Seasonal dynamics of bioenergy characteristics in grassland functional groups.

Semi-natural grasslands (SNG) are considered as bioenergy resources, either for combustion or for biogas production. To provide information on the seasonal (June–August) dynamics of herbage quantity and chemical composition, we studied the effects of factors on the proportions of four plant functional groups, chemical composition and total biomass yield at twelve sites representing three Estonian SNG types. Biomass yield increased only in alluvial meadows with the largest contribution of sedges/rushes (39%). Grasses dominated in dry-to-mesic meadows (62%) and other forbs in wooded meadows (55%). Concentration of fibre was highest in grasses and sedges/rushes (682 and 645 g NDF kg−1), and lignin was highest in legumes and other forbs (103 and 113 g kg−1). Legumes contained more C and N (447 and 25 g kg−1) and grasses more Cl (1 g kg−1). The highest concentrations of Mg, K and ash were in other forbs (4, 18 and 80 g kg−1), and Ca in legumes (16 g kg−1). The results are discussed in the context of suitability of different functional groups and SNG types for bioenergy conversion. Grasses and sedge-/rush-rich biomass are considered suitable for methane production. Biomass harvesting in July and minimizing the problems with N and Cl during combustion need to be considered. Forb-rich biomass should be pre-treated before direct combustion.