Interference effects of the invasive plantCarduus nutans L. against the nitrogen fixation ability ofTrifolium repens L.

Abstract

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.

Biomassa microbiana e matéria orgânica em solo de Caatinga, cultivado com melão na Chapada do Apodi, Ceará

“…Trabalhos têm demonstrado que os quocientes microbianos (qMic) e metabóli-cos (qCO 2 ) são sensíveis aos efeitos ambientais e antropogênicos sobre a comunidade microbiana do solo, podendo-se constituir indicadores das perturbações dos ecossistemas (Andréa et al, 2004;Harris, 2003). Wardle et al (1994) afirma que os baixos teores de qMIC podem ocorrer por estar a microbiota sob algum fator de estresse, ou, como propõem Gama-Rodrigues et al (2008), por causa da baixa qualidade nutricional da matéria orgâ-nica, fazendo com o que a biomassa microbiana torne-se incapaz de utilizar totalmente o carbono orgânico total. …”

“…O quociente microbiano (qMIC) foi calculado pela razão entre o carbono da biomassa microbiana e o carbono orgânico do solo, afim de refletir os aportes de carbono e a conversão de substratos orgânicos para o carbono da biomassa microbiana (Sparling, 1997). Para Wardle et al (1994), este quociente pode avaliar a qualidade da matéria orgânica, enquanto, para Ferreira (2008), trata-se de um indicador da sua dinâmica, expressando a eficiên-cia da biomassa em utilizar C e N.…”

The Role of Plants in the Effects of Global Change on Nutrient Availability and Stoichiometry in the Plant-Soil System

“…In fact, allelopathic substances in the litter of alien plants can inhibit growth (Hata et al, 2010;Cipollini et al, 2012;Rashid and Reshi, 2012), mycorrhization and the capacities of nutrient uptake (Zhang et al, 2007), and N 2 fixation in native plants (Wardle et al, 1994) and change soil nutrient decomposition rates (Chen et al, 2007a), frequently stimulating soil nutrient cycling (Chen et al, 2007b). Allelopathic compounds in the litter of native plants, however, can also inhibit the growth of alien plants (Hou et al, 2012).…”

A Novel Approach to Grass–Legume Management

“…White clover is vulnerable to both root and shoot competition and seedlings even compete with each other and can be more susceptible to this kind of competition than with some weed seedlings (Wardle and Nicholson, 1994 (Sturite et al, 2007). About 60% of the leaves produced had turned over by the end of the growing season and another 30% had died or disappeared by subsequent spring (Sturite et al, 2007).…”