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 (CO), methane (CH) and nitrous oxide (NO) 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 COeq ha; this indicated that soil trace gas emissions offset the CO uptake by the plantation. CH and NO contributed almost equally to offset the CO uptake of -5.28 (±0.67) Mg COeq ha with an overall emission of 3.56 (±0.35) Mg COeq ha of NO and of 3.53 (±0.85) Mg COeq ha of CH. NO emissions mostly occurred during one single peak a few months after the site was converted to SRC; this peak comprised 44% of the total NO 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 NO and CH respectively. This study underlines the importance of the 'non-CO 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.