Root hydrocarbons as potential markers for determining species composition

Grasslands can be a complex mixture of plant species. A method is described to allow the identification of both roots and shoots of five different grass species, thus permitting greater knowledge about whole plant allocation and competition in mixed pastures. The five species were Lolium perenne, Festuca ovina, Festuca rubra, Poa trivialis and Agrostis capillaris. N-alkanes with odd-numbers of carbon atoms in the chains predominate in plants and in the five grass species studied, concentrations of alkanes of chain length of C-29, C-31 and C-33 were highest. Average concentrations of C-27-C-33 alkanes in shoots and roots were 187 and 11 mg kg(-1), respectively. This wide range of values required considerable modifications to the method of analysis, including expressing concentrations on an organic matter basis and scaling-down the procedure. The n-alkane concentrations in roots are different from those in shoots and therefore values from shoots cannot be used to predict the composition in roots. Using a canonical variate analysis, all five grass species could be separated using concentrations of C-26, C-31 and C-33 values in the roots. The greatest difference occurred between A. capillaris and the others, whereas discrimination was least between the two Festuca species. Defoliation had contrasting effects on the concentration of a few n-alkanes, but not in the n-alkanes used to discriminate between grass species. Alkane analysis shows great potential as a method to quantify the species composition of the root biomass beneath mixed pasture species.