Seasonal Changes In Nitrogen and Fiber Fraction Concentrations Of Warm-Season Perennial Bioenergy Grasses In The USA Gulf Coast Region.

Understanding changes in plant chemical composition during the growing season aids in identifying optimal harvest management practices for perennial bioenergy grasses. Additionally, characterizing above-ground N allocation among plant parts provides information for management of N cycling. This research was conducted at the University of Florida during 2010 and 2011 to quantify bimonthly changes in compositional properties that might affect harvested biomass quality and N cycling efficiency. Biomass from ‘L79-1002’ energycane (Saccharum spp.), ‘Merkeron’ elephantgrass (Pennisetum purpureum Schum.), and an elephantgrass breeding line, ‘UF-1’, was collected from June to December bimonthly and analyzed for cellulose, acid detergent lignin (ADL), and total N concentrations. The two elephantgrass entries had greater average cellulose concentration than energycane (418 vs. 378 mg g^-1, respectively, in 2010, and 420 vs. 393 mg g^-1, respectively, in 2011) caused by relatively greater non-structural carbohydrate concentration of energycane than elephantgrass entries. Elephantgrass generally had greater ADL concentration than energycane (avg. 75 mg g^-1 vs. 63 mg g^-1). Nitrogen concentration in total biomass decreased over the season by 77% and 64% in 2010 and 2011, respectively, and it differed among plant parts. Across three entries, at season end,  although leaf constituted 19 to 23% of total biomass, it contributed 32 to 46% of total N harvested. Thus, the proportion of total N in leaf was approximately twice as great as the proportion of total biomass that was leaf. Biomass with a greater concentration of cellulose and less N concentration and content can be harvested in November and December, but late-season single harvests are associated with greater ADL concentration. Leaf removal and return to the field during harvesting may increase the efficiency of N cycling in late season and contribute to production of higher quality biomass for conversion processes in which N is undesirable.