Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 4, 2006
Publication Date: March 6, 2007
Citation: Davis, A.S. 2007. Nitrogen fertilizer and crop residue effects on seed mortality of eight annual weed species. Weed Science. 55(2):123-128. Interpretive Summary: Weed seeds can persist in the soil seedbank for years, contributing to ongoing weed infestations of crop fields. Soil carbon and nitrogen balance affects the decay of many types of organic residues in soil, and may also influence weed seed decay. In this study, soil carbon and nitrogen were manipulated to determine their impact on seed mortality of eight species of annual weeds common to the North Central region of the U.S. Three out of the eight species, velvetleaf, giant ragweed and wooly cupgrass, showed a response in seed decay to soil treatments. Increasing levels of soil nitrogen led to a direct increase in velvetleaf seed mortality. Changes in giant ragweed and wooly cupgrass seed mortality due to soil fertility treatments were mediated through changes in germination, rather than direct effects on seed mortality. Effects of soil fertility on weed seed fate appear to be species-specific. Differences in seed quality among weed species will need to be better understood in order to explain why some seeds survive longer in soil than others.
Technical Abstract: Weed seed persistence in the soil seedbank plays a central role in weed population dynamics, yet limited knowledge of mechanisms regulating weed seed survival in soil remains an obstacle to developing weed seedbank management practices. Weed seeds are rich in carbon and nitrogen, and therefore may represent an important nutritional resource to soil microbes. The objective of this study was to test the hypothesis that weed seed mortality due to microbial attack is limited by soil inorganic N availability and soil C:N ratio. In controlled environment bioassays, a factorial of N fertilizer rate (0, 88 and 150 mg N kg soil-1) and corn stover addition rate (0 and 3000 mg stover kg soil-1) was applied to experimental units containing Illinois field soil (silt loam, 3.8% soil organic matter) and seeds of one of eight annual weed species common to Illinois field crops: giant foxtail, green foxtail, yellow foxtail, wooly cupgrass, giant ragweed, redroot pigweed, velvetleaf and Venice mallow. Seeds were incubated for two months, after which they were recovered from the soil and tested for viability. Only three of the eight species, velvetleaf, giant ragweed and wooly cupgrass, responded to the experimental treatments. Velvetleaf seed mortality was 40% lower in the corn-stover amended treatment than in the unamended treatment. Both giant ragweed and wooly cupgrass showed a more complex interaction between the N fertilizer and corn stover treatments. Path analysis offered support for the hypothesis that the influence of soil N on seed mortality in velvetleaf was due to the direct effect of soil N on microbial attack of velvetleaf seeds, whereas for giant ragweed and wooly cupgrass, the effect on seed mortality appeared to be mediated through soil N effects on germination. Mechanisms underlying soil N fertility effects on weed seed mortality appear to be species-specific. Future investigations of this phenomenon should include quantitative measures of seed coat composition and quality.