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.     

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.     

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.     

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.     

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.     

Seasonal changes in the spatial structures of N2O, CO2, and CH4 fluxes from Acacia mangium plantation soils in Indonesia

We evaluated the spatial structures of nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄) fluxes in an Acacia mangium plantation stand in Sumatra, Indonesia, in drier (August) and wetter (March) seasons. A 60 × 100-m plot was established in an A. mangium plantation that included different topographical elements of the upper plateau, lower plateau, upper slope and foot slope. The plot was divided into 10 × 10-m grids and gas fluxes and soil properties were measured at 77 grid points at 10-m intervals within the plot. Spatial structures of the gas fluxes and soil properties were identified using geostatistical analyses. Averaged N₂O and CO₂ fluxes in the wetter season (1.85 mg N m⁻² d⁻¹ and 4.29 g C m⁻² d⁻¹, respectively) were significantly higher than those in the drier season (0.55 mg N m⁻² d⁻¹ and 2.73 g C m⁻² d⁻¹, respectively) and averaged CH₄ uptake rates in the drier season (−0.62 mg C m⁻² d⁻¹) were higher than those in the wetter season (−0.24 mg C m⁻² d⁻¹). These values of N₂O fluxes in A. mangium soils were higher than those reported for natural forest soils in Sumatra, while CO₂ and CH₄ fluxes were in the range of fluxes reported for natural forest soils. Seasonal differences in these gas fluxes appears to be controlled by soil water content and substrate availability due to differing precipitation and mineralization of litter between seasons. N₂O fluxes had strong spatial dependence with a range of about 18 m in both the drier and wetter seasons. Topography was associated with the N₂O fluxes in the wetter season with higher and lower fluxes on the foot slope and on the upper plateau, respectively, via controlling the anaerobic-aerobic conditions in the soils. In the drier season, however, we could not find obvious topographic influences on the spatial patterns of N₂O fluxes and they may have depended on litter amount distribution. CO₂ fluxes had no spatial dependence in both seasons, but the topographic influence was significant in the drier season with lowest fluxes on the foot slope, while there was no significant difference between topographic positions in the wetter season. The distributions of litter amount and soil organic matter were possibly associated with CO₂ fluxes through their effects on microbial activities and fine root distribution in this A. mangium plantation.     

Interactions between physical and biotic factors influence CO2 flux in Antarctic dry valley soils

Soil carbon dioxide (CO₂) flux is an integrative measure of ecosystem functioning representing both biotic and physical controls over carbon (C) balance. In the McMurdo Dry Valleys of Antarctica, soil CO₂ fluxes (approximately −0.1-0.15 μmol m⁻² s⁻¹) are generally low, and negative fluxes (uptake of CO₂) are sometimes observed. A combination of biological respiration and physical mechanisms, driven by temperature and mediated by soil moisture and mineralogy, determine CO₂ flux and, therefore, soil organic C balance. The physical factors important to CO₂ flux are being altered with climate variability in many ecosystems including arid forms such as the Antarctic terrestrial ecosystems, making it critical to understand how climate factors interact with biotic drivers to control soil CO₂ fluxes and C balances. We measured soil CO₂ flux in experimental field manipulations, microcosm incubations and across natural environmental gradients of soil moisture to estimate biotic soil respiration and abiotic sources of CO₂ flux in soils over a range of physical and biotic conditions. We determined that temperature fluctuations were the most important factor influencing diel variation in CO₂ flux. Variation within these diel CO₂ cycles was explained by differences in soil moisture. Increased temperature (as opposed to temperature fluctuations) had little or no effect on CO₂ flux if moisture was not also increased. We conclude that CO₂ flux in dry valley soils is driven primarily by physical factors such as soil temperature and moisture, indicating that future climate change may alter the dry valley soil C cycle. Negative CO₂ fluxes in arid soils have recently been identified as potential net C sinks. We demonstrate the potential for arid polar soils to take up CO₂, driven largely by abiotic factors associated with climate change. The low levels of CO₂ absorption into soils we observed may not constitute a significant sink of atmospheric CO₂, but will influence the interpretation of CO₂ flux for the dry valley soil C cycle and possibly other arid environments where biotic controls over C cycling are secondary to physical drivers.     

