Seed reserves in temperate Australian rice fields following pasture rotation and continuous cropping.
The size and composition of the soil seed banks of 9 rice fields in the Coleambally irrigation area of New South Wales are described. Continuously cropped fields were compared with pasture rotation sites. Flooded and non-flooded soil treatments were used to estimate seed populations and to determine preferred germination conditions. The majority of spp. germinated most prolifically in moist, drained soil. However, the 2 most abundant spp., Elatine gratioloides and Cyperus difformis, germinated best in flooded conditions. 42 spp. were identified, with E. gratioloides and C. difformis together accounting for 68-99% of the total. Total seed populations ranged from 17 300-646 000 seeds/m2 with a max. density of 334 000 E. gratioloides seeds/m2 at one site. A close relationship was found between the numerical importance of seed bank spp. and their occurrence in the rice weed flora. The 3 most abundant seed bank spp., E. gratioloides, C. difformis and Echinochloa crus-galli, were, with Damasonium minus, the 4 most commonly recorded weeds of rice in the region. The largest total viable seed population occurred at the site with the longest history of continuous rice cropping, and the lowest seed densities were recorded at sites following a pasture phase. These patterns were not consistent, however, suggesting that factors other than rotation are important in influencing seed bank size.