Impact of nitrogen fertilization and stubble burning on the downy brome seedbank in a winter wheat fallow rotation

A study was conducted to evaluate the effects of N fertilization, wheat (Triticum aestivum L.) stubble burning, and organic amendments on the density of downy brome (Bromus tectorum L.) seed in soil of a winter wheat-fallow crop rotation. The site was part of a long-term study that has been conducted since 1931 near Pendleton, OR. The study consists of a winter wheat-fallow crop rotation with both a fallow and cropped portion of each treatment present each year. The objective of the original long-term study was to document changes over time in soil physical and chemical properties due to the addition of inorganic and organic sources of N fertilizer, and repeated burning of crop residues. A weed seedbank component was added to this study in 1992. To measure effects on the weed seedbank, soil samples were taken in the fall after winter wheat harvest and after winter wheat planting in the fallow and cropped portions, respectively, from 1992 through 1994. From these samples downy brome seed content was analyzed using a sieving-flotation seed extraction technique. Long-term use of inorganic N fertilizer increased the downy brome seedbank compared with an unfertilized control. Crop residue burning reduced the downy brome seedbank and downy brome plants in the wheat crop. Addition of steer manure or green pea [Pisum sativum (L.) subsp. sativum] vines as an N source did not increase downy brome in the seedbank compared with use of inorganic N fertilizer. This was, perhaps, due to factors such as improved soil health resulting from use of organic soil amendments, or differences in application timing or source of inorganic and organic N fertilizer The organic form of N from manure may have been applied at a more opportune time for winter wheat than for downy brome, In addition, plots receiving steer manure produced a higher yielding wheat crop that was likely to be more competitive against downy brome, thereby limiting seed production.