Fluxes of climate‐relevant trace gases between a Norway spruce forest soil and atmosphere during repeated freeze–thaw cycles in mesocosms

For this century, an increasing frequency of extreme meteorological boundary conditions is expected, presumably resulting in a changing frequency of freezing and thawing of soils in higher‐elevation areas. Our current knowledge about the effects of these events on trace‐gas emissions from soils is scarce. In this study, the effects of freeze–thaw events on the fluxes of the trace gases CO₂, N₂O, and NO between soil and atmosphere were investigated in a laboratory experiment. Undisturbed soil columns were collected from a mature Norway spruce forest in the “Fichtelgebirge”, SE Germany. The influence of freezing temperatures (–3°C, –8°C, –13°C) on gas fluxes was studied during the thawing periods (+5°C) in three freeze–thaw cycles (FTCs) and compared to unfrozen controls (+5°C). Two different types of soil columns were examined in parallel—one consisting of O layer only (O columns) and one composed of O layer and mineral soil horizons (O+M columns)—to quantify the contribution of the organic layer and the top mineral soil to the production or consumption of these trace gases. During the thawing period, we observed increasing emissions of CO₂, N₂O, and NO from the spruce forest soil, but the cumulative emissions of these gases did mostly not exceed the level of the controls. The results show that the O layers were mainly involved in the gas production. Severe soil frost increased CO₂ fluxes during soil thawing, whereas repetition of the freeze–thaw events decreased CO₂ fluxes from the thawing soil. Fluxes of N₂O and NO were neither influenced by freezing temperature nor by freeze–thaw repetition. Stable‐isotope analysis indicated that denitrification was mostly responsible for the N₂O production in the FTC columns. Furthermore, isotope data demonstrated a consumption of N₂O through microbial denitrification to N₂. It was further shown, that production of N₂O also occurred in the mineral horizons. The NO emissions were mainly driven by increasing soil temperature during thawing. In this freeze–thaw experiment up to 20 times higher NO than N₂O fluxes were recorded. Our results suggest that topsoil thawing has little potential to increase the emissions of CO₂, N₂O, and NO in spruce forest soils.     

Compaction effects on CO2 and N2O production during drying and rewetting of soil

The effects of compaction on soil porosity and soil water relations are likely to influence substrate availability and microbial activity under fluctuating soil moisture conditions. We conducted a short laboratory incubation to investigate the effects of soil compaction on substrate availability and biogenic gas (CO₂ and N₂O) production during the drying and rewetting of a fine-loamy soil. Prior to initiating the drying and wetting treatments, CO₂ production (−10 kPa soil water content) from uncompacted soil was 2.3 times that of compacted soil and corresponded with higher concentrations of microbial biomass C (MBC) and dissolved organic C (DOC). In contrast, N₂O production was 67 times higher in compacted than uncompacted soil at field capacity. Soil aeration rather than substrate availability (e.g. NO₃⁻ and DOC) appeared to be the most important factor affecting N₂O production during this phase. The drying of compacted soil resulted in an initial increase in CO₂ production and a nearly two-fold higher average rate of C mineralization at maximum dryness (owing to a higher water-filled pore space [WFPS]) compared to uncompacted soil. During the drying phase, N₂O production was markedly reduced (by 93-96%) in both soils, though total N₂O production remained slightly higher in compacted than uncompacted soil. The increase in CO₂ production during the first 24 h following rewetting of dry soil was about 2.5 times higher in uncompacted soil and corresponded with a much greater release of DOC than in compacted soil. MBC appeared to be the source of the DOC released from uncompacted soil but not from compacted soil. The production of N₂O during the first 24 h following rewetting of dry soil was nearly 20 times higher in compacted than uncompacted soil. Our results suggest that N₂O production from compacted soil was primarily the result of denitrification, which was limited by substrates (especially NO₃⁻) made available during drying and rewetting and occurred rapidly after the onset of anoxic conditions during the rewetting phase. In contrast, N₂O production from uncompacted soil appeared to be primarily the product of nitrification that was largely associated with an accumulation of NO₃⁻ following rewetting of dry soil. Irrespective of compaction, the response to drying and rewetting was greater for N₂O production than for CO₂ production.     

