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. Research QuestionDowny brome is a major constraint to winter wheat production in most production areas of the USA. Wheat production practices such as N fertilization and stubble burning affect downy brome populations in winter wheat. Differences in downy brome populations were observed in a long-term N fertilization and stubble burning experiment and led to a study to determine the effects of inorganic and organic sources of N fertilization, and crop residue burning on downy brome seed in the soil in a dryland winter wheat-fallow crop rotation. Soil samples were taken and downy brome seed content was determined using a sieving-flotation seed extraction technique and counting. The effects of N fertilization rate and source, and crop residue burning on downy brome population dynamics were determined. Literature SummaryPractices that affect the distribution, number, or viability of seeds in the seed-bank have a direct impact on subsequent weed plant populations. For example, the primary tillage method used for seedbed preparation directly affects dawny brome seed distribution in the soil and subsequent plant density. Crop rotation has been shown to affect downy brome populations. Nitrogen fertilizer, if improperly timed, can indirectly increase downy brome populations by increasing downy brome growth and, presumably, seed production. Burning of wheat crop residue after harvest has commonly been used as a method of controlling downy brome. One objective of field burning is to destroy downy brome seed produced in the previous wheat crop. Study DescriptionA 3-yr field study was conducted near Pendleton, OR, to document the effects of repeated applications of inorganic and organic sources of fertilizer N, and repeated burning of crop residues on the downy brome seedbank. Soil samples were taken in the fall after winter wheat harvest and after winter wheat planting in the fallow and cropped portions from 1992 through 1994. From these samples, downy brome seed content was analyzed using a sieving-flotation seed extraction technique. Comparisons were made of downy brome seedbank counts between plots receiving repeated N fertilizer rates, organic amendments, or burning of wheat crop residue. Applied QuestionHow much does N fertilization and stubble burning affect downy brome seed numbers in the soil in a winter wheat-fallow crop rotation? From soil samples taken in a winter wheat-fallow crop rotation near Pendleton, OR, the number of downy brome seed in the soil and subsequent downy brome plant density were substantially higher in unburned plots than in burned plots where N fertilizer was applied. When no N fertilizer was applied, few differences occurred in the downy brome seedbank between burned and unburned treatments. Inorganic N fertilization in the absence of burning resulted in substantially increased downy brome seedbank counts. Organic N sources did not increase downy brome seedbank counts. The source and application rate of N fertilizer had a more pronounced effect on the downy brome seedbank than did burning of crop residue.