Ruth Burke, Iowa State University, Ames, IA and Emily A. Heaton, Department of Agronomy, Iowa State University, Ames, IA
Bioenergy feedstocks should be low in residual mineral nutrients that act as anti-quality factors during conversion processes. Previous research has shown delaying harvest of temperate perennial energy grasses until late winter or early spring reduces mineral nutrient content over winter, and typically attributes reductions primarily to translocation, but also to foliar nutrient leaching and other passive processes. However, while the magnitude of nutrient reduction from translocation has to some extent been quantified, foliar nutrient leaching has not. A better understanding of how foliar nutrient leaching contributes to end-season nutrient loss in perennial grasses could elucidate harvest time recommendations for improved feedstock quality and nutrient cycling. Current literature on foliar nutrient leaching is restricted to horticultural crops and perennial tree species or focuses on agricultural crops that have been damaged or conditioned in some way. In this study, we applied simulated rainfall in situ to switchgrass stands to determine if strong rainfall events can induce foliar nutrient leaching in senescing, but otherwise undamaged, foliage. Five hour-long rainfall events (each ~120 mm) were simulated every two weeks from the beginning of September to a killing frost using a modified soil erosion rainfall simulator. Foliar samples were taken before and after simulated rainfall and in a dry control, then analyzed for N, P, K, S, Mg and Ca. First year results indicate that strong rainfall events may induce slight foliar leaching of N and Mg, with no effect on other nutrients. The magnitude of leaching averaged 540 and 120 mg kg-1 for N and Mg, respectively. While statistically significant (α= 0.05), these values would suggest that foliar nutrient leaching is not a strong driver of nutrient loss during the senescent period.