Weed seed bank and vegetation at the beginning and end of the first cycle of a 4-course crop rotation with minimal weed control.
The weed vegetation and seed bank were studied at the initiation (1979) and after the first cycle (1983) of a 4-course crop rotation (oats, clover [Trifolium sp.] winter wheat, faba beans) in which no herbicides or inorganic fertilizers were used and high levels of weeds were tolerated except in the seedbed. Greatest abundance of summer annuals was found in spring-sown faba beans and oats. Cover of species of the winter wheat alliance (annuals, biennials and perennials that overwintered and flowered in winter wheat) and of perennial grasses was greatest in winter wheat. After one cycle of the rotation, weed cover in faba beans and winter wheat had increased by >50%. Perennials increased in both crops, as did weeds having life cycles similar to their respective crops. The increase in the seed bank after one cycle of the rotation in three fields ranged from 25 000 to 51 000 seeds/m2. The small-seeded Gnaphalium uliginosum, Plantago major and Juncus spp. increased greatly in both the surface (0-7.6 cm) and subsurface (7.6-15.2 cm) soil layers, but 83-100% of the increase in the seed banks of other species was in the surface soil. Most dominant weeds were well represented in the seed bank, but perennials were not. Many species had large seed banks but were absent or were minor elements in the vegetation.