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 rape's 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

Effects of grass species and grass growth on atmospheric nitrogen deposition to a bog ecosystem surrounded by intensive agricultural land use

We applied a N-15 dilution technique called Integrated Total Nitrogen Input (ITNI) to quantify annual atmospheric N input into a peatland surrounded by intensive agricultural practices over a 2-year period. Grass species and grass growth effects on atmospheric N deposition were investigated using Lolium multiflorum and Eriophorum vaginatum and different levels of added N resulting in increased biomass production. Plant biomass production was positively correlated with atmospheric N uptake (up to 102.7mg N pot(-1)) when using Lolium multiflorum. In contrast, atmospheric N deposition to Eriophorum vaginatum did not show a clear dependency to produced biomass and ranged from 81.9 to 138.2mgNpot(-1). Both species revealed a relationship between atmospheric N input and total biomass N contents. Airborne N deposition varied from about 24 to 55kgNha(-1)yr(-1). Partitioning of airborne N within the monitor system differed such that most of the deposited N was found in roots of Eriophorum vaginatum while the highest share was allocated in aboveground biomass of Lolium multiflorum. Compared to other approaches determining atmospheric N deposition, ITNI showed highest airborne N input and an up to fivefold exceedance of the ecosystem-specific critical load of 5-10kgNha(-1)yr(-1).Peer reviewe

Nitrification inhibitors reduce N2O emissions induced by application of biogas digestate to oilseed rape

Winter oilseed rape (WOSR) is the major oil crop cultivated in Europe and the most important feedstock for biodiesel. Up to 90% of the greenhouse gas (GHG) emissions from biodiesel production can occur during oilseed rape cultivation. Therefore, mitigation strategies are required and need to focus on direct nitrous oxide (N2O) emission as one of the largest GHG contributors in biodiesel production. Earlier studies show that nitrification inhibitors (NIs) can reduce N2O emissions derived from N-fertilization. Since information on the effect of biogas digestates with or without NIs on N2O emissions from WOSR fields is scarce, the aim of this study was to evaluate their effects on N2O emissions, mineral N dynamics, and oil yield in WOSR production fertilized with digestate. The study was conducted at five sites across Germany over three years resulting in 15 full site-years data sets. Across all sites and years, N2O emission from WOSR fertilized with biogas digestate (180 kg NH4+-N ha−1yr−1) ranged between 0.2 and 3.5 kg N2O–N ha−1 yr−1. Due to the reduction of the nitrate concentrations following digestate application, application of NI significantly reduced annual N2O emission by 36%. Our results demonstrate that NI can be an effective measure for reducing N2O emissions from digestate application, but its effectiveness depends on soil and weather conditions, and ultimately on the site-specific potential for N2O production and release. There was no effect of NI application on grain and oil yield.Bundesministerium für Ernährung und Landwirtschaft (DE)Universität Hohenheim (3153)Peer Reviewe

Nitrous oxide emissions from winter oilseed rape cultivation

Winter oilseed rape (Brassica napus L., WOSR) is the major oil crop cultivated in Europe. Rapeseed oil is predominantly used for production of biodiesel. The framework of the European Renewable Energy Directive requires that use of biofuels achieves GHG savings of at least 50% compared to use of fossil fuel starting in 2018. However, N2O field emissions are estimated using emission factors that are not specific for the crop and associated with strong uncertainty. N2O field emissions are controlled by N fertilization and dominate the GHG balance of WOSR cropping due to the high global warming potential of N2O. Thus, field experiments were conducted to increase the data basis and subsequently derive a new WOSR-specific emission factor. N2O emissions and crop yields were monitored for three years over a range of N fertilization intensities at five study sites representative of German WOSR production. N2O fluxes exhibited the typical high spatial and temporal variability in dependence on soil texture, weather and nitrogen availability. The annual N2O emissions ranged between 0.24 kg and 5.48 kg N2O-N ha−1 a−1. N fertilization increased N2O emissions, particularly with the highest N treatment (240 kg N ha−1). Oil yield increased up to a fertilizer amount of 120 kg N ha−1, higher N-doses increased grain yield but decreased oil concentrations in the seeds. Consequently oil yield remained constant at higher N fertilization. Since, yield-related emission also increased exponentially with N surpluses, there is potential for reduction of the N fertilizer rate, which offers perspectives for the mitigation of GHG emissions. Our measurements double the published data basis of annual N2O flux measurements in WOSR. Based on this extended dataset we modeled the relationship between N2O emissions and fertilizer N input using an exponential model. The corresponding new N2O emission factor was 0.6% of applied fertilizer N for a common N fertilizer amount under best management practice in WOSR production (200 kg N ha−1 a−1). This factor is substantially lower than the linear IPCC Tier 1 factor (EF1) of 1.0% and other models that have been proposed. © 201