N2O flux from plant-soil systems in polar deserts switch between sources and sinks under different light conditions

Production and consumption of greenhouse gases such as CO₂, CH₄ and N₂O are key factors driving climate change. While CO₂ sinks are commonly reported and the mechanisms relatively well understood, N₂O sinks have often been overlooked and the driving factors for these sinks are poorly understood. We examined CO₂, CH₄ and N₂O flux in three High Arctic polar deserts under both light (measured in transparent chambers) and dark (measured in opaque chambers) conditions. We further examined if differences in soil moisture, evapotranspiration, Photosynthetically Active Radiation (PAR), and/or plant communities were driving gas fluxes measured in transparent and opaque chambers at each of our sites. Nitrous oxide sinks were found at all of our sites suggesting that N₂O uptake can occur under extreme polar desert conditions, with relatively low soil moisture, soil temperature and limited soil N. Fluxes of CO₂ and N₂O switched from sources under dark conditions to sinks under light conditions, while CH₄ fluxes at our sites were not affected by light conditions. Neither evapotranspiration nor PAR were significantly correlated with CO₂ or N₂O flux, however, soil moisture was significantly correlated with both gas fluxes. The relationship between soil moisture and N₂O flux was different under light and dark conditions, suggesting that there are other factors, in addition to moisture, driving N₂O sinks. We found significant differences in N₂O and CO₂ flux between plant communities under both light and dark conditions and observed individual communities that shifted between sources and sinks depending on light conditions. Failure of many studies to include plant-mediated N₂O flux, as well as, N₂O soil sinks may account for the currently unbalanced global N₂O budget.     

Effect of wetting intensity on soil GHG fluxes and microbial biomass under a temperate forest floor during dry season

The increasing frequency of periodic droughts followed by heavy rainfalls is expected for this current century, but little is known about the effects of wetting intensity on the in situ biogenic greenhouse gas (GHG) fluxes of forest soils and soil microbial biomass. To gain new insights into the underlying mechanisms responsible for wetting-induced GHG fluxes in situ, rain simulation field experiments during a natural prolonged drought period were done under a temperate forest in northeast China. The intensity of rainfall-induced CO₂ pulses increased from 0.84 to 2.08 g CO₂–C m^? 2 d^? 1 with the intensity of wetting up to ca. 80% water-filled pore space, which coincided with an increase in soil microbial biomass and with a decrease in soil labile organic C following wetting. Methane uptake rates decreased from 1.76 to 0.87 mg CH₄–C m^? 2 d^? 1 with the intensity of wetting. Wetting dry forest floor increased N₂O fluxes from 6.2 to 25.9 μg N₂O–N m^? 2 d^? 1, but there was no significant difference between all experimental wetted plots. The rainfall-induced N₂O pulses with increasing wetting intensity were opposite to that of the CO₂ pulses, showing a maximum response at the lowest wetting intensity. An analysis of the temperature sensitivity of GHG fluxes indicated that temperature had an increased effect on the in situ CO₂ flux and CH₄ uptake, respectively, under wetted and dry conditions. The global warming potential of GHG fluxes and Q₁₀ value of the temperature response of CO₂ fluxes increased linearly with wetting intensity. The results indicate that the rainfall-induced soil CO₂ pulse is mainly due to enhanced microbial consumption on substrates and highlight the complex nature of belowground C-cycling responses to climate change in northeast China forests that normally experience periodic droughts followed by heavy rainfalls over the year.     

Influence of water depth and soil amelioration on greenhouse gas emissions from peat soil columns

Recently, large areas of tropical peatland have been converted into agricultural fields. To be used for agricultural activities, peat soils need to be drained, limed and fertilized due to excess water, low nutrient content and high acidity. Water depth and amelioration have significant effects on greenhouse gas (GHG) production. Twenty-seven soil samples were collected from Jabiren, Central Kalimantan, Indonesia, in 2014 to examine the effect of water depth and amelioration on GHG emissions. Soil columns were formed in the peatland using polyvinyl chloride (PVC) pipe with a diameter of 21 cm and a length of 100 cm. The PVC pipe was inserted vertically into the soil to a depth of 100 cm and carefully pulled up with the soil inside after sealing the bottom. The treatments consisting of three static water depths (15, 35 and 55 cm from the soil surface) and three ameliorants (without ameliorant/control, biochar+compost and steel slag+compost) were arranged using a randomized block design with two factors and three replications. Fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) from the soil columns were measured weekly. There was a linear relationship between water depth and CO₂ emissions. No significant difference was observed in the CH₄ emissions in response to water depth and amelioration. The ameliorations influenced the CO₂ and N₂O emissions from the peat soil. The application of biochar+compost enhanced the CO₂ and N₂O emissions but reduced the CH₄ emission. Moreover, the application of steel slag+compost increased the emissions of all three gases. The highest CO₂ and N₂O emissions occurred in response to the biochar+compost treatment followed by the steel slag-compost treatment and without ameliorant. Soil pH, redox potential (Eh) and temperature influenced the CO₂, CH₄ and N₂O fluxes. Experiments for monitoring water depth and amelioration should be developed using peat soil as well as peat soil–crop systems.     

