Contribution of volatile organic compound fluxes to the ecosystem carbon budget of a poplar short‐rotation plantation

Biogenic volatile organic compounds (BVOCs) are major precursors of both ozone and secondary organic aerosols (SOA) in the troposphere and represent a non‐negligible portion of the carbon fixed by primary producers, but long‐term ecosystem‐scale measurements of their exchanges with the atmosphere are lacking. In this study, the fluxes of 46 ions corresponding to 36 BVOCs were continuously monitored along with the exchanges of mass (carbon dioxide and water vapor) and energy (sensible and latent heat) for an entire year in a poplar (Populus) short‐rotation crop (SRC), using the eddy covariance methodology. BVOC emissions mainly consisted of isoprene, acetic acid, and methanol. Total net BVOC emissions were 19.20 kg C ha^?1 yr^?1, which represented 0.63% of the net ecosystem exchange (NEE), resulting from ?23.59 Mg C ha^?1 yr^?1 fixed as CO₂ and 20.55 Mg C ha^?1 yr^?1 respired as CO₂ from the ecosystem. Isoprene emissions represented 0.293% of NEE, being emitted at a ratio of 1 : 1709 mol isoprene per mol of CO₂ fixed. Based on annual ecosystem‐scale measurements, this study quantified for the first time that BVOC carbon emissions were lower than previously estimated in other studies (0.5–2% of NEE) on poplar trees. Furthermore, the seasonal and diurnal emission patterns of isoprene, methanol, and other BVOCs provided a better interpretation of the relationships with ecosystem CO₂ and water vapor fluxes, with air temperature, vapor pressure deficit, and photosynthetic photon flux density.     

Nitrous oxide emissions from grass swards during the eighth year of elevated atmospheric pCO2 (Swiss FACE)

Emissions of N₂O were measured during the growth season over a year from grass swards under ambient (360 μL L^?1) and elevated (600 μL L^?1) CO₂ partial pressures at the Free Air Carbon dioxide Enrichment (FACE) experiment, Eschikon, Switzerland. Measurements were made following high (56 g N m^?2 yr^?1) and low (14 g N m^?2 yr^?1) rates of fertilizer application, split over 5 re‐growth periods, to Lolium perenne, Trifolium repens and mixed Lolium/Trifolium swards. Elevated pCO₂ increased annual emissions of N₂O from the high fertilized Lolium and mixed Lolium/Trifolium swards resulting in increases in GWP (N₂O emissions) of 179 and 111 g CO₂ equivalents m^?2, respectively, compared with the GWP of ambient pCO₂ swards, but had no significant effect on annual emissions from Trifolium monoculture swards. The greater emissions from the high fertilized elevated pCO₂Lolium swards were attributed to greater below‐ground C allocation under elevated pCO₂ providing the energy for denitrification in the presence of excess mineral N. An annual emission of 959 mg N₂O‐N m^?2 yr^?1 (1.7% of fertilizer N applied) was measured from the high fertilized Lolium sward under elevated pCO₂. The magnitude of emissions varied throughout the year with 84% of the total emission from the elevated pCO₂Lolium swards measured during the first two re‐growths (April–June 2001). This was associated with higher rainfall and soil water contents at this time of year. Trends in emissions varied between the first two re‐growths (April–June 2001) and the third, fourth and fifth re‐growths (late June–October 2000), with available soil NO₃^? and rainfall explaining 70%, and soil water content explaining 72% of the variability in N₂O in these periods, respectively. Caution is therefore required when extrapolating from short‐term measurements to predict long‐term responses to global climate change. Our findings are of global significance as increases in atmospheric concentrations of CO₂ may, depending on sward composition and fertilizer management, increase greenhouse gas emissions of N₂O, thereby exacerbating the forcing effect of elevated CO₂ on global climate. Our results suggest that when applying high rates of N fertilizer to grassland systems, Trifolium repens swards, or a greater component of Trifolium in mixed swards, may minimize the negative effect of continued increasing atmospheric CO₂ concentrations on global warming.     