Elevated CO2 concentration and nitrogen fertilisation effects on N2O and CH4 fluxes and biomass production of Phleum pratense on farmed peat soil

The effects of elevated CO₂ supply on N₂O and CH₄ fluxes and biomass production of Phleum pratense were studied in a greenhouse experiment. Three sets of 12 farmed peat soil mesocosms (10 cm dia, 47 cm long) sown with P. pratense and equally distributed in four thermo-controlled greenhouses were fertilised with a commercial fertiliser in order to add 2, 6 or 10 g N m⁻². In two of the greenhouses, CO₂ concentration was kept at atmospheric concentration (360 μmol mol⁻¹) and in the other two at doubled concentration (720 μmol mol⁻¹). Soil temperature was kept at 15 °C and air temperature at 20 °C. Natural lighting was supported by artificial light and deionized water was used to regulate soil moisture. Forage was harvested and the plants fertilised three times during the basic experiment, followed by an extra fertilisations and harvests. At the end of the experiment CH₄ production and CH₄ oxidation potentials were determined; roots were collected and the biomass was determined. From the three first harvests the amount of total N in the aboveground biomass was determined. N₂O and CH₄ exchange was monitored using a closed chamber technique and a gas chromatograph. The highest N₂O fluxes (on average, 255 μg N₂O m⁻² h⁻¹ during period IV) occurred just after fertilisation at high water contents, and especially at the beginning of the growing season (on average, 490 μg N₂O m⁻² h⁻¹ during period I) when the competition of vegetation for N was low. CH₄ fluxes were negligible throughout the experiment, and for all treatments the production and oxidation potentials of CH₄ were inconsequential. Especially at the highest rates of fertilisation, the elevated supply of CO₂ increased above- and below-ground biomass production, but both at the highest and lowest rates of fertilisation, decreased the total amount of N in the aboveground dry biomass. N₂O fluxes tended to be higher under doubled CO₂ concentrations, indicating that increasing atmospheric CO₂ concentration may affect N and C dynamics in farmed peat soil.     

Influence of warming and enchytraeid activities on soil CO2 and CH4 fluxes

To determine the sum of ‘direct’ and ‘indirect’ effects of climatic change on enchytraeid activity and C fluxes from an organic soil we assessed the influence of temperature (4, 10 and 15 °C incubations) on enchytraeid populations and soil CO₂ and CH₄ fluxes over 116 days. Moisture was maintained at 60% of soil dry weight during the experimental period and measurements of enchytraeid biomass and numbers, and CO₂ and CH₄ fluxes were made after 3, 16, 33, 44, 65, 86 and 116 days. Enchytraeid population numbers and biomass increased in all temperature treatments with the greatest increase produced at 15 °C (to over threefold initial values by day 86). Results also showed that enchytraeid activity increased CO₂ fluxes by 10.7±4.5, 3.4±4.0 and 26.8±2.6% in 4, 10 and 15 °C treatments, respectively, with the greatest CO₂ production observed at 15 °C for the entire 116 day incubation period (P<0.05). The soil respiratory quotient analyses at lower temperatures (i.e. 4-10 °C) gave a Q₁₀ of 1.7 and 1.9 with and without enchytraeids, respectively. At temperatures above 10 °C (i.e. 10-15 °C) Q₁₀ significantly increased (P<0.01) and was 25% greater in the presence of enchytraeids (Q₁₀=3.4) than without (Q₁₀=2.6). In contrast to CO₂ production, no significant relationships were observed between net CH₄ fluxes and temperature and only time showed a significant effect on CH₄ production (P<0.01).Total soil CO₂ production was positively linked with enchytraeid biomass and mean soil CO₂-C production was 77.01±6.05 CO₂-C μg mg enchytraeid tissue⁻¹ day⁻¹ irrespective of temperature treatment. This positive relationship was used to build a two step regression model to estimate the effects of temperature on enchytraeid biomass and soil CO₂ respiration in the field. Predictions of potential CO₂ production were made using enchytraeid biomass data obtained in the field from two upland grassland sites (Sourhope and Great Dun Fell at the Moor House Nature Reserve, both in the UK). The findings of this work suggest that a 5 °C increase in atmospheric temperature above mean ambient temperature could have the potential to produce a significant increase in enchytraeid biomass resulting in a near twofold increase in soil CO₂ release from both soil types. The interaction between temperature and soil biology will clearly be an important determinant of soil respiration responses to global warming.     

