Julia Drewer

Direct nitrous oxide emissions from oilseed rape cropping – a meta-analysis

Oilseed rape is one of the leading feedstocks for biofuel production in Europe. The climate change mitigation effect of rape methyl ester (RME) is particularly challenged by the greenhouse gas (GHG) emissions during crop production, mainly as nitrous oxide (N2O) from soils. Oilseed rape requires high nitrogen fertilization and crop residues are rich in nitrogen, both potentially causing enhanced N2O emissions. However, GHG emissions of oilseed rape production are often estimated using emission factors that account for crop‐type specifics only with respect to crop residues. This meta‐analysis therefore aimed to assess annual N2O emissions from winter oilseed rape, to compare them to those of cereals and to explore the underlying reasons for differences. For the identification of the most important factors, linear mixed effects models were fitted with 43 N2O emission data points deriving from 12 different field sites. N2O emissions increased exponentially with N‐fertilization rates, but interyear and site‐specific variability were high and climate variables or soil parameters did not improve the prediction model. Annual N2O emissions from winter oilseed rape were 22% higher than those from winter cereals fertilized at the same rate. At a common fertilization rate of 200 kg N ha−1 yr−1, the mean fraction of fertilizer N that was lost as N2O‐N was 1.27% for oilseed rape compared to 1.04% for cereals. The risk of high yield‐scaled N2O emissions increased after a critical N surplus of about 80 kg N ha−1 yr−1. The difference in N2O emissions between oilseed rape and cereal cultivation was especially high after harvest due to the high N contents in oilseed rapes crop residues. However, annual N2O emissions of winter oilseed rape were still lower than predicted by the Stehfest and Bouwman model. Hence, the assignment of oilseed rape to the crop‐type classes of cereals or other crops should be reconsidered.

Measurement of the 13C isotopic signature of methane emissions from northern European wetlands

Isotopic data provide powerful constraints on regional and global methane emissions and their source profiles. However, inverse modeling of spatially resolved methane flux is currently constrained by a lack of information on the variability of source isotopic signatures. In this study, isotopic signatures of emissions in the Fennoscandian Arctic have been determined in chambers over wetland, in the air 0.3 to 3 m above the wetland surface and by aircraft sampling from 100 m above wetlands up to the stratosphere. Overall, the methane flux to atmosphere has a coherent δ13C isotopic signature of −71 ± 1‰, measured in situ on the ground in wetlands. This is in close agreement with δ13C isotopic signatures of local and regional methane increments measured by aircraft campaigns flying through air masses containing elevated methane mole fractions. In contrast, results from wetlands in Canadian boreal forest farther south gave isotopic signatures of −67 ± 1‰. Wetland emissions dominate the local methane source measured over the European Arctic in summer. Chamber measurements demonstrate a highly variable methane flux and isotopic signature, but the results from air sampling within wetland areas show that emissions mix rapidly immediately above the wetland surface and methane emissions reaching the wider atmosphere do indeed have strongly coherent C isotope signatures. The study suggests that for boreal wetlands (>60°N) global and regional modeling can use an isotopic signature of −71‰ to apportion sources more accurately, but there is much need for further measurements over other wetlands regions to verify this.

Simulation of greenhouse gases following land-use change to bioenergy crops using the ECOSSE model. A comparison between site measurements and model predictions

This article evaluates the suitability of the ECOSSE model to estimate soil greenhouse gas (GHG) fluxes from short rotation coppice willow (SRC‐Willow), short rotation forestry (SRF‐Scots Pine) and Miscanthus after land‐use change from conventional systems (grassland and arable). We simulate heterotrophic respiration (Rh), nitrous oxide (N2O) and methane (CH4) fluxes at four paired sites in the UK and compare them to estimates of Rh derived from the ecosystem respiration estimated from eddy covariance (EC) and Rh estimated from chamber (IRGA) measurements, as well as direct measurements of N2O and CH4 fluxes. Significant association between modelled and EC‐derived Rh was found under Miscanthus, with correlation coefficient (r) ranging between 0.54 and 0.70. Association between IRGA‐derived Rh and modelled outputs was statistically significant at the Aberystwyth site (r = 0.64), but not significant at the Lincolnshire site (r = 0.29). At all SRC‐Willow sites, significant association was found between modelled and measurement‐derived Rh (0.44 ≤ r ≤ 0.77); significant error was found only for the EC‐derived Rh at the Lincolnshire site. Significant association and no significant error were also found for SRF‐Scots Pine and perennial grass. For the arable fields, the modelled CO2 correlated well just with the IRGA‐derived Rh at one site (r = 0.75). No bias in the model was found at any site, regardless of the measurement type used for the model evaluation. Across all land uses, fluxes of CH4 and N2O were shown to represent a small proportion of the total GHG balance; these fluxes have been modelled adequately on a monthly time‐step. This study provides confidence in using ECOSSE for predicting the impacts of future land use on GHG balance, at site level as well as at national level.