Gas chromatography vs. quantum cascade laser-based N₂O flux measurements using a novel chamber design

Recent advances in laser spectrometry offer new opportunities to investigate the soil–atmosphere exchange of nitrous oxide. During two field campaigns conducted at a grassland site and a willow field, we tested the performance of a quantum cascade laser (QCL) connected to a newly developed automated chamber system against a conventional gas chromatography (GC) approach using the same chambers plus an automated gas sampling unit with septum capped vials and subsequent laboratory GC analysis. Through its high precision and time resolution, data of the QCL system were used for quantifying the commonly observed nonlinearity in concentration changes during chamber deployment, making the calculation of exchange fluxes more accurate by the application of exponential models. As expected, the curvature values in the concentration increase was higher during long (60 min) chamber closure times and under high-flux conditions (FN2O > 150 µg N m−2 h−1) than those values that were found when chambers were closed for only 10 min and/or when fluxes were in a typical range of 2 to 50 µg N m−2 h−1. Extremely low standard errors of fluxes, i.e., from  ∼  0.2 to 1.7 % of the flux value, were observed regardless of linear or exponential flux calculation when using QCL data. Thus, we recommend reducing chamber closure times to a maximum of 10 min when a fast-response analyzer is available and this type of chamber system is used to keep soil disturbance low and conditions around the chamber plot as natural as possible. Further, applying linear regression to a 3 min data window with rejecting the first 2 min after closure and a sampling time of every 5 s proved to be sufficient for robust flux determination while ensuring that standard errors of N2O fluxes were still on a relatively low level. Despite low signal-to-noise ratios, GC was still found to be a useful method to determine the mean the soil–atmosphere exchange of N2O on longer timescales during specific campaigns. Intriguingly, the consistency between GC and QCL-based campaign averages was better under low than under high N2O efflux conditions, although single flux values were highly scattered during the low efflux campaign. Furthermore, the QCL technology provides a useful tool to accurately investigate the highly debated topic of diurnal courses of N2O fluxes and its controlling factors. Our new chamber design protects the measurement spot from unintended shading and minimizes disturbance of throughfall, thereby complying with high quality requirements of long-term observation studies and research infrastructures

Sustainable and resource efficient intensivation of crop production – Perspectives of agro-ecosystem researchPolicy paper of the DFG Senate Commission on Agroecosystem Research

