The growth of external AM fungal mycelium in sand dunes and in experimental systems

We estimated the biomass and growth of arbuscular mycorrhizal (AM) mycelium in sand dunes using signature fatty acids. Mesh bags and tubes, containing initially mycelium-free sand, were buried in the field near the roots of t he dune grass Ammophila arenaria L. AM fungal mycelia were detected at a di stance of about 8.5 cm from the roots after 68 days of growth by use of neu tral lipid fatty acid (NLFA) 16:1 omega5. The average rate of mycelium exte nsion during September and October was estimated as 1.2 mm day(-1). The lip id and fatty acid compositions of AM fungal mycelia of isolates and from sa nd dunes were analysed and showed all to be of a similar composition. Phosp holipid fatty acids (PLFAs) can be used as indicators of microbial biomass. The mycelium of G. intraradices growing in glass beads contained 8.3 nmol PLFAs per mg dry biomass, and about 15% of the PLFAs in G. intraradices, G. claroideum and AM fungal mycelium extracted from sand dunes, consisted of the signature PLFA 16:1 omega5. We thus suggest a conversion factor of 1.2 nmol PLFA 16:1 omega5 per mg dry biomass. Calculations using this conversio n factor indicated up to 34 mug dry AM fungal biomass per g sand in the san d dunes, which was less than one tenth of that found in an experimental sys tem with Glomus spp. growing with cucumber as plant associate in agricultur al soil. The PLFA results from different systems indicated that the biomass of the AM fungi constitutes a considerable part of the total soil microbia l biomass. Calculations based on ATP of AM fungi in an experimental growth system indicated that the biomass of the AM fungi constituted approximately 30% of the total microbial biomass.