Mycorrhizal inoculum potential of arbuscular mycorrhizal fungi in soils irrigated with wastewater for various lengths of time, as affected by heavy metals and available P

Sewage water is widely used for irrigation in dry countries, but the practice can lead to the accumulation of heavy metals in the soil and consequent poisoning of the soil's microorganisms. The area irrigated with sewage water in the Mezquital Valley in central Mexico has steadily expanded during the last 100 years, and it provides the opportunity to assess the effects of the practice on the mycorrhiza in particular.We sampled the topsoil of the two main kinds of soil, Vertisol and Leptosol, in fields irrigated for 5, 35, 65 and 95 years in the Valley. We measured the concentrations of zinc, lead, copper and cadmium, all of which appeared to have increased linearly with time. We also determined the abundances of arbuscular mycorrhizal morphotypes in the soil both at the time of sampling and after incubation in association with Allium cepa L. in the greenhouse.Spores decreased in abundance with increasing duration of irrigation in the Vertisol, whereas in the Leptosol the numbers after only 5 years of irrigation were small, increased after 35 years of irrigation and decreased again thereafter. In the greenhouse the production of spores and sporocarps was maximal in soil irrigated for between 35 and 65 years, as were the intraradical hyphae, spores and vesicles. Glomus species dominate the morphotypes, and the spores of Glomus mosseae were the most abundant in both soils and after all times.The total root colonization potential seemed unaffected by duration of irrigation, and was substantially greater in the Leptosol than in the Vertisol. However, species of the Glomus genus tend to dominate over other genera as irrigation proceeds, since Glomus species spread not only by spores, but also by roots. We conclude that irrigation with sewage water in the Mezquital Valley is decreasing the mycorrhizas’ diversity in the long term.