Simulated Rainfall Events Induce Real, but Minimal, Foliar Nutrient Leaching in Senescing Switchgrass Biomass in Iowa.

Ideal bioenergy feedstocks are low in nutrients that act as anti-quality factors during conversion processes. Research has shown that delaying harvest of temperate perennial energy grasses until late winter reduces nutrient content, primarily due to end-season resorption, but indicates a role for foliar nutrient leaching. While end-season resorption has been estimated, foliar nutrient leaching has not, and is a factor that could refine harvest time recommendations and improve feedstock quality. Additionally, establishing a baseline of mineral loss during switchgrass senescence will improve current understanding of leaf-level nutrient resorption. We applied simulated rainfall in situ to switchgrass stands to determine if strong rainfall events can induce foliar nutrient leaching in senescing, unharvested, foliage. Five hour-long rainfall events (each ~120 mm) were simulated every two weeks from the beginning of September to a killing frost in 2014 and 2015. Foliar samples were taken before and after simulated rainfall and in dry controls, then analyzed for N, P, K, S, Mg and Ca. Nutrients tended to actively resorb from leaves, with resorption estimates ranging from 34% to 81.6%. Results from the comparison of rainfall plots to the control plot baselines indicate that lower canopy leaves, upon reaching ≥50% senescence, are slightly susceptible to foliar nutrient leaching, with losses ranging from 0.30 to 2.76 g kg^-1 (dm) for K, P, and Mg. Nitrogen, Ca and S were not susceptible to foliar leaching. While statistically significant (α= 0.05), these values suggest that foliar nutrient leaching is not a strong driver of nutrient loss during senescence.