The European carbon balance. Part 2: croplands

We estimated the long‐term carbon balance [net biome production (NBP)] of European (EU‐25) croplands and its component fluxes, over the last two decades. Net primary production (NPP) estimates, from different data sources ranged between 490 and 846 gC m^?2 yr^?1, and mostly reflect uncertainties in allocation, and in cropland area when using yield statistics. Inventories of soil C change over arable lands may be the most reliable source of information on NBP, but inventories lack full and harmonized coverage of EU‐25. From a compilation of inventories we infer a mean loss of soil C amounting to 17 g m^?2 yr^?1. In addition, three process‐based models, driven by historical climate and evolving agricultural technology, estimate a small sink of 15 g C m^?2 yr^?1 or a small source of 7.6 g C m^?2 yr^?1. Neither the soil C inventory data, nor the process model results support the previous European‐scale NBP estimate by Janssens and colleagues of a large soil C loss of 90 ± 50 gC m^?2 yr^?1. Discrepancy between measured and modeled NBP is caused by erosion which is not inventoried, and the burning of harvest residues which is not modeled. When correcting the inventory NBP for the erosion flux, and the modeled NBP for agricultural fire losses, the discrepancy is reduced, and cropland NBP ranges between ?8.3 ± 13 and ?13 ± 33 g C m^?2 yr^?1 from the mean of the models and inventories, respectively. The mean nitrous oxide (N₂O) flux estimates ranges between 32 and 37 g C Eq m^?2 yr^?1, which nearly doubles the CO₂ losses. European croplands act as small CH₄ sink of 3.3 g C Eq m^?2 yr^?1. Considering ecosystem CO₂, N₂O and CH₄ fluxes provides for the net greenhouse gas balance a net source of 42–47 g C Eq m^?2 yr^?1. Intensifying agriculture in Eastern Europe to the same level Western Europe amounts is expected to result in a near doubling of the N₂O emissions in Eastern Europe. N₂O emissions will then become the main source of concern for the impact of European agriculture on climate.     

Response of isoprene emission to ambient CO2 changes and implications for global budgets

We explore the potential role of atmospheric carbon dioxide (CO₂) on isoprene emissions using a global coupled land–atmosphere model [Community Atmospheric Model–Community Land Model (CAM–CLM)] for recent (year 2000, 365 ppm CO₂) and future (year 2100, 717 ppm CO₂) conditions. We incorporate an empirical model of observed isoprene emissions response to both ambient CO₂ concentrations in the long‐term growth environment and short‐term changes in intercellular CO₂ concentrations into the MEGAN biogenic emission model embedded within the CLM. Accounting for CO₂ inhibition has little impact on predictions of present‐day global isoprene emission (increase from 508 to 523 Tg C yr^?1). However, the large increases in future isoprene emissions typically predicted in models, which are due to a projected warmer climate, are entirely offset by including the CO₂ effects. Projected global isoprene emissions in 2100 drop from 696 to 479 Tg C yr^?1 when this effect is included, maintaining future isoprene sources at levels similar to present day. The isoprene emission response to CO₂ is dominated by the long‐term growth environment effect, with modulations of 10% or less due to the variability in intercellular CO₂ concentration. As a result, perturbations to isoprene emissions associated with changes in ambient CO₂ are largely aseasonal, with little diurnal variability. Future isoprene emissions increase by more than a factor of two in 2100 (to 1242 Tg C yr^?1) when projected changes in vegetation distribution and leaf area density are included. Changing land cover and the role of nutrient limitation on CO₂ fertilization therefore remain the largest source of uncertainty in isoprene emission prediction. Although future projections suggest a compensatory balance between the effects of temperature and CO₂ on isoprene emission, the enhancement of isoprene emission due to lower ambient CO₂ concentrations did not compensate for the effect of cooler temperatures over the last 400 thousand years of the geologic record (including the Last Glacial Maximum).     