Effects of elevated atmospheric CO2, prolonged summer drought and temperature increase on N2O and CH4 fluxes in a temperate heathland

In temperate regions, climate change is predicted to increase annual mean temperature and intensify the duration and frequency of summer droughts, which together with elevated atmospheric carbon dioxide (CO₂) concentrations, may affect the exchange of nitrous oxide (N₂O) and methane (CH₄) between terrestrial ecosystems and the atmosphere. We report results from the CLIMAITE experiment, where the effects of these three climate change parameters were investigated solely and in all combinations in a temperate heathland. Field measurements of N₂O and CH₄ fluxes took place 1-2 years after the climate change manipulations were initiated. The soil was generally a net sink for atmospheric CH₄. Elevated temperature (T) increased the CH₄ uptake by on average 10 μg C m⁻² h⁻¹, corresponding to a rise in the uptake rate of about 20%. However, during winter elevated CO₂ (CO₂) reduced the CH₄ uptake, which outweighed the positive effect of warming when analyzed across the study period. Emissions of N₂O were generally low (<10 μg N m⁻² h⁻¹). As single experimental factors, elevated CO₂, temperature and summer drought (D) had no major effect on the N₂O fluxes, but the combination of CO₂ and warming (TCO₂) stimulated N₂O emission, whereas the N₂O emission ceased when CO₂ was combined with drought (DCO₂). We suggest that these N₂O responses are related to increased rhizodeposition under elevated CO₂ combined with increased and reduced nitrogen turnover rates caused by warming and drought, respectively. The N₂O flux in the multifactor treatment TDCO₂ was not different from the ambient control treatment. Overall, our study suggests that in the future, CH₄ uptake may increase slightly, while N₂O emission will remain unchanged in temperate ecosystems on well-aerated soils. However, we propose that continued exposure to altered climate could potentially change the greenhouse gas flux pattern in the investigated heathland.     

Short-term effects of clearfelling on soil CO2, CH4, and N2O fluxes in a Sitka spruce plantation

We examined the effects of forest clearfelling on the fluxes of soil CO₂, CH₄, and N₂O in a Sitka spruce (Picea sitchensis (Bong.) Carr.) plantation on an organic-rich peaty gley soil, in Northern England. Soil CO₂, CH₄, N₂O as well as environmental factors such as soil temperature, soil water content, and depth to the water table were recorded in two mature stands for one growing season, at the end of which one of the two stands was felled and one was left as control. Monitoring of the same parameters continued thereafter for a second growing season. For the first 10 months after clearfelling, there was a significant decrease in soil CO₂ efflux, with an average efflux rate of 4.0 g m⁻² d⁻¹ in the mature stand (40-year) and 2.7 g m⁻² d⁻¹ in clearfelled site (CF). Clearfelling turned the soil from a sink (−0.37 mg m⁻² d⁻¹) for CH₄ to a net source (2.01 mg m⁻² d⁻¹). For the same period, soil N₂O fluxes averaged 0.57 mg m⁻² d⁻¹ in the CF and 0.23 mg m⁻² d⁻¹ in the 40-year stand. Clearfelling affected environmental factors and lead to higher daily soil temperatures during the summer period, while it caused an increase in the soil water content and a rise in the water table depth. Despite clearfelling, CO₂ remained the dominant greenhouse gas in terms of its greenhouse warming potential.     