Enhanced isoprene emission capacity and altered light responsiveness in aspen grown under elevated atmospheric CO2 concentration

Controversial evidence of CO₂‐responsiveness of isoprene emission has been reported in the literature with the response ranging from inhibition to enhancement, but the reasons for such differences are not understood. We studied isoprene emission characteristics of hybrid aspen (Populus tremula x P. tremuloides) grown under ambient (380 μmol mol^?1) and elevated (780 μmol mol^?1) [CO₂] to test the hypothesis that growth [CO₂] effects on isoprene emission are driven by modifications in substrate pool size, reflecting altered light use efficiency for isoprene synthesis. A novel in vivo method for estimation of the pool size of the immediate isoprene precursor, dimethylallyldiphosphate (DMADP) and the activity of isoprene synthase was used. Growth at elevated [CO₂] resulted in greater leaf thickness, more advanced development of mesophyll and moderately increased photosynthetic capacity due to morphological “upregulation”, but isoprene emission rate under growth light and temperature was not significantly different among ambient‐ and elevated‐[CO₂]‐grown plants independent of whether measured at 380 μmol mol^?1 or 780 μmol mol^?1 CO₂. However, DMADP pool size was significantly less in elevated‐[CO₂]‐grown plants, but this was compensated by increased isoprene synthase activity. Analysis of CO₂ and light response curves of isoprene emission demonstrated that the [CO₂] for maximum isoprene emission was shifted to lower [CO₂] in elevated‐[CO₂]‐grown plants. The light‐saturated isoprene emission rate (I_max,Q) was greater, but the quantum efficiency at given I_max,Q was less in elevated‐[CO₂]‐grown plants, especially at higher CO₂ measurement concentration, reflecting stronger DMADP limitation at lower light and higher [CO₂]. These results collectively demonstrate important shifts in light and CO₂‐responsiveness of isoprene emission in elevated‐[CO₂]‐acclimated plants that need consideration in modeling isoprene emissions in future climates.     

Impact of rising CO2 on emissions of volatile organic compounds: isoprene emission from Phragmites australis growing at elevated CO2 in a natural carbon dioxide spring†

Isoprene basal emission (the emission of isoprene from leaves exposed to a light intensity of 1000 µmol m^?2 s^?1 and maintained at a temperature of 30 °C) was measured in Phragmites australis plants growing under elevated CO₂ in the Bossoleto CO₂ spring at Rapolano Terme, Italy, and under ambient CO₂ at a nearby control site. Gas exchange and biochemical measurements were concurrently taken. Isoprene emission was lower in the plants growing at elevated CO₂ than in those growing at ambient CO₂. Isoprene emission and isoprene synthase activity (IsoS) were very low in plants growing at the bottom of the spring under very rich CO₂ and increased at increasing distance from the spring (and decreasing CO₂ concentration). Distance from the spring did not significantly affect photosynthesis making it therefore unlikely that there is carbon limitation to isoprene formation. The isoprene emission rate was very quickly reduced after rapid switches from elevated to ambient CO₂ in the gas‐exchange cuvette, whereas it increased when switching from ambient to elevated CO₂. The rapidity of the response may be consistent with post‐translational modifications of enzymes in the biosynthetic pathway of isoprene formation. Reduction of IsoS activity is interpreted as a long‐term response. Basal emission of isoprene was not constant over the day but showed a diurnal course opposite to photosynthesis, with a peak during the hottest hours of the day, independent of stomatal conductance and probably dependent on external air temperature or temporary reduction of CO₂ concentration. The present experiments show that basal emission rate of isoprene is likely to be reduced under future elevated CO₂ levels and allow improvement in the modelling of future isoprene emission rates.     

Isoprenoid emission in trees of Quercus pubescens and Quercus ilex with lifetime exposure to naturally high CO2 environment†

The long‐term effect of elevated atmospheric CO₂ on isoprenoid emissions from adult trees of two Mediterranean oak species (the monoterpene‐emitting Quercus ilex L. and the isoprene‐emitting Quercus pubescens Willd.) native to a high‐CO₂ environment was investigated. During two consecutive years, isoprenoid emission was monitored both at branch level, measuring the actual emissions under natural conditions, and at leaf level, measuring the basal emissions under the standard conditions of 30 °C and at light intensity of 1000 µmol m^?2 s^?1. Long‐term exposure to high atmospheric levels of CO₂ did not significantly affect the actual isoprenoid emissions. However, when leaves of plants grown in the control site were exposed for a short period to an elevated CO₂ level by rapidly switching the CO₂ concentration in the gas‐exchange cuvette, both isoprene and monoterpene basal emissions were clearly inhibited. These results generally confirm the inhibitory effect of elevated CO₂ on isoprenoid emission. The absence of a CO₂ effect on actual emissions might indicate higher leaf temperature at elevated CO₂, or an interaction with multiple stresses some of which (e.g. recurrent droughts) may compensate for the CO₂ effect in Mediterranean ecosystems. Under elevated CO₂, isoprene emission by Q. pubescens was also uncoupled from the previous day's air temperature. In addition, pronounced daily and seasonal variations of basal emission were observed under elevated CO₂ underlining that correction factors may be necessary to improve the realistic estimation of isoprene emissions with empirical algorithms in the future. A positive linear correlation of isoprenoid emission with the photosynthetic electron transport and in particular with its calculated fraction used for isoprenoid synthesis was found. The slope of this relationship was different for isoprene and monoterpenes, but did not change when plants were grown in either ambient or elevated CO₂. This suggests that physiological algorithms may usefully predict isoprenoid emission also under rising CO₂ levels.     

