Cattle Slurry Fertilization to Giant Reed ( Arundo donax L.): Biomass Yield and Nitrogen Use Efficiency

Although giant reed (Arundo donax L.) is regarded as a promising energy crop for Mediterranean environments, its efficiency in utilizing nitrogen (N) fertilizers is still largely unexplored. This study addresses the following research questions: (i) What is the effect of cattle slurry application on giant reed productivity? (ii) does giant reed use efficiently the N applied in the form of cattle slurry? (iii) does the ratio of carbon (C) to N (C/N) in harvested biomass vary with rates N supply? To explore these questions, a field experiment was carried out over 5 years in the low Po Valley, Northern Italy. Fertilization treatments annually applied were the following: (i) no fertilization (control); (ii) cattle slurry 10 mm, i.e., 10 L m−2 (CS10); and (iii) cattle slurry 20 mm, i.e., 20 L m−2 (CS20). Compared to the control, the cattle slurry supply substantially increased yield and yield stability of giant reed. The physiological efficiency of applied N (PEN) was 216 kg aboveground DM per kg N uptake for the favorable years 2009, 2010, and 2011. Giant reed produced a lot of biomass per kg of N uptake, therefore limiting the amount of fertilizers needed for its cultivation. The recovery efficiency of applied N (REN) varied from 0.12 to 0.39. The agronomic efficiency of applied N (AEN) varied from 18 to 51 kg aboveground DM per kg N applied, with considerable year-to-year variation. Finally, the N content of harvested biomass always remained below the threshold of 0.6 % N, which is considered the guide value for high-quality biomass feedstock for combustion.