Changes in germinable seed levels in soil beneath pastures near Townsville, North Queensland

Abstract

Germinable seed levels in the soil at the start of the growing season were measured in pasture plots near Townsville for 5 years from 1978. The plots were sown to Urochloa and Stylosanthes species and were either grazed by steers throughout the year or during the dry season only. For 1 pasture, additional measurements were made of the distribution of seeds with depth in the soil and of variation throughout the year in seed levels. There were seeds of 88 species in the soil, although many (49%) were present in only small numbers (< 10/m2). The most common were Fimbristylis spp., Stylosanthes hamata, Hedyotis galioides, Echinochloa colona, Sporobolus australasicus, Eragrostis spp., Epaltes australis, and Phyllanthus tenellus. Total seed numbers ranged from 11 000 to 36 000 per m2; the plots grazed throughout the year had more seeds in 1981, but all other year-grazing regime combinations were not significantly different. For individual species there was more variation, and the differences between years were much greater than were those between grazing regimes. There was little or no relationship between seed numbers and pasture composition and yield. More than 90% of the seeds were in the top 5 cm for all plant groups (legumes, grasses, sedges and forbs). Patterns of change during the year in germinable seed numbers were observed in 7 species. Seed numbers were at their maximum in all species at the start of the growing season. In 2 species (S. hamata and E. australis), there were always some seeds present, but, in the other 5 (E. colona, Alternanthera sp., Bacopa floribunda, Fimbristylis spp. and H. galioides), seed was not detected in at least 1 of the years.