Biofuel feedstock production potential in stream buffers of the inland Pacific Northwest: Productivity and management issues with invasive plants

Abstract

Stream buffers increase the complexity of agroecosystems, providing wildlife habitat, sediment and nutrient sinks to improve water quality, and potentially lignocellulosic feedstock for biofuels. The establishment of stream buffers can be difficult, with weeds competing with desired species and increasing weed pressure in the production fields and contaminating feedstocks. Our objective was to evaluate the success of establishing two different feedstock alternatives, alfalfa (ALF; Medicago sativa L., cultivar Ladak) and tall wheatgrass (TWG; Agropyron elongatum Podp., cultivar Alkar), plus a mix of alfalfa and tall wheatgrass (MIX) at two sites within each of three precipitation zones in the Pacific Northwest (PNW). We evaluated productivity, stand establishment, and weed infestation using a split plot, randomized complete block design. Establishment of targeted species was most successful and productivity was highest in the intermediate precipitation zone. Lignocellulosic feedstock produced by TWG (7.52 ± 2.07 Mg ha⁻¹ [3.35 ± 0.92 tn ac⁻¹]) and MIX (7.45 ± 2.00 Mg ha⁻¹ [3.32 ± 0.89 tn ac⁻¹]) produced significantly more lignocellulosic feedstock than ALF (1.68 ± 0.79 Mg ha⁻¹ [0.75 ± 0.35 tn ac⁻¹]) and SG (2.62 ± 0.99 Mg ha⁻¹ [1.17 ± 0.44 tn ac⁻¹]). The best competition against invasive annual species was obtained in the TWG and MIX treatments, with tall wheatgrass providing the bulk of biomass in either treatment. Productivity was acceptable where naturally occurring subirrigation was present and contributed to crop germination and establishment in the low precipitation zone, but weed pressure was substantial and caused near to complete crop failure in the driest plots. A high level of weed management will be required to produce monocultures of lignocellulosic feedstocks in stream buffers.