The greenhouse gas budgets of 15 European crop sites covering a large climatic gradient and corresponding
to 41 site-years were estimated. The sites included a wide range of management practices (organic
and/or mineral fertilisation, tillage or ploughing, with or without straw removal, with or without irrigation,
etc.) and were cultivated with 15 representative crop species common to Europe. At all sites, carbon
inputs (organic fertilisation and seeds), carbon exports (harvest or fire) and net ecosystem production
(NEP), measured with the eddy covariance technique, were calculated. The variability of the different
terms and their relative contributions to the net ecosystem carbon budget (NECB) were analysed for all
site-years, and the effect of management on NECB was assessed. To account for greenhouse gas (GHG)
fluxes that were not directly measured on site, we estimated the emissions caused by field operations
(EFO) for each site using emission factors from the literature. The EFO were added to the NECB to calculate
the total GHG budget (GHGB) for a range of cropping systems and management regimes. N2O emissions
were calculated following the IPCC (2007) guidelines, and CH4 emissions were estimated from the literature
for the rice crop site only. At the other sites, CH4 emissions/oxidation were assumed to be negligible
compared to other contributions to the net GHGB. Finally, we evaluated crop efficiencies (CE) in relation
to global warming potential as the ratio of C exported from the field (yield) to the total GHGB. On average,
NEP was negative (-284±228gCm-2 year-1), and most cropping systems behaved as atmospheric
sinks, with sink strength generally increasing with the number of days of active vegetation. The NECB
was, on average, 138±239gCm-2 year-1, corresponding to an annual loss of about 2.6±4.5% of the soil
organic C content, but with high uncertainty. Management strongly influenced the NECB, with organic
fertilisation tending to lower the ecosystem carbon budget. On average, emissions caused by fertilisers
(manufacturing, packaging, transport, storage and associated N2O emissions) represented close to 76% of EFO. The operation of machinery (use and maintenance) and the use of pesticides represented 9.7 and
1.6% of EFO, respectively. On average, the NEP (through uptake of CO2) represented 88% of the negative
radiative forcing, and exported C represented 88% of the positive radiative forcing of a mean total GHGB of
203±253 g C-eqm-2 year-1. Finally, CE differed considerably among crops and according to management
practices within a single crop. Because the CE was highly variable, it is not suitable at this stage for use as
an emission factor for management recommendations, and more studies are needed to assess the effects
of management on crop efficiency.