Grasslands for bioenergy production. A review

The promise of low-input high-diversity prairies to provide sustainable bioenergy production has recently been emphasized. This review article presents a critical discussion of some controversial points of using grasslands to produce bioenergy. The following issues are addressed: proteins versus biofuels; reactive nitrogen emissions; biodiversity; and effective land use. Two major disadvantages in deriving bioenergy from grasslands are identified: (i) marginal lands are displaced from their fundamental role of producing meat and milk foods, in contrast with the rising worldwide demand for high-quality food; and (ii) the combustion of N-rich grassland biomass, or by-products, results in emission of reactive N into the atmosphere and dramatically reduces the residence time of biologically-fixed nitrogen in the ecosystems. Nitrogen oxides, released during atmospheric combustion of fossil fuels and biomass, have a detrimental effect on global warming. Since intensively managed crops on fertile soils need to be cultivated to fulfil the dietary needs of populations, the potential role of inedible cereal crop residues in providing bioenergy merits consideration. This might spare more marginal land area for forage production or even for full natural use, in order to sustain high levels of biodiversity. Owing to the complexity of terrestrial systems, and the complexity of interactions, a modeling effort is needed in order to predict and quantify outcomes of specific combination of land use at higher integration levels.