Net fluxes of CO2, but not N2O or CH4, are affected following agronomic-scale additions of urea to prairie and arable soils

While experimental addition of nitrogen (N) tends to enhance soil fluxes of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), it is not known if lower and agronomic-scale additions of urea-N applied also enhance trace gas fluxes, particularly for semi-arid agricultural lands in the northern plains. We aimed to test if this were true at agronomic rates [low (11 kg N ha⁻¹), moderate (56 kg N ha⁻¹), and high (112 kg N ha⁻¹)] for central North Dakota arable and prairie soils using intact soil cores to minimize disturbance and simulate field conditions. Additions of urea to cores incubated at 21 °C and 57% water-filled pore space enhanced fluxes of CO₂ but not CH₄ and N₂O. At low, moderate, and high urea-N, CO₂ fluxes were significantly greater than control but not fluxes of CH₄ and N₂O. The increases in CO₂ emission with rate of urea-N application indicate that agronomic-scale N inputs may stimulate microbial carbon cycling in these soils, and that the contribution of CO₂ to net greenhouse gas source strength following fertilization of semi-arid agroecosystems may at times be greater than contributions by N₂O and CH₄.     

The effect of increased atmospheric carbon dioxide concentration on emissions of nitrous oxide, carbon dioxide and methane from a wheat field in a semi-arid environment in northern China

There are no reports on the effects of elevated carbon dioxide [CO₂] on the fluxes of N₂O, CO₂ and CH₄ from semi-arid wheat cropping systems. These three soil gas fluxes were measured using closed chambers under ambient (420 ± 18 μmol mol⁻¹) and elevated (565 ± 37 μmol mol⁻¹) at the Free-Air Carbon dioxide Enrichment experimental facility in northern China. Measurements were made over five weeks on a wheat crop (Triticum aestivum L. cv. Zhongmai 175). Elevated [CO₂] increased N₂O and CO₂ emission from soil by 60% and 15%, respectively, but had no significant effect on CH₄ flux. There was no significant interaction between [CO₂] and N application rate on these gas fluxes, probably because soil N was not limiting. At least 22% increase in C storage is required to offset the observed increase in greenhouse gas emissions under elevated [CO₂].     

Dinitrogen and N2O emissions in arable soils: Effect of tillage, N source and soil moisture

A laboratory investigation was performed to compare the fluxes of dinitrogen (N₂), N₂O and carbon dioxide (CO₂) from no-till (NT) and conventional till (CT) soils under the same water, mineral nitrogen and temperature status. Intact soil cores (0-10 cm) were incubated for 2 weeks at 25 °C at either 75% or 60% water-filled pore space (WFPS) with ¹⁵N-labeled fertilizers (100 mg N kg⁻¹ soil). Gas and soil samples were collected at 1-4 day intervals during the incubation period. The N₂O and CO₂ fluxes were measured by a gas chromatography (GC) system while total N₂ and N₂O losses and their ¹⁵N mole fractions in the soil mineral N pool were determined by a mass spectrometer. The daily accumulative fluxes of N₂ and N₂O were significantly affected by tillage, N source and soil moisture. We observed higher (P<0.05) fluxes of N₂+N₂O, N₂O and CO₂ from the NT soils than from the CT soils. Compared with the addition of nitrate (NO₃⁻), the addition of ammonium (NH₄⁺) enhanced the emissions of these N and C gases in the CT and NT soils, but the effect of NH₄⁺ on the N₂ and/or N₂O fluxes was evident only at 60% WFPS, indicating that nitrification and subsequent denitrification contributed largely to the gaseous N losses and N₂O emission under the lower moisture condition. Total and fertilizer-induced emissions of N₂ and/or N₂O were higher (P<0.05) at 75% WFPS than with 60% WFPS, while CO₂ fluxes were not influenced by the two moisture levels. These laboratory results indicate that there is greater potential for N₂O loss from NT soils than CT soils. Avoiding wet soil conditions (>60% WFPS) and applying a NO₃⁻ form of N fertilizer would reduce potential N₂O emissions from arable soils.     