Marshland conversion to cropland in northeast China from 1950 to 2000 reduced the greenhouse effect

It has been well recognized that converting wetlands to cropland results in loss of soil organic carbon (SOC), while less attention was paid to concomitant changes in methane (CH₄) and nitrous oxide (N₂O) emissions. Using datasets from the literature and field measurements, we investigated loss of SOC and emissions of CH₄ and N₂O due to marshland conversion in northeast China. Analysis of the documented crop cultivation area indicated that 2.91 Mha of marshland were converted to cropland over the period 1950–2000. Marshland conversion resulted in SOC loss of ～240 Tg and introduced ～1.4 Tg CH₄ and ～138 Gg N₂O emissions in the cropland, while CH₄ emissions reduced greatly in the marshland, cumulatively ～28 Tg over the 50 years. Taking into account the loss of SOC and emissions of CH₄ and N₂O, the global warming potential (GWP) at a 20‐year time horizon was estimated to be ～180 Tg CO₂_eq. yr^?1 in the 1950s and ～120 Tg CO₂_eq. yr^?1 in the 1990s, with a ～33% reduction. When calculated at 100‐year time horizon, the GWP was ～73 Tg CO₂ _eq. yr^?1 in the 1950s and ～58 Tg CO₂_eq. yr^?1 in the 1990s, with a ～21% reduction. It was concluded that marshland conversion to cropland in northeast China reduced the greenhouse effect as far as GWP is concerned. This reduction was attributed to a substantial decrease in CH₄ emissions from the marshland. An extended inference is that the declining growth rate of atmospheric CH₄ since the 1980s might be related to global loss of wetlands, but this connection needs to be confirmed.     

Interactive effects of elevated CO2 and soil fertility on isoprene emissions from Quercus robur

The effects of global change on the emission rates of isoprene from plants are not clear. A factor that can influence the response of isoprene emission to elevated CO₂ concentrations is the availability of nutrients. Isoprene emission rate under standard conditions (leaf temperature: 30°C, photosynthetically active radiation (PAR): 1000 μmol photons m^?2 s^?1), photosynthesis, photosynthetic capacity, and leaf nitrogen (N) content were measured in Quercus robur grown in well‐ventilated greenhouses at ambient and elevated CO₂ (ambient plus 300 ppm) and two different soil fertilities. The results show that elevated CO₂ enhanced photosynthesis but leaf respiration rates were not affected by either the CO₂ or nutrient treatments. Isoprene emission rates and photosynthetic capacity were found to decrease with elevated CO₂, but an increase in nutrient availability had the converse effect. Leaf N content was significantly greater with increased nutrient availability, but unaffected by CO₂. Isoprene emission rates measured under these conditions were strongly correlated with photosynthetic capacity across the range of different treatments. This suggests that the effects of CO₂ and nutrient levels on allocation of carbon to isoprene production and emission under near‐saturating light largely depend on the effects on photosynthetic electron transport capacity.     

The positive net radiative greenhouse gas forcing of increasing methane emissions from a thawing boreal forest‐wetland landscape

At the southern margin of permafrost in North America, climate change causes widespread permafrost thaw. In boreal lowlands, thawing forested permafrost peat plateaus (‘forest’) lead to expansion of permafrost‐free wetlands (‘wetland’). Expanding wetland area with saturated and warmer organic soils is expected to increase landscape methane (CH₄) emissions. Here, we quantify the thaw‐induced increase in CH₄ emissions for a boreal forest‐wetland landscape in the southern Taiga Plains, Canada, and evaluate its impact on net radiative forcing relative to potential long‐term net carbon dioxide (CO₂) exchange. Using nested wetland and landscape eddy covariance net CH₄ flux measurements in combination with flux footprint modeling, we find that landscape CH₄ emissions increase with increasing wetland‐to‐forest ratio. Landscape CH₄ emissions are most sensitive to this ratio during peak emission periods, when wetland soils are up to 10 °C warmer than forest soils. The cumulative growing season (May–October) wetland CH₄ emission of ~13 g CH₄ m^?2 is the dominating contribution to the landscape CH₄ emission of ~7 g CH₄ m^?2. In contrast, forest contributions to landscape CH₄ emissions appear to be negligible. The rapid wetland expansion of 0.26 ± 0.05% yr^?1 in this region causes an estimated growing season increase of 0.034 ± 0.007 g CH₄ m^?2 yr^?1 in landscape CH₄ emissions. A long‐term net CO₂ uptake of >200 g CO₂ m^?2 yr^?1 is required to offset the positive radiative forcing of increasing CH₄ emissions until the end of the 21st century as indicated by an atmospheric CH₄ and CO₂ concentration model. However, long‐term apparent carbon accumulation rates in similar boreal forest‐wetland landscapes and eddy covariance landscape net CO₂ flux measurements suggest a long‐term net CO₂ uptake between 49 and 157 g CO₂ m^?2 yr^?1. Thus, thaw‐induced CH₄ emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century.     