N2O and NO fluxes between a Norway spruce forest soil and atmosphere as affected by prolonged summer drought

Global change scenarios predict an increasing frequency and duration of summer drought periods in Central Europe especially for higher elevation areas. Our current knowledge about the effects of soil drought on nitrogen trace gas fluxes from temperate forest soils is scarce. In this study, the effects of experimentally induced drought on soil N₂O and NO emissions were investigated in a mature Norway spruce forest in the Fichtelgebirge (northeastern Bavaria, Germany) in two consecutive years. Drought was induced by roof constructions over a period of 46 days. The experiment was run in three replicates and three non-manipulated plots served as controls. Additionally to the N₂O and NO flux measurements in weekly to monthly intervals, soil gas samples from six different soil depths were analysed in time series for N₂O concentration as well as isotope abundances to investigate N₂O dynamics within the soil. N₂O fluxes from soil to the atmosphere at the experimental plots decreased gradually during the drought period from 0.2 to −0.0 μmol m⁻² h⁻¹, respectively, and mean cumulative N₂O emissions from the manipulated plots were reduced by 43% during experimental drought compared to the controls in 2007. N₂O concentration as well as isotope abundance analysis along the soil profiles revealed that a major part of the soil acted as a net sink for N₂O, even during drought. This N₂O sink, together with diminished N₂O production in the organic layers, resulted in successively decreased N₂O fluxes during drought, and may even turn this forest soil into a net sink of atmospheric N₂O as observed in the first year of the experiment. Enhanced N₂O fluxes observed after rewetting up to 0.1 μmol m⁻² h⁻¹ were not able to compensate for the preceding drought effect. During the experiment in 2006, with soil matric potentials in 20 cm depth down to −630 hPa, cumulative NO emissions from the throughfall exclusion plots were reduced by 69% compared to the controls, whereas cumulative NO emissions from the experimental plots in 2007, with minimum soil matric potentials of −210 hPa, were 180% of those of the controls. Following wetting, the soil of the throughfall exclusion plots showed significantly larger NO fluxes compared to the controls (up to 9 μmol m⁻² h⁻¹ versus 2 μmol m⁻² h⁻¹). These fluxes were responsible for 44% of the total emission of NO throughout the whole course of the experiment. NO emissions from this forest soil usually exceeded N₂O emissions by one order of magnitude or more except during wintertime.     

Fluxes of CO2, CH4 and N2O from drained organic soils in deciduous forests

We examined net greenhouse gas exchange at the soil surface in deciduous forests on soils with high organic contents. Fluxes of CO₂, CH₄ and N₂O were measured using dark static chambers for two consecutive years in three different forest types; (i) a drained and medium productivity site dominated by birch, (ii) a drained and highly productive site dominated by alder and (iii) an undrained and highly productive site dominated by alder. Although the drained sites had shallow mean groundwater tables (15 and 18 cm, respectively) their average annual rates of forest floor CO₂ release were almost twice as high compared to the undrained site (1.9±0.4 and 1.7±0.3, compared to 1.0±0.2 kg CO₂ m⁻² yr⁻¹). The average annual CH₄ emission was almost 10 times larger at the undrained site (7.6±3.1 compared to 0.9±0.5 g CH₄ m⁻² yr⁻¹ for the two drained sites). The average annual N₂O emissions at the undrained site (0.1±0.05 g N₂O m⁻² yr⁻¹) were lower than at the drained sites, and the emissions were almost five times higher at the drained alder site than at the drained birch site (0.9±0.35 compared to 0.2±0.11 g N₂O m⁻² yr⁻¹). The temporal variation in forest floor CO₂ release could be explained to a large extent by differences in groundwater table and air temperature, but little of the variation in the CH₄ and N₂O fluxes could be explained by these variables. The measured soil variables were only significant to explain for the within-site spatial variation in CH₄ and N₂O fluxes at the undrained swamp, and dark forest floor CO₂ release was not explained by these variables at any site. The between-site spatial variation was attributed to variations in drainage, groundwater level position, productivity and tree species for all three gases. The results indicate that N₂O emissions are of greater importance for the net greenhouse gas exchange at deciduous drained forest sites than at coniferous drained forest sites.     

