Produção de micélio externo por fungos micorrízicos arbusculares e crescimento da soja em função de doses de fósforo

The objective of this work was to study the association between soybean (Glycine max L. Merr. cv. IAC 8) and two different arbuscular-mycorrhizal (AM) fungi (Gigaspora margarita or Glomus intraradices), grown under increasing rates of phosphorus (0, 25, 50, 100 and 200 mg kg1) and harvested at four times (15, 30, 60 and 90 days). The assessed variables were: shoot dry weight, pod dry weight, root colonization by AM fungi, the concentrations of N, P, K, S, Ca, Mg, Fe, Cu, Mn, and Zn in shoots and pods, estimation of the length of total external mycelium (TEM) and the length of active external mycelium (AEM). There was a temporal growth reduction in the soybean-G. margarita association at 60 days, and shoot dry weight increase due to AM fungus colonization was more evident at 90 days. Increasing P rates decreased root colonization in both fungi, likewise with TEM and AEM. Colonization by G. margarita was less efficient in a soybean symbiosis because it grew slower than G. intraradices and presented less AEM. This may have been the reason for growth reduction in the soybean-G. margarita association at 60 days. Length of AEM in G. intraradices inoculated soils increased with time and decreased with increasing P rates. Changes in AEM were less evident for the soybean-G. margarita. Iron and manganese concentrations in pods at 90 days were lower in plants colonized by AM fungi at low P rates, where colonization levels were high.