Long‐term impacts of manure amendments on carbon and greenhouse gas dynamics of rangelands

Livestock manure is applied to rangelands as an organic fertilizer to stimulate forage production, but the long‐term impacts of this practice on soil carbon (C) and greenhouse gas (GHG) dynamics are poorly known. We collected soil samples from manured and nonmanured fields on commercial dairies and found that manure amendments increased soil C stocks by 19.0 ± 7.3 Mg C ha^?1 and N stocks by 1.94 ± 0.63 Mg N ha^?1 compared to nonmanured fields (0–20 cm depth). Long‐term historical (1700–present) and future (present–2100) impacts of management on soil C and N dynamics, net primary productivity (NPP), and GHG emissions were modeled with DayCent. Modeled total soil C and N stocks increased with the onset of dairying. Nitrous oxide (N₂O) emissions also increased by ~2 kg N₂O‐N ha^?1 yr^?1. These emissions were proportional to total N additions and offset 75–100% of soil C sequestration. All fields were small net methane (CH₄) sinks, averaging ?4.7 ± 1.2 kg CH₄‐C ha^?1 yr^?1. Overall, manured fields were net GHG sinks between 1954 and 2011 (?0.74 ± 0.73 Mg CO₂ e ha^?1 yr^?1, CO₂e are carbon dioxide equivalents), whereas nonmanured fields varied around zero. Future soil C pools stabilized 40–60 years faster in manured fields than nonmanured fields, at which point manured fields were significantly larger sources than nonmanured fields (1.45 ± 0.52 Mg CO₂e ha^?1 yr^?1 and 0.51 ± 0.60 Mg CO₂e ha^?1 yr^?1, respectively). Modeling also revealed a large background loss of soil C from the passive soil pool associated with the shift from perennial to annual grasses, equivalent to 29.4 ± 1.47 Tg CO₂e in California between 1820 and 2011. Manure applications increased NPP and soil C storage, but plant community changes and GHG emissions decreased, and eventually eliminated, the net climate benefit of this practice.     

The contributions of land-use change, CO2 fertilization, and climate variability to the Eastern US carbon sink

Atmospheric measurements and land‐based inventories imply that terrestrial ecosystems in the northern hemisphere are taking up significant amounts of anthropogenic cabon dioxide (CO₂) emissions; however, there is considerable disagreement about the causes of this uptake, and its expected future trajectory. In this paper, we use the ecosystem demography (ED) model to quantify the contributions of disturbance history, CO₂ fertilization and climate variability to the past, current, and future terrestrial carbon fluxes in the Eastern United States. The simulations indicate that forest regrowth following agricultural abandonment accounts for uptake of 0.11 Pg C yr^?1 in the 1980s and 0.15 Pg C yr^?1 in the 1990s, and regrowth following forest harvesting accounts for an additional 0.1 Pg C yr^?1 of uptake during both these decades. The addition of CO₂ fertilization into the model simulations increases carbon uptake rates to 0.38 Pg C yr^?1 in the 1980s and 0.47 Pg C yr^?1 in the 1990s. Comparisons of predicted aboveground carbon uptake to regional‐scale forest inventory measurements indicate that the model's predictions in the absence of CO₂ fertilization are 14% lower than observed, while in the presence of CO₂ fertilization, predicted uptake rates are 28% larger than observed. Comparable results are obtained from comparisons of predicted total Net Ecosystem Productivity to the carbon fluxes observed at the Harvard Forest flux tower site and in model simulations free‐air CO₂ enrichment (FACE) experiments. These results imply that disturbance history is the principal mechanism responsible for current carbon uptake in the Eastern United States, and that conventional biogeochemical formulations of plant growth overestimate the response of plants to rising CO₂ levels. Model projections out to 2100 imply that the carbon uptake arising from forest regrowth will increasingly be dominated by forest regrowth following harvesting. Consequently, actual carbon storage declines to near zero by the end of the 21st century as the forest regrowth that has occurred since agricultural abandonment comes into equilibrium with the landscape's new disturbance regime. Incorporating interannual climate variability into the model simulations gives rise to large interannual variation in regional carbon fluxes, indicating that long‐term measurements are necessary to detect the signature of processes that give rise to long‐term uptake and storage.     