Influence of forest disturbance on CO2, CH4 and N2O fluxes from larch forest soil in the permafrost taiga region of eastern Siberia

Abstract

To evaluate the effect of increasing forest disturbances on greenhouse gas budgets in a taiga forest in eastern Siberia, CO₂, CH₄ and N₂O fluxes from the soils were measured during the growing season in intact, burnt and clear-felled larch forests (4–5 years after the disturbance). Soil temperature and moisture were higher at the two disturbed sites than at the forest site. A 64–72% decrease in the Q ₁₀ value of soil CO₂ flux from the disturbed sites compared with the forest site (5.92) suggested a reduction in root respiration and a dominance of organic matter decomposition at the disturbed sites. However, the cumulative CO₂ emissions (May–August) were not significantly different among the sites (2.81–2.90 Mg C ha^?1 per 3 months). This might be because decreased larch root respiration was compensated for by increased organic matter decomposition resulting from an increase in the temperature and root respiration of invading vegetation at the disturbed sites. The CH₄ uptake (kg C ha^?1 per 4 months [May–September]) at the burnt site was significantly higher (–0.15) than the uptake at the forest (–0.045) and clear-felled sites (0.0027). Although there were no significant differences among the sites, N₂O emission (kg N ha^?1 per 4 months) was slightly lower at the burnt site (0.013) and higher at the clear-felled site (0.068) than at the forest site (0.038). This different influence of burning and tree felling on CH₄ and N₂O fluxes might result from changes in the physical and chemical properties of the soil with respect to forest fire.     

Spatial structures of N2O, CO2, and CH4 fluxes from Acacia mangium plantation soils during a relatively dry season in Indonesia

We investigated spatial structures of N₂O, CO₂, and CH₄ fluxes during a relatively dry season in an Acacia mangium plantation stand in Sumatra, Indonesia. The fluxes and soil properties were measured at 1-m intervals in a 1 × 30-m plot (62 grid points) and at 10-m intervals in a 40 × 100-m plot (55 grid points) at different topographical positions of the upper plateau, slope, and valley bottom in the plantation. Spatial structures of each gas flux and soil property were identified using geostatistical analysis. The means (±SD) of N₂O, CO₂, and CH₄ fluxes in the 10-m grids were 0.54 (±0.33) mg N m⁻² d⁻¹, 2.81 (±0.71) g C m⁻² d⁻¹, and −0.84 (±0.33) mg C m⁻² d⁻¹, respectively. This suggests that A. mangium soils function as a larger source of N₂O than natural forest soils in the adjacent province on Sumatra during the relatively dry season, while CO₂ and CH₄ emissions from the A. mangium soils were less than or consistent with those in the natural forest soils. Multiple spatial dependence of N₂O fluxes within 3.2 m (1-m grids) and 35.0 m (10-m grids), and CO₂ fluxes within 1.8 m (1-m grids) and over 65 m (10-m grids) was detected. From the relationship among N₂O and CO₂ gas fluxes, soil properties, and topographic elements, we suggest that the multiple spatial structures of N₂O and CO₂ fluxes are mainly associated with soil resources such as readily mineralizable carbon and nitrogen in a relatively dry season. The soil resource distributions were probably controlled by the meso- and microtopography. Meanwhile, CH₄ fluxes were spatially independent in the A. mangium soils, and the water-filled pore space appeared to mainly control the spatial distribution of these fluxes.     

NO, N2O, CH4 and CO2 fluxes in winter barley field of Japanese Andisol as affected by N fertilizer management

The study was carried out at the experimental station of the Japan International Research Center for Agricultural Sciences to investigate gas fluxes from a Japanese Andisol under different N fertilizer managements: CD, a deep application (8 cm) of the controlled release urea; UD, a deep application (8 cm) of the conventional urea; US, a surface application of the conventional urea; and a control, without any N application. NO, N₂O, CH₄ and CO₂ fluxes were measured simultaneously in a winter barley field under the maize/barley rotation. The fluxes of NO and N₂O from the control were very low, and N fertilization increased the emissions of NO and N₂O. NO and N₂O from N fertilization treatments showed different emission patterns: significant NO emissions but low N₂O emissions in the winter season, and low NO emissions but significant N₂O emissions during the short period of barley growth in the spring season. The controlled release of the N fertilizer decreased the total NO emissions, while a deep application increased the total N₂O emissions. Fertilizer-derived NO-N and N₂O-N from the treatments CD, UD and US accounted for 0.20±0.07%, 0.71±0.15%, 0.62±0.04%, and 0.52±0.04%, 0.50±0.09%, 0.35±0.03%, of the applied N, respectively, during the barley season. CH₄ fluxes from the control were negative on most sampling dates, and its net soil uptake was 33±7.1 mg m⁻² during the barley season. The application of the N fertilizer decreased the uptake of atmospheric CH₄ and resulted in positive emissions from the soil. CO₂ fluxes were very low in the early period of crop growth while higher emissions were observed in the spring season. The N fertilization generally increased the direct CO₂ emissions from the soil. N₂O, CH₄ and CO₂ fluxes were positively correlated (P<0.01) with each other, whereas NO and CO₂ fluxes were negatively correlated (P<0.05). The N fertilization increased soil-derived global warming potential (GWP) significantly in the barley season. The net GWP was calculated by subtracting the plant-fixed atmospheric CO₂ stored in its aboveground parts from the soil-derived GWP in CO₂ equivalent. The net GWP from the CD, UD, US and the control were all negative at −243±30.7, −257±28.4, −227±6.6 and −143±9.7 g C m⁻² in CO₂ equivalent, respectively, in the barley season.     

