All washed out? Foliar nutrient resorption and leaching in senescing switchgrass

Authors: BURKE, RUTH - Iowa State University; MOORE, KENNETH - Iowa State University; Shipitalo, Martin; MIGUEZ, FERNANDO - Iowa State University; HEATON, EMILY - Iowa State University

Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2017
Publication Date: 2/23/2017
Publication URL: http://handle.nal.usda.gov/10113/5728364
Citation: Burke, R.H., Moore, K.J., Shipitalo, M.J., Miguez, F.E., Heaton, E.A. 2017. All washed out? Foliar nutrient resorption and leaching in senescing switchgrass. BioEnergy Research. 12:22-23. doi: 10.1007/s12155-017-9819-6.

Interpretive Summary: Switchgrass has been identified as a model bioenergy crop by the US Department of Energy. Nutrients remaining in harvested switchgrass, however, reduce the amount available for future cuttings of this perennial grass and can cause problems when converting it to liquid biofuels. Nutrients move down to the roots when the grass matures (resorption) and are also leached from above ground portions by rainfall. Delaying harvest in the fall and winter to decrease nutrient content, however, can decrease the yield due to leaf drop and lodging. Thus, farmers need guidance regarding the best time to harvest this crop. Therefore, we investigated how fast nutrients are lost from switchgrass by resorption and leaching by monitoring nutrient levels for 10 weeks after maturity in plots under natural rainfall and plots subject to additional, simulated, heavy rainfall. Hour-long simulated rainfalls of ~5 inches were applied every two weeks from early September to a killing frost in 2014 and 2015. Leaf samples were taken from the upper and lower canopy before and after simulated rainfalls and from no rain controls and analyzed for elemental N, P, K, S, Mg, and Ca. Nutrient resorption estimates ranged from 33% to 82% in control plots. Comparison of rainfall plots to controls indicated that lower canopy leaves were slightly susceptible to foliar nutrient leaching of K, P, and Mg, but not N, Ca, and S. These results suggested that foliar nutrient leaching is not likely to significantly alter the mineral nutrient concentration of switchgrass, thus it may be best for farmers to harvest shortly after fall dieback in order to avoid harvest difficulties and loss of biomass.

Technical Abstract: 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 grasses until late winter reduces nutrient content, primarily due to end-season resorption, but also 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 recommendations. Additionally, establishing a baseline of mineral loss during switchgrass senescence will improve our understanding of leaf-level nutrient resorption. Therefore, we applied simulated rainfall to replicated (n=5), 2 by 3.5 m, established switchgrass plots to determine if heavy precipitation can induce nutrient leaching in senescing foliage. Hour-long simulated rainfalls of ~120 mm were applied every two weeks from early September to a killing frost in 2014 and 2015. Leaf samples were taken from the upper and lower canopy before and after simulated rainfalls and from no rain controls and analyzed for elemental N, P, K, S, Mg, and Ca. Nutrient resorption estimates ranged from 33% to 82% in control plots. Comparison of rainfall plots to controls indicated that lower canopy leaves, upon reaching = 50 % senescence, were slightly susceptible to foliar nutrient leaching, with losses ranging from 0.3 to 2.8 g kg-1 dry matter for K, P, and Mg. Nitrogen, Ca and S were not susceptible to foliar leaching. Although statistically significant (P = 0.05), these values suggested that foliar leaching was not a strong driver of nutrient loss during senescence.