Bioenergy from crops is expected to make a considerable contribution to climate change mitigation. However, bioenergy is not necessarily carbon neutral because emissions of CO 2 , N 2 O and CH 4 during crop production may reduce or completely counterbalance CO 2 savings of the substituted fossil fuels. These greenhouse gases (GHGs) need to be included into the carbon footprint calculation of different bioenergy crops under a range of soil conditions and management practices. This review compiles existing knowledge on agronomic and environmental constraints and GHG balances of the major European bioenergy crops, although it focuses on dedicated perennial crops such as Miscanthus and short rotation coppice species. Such second-generation crops account for only 3% of the current European bioenergy production, but field data suggest they emit 40% to >99% less N 2 O than conventional annual crops. This is a result of lower fertilizer requirements as well as a higher N-use efficiency, due to effective N-recycling. Perennial energy crops have the potential to sequester additional carbon in soil biomass if established on former cropland (0.44 Mg soil C ha À1 yr À1 for poplar and willow and 0.66 Mg soil C ha À1 yr À1 for Miscanthus). However, there was no positive or even negative effects on the C balance if energy crops are established on former grassland. Increased bioenergy production may also result in direct and indirect land-use changes with potential high C losses when native vegetation is converted to annual crops. Although dedicated perennial energy crops have a high potential to improve the GHG balance of bioenergy production, several agronomic and economic constraints still have to be overcome.Keywords: biofuel, carbon debt, carbon footprint, land management, methane, Miscanthus, nitrous oxide, short rotation coppice, soil organic carbon Greenhouse gas saving with bioenergy -a European perspectiveThe European Union has committed to increase the proportion of renewable energy from 9% in 2010 to 20% of Correspondence: Axel Don,

Over 13 million ha of former cropland are enrolled in the US Conservation Reserve Program (CRP), providing well-recognized biodiversity, water quality, and carbon (C) sequestration benefits that could be lost on conversion back to agricultural production. Here we provide measurements of the greenhouse gas consequences of converting CRP land to continuous corn, corn–soybean, or perennial grass for biofuel production. No-till soybeans preceded the annual crops and created an initial carbon debt of 10.6 Mg CO 2 equivalents (CO 2 e)·ha −1 that included agronomic inputs, changes in C stocks, altered N 2 O and CH 4 fluxes, and foregone C sequestration less a fossil fuel offset credit. Total debt, which includes future debt created by additional changes in soil C stocks and the loss of substantial future soil C sequestration, can be constrained to 68 Mg CO 2 e·ha −1 if subsequent crops are under permanent no-till management. If tilled, however, total debt triples to 222 Mg CO 2 e·ha −1 on account of further soil C loss. Projected C debt repayment periods under no-till management range from 29 to 40 y for corn–soybean and continuous corn, respectively. Under conventional tillage repayment periods are three times longer, from 89 to 123 y, respectively. Alternatively, the direct use of existing CRP grasslands for cellulosic feedstock production would avoid C debt entirely and provide modest climate change mitigation immediately. Incentives for permanent no till and especially permission to harvest CRP biomass for cellulosic biofuel would help to blunt the climate impact of future CRP conversion.

Summary paragraph 34Plants acquire carbon through photosynthesis to sustain biomass production, autotrophic 35 respiration, and production of non-structural compounds for multiple purposes 1 . The fraction 36 of photosynthetic production used for biomass production, the biomass production 37 efficiency 2 , is a key determinant of the conversion of solar energy to biomass. In forest 38 ecosystems, biomass production efficiency was suggested to be related to site fertility 2 . Here 39 we present a global database of biomass production efficiency from 131 sites compiled from 40 individual studies using harvest, biometric, eddy covariance, or process-based model 41 estimates of production -dominated, however, by data from Europe and North America. We 42show that instead of site fertility, ecosystem management is the key factor that controls 43 biomass production efficiency in terrestrial ecosystems. In addition, in natural forests, 44 grasslands, tundra, boreal peatlands and marshes biomass production efficiency is 45 independent of vegetation, environmental and climatic drivers. This similarity of biomass 46 production efficiency across natural ecosystem types suggests that the ratio of biomass 47 production to gross primary productivity is constant across natural ecosystems. We suggest 48 that plant adaptation results in similar growth efficiency in high and low fertility natural 49 systems, but that nutrient influxes under managed conditions favour a shift to carbon 50 investment from the belowground flux of non-structural compounds to aboveground biomass. 51 52 53 Main text 54The fraction of gross primary production (GPP) used for biomass production (BP) of 55 terrestrial ecosystems has recently been coined biomass production efficiency (BPE) 2 . BPE is 56 typically used as a proxy for the carbon-use efficiency or NPP-to-GPP ratio, where NPP refers 57 to net primary production i.e. BP plus the production of non-structural organic compounds 1 . 58 4 Current knowledge about BPE is mainly derived from research on forests. Earlier work 59 reported BPE to be conservative across forests 3 , whereas more recent syntheses suggest high 60 inter-site variability 2,4 . The variation in BPE was first attributed to vegetation properties 61 (forest age) and climate variables 4 . More recently, it was shown that forest BPE in a range of 62 natural and managed sites was correlated with site fertility, with management as a secondary 63 BPE driver 2 . 64Fertility and management are strongly correlated as management enhances 65 productivity by increasing plant-available resources, including nutrients. For instance, 66 fertilization of grasslands directly increases the ecosystem nutrient stock, whereas forest 67 thinning indirectly increases nutrient availability at the tree level by reducing plant-plant 68 competition. In addition, fertile sites are more likely than infertile sites to be managed. 69Atmospheric deposition of nutrients, especially nitrogen (N), might further complicate the 70 relationship between BPE, fertility and ...