Sixteen-Year Biomass Yield and Soil Carbon Storage of Giant Reed (Arundo donax L.) Grown Under Variable Nitrogen Fertilization Rates

High and stable biomass yields for long periods (15–20 years) are indispensable for the successful establishment of a well-developed bioenergy sector. However, the effects of management practices, particularly nitrogen fertilization, on productivity and soil organic carbon (SOC) are difficult to understand, especially when considering that continuous harvesting cycles may have cumulative effects on the crop and its resources use capacity. The objective of this study was to evaluate the effects of different N fertilization levels on biomass production and SOC accumulation of giant reed over 16 years. Every year, starting from the second one, two N fertilization rates were applied: 80 (N80) and 160 (N160) kg N ha−1. The control treatment (N0) was unfertilized. Nitrogen content and use capacity, and SOC gains were determined. Mean 16-year biomass yields were 16.2, 17.1, and 19.5 Mg ha−1 in the N0, N80, and N160 treatments, respectively. Variable yielding phases were observed in the N160 treatment with declining yields towards the last sampling season, whereas the N0 was characterized by increasing yields up to the fourth growing season; thereafter, declining yields were observed. Nitrogen concentration and removed N in the aboveground harvested biomass increased from N0 to N160 and as the stand become older. Mean total SOC stock gains were 1.0 and 0.6 Mg C ha−1 year−1 in the N160 and N0 treatments, respectively. The largest SOC stocks were found in the topsoil, with the largest amount (12 Mg C ha−1 in 16 years) in the N160 treatment. In conclusion, long-term high N fertilization rates result in marginal increments in biomass productivity (about 3 Mg ha−1 year−1), but in substantial increments in SOC, especially in surface soil layers. A farmer might prefer to grow giant reed without the burdens of fertilization despite the seemingly benefits on SOC and lower yields of unfertilized plots.