Soil drainage and distribution of VAM fungi in two toposequences

Abstract

Much remains unknown about the distribution and abundance of vesicular-arbuscular mycorrhizal (VAM) fungi in natural ecosystems. Our objective was to determine whether soil drainage and position in the toposequence affect VAM fungal spore production and colonization of soybean ( Glycine max L. Merr.) roots. Plant roots and rhizosphere soil from 24 soils, which consisted of sets of three soils each that varied in soil drainage class from four sites located in two toposequences [Clarion-Nicollet-Webster (CNW) and Sharpsburg-Macksburg-Winterset (SMW)], were sampled. Soils were analyzed for chemical properties, and numbers of VAM fungal spores per 50 g soil were determined by a wet-sieving-and-decanting technique. The percentages of root VAM fungal colonization were determined by the grid-line intersect method. The distribution of spores among locations, series and series within locations varied significantly ( P < 0.001). Soil from the CNW toposequence averaged 417 spores (50 g) −1 soil, whereas soil from the SMW toposequence averaged 147 spores (50 g) −1 soil. Within the two toposequences, the poorly to somewhat poorly drained soils had higher spore counts than the well-drained to moderately well-drained soils. In the CNW toposequence, the mean spore counts of Nicollet and Webster soils (poorer drained) were 1.37-fold higher than those of Clarion soils (better drained). In the SMW toposequence, the mean spore counts of Macksburg and Winterset soils (poorer drained) were 2.06 fold higher than those of Sharpsburg soils (better drained). VAM fungal colonization of roots ranged from 74 to 88% in the CNW toposequence and from 68 to 86% in the SMW toposequence. No correlation was found between spore count and VAM fungal colonization.