The impact of ploughing intensively managed temperate grasslands on N2O, CH4 and CO2 fluxes

Background and aimsTemperate grasslands are a globally important component of agricultural production systems and a major contributor to the exchange of greenhouse gases (GHG) between the biosphere and atmosphere. Many intensively managed grazed grasslands in NW Europe are ploughed and reseeded occasionally in order to improve their productivity. Here, we examined the impact of ploughing on the emission of GHGs a grassland.MethodsTo study these interactions we measured soil GHG fluxes using the static chamber method in addition to the net ecosystem exchange (NEE) of CO2 by eddy covariance from two adjacent fields. Until ploughing one field in 2012 and the other in 2014, management of these intensively grazed grasslands was almost the same and typical for the study region.ResultsThe effect on N2O is small, but distinguishable from the effects of N fertilisation, soil temperature and soil moisture. Tillage-induced N2O fluxes were close to expectations based on the IPCC default methodology. By far the dominant effect on the GHG balance was the temporary reduction in GPP.ConclusionsPloughing and reseeding can substantially influence short-term GHG emissions. Therefore tillage-induced fluxes ought to be considered when estimating greenhouse gas fluxes or budgets from grasslands that are periodically ploughed.

A comparison of isoprene and monoterpene emission rates from the perennial bioenergy crops short-rotation coppice willow and Miscanthus and the annual arable crops wheat and oilseed rape

Biogenic volatile organic compounds (BVOC) emissions from bioenergy crops may differ from those of conventional crops. We compared emission rates of isoprene and a number of monoterpenes from the lignocellulosic bioenergy crops short‐rotation coppice (SRC) willow and Miscanthus, with the conventional crops wheat and oilseed rape. BVOC emission rates were measured via dynamic vegetation enclosure and GC‐MS analysis approximately monthly between April 2010 and August 2012 at a location in England and from SRC willow at two locations in Scotland. The largest BVOC emission rates were measured from willow in England and varied between years. Isoprene emission rates varied between

Estimation of cumulative fluxes of nitrous oxide: uncertainty in temporal upscaling and emission factors

Summary Nitrous oxide (N2O) is a greenhouse gas produced mainly by the microbial breakdown of agricultural fertilizer. ‘Emission factors’ (EFs, the fraction of nitrogen added that is released as N2O) are based on flux chamber measurements following the application of fertilizer. These measurements are very variable in space and time so that EFs are often uncertain, but this is rarely quantified. We developed a method that simplifies the calculation of EFs, incorporates prior knowledge and quantifies the uncertainty with a B ayesian approach to fit the parameters of a lognormal model. We compared this with the standard method for interpolating, extrapolating and integrating fluxes of N2O (trapezoidal integration). We verified both methods against process‐based model output where the true integral was known and against eddy covariance data where the integral was estimated more accurately because of the greater spatial and temporal coverage. We used the process‐based model to simulate flux chamber data and added a lognormal spatial distribution to the model output. The lognormal model performed better than the standard method, in terms of estimating the true underlying cumulative flux more accurately. Estimates based on chamber and eddy covariance data were sometimes substantially different, but with no clear systematic bias. The B ayesian approach with the lognormal model enabled us to combine both chamber and eddy covariance data to constrain cumulative fluxes. The standard trapezoidal method typically underestimates emission factors to some extent if fluxes are lognormally distributed in space. The B ayesian approach with the lognormal model is a robust method for quantifying the uncertainty in cumulative fluxes of N2O. HighlightsEmission factors for N 2 O are based on sparse and variable measurements, and so are uncertain.We use a B ayesian approach to simplify the calculation and quantify the uncertainty.No observed systematic difference between eddy covariance and chamber measurement methods.The standard trapezoidal method will typically underestimate emission factors.

Difference in soil methane (CH4) and nitrous oxide (N2O) fluxes from bioenergy crops SRC willow and SRF Scots pine compared with adjacent arable and fallow in a temperate climate

Soil greenhouse gas (GHG) fluxes of methane (CH4) and nitrous oxide (N2O) were measured over a 2-year period from several land use systems on adjacent sites under the same soil and climatic conditions to assess the influence of the transition from arable agricultural (barley) and fallow to perennial bioenergy crops short rotation coppice (SRC) willow (Salix spp.) and short rotation forest (SRF) Scots pine (Pinus sylvestris). There were no significant differences between CH4 and N2O fluxes measured from the SRC, SRF and fallow, but the arable agricultural site showed an order of magnitude higher N2O emissions compared with the others. Fertiliser application to the arable crop was the major factor influencing N2O emissions, and both air and soil temperature showed no significant effects on fluxes between the different land use systems. Soil moisture was significantly different from the arable crop, showing a greater range than from SRF and SRC. Hence, these bioenergy crops might be viable options for water-stressed areas.

