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

Abstract

AbstractSustainable 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 California 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 15N 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 15N 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 15N translocation from AG to BG biomass during senescence. Furthermore, at the end of 3 years, the low‐input system had lower fertilizer 15N 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 Mediterranean climates.