The floristic diversity and the vertical distribution of the weed seedbank were 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 soil 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 perennial seedbank of RT fields at both the 0- to 5-cm and 5- to 15-cm depths. Annual 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 at the two sampled depths. In clay loams, the density of annual grass seeds in RT fields was six times greater than in CT fields in the top 5 cm of soil and two times greater at the 5- to 15-cm depth. The vertical distribution of total seeds in soil did not differ between tillage systems. The top 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 highest concentration of annual dicot seeds 5 to 15 cm deep, whereas in the RT system, the same depth contained the highest concentration of perennial seeds. These results confirm that tillage systems and soil types can regulate seedbanks. Weed management programs must take this information into account.
Nomenclature:Dicamba; SAN 582, 2-chloro-N-[(1-methyl-2-methoxy)ethyl]-N-(2,4-dimethyl-thien-3-yl)-acetamide; glyphosate; Abutilon theophrasti Medik ABUTH, velvetleaf; Ambrosia artemisiifolia L. AMBEL, common ragweed; Chenopodium album L. CHEAL, common lambsquarters; Echinochloa crus-galli (L.) Beauv. ECHCG, barnyard grass; Oxalis stricta L. OXAST, yellow woodsorrel; Panicum dichotomiflorum (L.) Michx. var. geniculatum (Wood) Fern. PANDI, fall panicum; Plantago major L. PLAMA, broad-leaved plantain; Setaria faberi Herrm. SETFA, giant foxtail; Setaria pumila (Poir) Roem et Schult SETLU, yellow foxtail; Setaria viridis (L.) Beauv. SETVI, green foxtail; Taraxacum officinale Weber in Wiggers TAROF, dandelion; Brassica napus L., oilseed rape; Glycine max (L.) Merr., soybean; Triticum aestivum L., wheat; Zea mays L., corn.