Potential contribution of Lumbricus terrestris L. to carbon dioxide, methane and nitrous oxide fluxes from a forest soil

 Potential effects of earthworms (Lumbricus terrestris L.) inoculated into soil on fluxes of CO₂, CH₄ and N₂O were investigated for an untreated and a limed soil under beech in open topsoil columns under field conditions for 120 days. Gas fluxes from L. terrestris, beech litter and mineral soil from soil columns were measured separately in jars at 17  °C. The inoculation with L. terrestris and the application of lime had no effect on cumulative CO₂ emissions from soil. During the first 3–4 weeks earthworms significantly (P<0.05) increased CO₂ emissions by 16% to 28%. In contrast, significantly lower (P<0.05) CO₂ emission rates were measured after 11 weeks. The data suggest that earthworm activity was high during the first weeks due to the creation of burrows and incorporation of beech litter into the mineral soil. Low cumulative CH₄ oxidation rates were found in all soil columns as a result of CH₄ production and oxidation processes. L. terrestris with fresh feces and the beech litter produced CH₄ during the laboratory incubation, whereas the mineral soil oxidised atmospheric CH₄. Inoculation with L. terrestris led to a significant reduction (P<0.02) in the CH₄ oxidation rate of soil, i.e. 53% reduction. Liming had no effect on cumulative CH₄ oxidation rates of soil columns and on CH₄ fluxes during the laboratory incubation. L. terrestris significantly increased (P<0.001) cumulative N₂O emissions of unlimed soil columns by 57%. The separate incubation of L. terrestris with fresh feces resulted in rather high N₂O emissions, but the rate strongly decreased from 54 to 2 μg N kg^–1 (dry weight) h^–1 during the 100 h of incubation. Liming had a marked effect on N₂O formation and significantly (P<0.001) reduced cumulative N₂O emissions by 34%. Although the interaction of liming and L. terrestris was not significant, N₂O emissions of limed soil columns with L. terrestris were 8% lower than those of the control. Received: 2 September 1999     

Applicability of the soil gradient method for estimating soil–atmosphere CO2, CH4, and N2O fluxes for steppe soils in Inner Mongolia

For evaluating the applicability of the soil gradient method as a substitute for CO₂‐, CH₄‐, and N₂O‐flux measurements in steppe, we carried out chamber measurements and determined soil gas concentration at an ungrazed (UG99) and a grazed (WG) site in Inner Mongolia, China. The agreement of the concentration‐based flux estimates with measured chamber‐based fluxes varied largely depending on the respective GHG in the sequence CO₂ > CH₄ >> N₂O. A calibration of the gas‐transport parameter used to calculate fluxes based on soil gas concentrations improved the results considerably for CO₂ and CH₄. After calibration, the average deviation from the chamber‐based annual cumulative flux for both sites was 11.5%, 10.5%, and 59% for CO₂, CH₄, and N₂O. The gradient method did not constitute an adequate stand‐alone substitute for greenhouse‐gas flux estimation since a calibration using chamber‐based measurements was necessary and vigorous production processes were confined to the uppermost, almost water‐saturated soil layer.     

Dissolved organic matter and inorganic N jointly regulate greenhouse gases fluxes from forest soils with different moistures during a freeze-thaw period

