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It is important to demonstrate that replacing fossil fuel with bioenergy crops can reduce the national greenhouse gas ( GHG) footprint. We compared field emissions of nitrous oxide (N2O), methane ( CH4) and soil respiration rates from the C4 grass Miscanthus × giganteus and willow ( salix) with emissions from annual arable crops grown for food production. The study was carried out in NE England on adjacent fields of willow , Miscanthus, wheat ( Triticum aetivum) and oilseed rape ( Brassica napus). N2O, CH4 fluxes and soil respiration rates were measured monthly using static chambers from June 2008 to November 2010. Net ecosystem exchange ( NEE) of carbon dioxide ( CO2) was measured by eddy covariance on Miscanthus from May 2008 and on willow from October 2009 until November 2010. The N2O fluxes were significantly smaller from the bioenergy crops than that of the annual crops. Average fluxes were 8 and 32 μg m−2 h−1 N2O-N from wheat and oilseed rape, and 4 and 0.2 μg m−2 h−1 N2O-N from Miscanthus and willow, respectively. Soil CH4 fluxes were negligible for all crops and soil respiration rates were similar for all crops. NEE of CO2 was larger for Miscanthus (−770 g C m−2 h−1) than willow (−602 g C m−2 h−1) in the growing season of 2010. N2O emissions from Miscanthus and willow were lower than for the wheat and oilseed rape which is most likely a result of regular fertilizer application and tillage in the annual arable cropping systems. Application of 15N-labelled fertilizer to Miscanthus and oil seed rape resulted in a fertilizer-induced increase in N2O emission in both crops. Denitrification rates (N2O + N2) were similar for soil under Miscanthus and oilseed rape. Thus, perennial bioenergy crops only emit less GHGs than annual crops when they receive no or very low rates of N fertilizer. [ABSTRACT FROM AUTHOR]

Abstract Well-documented models of soil respiration and C[sub 3] photosynthesis are combined to investigate the overall carbon dioxide balance of a high arctic polar semidesert site near Ny-Alesund, Svalbard. Measurements of carbon dioxide fluxes from an eddy correlation system and climate variables are used to calibrate this combined model for the snowmelt-summer period in 1995. Particular emphasis was put on modelling effectively the separate soil respiration and net photosynthesis of the moss/lichen, vascular plant and bare soil areas within the heterogeneous site, and ascertaining whether moss/lichen communities could be effectively modelled using a C[sub 3] photosynthetic model. The calibrated model of 1995 data was subsequently tested in predictive mode against eddy correlation flux measurements recorded in 1996. The model is shown to simulate effectively both years and highlights the important role that the timing of snowmelt, summer precipitation and soil temperature has upon the overall carbon dioxide balance of this site. A previous conjecture by the author from the limited flux measurements that the investigated period in 1995 was an overall sink for carbon dioxide and that the same period in 1996 was an overall source of carbon dioxide is borne out by the model runs with 3.9 +/- 3.7 gC m[sup -2] being assimilated into the vegetation during the 1995 period and 5.0 +/- 3.3 gC m[sup -2] being returned to the atmosphere during the same period in 1996. [ABSTRACT FROM AUTHOR]