top), back slope (Middle), and slope bottom (Bottom) positions were assessed in laboratory incubations. The CH4 production potential was determined under conditions which can occur in the field (relatively short-term flooding periods with initially ambient O2 concentrations), and the CH4 oxidation potential was estimated in wet soils enriched with CH4. None of the soils tested in this study emitted a significant amount of CH4. In fact, the Middle and Bottom soils, especially at the depth of 20–40 cm, were a consistent sink of methane. Soils collected at different slope positions significantly differed in their methanogenic, methanotrophic, and respiration activities. In comparison with the Top position (as reference soil), methane production and both CO2 production and O2 consumption under flooding were significantly stimulated in the soil from the Middle slope position (P < 0.001), while they were reduced in the Bottom soil (not significantly, by 6 to 57%). All upper soils (0–20 cm) completely oxidized the added methane (5 kPa) during 9–11 days of incubation. Soils collected from the 20–40 cm at the Middle and Bottom slope positions, however, consumed significantly more CH4 than the Top soil (P < 0.001).

How much oxygen is needed for acetylene to be consumed in soil?

Acetylene (C2H2) is employed for the quantification of important biological processes such as nitrogen fixation, nitrous oxide reduction, ammonium and methane oxidation, and methanogenesis. Although acetylene is not a natural product, the ability of bacteria to grow on C2H2 is a phenomenon common to soils and sediments. Our experiment was designed to study the modification of CO2 production, O2 uptake and microbial biomass (Cmic) in soil in response to the consumption of added acetylene. Two soils (peat-muck and Eutric Cambisol) were incubated under well aerated (60% water holding capacity [WHC]) or flooded conditions, and enriched with C2H2 in the range 0.1–10 kPa (initially, 0.434–63.4 mmol C2H2 kg−1) at a constant temperature of 20°C. Gases were measured chromatographically, while Cmic was measured by the physiological SIR (substrate-induced respiration) method based on the initial response of microorganisms to glucose amendment. We used a simple calculation of net CO2, net O2 and net Cmic (as differences between amended and not amended soils) to estimate the contribution of C2H2 to the total respiration and microbial growth during the incubation. Peat-muck soil consumed more C2H2 than Cambisol (maximum 54.03 vs. 19.25 mmol kg−1, p < 0.001). Acetylene utilization was faster and larger in flooded than in wet soils (16.2 and 7.81 mmol kg−1, respectively, p < 0.05), and followed the tendency observed for Cmic. Depending on C2H2 enrichment, air–water conditions, and soil tested, both reduction and stimulation of measured activities were observed in response to acetylene consumption. Low C2H2 uptake, especially in Cambisol incubated at 60% WHC, resulted in the reduction of soil respiration and biomass (by 1–29%). Large C2H2 consumption in flooded soils stimulated CO2 production, O2 uptake and Cmic, even by 78%, 72% and 43%, respectively. Net CO2, net O2, and net Cmic were linearly positively related to the quantity of consumed C2H2 (p < 0.001). Acetylene utilization was a combined effect of initial C2H2, soil properties, and air–water status. The amount of consumed C2H2 was the highest in flooded peat-muck soil enriched with 10 kPa, and lowest in Cambisol at 60% WHC with 0.1 kPa C2H2. Consumption of 1 mol acetylene induced production of 0.5 mol CO2, and uptake of 0.4 mol O2. Low acetylene consumption (<6 mmol kg−1) resulted in reduction of both soil respiration and microbial biomass.

Biogeochemical controls on methane, nitrous oxide, and carbon dioxide fluxes from deciduous forest soils in eastern Canada

[1] The exchange of the important trace gases, methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2), between forested soils and the atmosphere can show great temporal and spatial variability. We measured the flux of these three gases over 2 years along catenas at two forested sites, to determine the important controls. Well-drained soils consumed atmospheric CH4, while poorly drained swamp soils embedded in depressions were a source. CH4 fluxes could be predicted primarily by temperature and moisture, and tree cover exerted an influence mainly through the creation of large soil porosity, leading to increased consumption rates. In contrast, there were very poor relationships between N2O fluxes and environmental variables, reflecting the complex interactions of microbial, edaphic, and N cycling processes, such as nitrification in well-drained soils and denitrification in poorly drained soils, which led to N2O production (or consumption) in soils and hence larger variability. At the broad temporal and spatial scale, soil C:N ratio was a good predictor of N2O emission rates, through its influence upon N cycling processes. Soil CO2 emission rates showed less spatial and temporal variability, and were controlled by temperature and moisture. The source strength, in global warming potential of CH4 and N2O fluxes in CO2 equivalents, was reduced markedly when trace gas fluxes from 5 to 15% poorly drained soils were included in the net global warming potential calculation of whole forested watersheds. Soils drainage class integrates many of the biogeochemical processes controlling the flux of these gases providing a framework for extrapolating results.

Methane uptake in salt-affected soils shows low sensitivity to salt addition

Methane oxidation in aerated soils is a significant sink for atmospheric methane (CH4). Salt-affected soils are extensively present and constitute about 7% of total land surface. However, our knowledge about CH4 turnover between the atmosphere and the saline soils is very limited. In order to evaluate the potential of CH4 consumption in saline soils, CH4 fluxes were measured in intact cores of the slightly (ECe = 3.2 mS cm−1), moderately (ECe = 7.1 mS cm−1) and extremely (ECe = 50.7 mS cm−1 and 112.6 mS cm−1) saline soils from the Yellow River Delta, China. CH4 uptake of cores from the slightly saline soil ranged from 14 to 24 μg CH4–C m−2 h−1, comparable to those in the non-saline forest soils with similar texture. CH4 uptake of cores from the moderately saline soil was only about 6% of that in the slightly saline soil. CH4 uptake was too low to be measurable in the extremely saline soil. Compared with the non-saline soil, CH4 uptake in the saline soils was much less sensitive to salt, suggesting the higher salt-tolerance of CH4 oxidizers in the saline soil. The result also indicated an underestimate in CH4 uptake for the naturally-occurring saline soils by adding salt to non-saline soils. These results should be useful to study the global CH4 budget and to explore the physiological and ecological characteristics of methanotrophic bacteria in the salt-affected soils.