ABSTRACT

Antecedent soil moisture before freezing can affect greenhouse gases (GHG) fluxes from soils during thaw, but their critical threshold values for GHG fluxes and the underlying mechanisms are still not clear. By using packed soil-core incubation experiments, we have studied nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) fluxes from a mature broadleaf and Korean pine-mixed forest soil and an adjacent white birch forest soil with nine levels of soil moisture ranging from 10 to 90% water-filled pore space (WFPS) during a 2-month freezing at ?8°C and the following 10-day thaw at 10°C. The threshold values of soil moisture ranged from 50 to 70% WFPS for CH₄ uptake and from 70 to 90% WFPS for N₂O and CO₂ emissions from the two soils during the freeze-thaw period. Under the optimum soil moisture condition, fulvic-like compounds with high bioavailability contributed more than 60% of dissolved organic matter (DOM) in the soil. Cumulative N₂O emissions from forest soils during the freeze-thaw period were greatest when the concentration ratio of nitrate-N to dissolved organic carbon (DOC) was 0.04 g N g^?1 C. Cumulative soil CO₂ emissions and CH₄ uptake during the freeze-thaw period were both regulated by the interaction between soil DOC and net N mineralization. The activities of β-1,4-glucosidase and β-1,4-N-acetyl-glucosaminidase, microbial biomass C and N, and the microbial biomass C-to-N ratios, were all significantly correlated to the soil N₂O, CO₂, and CH₄ fluxes. Overall, upon a freeze-thaw period with different soil moistures, GHG fluxes from forest soils were jointly regulated by inorganic N and DOC concentrations, and related to the labile components of DOM released into the soil, which could be strictly controlled by the related microbial properties.     

Wavelet analysis of wintertime and spring thaw CO2 and N2O fluxes from agricultural fields

Fluxes of N₂O and CO₂ are not limited to the growing season; winter and spring thaw can represent a significant emission period. The objective of this study was to apply wavelet analysis to winter and spring thaw CO₂ and N₂O fluxes and soil temperatures, to yield additional information about underlying processes, examining temporal patterns and relationships among them. Fluxes used in this analysis were measured over 4 years using micrometeorological methods, in a study comparing two agricultural management practices, best management (BM) and conventional (CONV) practices. Cross-wavelet transform (XWT) and wavelet coherence (WCO) were applied to daily mean time series of N₂O fluxes for BM and CONV replicates and treatments, CO₂ vs. N₂O fluxes, CO₂ flux vs. air and soil temperatures, and N₂O flux vs. air and soil temperatures. N₂O fluxes for replicate plots had small differences in temporal variation while N₂O fluxes from BM and CONV treatments showed a large difference in their time series. XWT and WCO analysis confirmed differences in N₂O fluxes between management practices due to differences in temporal trends in the time series. Field emissions of N₂O and CO₂ fluxes showed times of common high fluxes, such as thaw events. Nitrous oxide and CO₂ flux time series showed a strong coherence with surface (air) temperatures. The relationship between N₂O fluxes and temperature decreased with depth but the relationship between CO₂ flux and temperature was similar for surface and at depth. The strong coherence between emissions and surface conditions does not support the suggested mechanism of trapped gas release. A release of trapped gases from below the ice formation would have been indicated by a strong coherence from CO₂ and N₂O with temperatures at depth as the trapping ice barrier melted. This study demonstrates the effectiveness of wavelets as a tool to investigate temporal relationships in GHG emissions, which is a relatively new application for this type of analysis.     

Emissions of CH4, CO2, and N2O from soil at a cattle overwintering area as affected by available C and N

Relationships between CH₄, CO₂, and N₂O emissions were studied in soil that had been freshly amended with large deposits of cattle wastes. Dynamics of CH₄, CO₂, and N₂O emissions were investigated with flux chambers from early April to late June 2011, during the 3 months following cattle overwintering at the site. This 81-day field study was supplemented with soil analyses of available C and N content and measurement of denitrification activity. In a more detailed field investigation, the daily time course of emissions was determined. The field research was complemented with a laboratory experiment that focused on the short-term time course of N₂O and CH₄ production in artificially created anoxic soil microsites. The following hypotheses were tested: (i) a large input of C (and N and other nutrients) in cattle manure creates conditions suitable for methanogenesis, and therefore overwintering areas can produce large amounts of CH₄; (ii) N₂O is produced and emitted until the level of mineral N decreases, while the level of CH₄ production is low; and (iii) production of CH₄ is greater when N immobilization decreases the level of NO₃⁻ in soil. N₂O emissions were relatively large during the first 3 weeks, then peaked (at ca. 4000 μg N₂ON m⁻² h⁻¹) and soon decreased to almost zero; the changes were related to the mineral and soluble organic N content in soil. CH₄ fluxes were large, though variable, in the first 2 months (600–3000 μg CH₄C m⁻² h⁻¹) and were independent of C and N availability. Although time courses differed for CH₄ and N₂O, a negative relationship between N₂O and CH₄ emissions was not detected. Contrary to CH₄ and N₂O fluxes, CO₂ emissions progressively increased to ca. 300 mg CO₂C m⁻² h⁻¹ at the end of the field study and were closely related to air and soil temperatures. Diurnal measurements revealed significant correlations between temperature and emissions of CH₄, N₂O, and CO₂. Addition of C to soil during anaerobic incubation increased the production and consumption of N₂O and supported the emission of CH₄. The results suggest that rapid denitrification significantly contributes to the exhaustion of oxidizing agents and helps create microsites supporting methanogenesis in otherwise N₂O-producing upland soil. The results also indicate that accurate estimate of gas fluxes in animal-impacted grassland areas requires assessment of both diurnal and long-term changes in CH₄, CO₂, and N₂O emissions.     

