The Effect of Nitrogen, Phosphorus, and Potassium Fertilizers on Prairie Biomass Yield, Ethanol Yield, and Nutrient Harvest

Native prairie plants can be managed to provide biomass for cellulosic ethanol production; however, there is inadequate information in northern latitudes regarding the effects of fertilizers on biomass and ethanol yields. We evaluated biomass yield, land ethanol yield (theoretical ethanol production per unit area), and nutrient harvest in grasslands managed across a gradient of nitrogen (N), phosphorus (P), and potassium (K) fertilizers at three locations in MN, USA, from 2008 to 2009. The Austin and Lamberton locations were planted with a mixture of prairie plants, while the Rosemount location was solely switchgrass (Panicum virgatum L.). Model-based estimations of agronomically optimum nitrogen rates (AONRs) for land ethanol yield were determined for five of six site-year environments. Five response functions were modeled for land ethanol yield, each predicting a unique AONR with varying degrees of confidence. The linear plateau function was best-supported for four of six environments. Agronomically optimum nitrogen rates ranged from 61 to 87 kg N ha⁻¹ and, on average, yielded 3,160, 2,090, and 3,180 L ethanol ha⁻¹ at Austin, Lamberton, and Rosemount, respectively. On average, predicted ethanol yields increased 52 % when fertilized at AONRs compared to yields without fertilizer. Phosphorus and K fertilizers did not affect land ethanol yield. Nitrogen, P, and K removed during biomass harvest increased with N fertilization and averaged 31, 6, and 20 kg ha⁻¹ at the AONRs. Nitrogen use efficiency declined with N fertilization during drier years. Modest rates of N fertilizer (between 60 and 90 kg N ha⁻¹) can maximize cellulosic ethanol production in established northern latitude grasslands. Soil P and K should be monitored as nutrients are removed during repeated biomass harvests.