Mit dem vorliegenden Grundsatzpapier zeigt die Senatskommission für Agrarökosystemforschung Perspektiven für die Grundlagenforschung zur nachhaltigen Erhöhung der Kulturpflanzenproduktion auf.Agrarsysteme stehen im Spannungsfeld zwischen steigendem Bedarf an landwirtschaftlichen Produkten, der Verknappung der Ressourcen, dem Verlust der Biodiversität und dem Klimawandel. Die für das Jahr 2050 prognostizierte notwendige Ertragssteigerung zur Sicherstellung des Bedarfs an Nahrungsmitteln kann, ohne die Belastbarkeitsgrenzen ökologischer Systeme zu überschreiten, nur durch wissenschaftlichen Fortschritt bewältigt werden (Abb. 1), der eine nachhaltige und ressourcen­effiziente Steigerung der Agrarproduktion ermöglicht (FAO, 2011; Dobermann und Nelson, 2013). Die nachhaltige Intensivierung stellt die Agrarwissenschaften vor neue Aufgaben, die weit über ihre klassischen Grenzen hinausgehen.Die Senatskommission plädiert daher für eine Erweiterung der agrarwissenschaftlichen Perspektive. Die meist auf einzelne Feldfrüchte bezogene Bewertung der Rela­tion zwischen Input und Ertrag muss ergänzt werden um die Optionen, die sich aus der räumlichen und zeitlichen Diversifikation der Produktionssysteme unter Einbeziehung der standörtlichen Eigenschaften, des Landschaftskontextes sowie des Klimawandels ergeben. Um Ökosystemleistungen einzubeziehen, müssen Produktionsstrategien entwickelt werden, die sich auf ganze Landschaften und Regionen richten und auch entsprechende sozio­öko­no­mische und agrarpolitische Rahmenbedingungen berücksichtigen.Vor diesem Hintergrund schlägt die Senatskommission drei interdisziplinäre Forschungsschwerpunkte zur ressourceneffizienten Erhöhung der Flächenproduktivität vor:(1) Ausnutzung des Potentials von Kulturpflanzen zur umweltschonenden Ertragssteigerung im Kontext öko­systemarer Bedingungen.(2) Nachhaltige Steigerung der Pflanzenproduktion im Landschaftskontext.(3) Ökonomische, gesellschaftliche und politische Dimensionen der Ertragssteigerung von Kulturpflanzen. DOI: 10.5073/JfK.2014.07.01, https://doi.org/10.5073/JfK.2014.07.01With its policy paper the Senate Commission on Agro-ecosystem Research of the Deutsche Forschungsgemeinschaft (DFG) summarizes potential benefits of basic research for the sustainable intensification of crop production. Agro-ecosystems critically contribute to fulfilling the need for increasing food and fiber production, diminishing resource depletion as well as counteracting biodiversity loss and climate change. Yield demands that are needed to ensure the food supply predicted for the year 2050 can only be achieved by scientific progress that allows the intensive yet environmentally friendly production of plant biomass (Figure 1), (FAO, 2011; Dobermann und Nelson, 2013; Ray et al., 2013). Sustainable intensification requires a scientific realignment that allows for broadening the scope of agricultural research. The productivity of farming systems should be evaluated with regard to their efficiency (input-output relation). In addition, the spatial and temporal variability of these systems must be considered by addressing local conditions, the landscape context and climate change. With respect to ecosystem services, new production strategies must be developed that take all aspects of landscape and regional complexity as well as socio-economic conditions and agricultural policy into account.Against this background, the Senate Commission on Agro-ecosystem Research proposes three priority areas of interdisciplinary research on resource efficient intensification of crop production:(1) Exploiting the biological potential of the individual crop plants for an environmentally friendly intensification in an ecosystem approach(2) Exploring sustainable intensification of crop production within a landscape context(3) Taking full account of the economic, social and political dimensions of sustainable intensification of crop production DOI: 10.5073/JfK.2014.07.01, https://doi.org/10.5073/JfK.2014.07.0

Isotope fractionations factors of N2O production and reduction by denitrification: a. Laboratory incubation studies using N2O reductase inhibition

