Seasonal nutrient dynamics and biomass quality of giant reed (Arundo donax L.) and miscanthus (Miscanthus x giganteus greef et deuter) as energy crops

The importance of energy crops in displacing fossil fuels within the energy sector in Europe is growing. Among energy crops, the use of perennial rhizomatous grasses (PRGs) seems promising owing to their high productivity and their nutrient recycling that occurs during senescence. In particular, nutrient requirements and biomass quality have a fundamental relevance to biomass systems efficiency. The objective of our study was to compare giant reed (Arundo donax L.) and miscanthus (Miscanthus × giganteus Greef et Deuter) in terms of nutrient requirements and cellulose, hemicellulose and lignin content. Hence, the aim was to identify, in the Mediterranean environment, the optimal harvest time that may combine, besides a high biomass yield, high nutrient use efficiency and a good biomass quality for second generation biofuel production. The research was carried out in 2009, in San Piero a Grado, Pisa (Central Italy; latitude 43°41’ N, longitude 10°21’ E), on seven-year-old crops in a loam soil characterised by good water availability. Maximum above-ground nutrients content were generally found in summer. Subsequently, a decrease was recorded; this suggested a nutrient remobilisation from above-ground biomass to rhizomes. In addition, miscanthus showed the highest N, P, and K use efficiency, probably related to its higher yield and its C4 pathway. Regarding biomass quality, stable values of cellulose (38%), hemicellulose (25%) and lignin (8%) were reported from July onwards in both crops. Hence, these components appear not to be discriminative parameters in the choice of the harvest time in the Mediterranean environment. In conclusion, our results highlighted that, in our environment, a broad harvest period (from late autumn to winter) seems suitable for these PRGs. However, further research is required to evaluate the role of rhizomes in nutrient storage and supply during the growing season, as well as ecological and productive performances in marginal lands, in particular where water availability may be a limiting factor.