Soil organic carbon dust emission: an omitted global source of atmospheric CO2

Soil erosion redistributes soil organic carbon (SOC) within terrestrial ecosystems, to the atmosphere and oceans. Dust export is an essential component of the carbon (C) and carbon dioxide (CO₂) budget because wind erosion contributes to the C cycle by removing selectively SOC from vast areas and transporting C dust quickly offshore; augmenting the net loss of C from terrestrial systems. However, the contribution of wind erosion to rates of C release and sequestration is poorly understood. Here, we describe how SOC dust emission is omitted from national C accounting, is an underestimated source of CO₂ and may accelerate SOC decomposition. Similarly, long dust residence times in the unshielded atmospheric environment may considerably increase CO₂ emission. We developed a first approximation to SOC enrichment for a well‐established dust emission model and quantified SOC dust emission for Australia (5.83 Tg CO₂‐e yr^?1) and Australian agricultural soils (0.4 Tg CO₂‐e yr^?1). These amount to underestimates for CO₂ emissions of ≈10% from combined C pools in Australia (year = 2000), ≈5% from Australian Rangelands and ≈3% of Australian Agricultural Soils by Kyoto Accounting. Northern hemisphere countries with greater dust emission than Australia are also likely to have much larger SOC dust emission. Therefore, omission of SOC dust emission likely represents a considerable underestimate from those nations’ C accounts. We suggest that the omission of SOC dust emission from C cycling and C accounting is a significant global source of uncertainty. Tracing the fate of wind‐eroded SOC in the dust cycle is therefore essential to quantify the release of CO₂ from SOC dust to the atmosphere and the contribution of SOC deposition to downwind C sinks.     

Estimation of nitrous oxide, nitric oxide and ammonia emissions from croplands in East, Southeast and South Asia

Agricultural activities have greatly altered the global nitrogen (N) cycle and produced nitrogenous gases of environmental significance. More than half of all chemical N fertilizer produced globally is used in crop production in East, Southeast and South Asia, where rice is central to nutrition. Emissions of nitrous oxide (N₂O), nitric oxide (NO) and ammonia (NH₃) from croplands in this region were estimated by considering background emission and emissions resulting from N added to croplands, including chemical N, animal manure, biologically fixed N and N in crop residues returned to fields. Background emission fluxes of N₂O and NO from croplands were estimated to be 1.22 and 0.57 kg N ha^?1 yr^?1, respectively. Separate fertilizer‐induced emission factors were estimated for upland fields and rice fields. Total N₂O emission from croplands in the study region was estimated to be 1.19 Tg N yr^?1, with 43% contributed by background emissions. The average fertilizer‐induced N₂O emission, however, accounts for only 0.93% of the applied N, which is less than the default IPCC value of 1.25%, because of the low emission factor from paddy fields. Total NO emission was 591 Gg N yr^?1 in the study region, with 40% from background emissions. The average fertilizer‐induced NO emission factor was 0.48%. Total NH₃ emission was estimated to be 11.8 Tg N yr^?1. The use of urea and ammonium bicarbonate and the cultivation of rice led to a high average NH₃ loss rate from chemical N fertilizer in the study region. Emissions were displayed at a 0.5° × 0.5° resolution with the use of a global landuse database.     

Importance of changing CO2, temperature, precipitation, and ozone on carbon and water cycles of an upland-oak forest: incorporating experimental results into model simulations