Isotopologue signatures of N2O such as 18O, average 15N (15Nbulk) and 15N site preference (SP = difference in 15N between the central and peripheral N positions of the asymmetric N2O molecule) can be used to constrain the atmospheric N2O budget and to characterize N2O turnover processes. However, the use of this approach to study N2O dynamics in soils requires knowledge of isotopologue fractionation factors (") for the various partial processes involved, e.g. N2O production by nitrification or denitrification, and N2O reduction by denitrification. Here we present results from laboratory incubations of soils and aquifer material to determine "gf N2O production ("prod) and N2O reduction to N2 ("red) during denitrification. "prod for 18O, 15Nbulk and SP was obtained by anaerobic incubation of NO 3 amended soils when N2O reduction was inhibited by 10 kPa acetylene. "red of the respective signatures was derived by comparing treatments with and without inhibition of N2O reduction. We investigated samples from 4 mineral soils, one organic soil and from a sandy aquifer. The mineral soils were incubated under unsaturated conditions in closed or open systems, the organic and aquifer samples as homogenized slurries in a closed system. Results of fractionation factors, process rates and incubation conditions will be presented and discussed in view of previous studies and theoretical considerations

Do farmers in Germany exploit the potential yield and nitrogen benefits from preceding oilseed rape in winter wheat cultivation?

<p>Field experiments show that wheat grown after oilseed rape (OSR) achieves higher yield levels, while the nitrogen (N) application is reduced. However, field experiment data are based on few locations with optimised management. We analysed a large dataset based on farm data to assess the true extent of break crop benefits (BCB) for yield and N fertilisation within German commercial farming.</p> <p>Across all German states and years, average yield of wheat preceded by OSR was 0.56 Mg ha⁻¹ higher than yield of wheat preceded by cereals (7.09 Mg ha⁻¹), although considerable variation between regions was observed. Mean N application across all states to wheat after OSR was 5 kg ha⁻¹ lower than to wheat after cereals. Choice of wheat types for different end uses (bread flour or animal feed) showed higher (0.77 Mg ha⁻¹) or lower (0.44 Mg ha⁻¹) BCB for yield of wheat cultivated after OSR compared with after cereals. The calculated BCB for yield and N fertilisation were lower than expected from dedicated field experiments and fertiliser recommendations. Thus the advantages of OSR as a preceding crop are generally utilised by commercial farmers in Germany but there is room for improvement.</p

Isotope fractionations factors of N2O production and reduction by denitrification derived from laboratory incubation studies and modeling

Quantifying denitrification in arable soils is crucial in predicting the microbial consumption of nitrogen fertilizers as well as N2O emissions. Stable isotopologue analyses of denitrification substrates (15NNO3, 18ONO3) and products (15NN2O, 18ON2O and SPN2O =Site Preference, i.e. difference in δ15N between the central and peripheral N positions of the asymmetric N2O molecule) can help to distinguish production pathways and to identify N2O reduction to N2. However, such interpretations are often ambiguous due to insufficient knowledge on isotopic fractionation mechanisms and wide differences in isotope fractionation factors determined by various studies for N2O production and reduction. Here we present results from laboratory incubations of soils and aquifer material to determine the net isotopic effect (η of N2O production (ηNO3-N2O) and N2O reduction to N2 (ηN2O-N2) during denitrification. ηNO3-N2O for 18O, 15Nbulk and SP was obtained by anaerobic incubation of NO3- amended soils when N2O reduction was inhibited by 10 kPa acetylene. ηN2O-N2 of the respective signatures was derived by comparing treatments with and without inhibition of N2O reduction. Furthermore, we present an original approach to determine ηprod and ηred by modeling. This determination is based on simultaneous modeling of both reaction steps (N2O production and reduction) and comparison of the results with experimental data from a laboratory incubation experiment carried out under N2-free atmosphere. For two analyzed arable soils (clay and sandy loam), the isotopic fractionation factors were very consistent. For N2O production mean net isotope effects of η15NNO3-N2O ~ -41‰, ηSPNO3-N2O ~ 2‰ and η18OH2O-N2O ~ +40‰ have been found. For N2O reduction mean net isotope effects of η15NN2O-N2 ~ +1‰, ηSPN2O-N2 ~ -7‰ and η18ON2O-N2 ~ -5‰ have been found