Switchgrass growth and effects on biomass accumulation, moisture content, and nutrient removal

Temporal patterns of plant growth, composition, and nutrient removal impact development of models for predicting optimal switchgrass (Panicum virgatum L.) harvest times for bioenergy. Original time-course data are needed to construct useful models. Objectives were to characterize seasonal trends in yield, tissue moisture, ash content, leaf area index (LAI), interception of photosynthetically active radiation (PAR), and macronutrient accumulation and losses. Plots were subjected to 12 single harvests May to February in 2009–2010 and 2010–2011. Biomass accumulation into late summer followed a sigmoidal function, reaching an asymptote at 14 Mg ha^–1, concurrent with early seed filling. During both seasons, yields decreased 26 to 32% from September to February. Moisture content declined to levels appropriate for storage (≤200 g kg^–1, wet basis) by 15 December. Interception of PAR plateaued at 96% in late July 2009 and mid-June 2010, whereas LAI declined in early July both years. Peak N uptake occurred in August 2009 and July 2010 (80 and 141 kg N ha^–1, respectively). Potassium uptake peaked in July 2009 (136 kg ha^–1) and in June 2010 (180 kg ha^–1). Phosphorus peaked in July and August 2009 and 2010 at 16 and 17 kg ha^–1, respectively. Peak biomass yields occurred in August–Septem-ber when N and K uptake and moisture contents were still elevated. Delaying harvests until late fall or winter reduced fertilizer replacement needs and moisture content, but a trade-off in yield occurred. Quantifying these intra-seasonal changes allows for simulating productivity trade-offs and applying those to economic and environmental analyses.