Dynamics of upward and downward N2O and CO2 fluxes in ploughed or no-tilled soils in relation to water-filled pore space, compaction and crop presence

Sharp peaks in nitrous oxide (N₂O) fluxes under no-tillage in wet conditions appear to be related to near surface soil and crop cover conditions. Here we explored some of the factors influencing tillage effects on short-term variations in gas flux so that we could learn about the mechanisms involved. Field investigations revealed that a cumulative emission of 13 kg N₂O–N ha⁻¹ over a 12-week period was possible under no-tillage for spring barley. We investigated how reducing crop cover and changing the structural arrangement of the water-filled pore space (WFPS) by short-term laboratory compaction influenced N₂O and carbon dioxide (CO₂) fluxes in upward and downward directions in core samples from tilled and untilled soil. Increasing the downward flux of N₂O within a soil profile by changing soil or moisture conditions may increase the likelihood of its further reduction to N₂ or dissolution. We took undisturbed cores from 3 to 8 cm depth, equilibrated them to −1 or −6 kPa matric potential, incubated them and measured N₂O and CO₂ fluxes from the upper and lower surfaces in a purpose-designed apparatus before and after compaction in an uniaxial tester. We also measured WFPS, air permeability, bulk density and air-filled porosity before and after compaction. Spring barley was tested in 1999 and winter barley in 2000.Fluxes of N₂O were from 1.5 to 35 times higher from no-tilled than ploughed even where the soil was of similar bulk density. Reduction of the crop cover increased CO₂ flux and could reduce N₂O flux. The effects of structural changes induced by laboratory compaction on the fluxes of N₂O and CO₂ were not influenced greatly by the tillage and crop cover treatments. Fluxes from the upper surfaces of cores (corresponding to 3 cm soil depth, upwards direction) could be up to 100 times greater (N₂O) or 8 times (CO₂) than from the lower surfaces (8 cm depth, downwards direction). These differences between surfaces were greatest when N₂O fluxes were very high in no-tilled soil (4.2 mg N₂O–N m⁻² h⁻¹) as occurred when WFPS exceeded 80% or became blocked with water, an effect that was increased by our compaction treatment. In general N₂O fluxes increased with WFPS. The production and emission of N₂O were strongly influenced by the soil physical environment, the magnitude of the water-filled pore space and continuity of the air-filled pore space in particular, produced in no-till versus plough cultivation.     

Nitrogen fixation by biological soil crusts and heterotrophic bacteria in an intact Mojave Desert ecosystem with elevated CO2 and added soil carbon

Fixation of N by biological soil crusts and free-living heterotrophic soil microbes provides a significant proportion of ecosystem N in arid lands. To gain a better understanding of how elevated CO₂ may affect N₂-fixation in aridland ecosystems, we measured C₂H₂ reduction as a proxy for nitrogenase activity in biological soil crusts for 2 yr, and in soils either with or without dextrose-C additions for 1 yr, in an intact Mojave Desert ecosystem exposed to elevated CO₂. We also measured crust and soil δ¹⁵N and total N to assess changes in N sources, and δ¹³C of crusts to determine a functional shift in crust species, with elevated CO₂. The mean rate of C₂H₂ reduction by biological soil crusts was 76.9±5.6 μmol C₂H₄ m⁻² h⁻¹. There was no significant CO₂ effect, but crusts from plant interspaces showed high variability in nitrogenase activity with elevated CO₂. Additions of dextrose-C had a positive effect on rates of C₂H₂ reduction in soil. There was no elevated CO₂ effect on soil nitrogenase activity. Plant cover affected soil response to C addition, with the largest response in plant interspaces. The mean rate of C₂H₂ reduction in soils either with or without C additions were 8.5±3.6 μmol C₂H₄ m⁻² h⁻¹ and 4.8±2.1 μmol m⁻² h⁻¹, respectively. Crust and soil δ¹⁵N and δ¹³C values were not affected by CO₂ treatment, but did show an effect of cover type. Crust and soil samples in plant interspaces had the lowest values for both measurements. Analysis of soil and crust [N] and δ¹⁵N data with the Rayleigh distillation model suggests that any plant community changes with elevated CO₂ and concomitant changes in litter composition likely will overwhelm any physiological changes in N₂-fixation.     

