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Impact of poplar bioenergy cultivation on ozone and volatile organic compound emissions

Final Report Summary - SRF-OZO (Impact of poplar bioenergy cultivation on ozone and volatile organic compound emissions)

The project "SRF-OZO: Impact of poplar cultivation on ozone and volatile organic compound emissions" made the first simultaneous measurements of the fluxes of ozone (O3) and of the emissions of biogenic volatile organic compounds (BVOCs) as well as of the fluxes of the main greenhouse gases (GHG) as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from a short rotation forestry (SRF) of poplar to determine their impact on O3 formation. The project started on 1 November 2014 and ended on 31 October 2016, and was conducted by Dr. Terenzio Zenone (hereby referred to as “researcher”) under supervision of the project's coordinator / scientist in charge, Prof.dr. Reinhart Ceulemans at the University of Antwerp.

The need for renewable energy sources to meet EU Directive 2009/28/EC is expected to lead to a considerable expansion in the planting of dedicated fast-growing biomass crops managed as SRF. Poplar (Populus spp) is currently the most widely planted SRF species and an increase in large-scale SRF poplar plantations might be expected in the future. Poplars are characterized by a considerably high isoprene emission coefficient, and they are susceptible to O3 pollution. The emission of isoprene and other BVOCs from vegetation plays an important role in tropospheric O3 formation. The potentially large expansion of isoprene emitting poplars for bioenergy production might, therefore, impact tropospheric O3 formation. The SRF-OZO project tackled the above mentioned environmental issue. SRF-OZO was closely linked to and benefited from the POPFULL project (European Research Council Advanced Grant № 233366). The collected data were used to scale up the measured BVOC emissions through the chemistry transport model LOTOS-EUROS in combination with the current surface area planted with poplar-for-bioenergy in Europe. This combination allowed to quantify the potential impact of the BVOC emissions on ground-level O3 concentrations. The research site was a poplar SRF plantation for bioenergy production located in Lochristi, East-Flanders (Belgium). The plantation was established in April 2010 with 12 selected clones of Populus spp on a surface of 18.4 ha.

The experimental observations indicated that the soil trace gas emissions of CH4 and N2O offset the CO2 uptake by the poplar SRF and made the total GHG budget of the plantation (excluding the end use of the harvested biomass) slightly negative. Ecosystem level fluxes of N2O were highly variable with a non-negligible proportion of N2O uptake. On the other hand, ecosystem level CH4 fluxes mainly showed emissions with only a sporadic uptake by the soil. Measured isoprene fluxes showed a well-defined daily cycle with maximum emission rates during the afternoon, corresponding with maximum values of incoming solar radiation. Total O3 uptake also showed a diurnal trend that mirrored the isoprene fluxes and occurred in parallel with the increase of the O3 concentration. The uptake of O3 was dominated by the stomatal component that represented 70% of the total O3 uptake.

When simulated isoprene and O3 fluxes were validated against observations by running the LOTOS-EUROS model, there was a reasonable agreement. By incorporating the areas covered by poplar plantations – as reported by the Food and Agriculture Organization (FAO) of the United Nations for different countries in Europe – into the model, the rise of isoprene fluxes ranged from 2.4% to 11.6% depending on the emission factor used. The increase in O3 concentrations in the air due to the existing poplar plantations was very limited and ranged between 0.1 to 0.5%. Despite current policies encouraging the use of biomass for energy, data from the literature confirmed that the area covered by poplar plantations did not significantly change over the last 20 years. The current area of poplar SRF plantations in Europe is far too small to create a threat for O3 formation, as they would only create 1% of extra isoprene emissions, as demonstrated by our simulations.

The researcher has presented the results of the project at the General Assembly of the European Geosciences Union (EGU; 12 – 17 April 2015; Vienna, Austria), at the 28th Task Force Meeting of the International Cooperative Program on Effects of Air Pollution on Natural Vegetation and Crops (3-5 February 2015; Rome, Italy), as well as at the fall meeting of the American Geophysical Union (AGU; 14-18 December 2015, San Francisco, USA).

The researcher published a total of 8 scientific papers in peer reviewed journals and one book chapter during the two years of the project. The main results of the SRF-OZO project were reported in the paper on “Interaction between isoprene and ozone fluxes and its impact at European level” published in Scientific Reports, 2016, 6,32676 (Zenone et al. 2016). The experimental observations and model simulations of this publication provide an important guideline for scientists and policy-makers involved in the bioenergy sector. The paper “CO2 uptake is offset by CH4 and N2O emissions in a poplar short rotation coppice” published in Global Change Biology Bioenergy, 2016, 8, 524–538 (Zenone et al. 2016) is one of the few publications that report the full greenhouse gas balance measured at ecosystem level using the eddy covariance technique. The publication highlights the importance of continuous and simultaneous observations on trace gases as N2O and CH4 in addition to the traditional observations of CO2 exchange. In the other publications “Biophysical drivers of the carbon dioxide, water vapor, and energy exchanges of a short-rotation poplar coppice” (Agricultural and Forest Meteorology, 2016, 209, 22-35), “Simultaneous leaf-and ecosystem-level fluxes of volatile organic compounds from a poplar-based SRC plantation” (Brilli et al., Agricultural and Forest Meteorology, 2014, 187, 22-35) and “Rapid leaf development drives the seasonal pattern of volatile organic compound (VOC) fluxes in a coppiced bioenergy poplar plantation” (Brilli et al., Plant, Cell and Environment, 2016 39, 539–555) the researcher and co-authors focused on the environmental variables that drive the exchanges of CO2 and BVOCs at ecosystem level. The researcher served as co-editor of the book Sustainable Biofuels. An Ecological Assessment of the Future Energy (De Gruyter Publisher, Berlin, Boston) and contributed to write a book chapter entitled ”Short rotation forestry for energy production in Italy: environmental aspects and new perspectives of use in the biofuel industry”.

After achieving its goals, and in several cases going beyond the intentions at the onset, the work conducted during the project represents an important tool for policy makers involved in the bioenergy sector. The project has been a unique case where the fluxes of the principal greenhouse gases and of BVOCs were measured at ecosystem level providing a complete overview of the gaseous exchanges between plants and the atmosphere.