Impact of a century of wastewater irrigation on the abundance of arbuscular mycorrhizal spores in the soil of the Mezquital Valley of Mexico

Phosphorus (P) and heavy metals introduced into soil by irrigating with sew age effluent can affect the soil's microflora and, in particular, the abund ance of arbuscular mycorrhizal fungi. We have studied the effect in the Mez quital Valley of Mexico, on two kinds of soil Leptosol and Vertisol) after both short-term (5 years) and long-term (90 years) irrigation. We did so by replicated sampling of all four combinations of soil type and length of ir rigation and then estimating the main effects and interactions by analysis of variance. Both kinds of soil appear to have accumulated large quantities of P and hea vy metals as a result of the irrigation so that concentrations have approxi mately doubled and now approach the thresholds for healthy soil. The differ ences between soil types were comparatively small. Phosphorus was the only element for which there was an interaction between soil type and time under irrigation. The Leptosol contained more spores than the Vertisol, irrespec tive of the length of the irrigation, and both types contained significantl y fewer spores after 90 years of irrigation than after only 5 years. The ef fect was more evident in Vertisols. Zn contrast, the sporocarpic morphotype s seemed unaffected by the accumulated phosphorus and metals. The sporocarp s of Glomus mosseae were twice as abundant in the Leptosol as in the Vertis ol, but those of Sclerocystis were equally prevalent in the two types of so il. (C) 2001 Elsevier Science B.V. All rights reserved.