Nitrous Oxide Emissions from Fertilized Fields of Biofuel Crops: Intercropping Switchgrass and Alfalfa Reduces Emissions.

Biofuel crops are proposed as an alternative to fossil fuels. Nitrogen fertilizer is the highest energy input for biofuel crop production and is applied in excess to enhance crop yields. Excess nitrogen application leads to increased emissions of greenhouse gas nitrous oxide (N₂O). Our primary research objective was to quantify the effects of plant species, soil type and nitrogen source on N₂O emissions. Our studies focused on two experimental plots of irrigated switchgrass, a potential biofuels feedstock, in Eastern Washington at two sites, near Prosser and Paterson. Experimental treatments included varying rates of chemical fertilizer and municipal biosolids application, and delivery of nitrogen by N-fixation by intercropping switchgrass with alfalfa. Gas samples were collected from static chambers immediately after irrigation at monthly intervals and the flux of N₂O was calculated using Fick’s law. A greater N₂O flux was measured at the Paterson site (sandy, pH ~6) than at the Prosser site (silt loam, pH ~8), suggesting an effect of soil texture and/or pH on N₂O emissions. Fluxes were elevated immediately after fertilization and irrigation: 12.5 g N₂O-N /ha/d at Prosser and 15 g N₂O-N /ha/d at Paterson. Unfertilized plots had the lowest emissions: 1.2 g N₂O-N at Prosser and 0.4 g N₂O-N at Paterson. N₂O emissions were 70% higher in plots treated with biosolids (25 g N₂O-N /ha/d) compared to plots treated with inorganic fertilizer.  Intercropping with alfalfa (at Prosser) resulted in a maximum flux of 4 g N₂O-N /ha/d, similar to the emissions from the unfertilized control plot.  Our results clearly demonstrated that soil type, moisture content, and the form and rate of nitrogen application, together greatly influence N₂O emission rates.