Emission of CO2, CH4 and N2O from lakeshore soils in an Antarctic dry valley

We measured soil profile concentrations and emission of CO₂, CH₄ and N₂O from soils along a lakeshore in Garwood Valley, Antarctica, to assess the extent and biogeochemical significance of biogenic gas emission to C and N cycling processes. Simultaneous emission of all three gases from the same site indicated that aerobic and anaerobic processes occurred in different layers or different parts of each soil profile. The day and location of high gas concentrations in the soil profile corresponded to those having high gas emission, but the pattern of concentration with depth in the soil profile was not consistent across sites. That the highest gas concentrations were not always in the deepest soil layer suggests either limited production or gas diffusion in the deeper layers. Emission of CO₂ was as high as 47 μmol m⁻² min⁻¹ and was strongly related to soil temperature. Soil respiration differed significantly according to location on the lakeshore, suggesting that factors other than environmental variables, such as the amount and availability of O₂ and nutrients, play an important role in C mineralization processes in these soils. High surface emission (maximum: 15 μmol m⁻² min⁻¹) and profile gas concentration (maximum: 5780 μL L⁻¹) of CH₄ were at levels comparable to those in resource-rich temperate ecosystems, indicating an active indigenous population of methanogenic organisms. Emission of N₂O was low and highly variable, but the presence of this gas and NO₃ in some of the soils suggest that denitrification and nitrification occur there. No significant relationships between N₂O emission and environmental variables were found. It appears that considerable C and N turnover occurs in the lakeshore soils, and accurate accounting will require measurements of aerobic and anaerobic mineralization. The production and emission of biogenic gases confirm the importance of these soils as hotspots of biological activity in the dry valleys and probable reservoirs of biological diversity.     

Increased moisture and methanogenesis contribute to reduced methane oxidation in elevated CO2 soils

Awareness of global warming has stimulated research on environmental controls of soil methane (CH₄) consumption and the effects of increasing atmospheric carbon dioxide (CO₂) on the terrestrial CH₄ sink. In this study, factors impacting soil CH₄ consumption were investigated using laboratory incubations of soils collected at the Free Air Carbon Transfer and Storage I site in the Duke Forest, NC, where plots have been exposed to ambient (370 μL L⁻¹) or elevated (ambient + 200 μL L⁻¹) CO₂ since August 1996. Over 1 year, nearly 90% of the 360 incubations showed net CH₄ consumption, confirming that CH₄-oxidizing (methanotrophic) bacteria were active. Soil moisture was significantly (p < 0.01) higher in the 25–30 cm layer of elevated CO₂ soils over the length of the study, but soil moisture was equal between CO₂ treatments in shallower soils. The increased soil moisture corresponded to decreased net CH₄ oxidation, as elevated CO₂ soils also oxidized 70% less CH₄ at the 25–30 cm depth compared to ambient CO₂ soils, while CH₄ consumption was equal between treatments in shallower soils. Soil moisture content predicted (p < 0.05) CH₄ consumption in upper layers of ambient CO₂ soils, but this relationship was not significant in elevated CO₂ soils at any depth, suggesting that environmental factors in addition to moisture were influencing net CH₄ oxidation under elevated CO₂. More than 6% of the activity assays showed net CH₄ production, and of these, 80% contained soils from elevated CO₂ plots. In addition, more than 50% of the CH₄-producing flasks from elevated CO₂ sites contained deeper (25–30 cm) soils. These results indicate that subsurface (25 cm+) CH₄ production contributes to decreased net CH₄ consumption under elevated CO₂ in otherwise aerobic soils.     

