Miscanthus x Giganteus Response to Nitrogen Fertilization Across 3 Sites in Iowa.

Miscanthus x giganteus is a sterile perennial grass used for biomass production in temperate environments. Though a productive crop, M. × giganteus shows inconsistent yield responses to N fertilizer across environments, particularly as the crop ages. While most studies recognize that environmental conditions during planting year may have a lasting effect in this long-lived crop, few experiments are designed to separate the confounding effects of growth year (weather, management) and stand age, particularly when considering N responses. Here we use a chronosequence experiment in which M. × giganteus Freedom was planted in split-plot design at three locations with planting year (2015, 2016) as the main plot and N fertilization (0, 112, 224, 336, 448 kg ha^-1) as the subplot (n=4). Plant development, leaf area index (LAI), and net CO₂ assimilation (A_sat) were measured over the growing season at one site in Central Iowa, USA, while end-season above-ground dry matter (DM) production was measured at all three sites (NW, Central and SE Iowa, USA). Early results show a strong interaction between N application rate and experimental site: DM did not change with N fertilization at two sites (P= 0.635 and P = 0.551 at the NW and SE site, respectively), but significantly increased at the central location (DM = 4.9, 9.7, 12.8, 11.9, and 14.3 Mg ha^-1, for N rates 0-448 kg ha^-1, respectively; P< 0.001). This positive yield response to N was supported by consistently higher A_sat (P = 0.003) and higher LAI (P = 0.0042) in fertilized plots. These results indicate that N fertilizer can, under some conditions, increase photosynthetic efficiency and yield, and will help to unravel how this response changes with space and time.