Floristic diversity, size, and vertical distribution of the weed seedbank in ridge and conventional tillage systems

The Aoristic diversity and the vertical distribution of the weed seedbank w ere studied in ridge tillage (RT) and conventional tillage (CT) systems in clay and clay loam soils. Viable seedbank populations were monitored during 3 yr using germination in a greenhouse. Ridge-tilled fields had a larger s oil seedbank (2,992 seeds m(-2)) than moldboard-plowed fields (1,481 seeds m(-2)) in the top 15 cm. This result can be explained by the larger perenni al seedbank of RT fields at both the 0- to 5-cm and 5- to 15-cm depths. Ann ual dicot seeds were more abundant in clay soils than in clay loams at the two soil depths. Annual grass seeds were more abundant under CT than under RT in clay soils ar the two sampled depths. In clay loams, the density of a nnual grass seeds in RT fields was six times greater than in CT fields in t he top 5 cm of soil and two times greater at the 5- to 15-cm depth. The ver tical distribution of total seeds in soil did not differ between tillage sy stems. The cop 5 cm of the 15-cm soil core contained 35 and 46% of all weed seed in CT and RT systems, respectively. However, the CT system had the hi ghest concentration of annual dicot seeds 5 to 15 cm deep, whereas in the R T system, the same depth contained the highest concentration of perennial s eeds. These results confirm that tillage systems and soil types can regulat e seedbanks. Weed management programs must take this information into accou nt.