Quantifying senescence-mediated nutrient loss processes in switchgrass (Panicum virgatum L.) biomass
The annual senescence that occurs in many perennial grass species is not a terminal event, and multiple internal processes are occurring as a plant enters winter or dry-season dormancy. When grown as bioenergy feedstocks, end-season nutrient conservation and loss processes taking place in perennial grasses are of interest to producers attempting to maximize yield while reducing residual nutrient content in the harvestable biomass. The residual nutrient content in these bioenergy feedstocks can reduce conversion efficiency, damage biorefinery equipment, and even cause pollution. In order to better understand nutrient cycling in perennial grasses as well as improve harvest management decisions for bioenergy feedstock producers, I quantified three nutrient loss processes in the model perennial grass species Panicum virgatum L.: 1) end-season nutrient resorption, 2) biomass nutrient leaching, and 3) mass loss due to overwinter leaf drop (litterfall). Over two autumn seasons in 2014 and 2015, I established a baseline of macronutrient reduction in senescing, undamaged in situ switchgrass plots and compared those baselines to plots exposed to heavy simulated rainfall in order to quantify potential foliar nutrient leaching. I found that leaf-level resorption may drive the bulk of phloem mobile nutrient (i.e., N, P, K) reduction from aboveground biomass during senescence, but phloem immobile nutrients (i.e., Ca) tend to remain behind in the standing biomass. From a practical point of view, foliar leaching was not observed to be a significant driver of nutrient loss during senescence. During the winter of 2015 – 2016, in situ switchgrass biomass samples were harvested monthly from undamaged stands. The harvested material was analyzed by aboveground morphological component (stem, leaf, panicle) for mass and macronutrient content to quantify nutrient losses and the passive process driving them. I observed that losses of non-leachable nutrients (i.e., N and Ca) were primarily due to overwinter leaf drop and losses of the more water-soluble nutrients (i.e., K and P) were primarily due to biomass leaching. These studies will improve knowledge of end-season nutrient cycling in perennial grasses and help inform management decisions for producers seeking to diversify their operations with switchgrass for bioenergy.