Can N2O stable isotopes and isotopomers be useful tools to characterize sources and microbial pathways of N2O production and consumption in tropical soils?

Nitrous oxide (N2O) is an important greenhouse gas in which the main sources are tropical rainforest and agricultural soils. N2O is produced in soils by microbial processes, which are enhanced by the application of nitrogenous fertilizers. The soil N2O bulk isotopic composition (δ 15Nbulk and δ 18O) and the “site-specific,” or intramolecular, 15N isotopic composition, i.e., the 15N/14N ratio at the cenral (α) or terminal (β) nitrogen position, expressed in this study as δ 15N α and δ 15N β could help identify both the sources (natural and anthropogenic) and microbial pathways of N2O production and consumption prior to emission.We report new isotope measurements of soil N2O emissions and from soil air collected during the rainy season in a mature tropical forest (Tapajos National Forest, Para, Brazil) and in a tropical agricultural corn field (“Fundo Tierra Nueva,” Guarico State, Venezuela). The statistically different δ 15Nbulk emission weighted average between the mature forest (−18.0‰ ± 4.0‰, n = 6) and agricultural corn field (−34.3‰ ± 12.4‰, n = 17) suggest that theδ 15Nbulk data are useful for distinguishing N2O fluxes from fertilized agricultural and natural “background” soils. They also demonstrate that the site-specific δ 15N measurements have the potential to provide a new tool to differentiate between the production and consumption N2O microbiological processes in soils. This study further demonstrates that the observed correlations (or lack thereof) between δ 15N α ,δ 15N β , and δ 18O can be used to estimate the relative proportion of N2O that would have been emitted to the air but was consumed via reduction of N2O to N2 within the soil.

Methane flux and high-affinity methanotrophic diversity along the chronosequence of a receding glacier in Greenland

AbstractMethane consumption in upland soils represents an important part of the biologically mediated sink of tropospheric methane. The present study focuses on the role of glacier forefields as a potential methane sink. The role of these environments, though increasing in size, has not yet been taken into account in the global methane budget. Net methane fluxes were analysed based on a static chamber method on a proglacial chronosequence from the Mittivakkat valley, southeast Greenland. Methane uptake could be measured in 7of the 12 study sites, with highest rates in the oldest materials from the chronosequence, suggesting that methane oxidation potential may increase during glacier recession (80–150 years). In the chamber located at the glacier front, net methane production was observed, indicating that the microbial community changes after glacial recession from being net methanogenic to becoming net methanotrophic. Diversity analyses based on denaturing gradient gel electrophoresis (DGGE) from the methanotrophic communities responsible for methane uptake at atmospheric levels demonstrate that methanotrophic microbial diversity changes along the chronosequence and show that there is a tendency to a larger diversity in the oldest part of the chronosequence. Sequencing of DNA retrieved from the DGGE revealed a restricted diversity of the methanotrophic community: GenBank accession numbers HM534684–HM534736.

Responses of ethylene and methane consumption to temperature and pH in temperate volcanic forest soils

SummaryThere is limited knowledge about the consumption and interaction of methane (CH4) and ethylene (C2H4) in forest soils under disturbances of temperature and acidification. Temperate volcanic forest topsoils (0‐5 cm) sampled under different tree species (e.g. Pinus sylvestris, Cryptomeria japonica and Quercus serrata) were used to study the capacities for CH4 and C2H4 consumption and their sensitivity to temperature and pH. We also studied the responses of soil nitrogen (N) transformations to temperature and relationships to consumption of both CH4 and C2H4. The C2H4 consumption rates increased with temperature up to 35oC, whereas the optimum temperature for CH4 consumption rates was approximately 25oC. Both Q10 values and activation energies for CH4 consumption rates over the range 5 to 25oC were larger than corresponding values for C2H4 consumption rates. The rates of nitrous oxide (N2O) and nitric oxide (NO) evolution and net N mineralization in the soils increased exponentially with temperature up to 35oC, with relatively large Q10 values and activation energies for NO evolution. In these forest topsoils, rates of CH4 and C2H4 consumption at pH &lt; 4.0 were negligible, and the pH optimum for both consumptions varied from 5.5 to 6.2. Most of the tested forest soils had an optimum pH for CH4 and C2H4 consumption that was above natural pH values, which indicated that soil acidification would inhibit CH4 and C2H4 consumption in situ. There was a high rate of net C2H4 evolution from forest soils acidified experimentally to pH &lt; 4.0, particularly from Cryptomeria japonica forest soil, and 67% of the variation in C2H4 evolution rates could be accounted for by the increase in soil water‐soluble organic carbon concentrations. Previous studies have shown that addition of C2H4 in headspace gases can inhibit atmospheric CH4 consumption in such forest soils. Hence, the evolution of C2H4 from temperate volcanic forest soils at decreasing pH can exacerbate inhibition of the soil atmospheric CH4 consumption in situ.