Organic matter quality of a forest soil subjected to repeated drying and different re‐wetting intensities

Extended drought periods followed by heavy rainfall may increase in many regions of the Earth, but the consequences for the quality of soil organic matter and soil microbial communities are poorly understood. Here, we investigated the effect of repeated drying and re‐wetting on microbial communities and the quality of particulate and dissolved organic matter in a Haplic Podzol from a Norway spruce stand. After air‐drying, undisturbed soil columns were re‐wetted at different intensities (8, 20 and 50 mm per day) and time intervals, so that all treatments received the same amount of water per cycle (100 mm). After the third cycle, SOM pools of the treatments were compared with those of non‐dried control columns. Lignin phenols were not systematically affected in the O horizons by the treatments whereas fewer lignin phenols were found in the A horizon of the 20‐ and 50‐mm treatments. Microbial biomass and the ratio of fungi to bacteria were generally not altered, suggesting that most soil microorganisms were well adapted to drying and re‐wetting in this soil. However, gram‐positive bacteria and actinomycetes were reduced whereas gram‐negative bacteria and protozoa were stimulated by the treatments. The increase in the (cy 17: 0 + cy 19: 0)/(16:1ω7c + 18:1ω7c) ratio indicates physiological or nutritional stress for the bacterial communities in the O, A and B horizons with increasing re‐wetting intensity. Drying and re‐wetting reduced the amount of hydrolysable plant and microbial sugars in all soil horizons. However, CO₂ and dissolved organic carbon fluxes could not explain these losses. We postulate that drying and re‐wetting triggered chemical alterations of hydrolysable sugar molecules in organic and mineral soil horizons.     

Aeration effects on CO2, N2O,and CH4 emission and leachate composition of a forest soil

The availability of O₂ is one of the most important factors controlling the chemical and biological reactions in soils. In this study, the effects of different aeration conditions on the dynamics of the emission of trace gases (CO₂, N₂O, CH₄) and the leachate composition (NO₃^‐, DOC, Mn, Fe) were determined. The experiment was conducted with naturally structured soil columns (silty clay, Vertisol) from a well aerated forest site. The soil monoliths were incubated in a microcosm system at different O₂ concentrations (0, 0.001, 0.005, 0.01, 0.05, and 0.205 m³ m^‐3 in the air flow through the headspace of the microcosms) for 85 days. Reduced O₂ availability resulted in a decreased CO₂ release but in increased N₂O emission rates. The greatest cumulative N₂O emissions (= 1.6 g N₂O‐N m^‐2) were observed at intermediate O₂ concentrations (0.005 and 0.01 m³ m^‐3) when both nitrification and denitrification occurred simultaneously in the soil. Cumulative N₂O emissions were smallest (= 0.05 g N₂O‐N m^‐2) for the aeration with ambient air (O₂ concentration: 0.205 m³ m^‐3), although nitrate availability was greatest in this treatment. The emission of CH₄ and leaching of Mn and Fe were restricted to the soil columns incubated under completely anoxic conditions. The sequence of the reduction processes under completely anoxic conditions complied with the thermodynamic theory: soil nitrate was reduced first, followed by the reduction of Mn(IV) and Fe(III) and finally CO₂ was reduced to CH₄. The re‐aeration of the soil columns after 85 days of anoxic incubation terminated the production of CH₄ and dissolved Fe and Mn in the soil but strongly increased the emission rates of CO₂ and N₂O and the leaching of NO₃^‐ probably because of the accumulation of DOC and NH₄⁺ during the previous anoxic period.     

