Soil Nitrate Following Biofuel Feedstock Grasses and Irrigation in the Pacific Northwest.

Managing the soil profile NO3–N through crop selection and irrigation is an important consideration for the sustainable production of cellulosic biofuel feedstock crops. Two seasons data were collected and analyzed from a four–year study conducted in Prosser, WA, USA to assess the effect of biofuel feedstock grasses and irrigation levels on soil profile NO3–N. The experimental design was a split plot with four replications. The main plots had three irrigation levels (60, 80 and 100 % ET), and the subplots contained three cultivars of switchgrass (Panicum virgatum L.), gamagrass (Tripsacum dactyloides L.) and sorghum (Sorghum bicolor L.) cultivars. Soil and root samples were collected in fall 2011 (season1) and winter of 2012 (Season2). Soil profile NO3–N concentrations were highest at the lowest irrigation levels. In Season1, the soil profile NO3–N concentration averaged over depths was 1.7 mg kg-1for the switchgrass cultivars, 5.6 mg kg-1 for gamagrass, and 13.1 mg kg-1for sorghum. In Season2, the concentrations were 1.4, 2.2 and 11.7 mg kg-1 for the switchgrass cultivars, gamagrass and sorghum, respectively. We found a significant correlation between switchgrass root mass and soil profile NO3–N; r=0.63-Kanlow; r =0.58- Blackwell; and r = 0.46- Shawnee. Our results suggest that soil profile NO3–N was lower under the switchgrass cultivars than either gamagrass or sorghum. More root mass and NO3–N were accumulated at lower irrigation levels. The results reported here can help in developing practical decision tools for managing fertilizer nitrogen in biomass biofuel crops.