Nutrient Dynamics in Switchgrass as a Function of Time

There are wide variations in fertilizer recommendations for switchgrass (Panicum virgatum L.) as biofuel feedstock or forage. Inconsistent yield responses to fertilization are common. Nutrient translocation, from aboveground leaves and stems to belowground roots, is a contributor to variable, and sometimes the absence of, fertilizer response. A field study evaluated how major nutrients are cycled within switchgrass during the growing season. Aboveground (AG) and belowground (BG) biomasses were harvested and analyzed separately for yield (AG) and nutrient concentrations (AG and BG). Maximum yields were 26.3 (2008), 17.5 (2009) and 29.3 (2010) Mg ha(-1) until senescence. In all years, the N concentration of the AG biomass decreased as the season progressed (p<0.0001). The belowground biomass N concentration increased over time in 2008 and 2010 (p<0.05). Phosphorus (P) and potassium (K) displayed similar trends to nitrogen concentration. Temporal changes of other nutrients in the AG and BG biomass concentrations were inconsistent and varied compared with N. The dynamics of the macronutrients suggested translocation from AG to BG as the switchgrass matured. Nutrients stored in roots can be beneficial for regrowth in the following growing season, which may reduce response to fertilizers. A greater understanding of nutrient cycling and harvest timing is needed to better manage different switchgrass production systems.