Conventional food, feed, and fiber agroecosystems are already being converted to biofuel cropping systems, and this is expected to intensify as demand for bioenergy grows. Given their relatively high inputs and complete removal of aboveground biomass, these bioenergy production systems can adversely affect ecosystem services. The objectives of this project were to quantify and compare the effects on biomass production, soil greenhouse gas flux, soil carbon accumulation, and soil nutrient balance among low input (50 kg fertilizer N ha-1 yr-1), low input plus biochar (7 t ha-1 yr-1), and high input (250 kg fertilizer N ha-1 yr-1) napiergrass [Pennisetum purpureum (L.) Schum.] agroecosystems over four years in North Central Florida. Plots were harvested annually in November at the end of the growing season and prior to first frost. After harvest soil samples were collected from each of the plots and analyzed for soil carbon and nutrient status. Greenhouse gas fluxes from the soil were measured in situ during year 4 of the study using an FTIR gas analyzer.
Annual aboveground biomass production ranged from about 20 t ha-1 to over 40 t ha-1. Biochar amendment did not positively affect yield during the course of the study. Despite greater C inputs, biochar amended soils did not show greater soil CO2 flux during year 4 of the study, which was consistent with significantly greater soil C accumulation over time in biochar amended plots. Although, biochar did not increase aboveground biomass yield during the course of the study, it did result in relatively greater soil P (135%), Mg (360%), Ca (100%), B (220%), Zn (3870%), and Mn (230%), and almost a 100% increase in soil CEC. Soil pH, total N (9%), K (50%), S (3%), and Fe (-7%) were relatively unaffected at the end of the study. Taken together these results support the hypothesis that biochar amendment can help to ameliorate the adverse consequences of perennial grass biomass production on ecosystem services. However, since biochar amendment did not contribute to increased yields, adoption of this practice by growers may be limited at present by the economics of biochar use.