The impact of extensive planting of Miscanthus as an energy crop on future CO2 atmospheric concentrations

A process‐based model of the energy crop Miscanthus×giganteus is integrated into the global climate impact model IMOGEN, simulating the potential of large‐scale Miscanthus plantation to offset fossil fuel emissions during the 21st century. This simulation produces spatially explicit, annual projections of Miscanthus yields from the present day to the year 2100 under an SRES A2 anthropogenic emissions scenario and includes the effects of climate change. IMOGEN also simulates natural vegetation and soil carbon storage throughout the 21st century. The benefit of Miscanthus cultivation (avoiding fossil fuel emissions of CO₂) is then compared with the cost of displacing natural vegetation (carbon emissions from vegetation and soil). The time taken for these effects to cancel out, the pay‐back time, is calculated regionally. The effects of large‐scale Miscanthus plantation are then integrated globally to produce an estimate of atmospheric CO₂ concentrations throughout the 21st century. Our best estimate of the pay‐back time for Miscanthus plantation is 30 years. We project a maximum possible reduction in atmospheric CO₂ of 323 ppmv by the end of 21st century, with a reduction of 162 ppmv corresponding to the best estimate scenario.     

Punching above their weight: Large release of greenhouse gases from small agricultural dams

Freshwater ecosystems play a major role in global carbon cycling through the breakdown of organic material and release of greenhouse gases (GHGs). Carbon dioxide (CO₂) and methane (CH₄) emissions from lakes, wetlands, reservoirs and small natural ponds have been well studied, however, the GHG emissions of highly abundant, small‐scale (<0.01 km²) agricultural dams (small stream and run‐off impoundments) are still unknown. Here, we measured the diffusive CO₂ and CH₄ flux of 77 small agricultural dams within south‐east Australia. The GHG emissions from these waterbodies, which are currently unaccounted for in GHG inventories, amounted to 11.12 ± 2.59 g CO₂‐equivalent m²/day, a value 3.43 times higher than temperate reservoir emissions. Upscaling these results to the entire state of Victoria, Australia, resulted in a farm dam CO₂‐equivalent/day emission rate of 4,853 tons, 3.1 times higher than state‐wide reservoir emissions in spite of farm dams covering only 0.94 times the comparative area. We also show that CO₂ and CH₄ emission rates were both significantly positively correlated with dissolved nitrate concentrations, and significantly higher in livestock rearing farm dams when compared to cropping farm dams. The results from this study demonstrate that small agricultural farm dams can be a major source of greenhouse gas emissions, thereby justifying their inclusion in global carbon budgets.     

CO2 uptake is offset by CH4 and N2O emissions in a poplar short‐rotation coppice

The need for renewable energy sources will lead to a considerable expansion in the planting of dedicated fast‐growing biomass crops across Europe. These are commonly cultivated as short‐rotation coppice (SRC), and currently poplar (Populus spp.) is the most widely planted. In this study, we report the greenhouse gas (GHG) fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) measured using eddy covariance technique in an SRC plantation for bioenergy production. Measurements were made during the period 2010–2013, that is, during the first two rotations of the SRC. The overall GHG balance of the 4 years of the study was an emission of 1.90 (±1.37) Mg CO₂eq ha^?1; this indicated that soil trace gas emissions offset the CO₂ uptake by the plantation. CH₄ and N₂O contributed almost equally to offset the CO₂ uptake of ?5.28 (±0.67) Mg CO₂eq ha^?1 with an overall emission of 3.56 (±0.35) Mg CO₂eq ha^?1 of N₂O and of 3.53 (±0.85) Mg CO₂eq ha^?1 of CH₄. N₂O emissions mostly occurred during one single peak a few months after the site was converted to SRC; this peak comprised 44% of the total N₂O loss during the two rotations. Accurately capturing emission events proved to be critical for deriving correct estimates of the GHG balance. The nitrogen (N) content of the soil and the water table depth were the two drivers that best explained the variability in N₂O and CH_4, respectively. This study underlines the importance of the ‘non‐CO₂ GHGs’ on the overall balance. Further long‐term investigations of soil trace gas emissions should monitor the N content and the mineralization rate of the soil, as well as the microbial community, as drivers of the trace gas emissions.     

