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

Abstract

Managing the soil profile NO3–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 NO3–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 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−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 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 gamagrass. 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 N in biomass biofuel crops.