Mycorrhiza and phosphate interactions as affecting plant development dinitrogen fixation nitrogen transfer and nitrogen uptake from soil in legume grass mixtures by using a nitrogen 15 dilution technique
Barea J.M.; El Atrach F.; Azcon R.
Soil Biology and Biochemistry 21(4): 581-590
ISSN/ISBN: 0038-0717 Accession: 007579890
The legume Medicago sativa (alfalfa)(+ Rhizobium meliloti) and Lolium perenne (ryegrass) were grown, in a greenhouse, either alone or together in a soil and supplied with increasing amounts of soluble phosphate (P) with or without a vesicular-arbuscular inoculum (VAM). A small amount of 15N-labelled ammonium sulphate was added to each pot to distinguish the sources of N in the plants. The more mycotrophic legume enhance VAM formation by the grass in the mixture at all rates of P additions. Regardless of the cropping system and the P concentration in soil VAM improved, in most cases, dry matter production and the competitive ability to the legume. In spite of that competition from ryegrass reduced alfalfa development with increasing P concentration in the soil. In general VAM increased nodulation and the concentrations of N and P in alfalfa. The 15N enrichment of plant shoots indicated that VAM improved N2-fixation in alfalfa at all rates of P. In mixed cropping, alfalfa derived almost all its N from fixation, but the total amount fixed was decreased by competition from ryegrass in the same pot. The apparent soil N pool size (A-value) for the grass growing alone was significantly higher in mycorrhizal pots and VAM actually increased the total amount of N that the grass derived from soil, supporting a role of VAM in N-uptake. In mixed cropping the various interactions acting on N nutrition of the grass probably mask observations of the actual mechanisms involved, but there was clear isotopic evidence of N-transfer from the legume to the grass in non-mycorrhizal, P-supplemented plants. Apparently VAM enhanced N-transfer in one out of four cases.