A new baseline of organic carbon stock in European agricultural soils using a modelling approach

Proposed European policy in the agricultural sector will place higher emphasis on soil organic carbon (SOC), both as an indicator of soil quality and as a means to offset CO₂ emissions through soil carbon (C) sequestration. Despite detailed national SOC data sets in several European Union (EU) Member States, a consistent C stock estimation at EU scale remains problematic. Data are often not directly comparable, different methods have been used to obtain values (e.g. sampling, laboratory analysis) and access may be restricted. Therefore, any evolution of EU policies on C accounting and sequestration may be constrained by a lack of an accurate SOC estimation and the availability of tools to carry out scenario analysis, especially for agricultural soils. In this context, a comprehensive model platform was established at a pan‐European scale (EU + Serbia, Bosnia and Herzegovina, Croatia, Montenegro, Albania, Former Yugoslav Republic of Macedonia and Norway) using the agro‐ecosystem SOC model CENTURY. Almost 164 000 combinations of soil‐climate‐land use were computed, including the main arable crops, orchards and pasture. The model was implemented with the main management practices (e.g. irrigation, mineral and organic fertilization, tillage) derived from official statistics. The model results were tested against inventories from the European Environment and Observation Network (EIONET) and approximately 20 000 soil samples from the 2009 LUCAS survey, a monitoring project aiming at producing the first coherent, comprehensive and harmonized top‐soil data set of the EU based on harmonized sampling and analytical methods. The CENTURY model estimation of the current 0–30 cm SOC stock of agricultural soils was 17.63 Gt; the model uncertainty estimation was below 36% in half of the NUTS2 regions considered. The model predicted an overall increase of this pool according to different climate‐emission scenarios up to 2100, with C loss in the south and east of the area (involving 30% of the whole simulated agricultural land) compensated by a gain in central and northern regions. Generally, higher soil respiration was offset by higher C input as a consequence of increased CO₂ atmospheric concentration and favourable crop growing conditions, especially in northern Europe. Considering the importance of SOC in future EU policies, this platform of simulation appears to be a very promising tool to orient future policymaking decisions.     

Toward a Carbon Dioxide Neutral Industrial Park

The industrial park of Herdersbrug (Brugge, Flanders, Belgium) comprises 92 small and medium‐sized enterprises, a waste‐to‐energy incinerator, and a power plant (not included in the study) on its site. To study the carbon dioxide (CO₂) neutrality of the park, we made a park‐wide inventory for 2007 of the CO₂ emissions due to energy consumption (electricity and fossil fuel) and waste incineration, as well as an inventory of the existing renewable electricity and heat generation. The definition of CO₂ neutrality in Flanders only considers CO₂ released as a consequence of consumption or generation of electricity, not the CO₂ emitted when fossil fuel is consumed for heat generation. To further decrease or avoid CO₂ emissions, we project and evaluate measures to increase renewable energy generation. The 21 kilotons (kt) of CO₂ emitted due to electricity consumption are more than compensated by the 25 kt of CO₂ avoided by generation of renewable electricity. Herdersbrug Industrial Park is thus CO₂ neutral, according to the definition of the Flemish government. Only a small fraction (6.6%) of the CO₂ emitted as a consequence of fossil fuel consumption (heat generation) and waste incineration is compensated by existing and projected measures for renewable heat generation. Of the total CO₂ emission (149 kt) due to energy consumption (electricity + heat generation) and waste incineration on the Herdersbrug Industrial Park in 2007, 70.5% is compensated by existing and projected renewable energy generated in the park. Forty‐seven percent of the yearly avoided CO₂ corresponds to renewable energy generated from waste incineration and biomass fermentation.     

Isoprene emissions from plants are mediated by atmospheric CO2 concentrations

The tropical African tree species Acacia nigrescens Oliv. was grown in environmentally controlled growth chambers at three CO₂ concentrations representative of the Last Glacial Maximum (～180 ppmv), the present day (～380 ppmv), and likely mid‐21st century (～600 ppmv) CO₂ concentrations. Isoprene (C₅H₈) emissions, per unit leaf area, were greater at lower‐than‐current CO₂ levels and lower at higher‐than‐current CO₂ levels relative to controls grown at 380 ppmv CO₂. Changes in substrate availability and isoprene synthase (IspS) activity were identified as the mechanisms behind the observed leaf‐level emission response. In contrast, canopy‐scale emissions remained unaltered between the treatments as changes in leaf‐level emissions were offset by changes in biomass and leaf area. Substrate concentration and IspS activity‐CO₂ responses were used in a biochemical model, coupled to existing isoprene emission algorithms, to model isoprene emissions from A. nigrescens grown for over 2 years at three different CO₂ concentrations. The addition of the biochemical model allowed for the use of emission factors measured under present day CO₂ concentrations across all three CO₂ treatments. When isoprene emissions were measured from A. nigrescens in response to instantaneous changes in CO₂ concentration, the biochemical model satisfactorily represented the observed response. Therefore, the effect of changes in atmospheric CO₂ concentration on isoprene emission at any timescale can be modelled and predicted.     

