Soil Profile Nitrogen Under Different Biofuel Feedstock Grasses and Irrigation Regimes in the Irrigated Arid Pacific Northwest.

Managing soil profile NO₃-N through crop selection and irrigation is a fundamental requirement for sustainable cellulosic biofuel feedstock crop production.  A four-year study was initiated in 2008 to assess the effect of biofuel feedstock grasses on soil profile NO₃. The experimental design was a split plot with four blocks. The main plots were  irrigation levels (60%, 80%, and 100% ET). The subplots constituted three species of cellulosic biofuel feedstock crops and soil sampling depths.  Kanlow, Shawnee, and Blackwell switchgrass varieties (Panicum virgatum) , Sugar T sorghum (Sorghum bicolor), and Pete gamagrass (Tripsacum dactyloides) were evaluated.  Soil and root samples were collected from 0-105 cm depth at 15-cm intervals following harvest in October 2011.  The soil and root mixture was dried in a forced air oven at 60⁰C for 7-days and separated mechanically using a 1-mm sieve. The soil samples were grounded and used to determine soil NO₃ levels. The root samples were washed mechanically, sieved using a 0.125-mm sieve, dried in a forced air oven at 60⁰C for 7-days, and weighed.  Soil profile NO₃ was 1.7 mg/kg in switchgrass, 5.6 mg/kg in gamagrass, and 13.1 mg/kg in sorghum averaged over depths.   Sorghum exhibited increased soil profile NO₃ levels at 0-60 cm depth and decreased as depth increases. Nitrate accumulated in the soil profile of switchgrass was significantly lower than the other species across all depths . Sorghum exhibited higher soil-NO₃ at different depths than switchgrass or Pete gamagrass. The switchgrass cultivars had the lowest soil profile NO₃ across all soil depths . Similarly, the correlation between root biomass and soil profile NO₃ for switchgrass (r=0.63, p<0.001 for Kanlow; r=0.58, p<0.001 for Blackwell; and r=0.46, p<0.05 for Shawnee) was significant, unlike sorghum (r= -0.05, p>0.1).  This suggests that the switchgrass cultivars were superior in scavenging nitrogen when compared with either gamagrass or sorghum.