Temperature dependence of soil CO2 efflux is strongly modulated by seasonal patterns of moisture availability in a Mediterranean ecosystem

Extensive research has focused on the temperature sensitivity of soil respiration. However, in Mediterranean ecosystems, soil respiration may have a pulsed response to precipitation events, especially during prolonged dry periods. Here, we investigate temporal variations in soil respiration (R_s), soil temperature (T) and soil water content (SWC) under three different land uses (a forest area, an abandoned agricultural field and a rainfed olive grove) in a dry Mediterranean area of southeast Spain, and evaluate the relative importance of soil temperature and water content as predictors of R_s. We hypothesize that soil moisture content, rather than soil temperature, becomes the major factor controlling CO₂ efflux rates in this Mediterranean ecosystem during the summer dry season. Soil CO₂ efflux was measured monthly between January 2006 and December 2007 using a portable soil respiration instrument fitted with a soil respiration chamber (LI-6400-09). Mean annual soil respiration rates were 2.06 ± 0.07, 1.71 ± 0.09, and 1.12 ± 0.12 μmol m⁻² s⁻¹ in the forest, abandoned field and olive grove, respectively. R_s was largely controlled by soil temperature above a soil water content threshold value of 10% at 0-15 cm depth for forest and olive grove, and 15% for abandoned field. However, below those thresholds R_s was controlled by soil moisture. Exponential and linear models adequately described R_s responses to environmental variables during the growing and dry seasons. Models combining abiotic (soil temperature and soil rewetting index) and biotic factors (above-ground biomass index and/or distance from the nearest tree) explained between 39 and 73% of the temporal variability of R_s in the forest and olive grove. However, in the abandoned field, a single variable - either soil temperature (growing season) or rewetting index (dry season) - was sufficient to explain between 51 and 63% of the soil CO₂ efflux. The fact that the rewetting index, rather than soil water content, became the major factor controlling soil CO₂ efflux rates during the prolonged summer drought emphasizes the need to quantify the effects of rain pulses in estimates of net annual carbon fluxes from soil in Mediterranean ecosystems.     

Soil CO2 efflux in a temperate deciduous forest: Environmental drivers and component contributions

Soil CO₂ efflux is a large component of total respiration in many ecosystems. It is important to understand the environmental controls on soil CO₂ efflux, in order to evaluate potential responses of ecosystems to climate change. This study investigated the relationship between total soil CO₂ efflux and soil temperature, soil moisture and solar radiation on an interannual basis for a plot of temperate deciduous ancient semi-natural woodland at Wytham Woods in central southern England. We also aimed to quantify the contribution of soil organic matter decomposition (SOM), root-and-rhizosphere respiration, and mycorrhizal respiration components to total soil CO₂ efflux, and determine their environmental correlates. Total soil CO₂ efflux was measured regularly from April 2006 to December 2008 and found to average 4.1 Mg C ha⁻¹ yr⁻¹ in both 2007 and 2008. In addition, we applied a recently developed approach to partition the efflux into SOM, root-and-rhizosphere, and mycorrhizal components in situ using mesh bags. SOM decomposition, root-and-rhizosphere, and mycorrhizal respiration were estimated to contribute 70 ± 6%, 22 ± 6% and 8 ± 3% of total soil CO₂ efflux respectively, equating to 3.0 ± 0.3, 0.9 ± 0.2 and 0.3 ± 0.1 Mg C ha⁻¹ yr⁻¹. In order to avoid the effect of temporal correlation between variables caused by seasonality, we investigated interannual variability by examining the relationship between CO₂ flux anomalies and anomalies in environmental variables. Variation in soil temperature explained 50% of the interannual variance in soil CO₂ efflux, and soil moisture a further 18% of the residual variance. Solar radiation, as a proxy for plant photosynthesis, had no significant effect on total soil CO₂ efflux, but was positively correlated with root-and-rhizosphere respiration, and mycorrhizal respiration. The relationship between anomalies in soil CO₂ efflux and soil temperature was highly significant, with a sensitivity of 0.164 ± 0.023 μmol CO₂ m⁻² s⁻¹ °C⁻¹. For mean peak summer efflux rates (2.03 μmol CO₂ m² s⁻¹), this is equivalent to 8% per °C, or a Q₁₀ temperature sensitivity of 2.2 ± 0.2. We demonstrate the utility of an anomaly analysis approach and conclude that soil temperature is the key driver of total soil CO₂ efflux primarily through its positive relationship with SOM-decomposition rate.