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.     

CO2 emission from mineral soils following land‐cover change in Brazil

This paper presents a first approximation of net CO₂ fluxes from mineral soil due to land use, land‐use changes and forestry (LULUCF) activities in Brazil for the periods 1970–90 and 1975–95. The methodology employed is an adaptation of the approach proposed by the IPCC in ‘Revised 1996 guidelines for national greenhouse gas inventories’, which is based on the variations in soil C stocks as a function of changes in land‐use activities. The calculation was done separately for each Brazilian state and subsequently summarized for all of Brazil. The annual fluxes for Brazil indicate a net emission of CO₂ to the atmosphere, which decreased from 93.3 Tg CO₂ for the period 1970–90 to 46.4 Tg CO₂ for the period 1975–95. This corresponded to yearly net emission rates of 11.0 g CO₂ m^?2 year^?1 for the 1970–90 period and 5.5 g CO₂ m^?2 year^?1 for the 1975–95 period. Within each administrative region, considerable differences in the yearly emission rates between the states could be observed. Several sources of uncertainties could be identified. The most important uncertainties were linked to the impact factor values, which represent changes in native C stock associated with conversion of the native vegetation to agricultural use.     

A Monte Carlo approach to estimate the uncertainty in soil CO2 emissions caused by spatial and sample size variability

The soil CO₂ emission is recognized as one of the largest fluxes in the global carbon cycle. Small errors in its estimation can result in large uncertainties and have important consequences for climate model predictions. Monte Carlo approach is efficient for estimating and reducing spatial scale sampling errors. However, that has not been used in soil CO₂ emission studies. Here, soil respiration data from 51 PVC collars were measured within farmland cultivated by maize covering 25 km² during the growing season. Based on Monte Carlo approach, optimal sample sizes of soil temperature, soil moisture, and soil CO₂ emission were determined. And models of soil respiration can be effectively assessed: Soil temperature model is the most effective model to increasing accuracy among three models. The study demonstrated that Monte Carlo approach may improve soil respiration accuracy with limited sample size. That will be valuable for reducing uncertainties of global carbon cycle.     

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.     

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.     

Indirect carbon dioxide emissions from producing bioenergy from forest harvest residues

Forest harvest residues are important raw materials for bioenergy in regions practicing forestry. Removing these residues from a harvest site reduces the carbon stock of the forest compared with conventional stem‐only harvest because less litter in left on the site. The indirect carbon dioxide (CO₂) emission from producing bioenergy occur when carbon in the logging residues is emitted into the atmosphere at once through combustion, instead of being released little by little as a result of decomposition at the harvest sites. In this study (1) we introduce an approach to calculate this indirect emission from using logging residues for bioenergy production, and (2) estimate this emission at a typical target of harvest residue removal, i.e. boreal Norway spruce forest in Finland. The removal of stumps caused a larger indirect emission per unit of energy produced than the removal of branches because of a lower decomposition rate of the stumps. The indirect emission per unit of energy produced decreased with time since starting to collect the harvest residues as a result of decomposition at older harvest sites. During the 100 years of conducting this practice, the indirect emission from average‐sized branches (diameter 2 cm) decreased from 340 to 70 kg CO₂ eq. MWh^?1 and that from stumps (diameter 26 cm) from 340 to 160 kg CO₂ eq. MWh^?1. These emissions are an order of magnitude larger than the other emissions (collecting, transporting, etc.) from the bioenergy production chain. When the bioenergy production was started, the total emissions were comparable to fossil fuels. The practice had to be carried out for 22 (stumps) or four (branches) years until the total emissions dropped below the emissions of natural gas. Our results emphasize the importance of accounting for land‐use‐related indirect emissions to correctly estimate the efficiency of bioenergy in reducing CO₂ emission into the atmosphere.     

