The negative effects of litter of parent plants of Cirsium vulgare on their offspring: autotoxicity or immobilization?

It is shown that litter of flowering plants of Cirsium vulgare inhibits the growth of seedlings of the same species at concentrations as low as 0.4% (g litter/g soil). The inhibition of growth cannot be fully compensated by adding nutrients, which indicates that autotoxicity may occur. It must be concluded, however, from an analysis of the results that immobilization of nutrients by microorganisms is the main cause of growth reduction if litter is added to the soil. In a field experiment under nutrient rich conditions plant litter did not affect germination, survival or growth of seedlings. The hypothesis is put forward that under conditions of nutrient deficiency and at high population densities, immobilization of nitrogen by Cirsium vulgare litter may influence plant growth. Results are discussed in relation to those of Stachon and Zimdahl (1980) and Wilson (1981) who claimed allelopathy for the related Cirsium arvense.