N2O fluxes of a bio-energy poplar plantation during a two years rotation period

Nitrous oxide emissions are of critical importance for the assumed climate neutrality of bio-energy. In this study we report on the N2O fluxes from a bio-energy poplar plantation measured with eddy covariance for 2years, after conversion of agricultural fields to few months after harvesting of the plantation. A pulse peak of N2O was detected after the land use change and in the wake of the first heavy rainfall. The N2O-N emission during just a single week was 2.7kg N2O-Nha(-1) which represented approximately 42% of the total N2O-N emitted during the 2years of measurements. After this peak emission, N2O fluxes were constantly rather low, not increasing after rainfall events any longer. Lowest emissions (and even N2O sink) occurred mostly during the end of the second growing season with maximum canopy development, and water table deeper than 80cm. Gross primary production (GPP) explained 68% of the monthly averaged variability in N2O emission from August to December 2011. Probably N uptake by vegetation during the peak of the second growing season limited N2O emission, which in fact increased again after the plantation was coppiced. For the majority of the measuring period, N2O fluxes did not present a well-defined diurnal pattern, with the exception of two periods: (1) from 19-22 August 2010 and (2) from September-November 2011. In both cases wind speed played a major role in controlling the diurnal pattern in these fluxes (explaining up to 80% of the diurnal variability in N2O fluxes on 19-22 August 2010), whereas at the end of the second growing season (September-November 2011), GPP explained 73% of the diurnal pattern in N2O fluxes.