Seasonal variation and fire effects on CH4, N2O and CO2 exchange in savanna soils of northern Australia

Tropical savanna ecosystems are a major contributor to global CO₂, CH₄ and N₂O greenhouse gas exchange. Savanna fire events represent large, discrete C emissions but the importance of ongoing soil-atmosphere gas exchange is less well understood. Seasonal rainfall and fire events are likely to impact upon savanna soil microbial processes involved in N₂O and CH₄ exchange. We measured soil CO₂, CH₄ and N₂O fluxes in savanna woodland (Eucalyptus tetrodonta/Eucalyptus miniata trees above sorghum grass) at Howard Springs, Australia over a 16 month period from October 2007 to January 2009 using manual chambers and a field-based gas chromatograph connected to automated chambers. The effect of fire on soil gas exchange was investigated through two controlled burns and protected unburnt areas. Fire is a frequent natural and management action in these savanna (every 1-2 years). There was no seasonal change and no fire effect upon soil N₂O exchange. Soil N₂O fluxes were very low, generally between −1.0 and 1.0 μg N m⁻² h⁻¹, and often below the minimum detection limit. There was an increase in soil NH₄⁺ in the months after the 2008 fire event, but no change in soil NO₃⁻. There was considerable nitrification in the early wet season but minimal nitrification at all other times.Savanna soil was generally a net CH₄ sink that equated to between −2.0 and −1.6 kg CH₄ ha⁻¹ y⁻¹ with no clear seasonal pattern in response to changing soil moisture conditions. Irrigation in the dry season significantly reduced soil gas diffusion and as a consequence soil CH₄ uptake. There were short periods of soil CH₄ emission, up to 20 μg C m⁻² h⁻¹, likely to have been caused by termite activity in, or beneath, automated chambers. Soil CO₂ fluxes showed a strong bimodal seasonal pattern, increasing fivefold from the dry into the wet season. Soil moisture showed a weak relationship with soil CH₄ fluxes, but a much stronger relationship with soil CO₂ fluxes, explaining up to 70% of the variation in unburnt treatments. Australian savanna soils are a small N₂O source, and possibly even a sink. Annual soil CH₄ flux measurements suggest that the 1.9 million km² of Australian savanna soils may provide a C sink of between −7.7 and −9.4 Tg CO₂-e per year. This sink estimate would offset potentially 10% of Australian transport related CO₂-e emissions. This CH₄ sink estimate does not include concurrent CH₄ emissions from termite mounds or ephemeral wetlands in Australian savannas.     

Emissions of CO2, CH4 and N2O from Southern European peatlands

Peatlands play an important role in emissions of the greenhouse gases CO₂, CH₄ and N₂O, which are produced during mineralization of the peat organic matter. To examine the influence of soil type (fen, bog soil) and environmental factors (temperature, groundwater level), emission of CO₂, CH₄ and N₂O and soil temperature and groundwater level were measured weekly or biweekly in loco over a one-year period at four sites located in Ljubljana Marsh, Slovenia using the static chamber technique. The study involved two fen and two bog soils differing in organic carbon and nitrogen content, pH, bulk density, water holding capacity and groundwater level. The lowest CO₂ fluxes occurred during the winter, fluxes of N₂O were highest during summer and early spring (February, March) and fluxes of CH₄ were highest during autumn. The temporal variation in CO₂ fluxes could be explained by seasonal temperature variations, whereas CH₄ and N₂O fluxes could be correlated to groundwater level and soil carbon content. The experimental sites were net sources of measured greenhouse gases except for the drained bog site, which was a net sink of CH₄. The mean fluxes of CO₂ ranged between 139 mg m⁻² h⁻¹ in the undrained bog and 206 mg m⁻² h⁻¹ in the drained fen; mean fluxes of CH₄ were between −0.04 mg m⁻² h⁻¹ in the drained bog and 0.05 mg m⁻² h⁻¹ in the drained fen; and mean fluxes of N₂O were between 0.43 mg m⁻² h⁻¹ in the drained fen and 1.03 mg m⁻² h⁻¹ in the drained bog. These results indicate that the examined peatlands emit similar amounts of CO₂ and CH₄ to peatlands in Central and Northern Europe and significantly higher amounts of N₂O.     

Nitrification and Methane Oxidation in Forest Soil: Acid Deposition, Nitrogen Input and Plant Effects

In a laboratory incubation experiment, nitrification potential, methane oxidation, N₂O and CO₂ release were studied in the organic soil layer (0–10 cm) of field lysimeters containing re-established soil profiles from a 100-year-old Scots pine (Pinus sylvestris) forest of Norway. The experiment was designed as a full factorial (3 factors; N fertilisation rates, soil acidification, and plants), with three replicates. The more acidic irrigation (pH 3) significantly reduced nitrification potential and N₂O fluxes, methane oxidation and CO₂ release. We concluded that the reduction in soil N₂O release by severe acid deposition is partly due to reduction in nitrification potential. The highest N₂O fluxes were observed in the combination of fertilised planted and less acidic pH treatment. N fertilisation (90 kg N ha^?1 y^?1 with NH₄NO₃) increased soil N₂O release by a factor of 8 and decreased CH₄ oxidation by 60–80%. Plant effects on soil nitrification potential and methane oxidation rates are discussed.