Productivity, 15 N dynamics and water use efficiency in low‐ and high‐input switchgrass systems

Sustainable and environmentally benign switchgrass production systems need to be developed for switchgrass to become a large‐scale dedicated energy crop. An experiment was conducted in C alifornia from 2009 to 2011 to determine the sustainability of low‐ and high‐input irrigated switchgrass systems as a function of yield, irrigation requirement, crop N removal, N translocation from aboveground ( AG ) to belowground ( BG ) biomass during senescence, and fertilizer 15 N recovery ( FNR ) in the AG and BG biomass (0–300 cm), and soil (0–300 cm). The low‐input system consisted of a single‐harvest (mid‐fall) irrigated until flowering (early summer), while the high‐input system consisted of a two‐harvest system (early summer and mid‐fall) irrigated throughout the growing season. Three N fertilization rates (0, 100, and 200 kg N ha −1  yr −1 ) were applied as subtreatments in a single application in the spring of each year. A single pulse of 15 N enriched fertilizer was applied in the first year of the study to micro‐plots within the 100 kg N ha −1 subplots. Average yields across years under optimal N rates (100 and 200 kg ha −1  yr −1 for low‐ and high‐input systems, respectively) were 20.7 and 24.8 Mg ha −1 . However, the low input (372 ha mm) required 47% less irrigation than the high‐input system (705 ha mm) and achieved higher irrigation use efficiency. In addition, the low‐input system had 46% lower crop N removal, 53% higher N stored in BG biomass, and a positive N balance, presumably due to 49% of 15 N translocation from AG to BG biomass during senescence. Furthermore, at the end of 3 years, the low‐input system had lower fertilizer 15 N removed by harvest (26%) and higher FNR remaining in the system in BG biomass plus soil (31%) than the high‐input system (45% and 21%, respectively). Based on these findings, low‐input systems are more sustainable than high‐input systems in irrigated M editerranean climates.