Carduus nutans L. is an invasive pasture/grassland species which may undergo rapid population growth through positive feedback. Plants of C. nutans produce a vegetative rosette, and after several months produce stems containing flower-heads, during which time the rosette leaves die and decompose. We investigated the influence of C. nutans on the nitrogen-fixation ability of Trifolium repens L. in three experiments. The first experiment was set up in a "mixture" design, and demonstrated that seedlings of T. repens were more susceptible to competition with other T. repens seedlings than to C. nutans seedlings. Nodule numbers and acetylene reduction per unit root, and acetylene reduction per unit nodules were adversely affected by increasing T. repens, but not C. nutansdensities. The second experiment was of an additive design, with separate partitions to isolate above-ground and belowground interference. Flowering C. nutans plants strongly inhibited T. repens root growth, nodulation and acetylene reduction, but usually only when shoot interference was permitted. This appears to be due to decomposition of rosette leaves, which was maximal at this stage. The third experiment involved monitoring effects of tagged C. nutans individuals against T. repens in the field. This experiment showed that acetylene reduction was severely influenced by flowering C. nutans (when rosette leaves were decomposing), even when only mild reduction of T. repens growth was observed, and these effects persisted for some months after the C. nutans plants had died. The results of these experiments in combination suggest that decomposing rosette leaves have a strong potential to inhibit T. repens nitrogen fixation. It appears that allelopathy is involved, since alternative explanations (e.g. root competition by C. nutans; effects of C. nutans on soil moisture, microbial nutrient immobilisation and light availability; facilitation of herbivores by C. nutans) can be effectively discounted. Although invasive species are often assumed to be associated with soil nitrogen build-up, we believe that some invasive species such as C. nutans have the potential to induce long-term decline of soil nitrogen input.

No abstract

The influence of osmotic effects of aqueous plant extracts on the results of ailelopathy bioassays was investigated for four pasture grass species. It was found that allelopathic effects were substantially overestimated when osmotic effects were not taken into account. It is concluded that bioassays using aqueous plant extracts are more realistic when osmotically adjusted control values (with the same osmotic potential as the plant extract being tested) are used.

The allelopathic effects of vegetative and flowering plants of the annual/biennial pasture weed Seneciojacobaea L. (ragwort) against Lolium perenne L. (perennial ryegrass) and four pasture legumes were investigated using a range of bioassays. Bioassays based on shoot and root leachates demonstrated detectable, although usually slight, allelopathic effects, and these did not usually differ between the two developmental stages of S. jacobaea. However, aqueous extract and tissue decomposition bioassays demonstrated stronger allelopathic effects, particularly for flowering plants, and this was in general agreement with toxicity assessments of soil collected from under S. jacobaea plants in the field. According to our study, flowering plants have the potential to weaken pasture through allelopathy, and decomposition of above-ground litter appears as the most likely mechanism facilitating this. The aqueous extract and tissue decomposition bioassays also revealed that L. perenne was less susceptible to S. jacobaea allelopathy than were the legumes, suggesting that encouraging a strong L. perenne component in pastures has potential for reducing the overall inhibitory effects of S. jacobaea on pasture production.

No abstract