Observed responses of upland‐oak vegetation of the eastern deciduous hardwood forest to changing CO₂, temperature, precipitation and tropospheric ozone (O₃) were derived from field studies and interpreted with a stand‐level model for an 11‐year range of environmental variation upon which scenarios of future environmental change were imposed. Scenarios for the year 2100 included elevated [CO₂] and [O₃] (+385 ppm and +20 ppb, respectively), warming (+4°C), and increased winter precipitation (+20% November–March). Simulations were run with and without adjustments for experimentally observed physiological and biomass adjustments. Initial simplistic model runs for single‐factor changes in CO₂ and temperature predicted substantial increases (+191% or 508 g C m^?2 yr^?1) or decreases (?206% or ?549 g C m^?2 yr^?1), respectively, in mean annual net ecosystem carbon exchange (NEE_a≈266±23 g C m^?2 yr^?1 from 1993 to 2003). Conversely, single‐factor changes in precipitation or O₃ had comparatively small effects on NEE_a (0% and ?35%, respectively). The combined influence of all four environmental changes yielded a 29% reduction in mean annual NEE_a. These results suggested that future CO₂‐induced enhancements of gross photosynthesis would be largely offset by temperature‐induced increases in respiration, exacerbation of water deficits, and O₃‐induced reductions in photosynthesis. However, when experimentally observed physiological adjustments were included in the simulations (e.g. acclimation of leaf respiration to warming), the combined influence of the year 2100 scenario resulted in a 20% increase in NEE_a not a decrease. Consistent with the annual model's predictions, simulations with a forest succession model run for gradually changing conditions from 2000 to 2100 indicated an 11% increase in stand wood biomass in the future compared with current conditions. These model‐based analyses identify critical areas of uncertainty for multivariate predictions of future ecosystem response, and underscore the importance of long term field experiments for the evaluation of acclimation and growth under complex environmental scenarios.     

Greenhouse gas fluxes from an Australian subtropical cropland under long‐term contrasting management regimes

The long‐term effects of conservation management practices on greenhouse gas fluxes from tropical/subtropical croplands remain to be uncertain. Using both manual and automatic sampling chambers, we measured N₂O and CH₄ fluxes at a long‐term experimental site (1968–present) in Queensland, Australia from 2006 to 2009. Annual net greenhouse gas fluxes (NGGF) were calculated from the 3‐year mean N₂O and CH₄ fluxes and the long‐term soil organic carbon changes. N₂O emissions exhibited clear daily, seasonal and interannual variations, highlighting the importance of whole‐year measurement over multiple years for obtaining temporally representative annual emissions. Averaged over 3 years, annual N₂O emissions from the unfertilized and fertilized soils (90 kg N ha^?1 yr^?1 as urea) amounted to 138 and 902 g N ha^?1, respectively. The average annual N₂O emissions from the fertilized soil were 388 g N ha^?1 lower under no‐till (NT) than under conventional tillage (CT) and 259 g N ha^?1 higher under stubble retention (SR) than under stubble burning (SB). Annual N₂O emissions from the unfertilized soil were similar between the contrasting tillage and stubble management practices. The average emission factors of fertilizer N were 0.91%, 1.20%, 0.52% and 0.77% for the CT‐SB, CT‐SR, NT‐SB and NT‐SR treatments, respectively. Annual CH₄ fluxes from the soil were very small (?200–300 g CH₄ ha^?1 yr^?1) with no significant difference between treatments. The NGGF were 277–350 kg CO₂‐e ha^?1 yr^?1 for the unfertilized treatments and 401–710 kg CO₂‐e ha^?1 yr^?1 for the fertilized treatments. Among the fertilized treatments, N₂O emissions accounted for 52–97% of NGGF and NT‐SR resulted in the lowest NGGF (401 kg CO₂‐e ha^?1 yr^?1 or 140 kg CO₂‐e t^?1 grain). Therefore, NT‐SR with improved N fertilizer management practices was considered the most promising management regime for simultaneously achieving maximal yield and minimal NGGF.     

Effects of seasonality,transport pathway,and spatial structure on greenhouse gas fluxes in a restored wetland

Wetlands can influence global climate via greenhouse gas (GHG) exchange of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Few studies have quantified the full GHG budget of wetlands due to the high spatial and temporal variability of fluxes. We report annual open‐water diffusion and ebullition fluxes of CO₂, CH₄, and N₂O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy‐covariance measurements of whole‐ecosystem CO₂ and CH₄ exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open‐water and vegetated cover types. Annual open‐water CO₂, CH₄, and N₂O emissions were 915 ± 95 g C‐CO₂ m^?2 yr^?1, 2.9 ± 0.5 g C‐CH₄ m^?2 yr^?1, and 62 ± 17 mg N‐N₂O m^?2 yr^?1, respectively. Diffusion dominated open‐water GHG transport, accounting for >99% of CO₂ and N₂O emissions, and ~71% of CH₄ emissions. Seasonality was minor for CO₂ emissions, whereas CH₄ and N₂O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open‐water fluxes (3.5 ± 0.3 kg CO₂‐eq m^?2 yr^?1) exceeded that of vegetated zones (1.4 ± 0.4 kg CO₂‐eq m^?2 yr^?1) due to high ecosystem respiration. After scaling results to the entire wetland using object‐based cover classification of remote sensing imagery, net uptake of CO₂ (?1.4 ± 0.6 kt CO₂‐eq yr^?1) did not offset CH₄ emission (3.7 ± 0.03 kt CO₂‐eq yr^?1), producing an overall positive radiative forcing effect of 2.4 ± 0.3 kt CO₂‐eq yr^?1. These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.     

