Vertical movement of weed seed surrogates by tillage implements and natural processes

Vertical position of weed seeds in the soil column is one of the critical factors governing the density of emerged seedlings, but data on movement of subsurface seeds and seed surrogates by tillage are limited. In this experiment ceramic beads were seeded at 0, 4, 8, 12, 16 and 20 cm using specially constructed equipment. Plots were then tilled at right angles to the bead slots with either a moldboard plow, a chisel plow with curved blades, a chisel plow with straight blades, heavy tandem disks, a rotary tiller, or left untilled. Soil was sampled to determine bead positions. The probability matrix describing movement of beads from each soil layer to any other was estimated for each tillage regimen by maximum likelihood. Movement patterns for moldboard plow, the two chisel plow treatments, and disk and rotary tillage differed significantly. The difference between disking and rotary tillage was only marginally significant (0.05 < P < 0.10), as was the difference between the two chisel treatments. The chisel, disk and rotary tillage treatments all buried at least 70% of surface beads to below 2 cm but they showed little vertical displacement of beads below 10 cm. For example, beads below 14 cm in the chisel treatments had a >80% probability of remaining in at their original depth. Tillage by moldboard plow showed the expected soil inversion, but burial of surface beads was greater than return of buried beads to the surface. For example, the probability of a surface bead ending up below 10 cm was over 77% whereas the probability of a bead originally in the 14–18 cm layer moving to the top 10 cm was only 44%. Sampling of the no tillage and moldboard plow treatments the following spring showed an upward movement of beads by natural causes for beads below 14 cm. To allow flexibility in vertically structured population models, the movement probability matrices were fitted with a continuous model based on the beta distribution. The model showed that for a hypothetical uniformly distributed seed bank, 97% of seeds in the top 4 cm following moldboard plowing had arrived there from greater depths. In contrast, only about one third of seeds in the top 4 cm of soil after tillage by chisel, disk or rotary tiller arrived from deeper layers.