Functional groups' performances as influenced by nitrogen, phosphorus and nodule inhibition of legumes

Abstract

Aims Nitrogen (N) and phosphorus (P) constitute essential elements for plant growth and their availability influence species diversity in herbaceous plant communities. Legumes exhibit relatively high abundance in N-limited soils. Moreover, the legumes' N:P ratios are much higher than those of the other plant species grown in the same site, probably because they are able to fix atmospheric N2. The objective of this study was to determine how the relative proportion in N and P availability and the restriction of legumes to fix atmospheric N2 affect: (i) the primary productivity of plant species, (ii) species composition and (iii) N and P concentrations of species. Methods In an outdoor experiment, mixtures containing grasses, legumes and non-legume forbs were established in 32 containers under four soil treatments (control, N addition, P addition and disinfected soil), in a completely randomized design with eight replicates. Plant growth was examined when N and P were limited in the control soil:sand mixture, in a pot experiment sown with Plantago lanceolata. Important Findings The pot experiment indicated that both N and P were limiting for the growth of P. lanceolata. Soil treatments affected primary productivity and species composition. Legumes had a relatively high abundance in the control and their growth was favoured, especially that of Medicago sativa, by P addition. Grasses' growth was increased by the addition of N. Inhibition of rhizobia resulted in poor growth of legumes and concomitant higher growth of grasses, in comparison to the control. The N:P ratios of non-legume species differed between treatments and were always higher in the legume species, even in the disinfected soil. The latter provides evidence that the high N concentrations found in legumes are a physiological characteristic of this specific group of plants.