Soil Nitrate following Biofuel Feedstock Grasses and Irrigation in the Pacific Northwest

Managing the soil profile NO 3 –N through crop selection and irrigation is an important consideration for the sustainable production of cellulosic biofuel feedstock crops. Data from two seasons were collected and analyzed from a 4–yr study conducted in Prosser, WA, to assess the effect of biofuel feedstock grasses and irrigation levels on soil profile NO 3 –N. The experimental design was a split plot with three replications. The main plots had three irrigation levels (60, 80, and 100% evapotranspiration, ET), and the subplots contained three cultivars of switchgrass ( Panicum virgatum L.) and gamagrass ( Tripsacum dactyloides L.) cultivars. Soil and root samples were collected in fall 2011 (Season1) and winter of 2012 (Season2). Soil profile NO 3 –N concentrations were highest at the lowest irrigation levels. In Season1, the soil profile NO 3 –N concentration averaged over depths was 1.7 mg kg −1 for the switchgrass cultivars and 5.6 mg kg −1 for gamagrass. In Season2, the concentrations were 1.4 and 2.2 mg kg −1 for the switchgrass cultivars and gamagrass, respectively. We found a significant correlation between switchgrass root mass and soil profile NO 3 –N; r = 0.63‐Kanlow; r = 0.58‐ Blackwell; and r = 0.46‐ Shawnee. Our results suggest that soil profile NO 3 –N was lower under the switchgrass cultivars than gamagrass. More root mass and NO 3 –N were accumulated at lower irrigation levels. The results reported here can help in developing practical decision tools for managing fertilizer N in biomass biofuel crops.