Greenhouse gas balance of cropland conversion to bioenergy poplar short-rotation coppice

Abstract. The production of bioenergy in Europe is one of the strategies conceived to reduce greenhouse gas (GHG) emissions. The suitability of the land use change from a cropland (REF site) to a short-rotation coppice plantation of hybrid poplar (SRC site) was investigated by comparing the GHG budgets of these two systems over 24 months in Viterbo, Italy. This period corresponded to a single rotation of the SRC site. The REF site was a crop rotation between grassland and winter wheat, i.e. the same management of the SRC site before the conversion to short-rotation coppice. Eddy covariance measurements were carried out to quantify the net ecosystem exchange of CO₂ (F_CO2), whereas chambers were used to measure N₂O and CH₄ emissions from soil. The measurements began 2 years after the conversion of arable land to SRC so that an older poplar plantation was used to estimate the soil organic carbon (SOC) loss due to SRC establishment and to estimate SOC recovery over time. Emissions from tractors and from production and transport of agricultural inputs (F_MAN) were modelled. A GHG emission offset, due to the substitution of natural gas with SRC biomass, was credited to the GHG budget of the SRC site. Emissions generated by the use of biomass (F_EXP) were also considered. Suitability was finally assessed by comparing the GHG budgets of the two sites. CO₂ uptake was 3512 ± 224 g CO₂ m⁻² at the SRC site in 2 years, and 1838 ± 107 g CO₂ m⁻² at the REF site. F_EXP was equal to 1858 ± 240 g CO₂ m⁻² at the REF site, thus basically compensating for F_CO2, while it was 1118 ± 521 g CO₂ m⁻² at the SRC site. The SRC site could offset 379.7 ± 175.1 g CO₂eq m⁻² from fossil fuel displacement. Soil CH₄ and N₂O fluxes were negligible. F_MAN made up 2 and 4 % in the GHG budgets of SRC and REF sites respectively, while the SOC loss was 455 ± 524 g CO₂ m⁻² in 2 years. Overall, the REF site was close to neutrality from a GHG perspective (156 ± 264 g CO₂eq m⁻²), while the SRC site was a net sink of 2202 ± 792 g CO₂eq m⁻². In conclusion the experiment led to a positive evaluation from a GHG viewpoint of the conversion of cropland to bioenergy SRC.