Ecosystem–atmosphere exchange of CH4 and N2O and ecosystem respiration in wetlands in the Sanjiang Plain, Northeastern China

Natural wetlands are critically important to global change because of their role in modulating atmospheric concentrations of CO₂, CH₄, and N₂O. One 4‐year continuous observation was conducted to examine the exchanges of CH₄ and N₂O between three wetland ecosystems and the atmosphere as well as the ecosystem respiration in the Sanjiang Plain in Northeastern China. From 2002 to 2005, the mean annual budgets of CH₄ and N₂O, and ecosystem respiration were 39.40 ± 6.99 g C m^?2 yr^?1, 0.124 ± 0.05 g N m^?2 yr^?1, and 513.55 ± 8.58 g C m^?2 yr^?1 for permanently inundated wetland; 4.36 ± 1.79 g C m^?2 yr^?1, 0.11 ± 0.12 g N m^?2 yr^?1, and 880.50 ± 71.72 g C m^?2 yr^?1 for seasonally inundated wetland; and 0.21 ± 0.1 g C m^?2 yr^?1, 0.28 ± 0.11 g N m^?2 yr^?1, and 1212.83 ± 191.98 g C m^?2 yr^?1 for shrub swamp. The substantial interannual variation of gas fluxes was due to the significant climatic variability which underscores the importance of long‐term continuous observations. The apparent seasonal pattern of gas emissions associated with a significant relationship of gas fluxes to air temperature implied the potential effect of global warming on greenhouse gas emissions from natural wetlands. The budgets of CH₄ and N₂O fluxes and ecosystem respiration were highly variable among three wetland types, which suggest the uncertainties in previous studies in which all kinds of natural wetlands were treated as one or two functional types. New classification of global natural wetlands in more detailed level is highly expected.     

Agricultural greenhouse gas mitigation potential globally,in Europe and in the UK: what have we learnt in the last 20 years?

Agricultural lands occupy about 40–50% of the Earth's land surface. Agricultural practices can make a significant contribution at low cost to increasing soil carbon sinks, reducing greenhouse gas (GHG) emissions and contributing biomass feedstocks for energy use. Considering all gases, the global technical mitigation potential from agriculture (excluding fossil fuel offsets from biomass) by 2030 is estimated to be ca. 5500–6000 Mt CO₂‐eq. yr^?1. Economic potentials are estimated to be 1500–1600, 2500–2700 and 4000–4300 Mt CO₂‐eq. yr^?1 at carbon prices of up to $US20, 50 and 100 t CO₂‐eq.^?1, respectively. The value of the global agricultural GHG mitigation at the same three carbon prices is $US32 000, 130 000 and 420 000 million yr^?1, respectively. At the European level, early estimates of soil carbon sequestration potential in croplands were ca. 200 Mt CO₂ yr^?1, but this is a technical potential and is for geographical Europe as far east as the Urals. The economic potential is much smaller, with more recent estimates for the EU27 suggesting a maximum potential of ca. 20 Mt CO₂‐eq. yr^?1. The UK is small in global terms, but a large part of its land area (11 Mha) is used for agriculture. Agriculture accounts for about 7% of total UK GHG emissions. The mitigation potential of UK agriculture is estimated to be ca. 1–2 Mt CO₂‐eq. yr^?1, accounting for less than 1% of UK total GHG emissions.     

High emissions of greenhouse gases from grasslands on peat and other organic soils

Drainage has turned peatlands from a carbon sink into one of the world's largest greenhouse gas (GHG) sources from cultivated soils. We analyzed a unique data set (12 peatlands, 48 sites and 122 annual budgets) of mainly unpublished GHG emissions from grasslands on bog and fen peat as well as other soils rich in soil organic carbon (SOC) in Germany. Emissions and environmental variables were measured with identical methods. Site‐averaged GHG budgets were surprisingly variable (29.2 ± 17.4 t CO₂‐eq. ha^?1 yr^?1) and partially higher than all published data and the IPCC default emission factors for GHG inventories. Generally, CO₂ (27.7 ± 17.3 t CO₂ ha^?1 yr^?1) dominated the GHG budget. Nitrous oxide (2.3 ± 2.4 kg N₂O‐N ha^?1 yr^?1) and methane emissions (30.8 ± 69.8 kg CH₄‐C ha^?1 yr^?1) were lower than expected except for CH₄ emissions from nutrient‐poor acidic sites. At single peatlands, CO₂ emissions clearly increased with deeper mean water table depth (WTD), but there was no general dependency of CO₂ on WTD for the complete data set. Thus, regionalization of CO₂ emissions by WTD only will remain uncertain. WTD dynamics explained some of the differences between peatlands as sites which became very dry during summer showed lower emissions. We introduced the aerated nitrogen stock (N_air) as a variable combining soil nitrogen stocks with WTD. CO₂ increased with N_air across peatlands. Soils with comparatively low SOC concentrations showed as high CO₂ emissions as true peat soils because N_air was similar. N₂O emissions were controlled by the WTD dynamics and the nitrogen content of the topsoil. CH₄ emissions can be well described by WTD and ponding duration during summer. Our results can help both to improve GHG emission reporting and to prioritize and plan emission reduction measures for peat and similar soils at different scales.     

