Nitrogen Fertilization Makes Miscanthus x Giganteus Grow Faster but Not Bigger: A Closer Look at Phenology.

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 nitrogen (N) fertilizer across environments, particularly as the crop ages. One limitation of these studies is that M. × giganteus N response is mainly assessed in terms of plant productivity (e.g. biomass) while the morphological development of the stand is ignored. This approach misses that different treatments, stand ages or locations may have reached different developmental stages or have matured at different rates. We compared M. × giganteus morphological development and stem biomass at 5 N fertilization rates (0, 112, 224, 336, 448 kg ha^-1) and three planting years (2015, 2016, 2017) during the 2016 and 2017 growing seasons. Our experiment allowed us to compare side-by-side the N response of different aged stands and stand age effect at different environments. Nitrogen fertilization had no effect on M. × giganteus development and biomass for 1-year-old stands (p-value = 0.63). However, in older stands, fertilized treatments emerged earlier and flowered later in the year. N fertilization also hastened vegetative development early in the growing season. Stem biomass was consistently higher in fertilized treatments across the growing season (p = 0.0028). However, differences between treatments were notably reduced (p-value = 0.0324) when compared in terms of their biomass at the same developmental stage (e.g. same number of leaves, same number of nodes). These results suggest that N fertilization may accelerate morphological development in established M. × giganteus stands and that part of the differences observed at specific sampling dates could be a result of plants being developmentally older, not necessarily bigger. Phenological study could help clarify the mechanisms behind the N response and allow better comparison across different environment and studies.