Change in terrestrial ecosystem water‐use efficiency over the last three decades

Defined as the ratio between gross primary productivity (GPP) and evapotranspiration (ET), ecosystem‐scale water‐use efficiency (EWUE) is an indicator of the adjustment of vegetation photosynthesis to water loss. The processes controlling EWUE are complex and reflect both a slow evolution of plants and plant communities as well as fast adjustments of ecosystem functioning to changes of limiting resources. In this study, we investigated EWUE trends from 1982 to 2008 using data‐driven models derived from satellite observations and process‐oriented carbon cycle models. Our findings suggest positive EWUE trends of 0.0056, 0.0007 and 0.0001 g C m^?2 mm^?1 yr^?1 under the single effect of rising CO₂ (‘CO₂’), climate change (‘CLIM’) and nitrogen deposition (‘NDEP’), respectively. Global patterns of EWUE trends under different scenarios suggest that (i) EWUE‐CO₂ shows global increases, (ii) EWUE‐CLIM increases in mainly high latitudes and decreases at middle and low latitudes, (iii) EWUE‐NDEP displays slight increasing trends except in west Siberia, eastern Europe, parts of North America and central Amazonia. The data‐driven MTE model, however, shows a slight decline of EWUE during the same period (?0.0005 g C m^?2 mm^?1 yr^?1), which differs from process‐model (0.0064 g C m^?2 mm^?1 yr^?1) simulations with all drivers taken into account. We attribute this discrepancy to the fact that the nonmodeled physiological effects of elevated CO₂ reducing stomatal conductance and transpiration (TR) in the MTE model. Partial correlation analysis between EWUE and climate drivers shows similar responses to climatic variables with the data‐driven model and the process‐oriented models across different ecosystems. Change in water‐use efficiency defined from transpiration‐based WUE_t (GPP/TR) and inherent water‐use efficiency (IWUE_t, GPP×VPD/TR) in response to rising CO₂, climate change, and nitrogen deposition are also discussed. Our analyses will facilitate mechanistic understanding of the carbon–water interactions over terrestrial ecosystems under global change.     

Are the benefits of yield responses to nitrogen fertilizer application in the bioenergy crop Miscanthus × giganteus offset by increased soil emissions of nitrous oxide?

A field trial was carried out on a 15 year old Miscanthus stand, subject to nitrogen fertilizer treatments of 0, 63 and 125 kg‐N ha^?1, measuring N₂O emissions, as well as annual crop yield over a full year. N₂O emission intensity (N₂O emissions calculated as a function of above‐ground biomass) was significantly affected by fertilizer application, with values of 52.2 and 59.4 g N₂O‐N t^?1 observed at 63 and 125 kg‐N ha^?1, respectively, compared to 31.3 g N₂O‐N t^?1 in the zero fertilizer control. A life cycle analyses approach was applied to calculate the increase in yield required to offset N₂O emissions from Miscanthus through fossil fuel substitution in the fuel chain. For the conditions observed during the field trial yield increases of 0.33 and 0.39 t ha^?1 were found to be required to offset N₂O emissions from the 63 kg‐N ha^?1 treatment, when replacing peat and coal, respectively, while increases of 0.71 and 0.83 t ha^?1 were required for the 125 kg‐N ha^?1 treatment, for each fuel. These values are considerably less than the mean above‐ground biomass yield increases observed here of 1.57 and 2.79 t ha^?1 at fertilization rates 63 and 125 kg‐N ha^?1 respectively. Extending this analysis to include a range of fertilizer application rates and N₂O emission factors found increases in yield necessary to offset soil N₂O emissions ranging from 0.26 to 2.54 t ha^?1. These relatively low yield increase requirements indicate that where nitrogen fertilizer application improves yield, the benefits of such a response will not be offset by soil N₂O emissions.     