The European carbon balance. Part 4: integration of carbon and other trace‐gas fluxes

Overviewing the European carbon (C), greenhouse gas (GHG), and non‐GHG fluxes, gross primary productivity (GPP) is about 9.3 Pg yr^?1, and fossil fuel imports are 1.6 Pg yr^?1. GPP is about 1.25% of solar radiation, containing about 360 × 10¹⁸ J energy – five times the energy content of annual fossil fuel use. Net primary production (NPP) is 50%, terrestrial net biome productivity, NBP, 3%, and the net GHG balance, NGB, 0.3% of GPP. Human harvest uses 20% of NPP or 10% of GPP, or alternatively 1‰ of solar radiation after accounting for the inherent cost of agriculture and forestry, for production of pesticides and fertilizer, the return of organic fertilizer, and for the C equivalent cost of GHG emissions. C equivalents are defined on a global warming potential with a 100‐year time horizon. The equivalent of about 2.4% of the mineral fertilizer input is emitted as N₂O. Agricultural emissions to the atmosphere are about 40% of total methane, 60% of total NO‐N, 70% of total N₂O‐N, and 95% of total NH₃‐N emissions of Europe. European soils are a net C sink (114 Tg yr^?1), but considering the emissions of GHGs, soils are a source of about 26 Tg CO₂ C‐equivalent yr^?1. Forest, grassland and sediment C sinks are offset by GHG emissions from croplands, peatlands and inland waters. Non‐GHGs (NH₃, NOx) interact significantly with the GHG and the C cycle through ammonium nitrate aerosols and dry deposition. Wet deposition of nitrogen (N) supports about 50% of forest timber growth. Land use change is regionally important. The absolute flux values total about 50 Tg C yr^?1. Nevertheless, for the European trace‐gas balance, land‐use intensity is more important than land‐use change. This study shows that emissions of GHGs and non‐GHGs significantly distort the C cycle and eliminate apparent C sinks.     

Assessment of the methane mitigation potentials of alternative water regimes in rice fields using a process‐based biogeochemistry model

Rice production is a substantial source of atmospheric CH₄, which is second only to CO₂ as a contributor to global warming. Since CH₄ is produced in anaerobic soil environments, water management is expected to be a practical measure to mitigate CH₄ emissions. In this study, we used a process‐based biogeochemistry model (DNDC‐Rice) to assess the CH₄ mitigation potentials of alternative water regimes (AWR) for rice fields at a regional scale. Before regional application, we tested DNDC‐Rice using site‐scale data from three rice fields in Japan with different water regimes. The observed CH₄ emissions were reduced by drainage of the fields, but were enhanced by organic amendments. DNDC‐Rice gave acceptable predictions of variation in daily CH₄ fluxes and seasonal CH₄ emissions due to changes in the water regime. For regional application, we constructed a GIS database at a 1 × 1 km mesh scale that contained data on rice field area, soil properties, daily weather, and farming management of each cell in the mesh, covering 3.2% of the rice fields in Japan's Hokkaido region. We ran DNDC‐Rice to simulate CH₄ emissions under five simulated water regimes: the conventional water regime and four AWR scenarios with gradually increasing drainage. We found that AWR can reduce CH₄ emission by up to 41% compared with the emission under conventional water regime. Including the changes in CO₂ and nitrous oxide emissions, potential mitigation of greenhouse gas (GHG) was 2.6 Mg CO₂ Eq. ha^?1 yr^?1. If this estimate is expanded to Japan's total rice fields, expected GHG mitigation is 4.3 Tg CO₂ Eq. yr^?1, which accounts for 0.32% of total GHG emissions from Japan. For a reliable national‐scale assessment, however, databases on soil, weather, and farming management must be constructed at a national scale, as these factors are widely variable between regions in Japan.