Nutrient Uptake by 'Alamo' Switchgrass Used as an Energy Crop

Field studies were conducted in 2000 and 2001 to examine yields and nutrient removal by Alamo switchgrass (Panicum virgatum L.) grown at eight locations within five states in the upper southeastern USA. Plots, which had been established for >5 years as part of a larger study, were cut either once (late fall) or twice (midsummer and late fall). Plots cut once received 50 kg N per hectare per year, while twice-cut plots received 100 kg N per hectare per year. Nutrient concentrations of and nutrient removal by harvested biomass were determined. Partitioning of nutrients into leaf and stem fractions was determined at the time of the midsummer harvest in 2000. Biomass production during 2000 and 2001 averaged 15.9 Mg/ha per year across all sites and was as high as 21.7 Mg/ha per year at one site. Two cuttings plus the additional 50 kg N per hectare did not generally increase seasonal yields; and, in one quite productive location, that management caused a yield reduction. Nitrogen removal with two cuts was much higher than with a single cut due largely to the higher N content in the midsummer harvest. Over the 2 years, twice as much N was removed with the two annual cuts as with one cut. Nitrogen removal exceeded the amounts of N applied in both managements, suggesting N was being supplied via mineralization or other processes. Phosphorus removal also increased significantly with the two-cut management. Seasonal K and Ca removals were more similar between the two managements. Nitrogen and P concentrations generally declined basipetally in tillers, with older leaves and internodes having lower concentrations of both nutrients. Potassium was more uniformly distributed than N throughout the tiller components (leaf and stem). Calcium was higher in older leaf blades. Levels of soil P, K, and Ca at most locations appeared not to be limiting biomass production and were adequate for long-term productivity.