INTERFERENCE EFFECTS OF THE INVASIVE PLANT CARDUUS-NUTANS L AGAINST THE NITROGEN-FIXATION ABILITY OF TRIFOLIUM-REPENS L

Carduus nutans L. is an invasive pasture/grassland species which may u ndergo 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 d ie and decompose. We investigated the influence of C. nutans on the ni trogen-fixation ability of Trifolium repens L. in three experiments. T he first experiment was set up in a ''mixture'' design, and demonstrat ed that seedlings of T. repens were more susceptible to competition wi th other T. repens seedlings than to C. nutans seedlings. Nodule numbe rs and acetylene reduction per unit root, and acetylene reduction per unit nodules were adversely affected by increasing T. repens, but not C. nutans densities. The second experiment was of an additive design, with separate partitions to isolate above-ground and belowground inter ference. Flowering C. nutans plants strongly inhibited T. repens root growth, nodulation and acetylene reduction, but usually only when shoo t interference was permitted. This appears to be due to decomposition of rosette leaves, which was maximal at this stage. The third experime nt involved monitoring effects of tagged C. nutans individuals against T. repens in the field. This experiment showed that acetylene reducti on 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 combinati on suggest that decomposing rosette leaves have a strong potential to inhibit T. repens nitrogen fixation. It appears that allelopathy is in volved, since alternative explanations (e.g. root competition by C. nu tans; effects of C. nutans on soil moisture, microbial nutrient immobi lisation and light availability; facilitation of herbivores by C. nuta ns) 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.