Changes in isotopic signatures of soil carbon and CO2 respiration immediately and one year after Miscanthus removal

The removal of perennial bioenergy crops, such as Miscanthus, has rarely been studied although it is an important form of land use change. Miscanthus is a C4 plant, and the carbon (C) it deposits during its growth has a different isotopic signature (12/13C) compared to a C3 plant. Identifying the proportion of C stored and released to the atmosphere is important information for ecosystem models and life cycle analyses. During a removal experiment in June 2011 of a 20‐year old Miscanthus field (Grignon, France), vegetation was removed mechanically and chemically. Two replicate plots were converted into a rotation of annual crops, two plots had Miscanthus removed with no soil disturbance, followed by bare soil (set‐aside), one control plot was left with continued Miscanthus cultivation, and an adjacent field was used as annual arable crops control. There was a significant difference in the isotopic composition of the total soil C under Miscanthus compared with adjacent annual arable crops in all three measured soil layers (0–5, 5–10 and 10–20 cm). Before Miscanthus removal, total C in the soil under Miscanthus ranged from 4.9% in the top layer to 3.9% in the lower layers with δ13C values of −16.3 to −17.8 while soil C under the adjacent arable crop was significantly lower and ranged from 1.6 to 2% with δ13C values of −23.2. This did not change much in 2012, suggesting the accumulation of soil C under Miscanthus persists for at least the first year. In contrast, the isotopic signals of soil respiration 1 year after Miscanthus removal from recultivated and set‐aside plots were similar to that of the annual arable control, while just after removal the signals were similar to that of the Miscanthus control. This suggests a rapid change in the form of soil C pools that are respired.

A Review of Stereochemical Implications in the Generation of Secondary Organic Aerosol from Isoprene Oxidation

The atmospheric reactions leading to the generation of secondary organic aerosol (SOA) from the oxidation of isoprene are generally assumed to produce only racemic mixtures, but aspects of the chemical reactions suggest this may not be the case. In this review, the stereochemical outcomes of published isoprene-degradation mechanisms contributing to high amounts of SOA are evaluated. Despite evidence suggesting isoprene first-generation oxidation products do not contribute to SOA directly, this review suggests the stereochemistry of first-generation products may be important because their stereochemical configurations may be retained through to the second-generation products which form SOA. Specifically, due to the stereochemistry of epoxide ring-opening mechanisms, the outcome of the reactions involving epoxydiols of isoprene (IEPOX), methacrylic acid epoxide (MAE) and hydroxymethylmethyl-α-lactone (HMML) are, in principle, stereospecific which indicates the stereochemistry is predefined from first-generation precursors. The products from these three epoxide intermediates oligomerise to form macromolecules which are proposed to form chiral structures within the aerosol and are considered to be the largest contributors to SOA. If conditions in the atmosphere such as pH, aerosol water content, relative humidity, pre-existing aerosol, aerosol coatings and aerosol cation/anion content (and other) variables acting on the reactions leading to SOA affect the tacticity (arrangement of chiral centres) in the SOA then they may influence its physical properties, for example its hygroscopicity. Chamber studies of SOA formation from isoprene encompass particular sets of controlled conditions of these variables. It may therefore be important to consider stereochemistry when upscaling from chamber study data to predictions of SOA yields across the range of ambient atmospheric conditions. Experiments analysing the stereochemistry of the reactions under varying conditions of the above variables would help elucidate whether there is stereoselectivity in SOA formation from isoprene and if the rates of SOA formation are affected.

Greenhouse gas (GHG) and biogenic volatile organic compound (bVOC) fluxes associated with land-use change to bioenergy crops

Abstract Greenhouse gases (GHGs) and biogenic volatile organic compounds (bVOCs) are released into the atmosphere by all ecosystems through natural processes. Mans activities tend to dominate in terms of scale of emissions, but the magnitude, timing, and location vary depending upon the organisms (including crops), physical environment (soil, hydrology, etc.), conditions (climate, season, etc.), and processes (respiration, photosynthesis, decay, management, etc.). How these emissions relate to bioenergy crops is essential in determining where they should be cultivated, which types to select, and how they are managed. Bioenergy crops are usually planted because the land manager sees an economic benefit to them and/or their organisation. However, trade-offs, usually non-financial, have to be made that can be seen as costs or benefits. The use of biomass to provide energy is commonly seen as a strategy to mitigate climate change and is even talked of as potentially being ‘carbon negative’. To make a full comparison between the different management options requires knowledge of the system prior to bioenergy planting and measures of impacts of the system in that location and beyond and across all components. In this chapter, we introduce the GHGs and bVOCs, detailing where they are generated, emitted, and absorbed by bioenergy crops; how they vary over different time frames and under different conditions; the overall balance of emissions to compare with other forms of land-use; how management can influence emissions; and finally the potential for future benefits from the production of bioenergy crop feedstocks. We also identify where the different gases are most important, the uncertainties, and where research is ongoing.