Effect of Change of Forest Carbon Storage on Net Carbon Dioxide Balance of Wood Use for Energy in Japan

This study analyzed the net carbon dioxide (CO₂) emission reductions between 2005 and 2050 by using wood for energy under various scenarios of forest management and energy conversion technology in Japan, considering both CO₂ emission reductions from replacement of fossil fuels and changes in carbon storage in forests. According to our model, wood production for energy results in a significant reduction of carbon storage levels in forests (by 46% to 77% in 2050 from the 2005 level). Thus, the net CO₂ emission reduction when wood is used for energy becomes drastically smaller. Conventional tree production for energy increases net CO₂ emissions relative to preserving forests, but fast‐growing tree production may reduce net CO₂ emissions more than preserving forests does. When wood from fast‐growing trees is used to generate electricity with gas turbines, displacing natural gas, the net CO₂ emission reduction from the combination of fast‐growing trees and electricity generation with gas turbines is about 58% of the CO₂ emission reduction from electricity generation from gas turbines alone in 2050, and an energy conversion efficiency of around 20% or more is required to obtain net reductions over the entire period until 2050. When wood is used to produce bioethanol, displacing gasoline, net reductions are realized after 2030, provided that heat energy is recovered from residues from ethanol production. These results show the importance of considering the change in carbon storage when estimating the net CO₂ emission reduction effect of the wood use for energy.     

Uncertainties in predicting rice yield by current crop models under a wide range of climatic conditions

Predicting rice (Oryza sativa) productivity under future climates is important for global food security. Ecophysiological crop models in combination with climate model outputs are commonly used in yield prediction, but uncertainties associated with crop models remain largely unquantified. We evaluated 13 rice models against multi‐year experimental yield data at four sites with diverse climatic conditions in Asia and examined whether different modeling approaches on major physiological processes attribute to the uncertainties of prediction to field measured yields and to the uncertainties of sensitivity to changes in temperature and CO₂ concentration [CO₂]. We also examined whether a use of an ensemble of crop models can reduce the uncertainties. Individual models did not consistently reproduce both experimental and regional yields well, and uncertainty was larger at the warmest and coolest sites. The variation in yield projections was larger among crop models than variation resulting from 16 global climate model‐based scenarios. However, the mean of predictions of all crop models reproduced experimental data, with an uncertainty of less than 10% of measured yields. Using an ensemble of eight models calibrated only for phenology or five models calibrated in detail resulted in the uncertainty equivalent to that of the measured yield in well‐controlled agronomic field experiments. Sensitivity analysis indicates the necessity to improve the accuracy in predicting both biomass and harvest index in response to increasing [CO₂] and temperature.     

Estimating annual N2O emissions from agricultural soils in temperate climates

The Kyoto protocol requires countries to provide national inventories for a list of greenhouse gases including N₂O. A standard methodology proposed by the Intergovernmental Panel on Climate Change (IPCC) estimates direct N₂O emissions from soils as a constant fraction (1.25%) of the nitrogen input. This approach is insensitive to environmental variability. A more dynamic approach is needed to establish reliable N₂O emission inventories and to propose efficient mitigation strategies. The objective of this paper is to develop a model that allows the spatial and temporal variation in environmental conditions to be taken into account in national inventories of direct N₂O emissions. Observed annual N₂O emission rates are used to establish statistical relationships between N₂O emissions, seasonal climate and nitrogen‐fertilization rate. Two empirical models, MCROPS and MGRASS, were developed for croplands and grasslands. Validated with an independent data set, MCROPS shows that spring temperature and summer precipitation explain 35% of the variance in annual N₂O emissions from croplands. In MGRASS, nitrogen‐fertilization rate and winter temperature explain 48% of the variance in annual N₂O emissions from grasslands. Using long‐term climate observations (1900–2000), the sensitivity of the models with climate variability is estimated by comparing the year‐to‐year prediction of the model to the precision obtained during the validation process. MCROPS is able to capture interannual variability of N₂O emissions from croplands. However, grassland emissions show very small interannual variations, which are too small to be detectable by MGRASS. MCROPS and MGRASS improve the statistical reliability of direct N₂O emissions compared with the IPCC default methodology. Furthermore, the models can be used to estimate the effects of interannual variation in climate, climate change on direct N₂O emissions from soils at the regional scale.