Effect of tillage, fungicide seed treatment, and soil fumigation on seed bank dynamics of wild oat (Avena fatua)

Abstract

No-tillage offers potential for improved soil quality, reduced erosion, and equal or increased crop yields. We hypothesized that, compared with conservation tillage (CT), no-tillage (NT) offers conditions more conducive to microbial decay of weed seed. In NT systems seed remain at or near the soil surface where crop residues, moisture, and lack of disturbance create an environment with greater soil microbial diversity. In late fall of 1998 and 1999, dormant seed of wild oat, either individually glued to plastic toothpicks or mixed with soil and placed in mesh bags, were buried (mean seed depth of 2.5 cm) in replicated field plots managed by NT or CT since 1982. Treatments including fungicide seed treatment (thiram + metalaxyl + captan) and soil fumigation (propylene oxide) provided estimates of the contribution of microorganisms to observed mortality. Seed were retrieved in May and August, 1999 and 2000. Contrary to our original hypothesis, the proportion of dead seed was generally similar in NT and CT systems. Lack of tillage system by seed or soil treatments affecting the proportion of dead or decayed seed suggests that the contribution of microorganisms to seed fate is similar in these tillage environments. However, the proportion of dormant seed was consistently lower in the NT compared with CT treatments; there was a corresponding increase in the proportion of germinated seed. Overall, more than half of the wild oat seed bank losses could be directly attributed to germination whereas losses due to decay were relatively minor by comparison. Despite favorable distribution of seed and improved quality of the surface-strata of soil in NT systems, this study fails to provide evidence that enhanced microbial decay will contribute to a “weed-suppressive” capacity in such cropping systems.