Distribution of mycorrhizal fungal spores in soils under agroforestry and monocultural coffee systems in Brazil

Summary

Deep-rooting trees in agroforestry systems may promote distribution of spores of arbuscular mycorrhizal fungi (AMF) at deeper soil levels. We investigated the vertical distribution of AMF spores in Oxisols under agroforestry and monocultural (unshaded) coffee systems in on-farm experiments ( Brazil). The number of AMF spores was considered as an indicator of mycorrhiza incidence in soil. Spores were extracted from 0-1, 2-3, 5-7.5, 10-15, 20-30, 40-60 cm soil-depths in agroforestry and monocultural coffee systems, of three different age groups (young, medium-aged and old), using centrifugation methods, and counted. Fine roots were collected and dry-weighed from 0-30 cm in young and old systems and from several depths in medium-aged systems. Soils were characterised with respect to texture, pH, organic matter, calcium, magnesium, phosphorus and potassium. Agroforestry had a higher percentage of spores (12-21% of the total number of spores) and roots (on average 1.5 g L-1 soil) in deeper layers (20-60 cm), and a lower percentage (79-88%) closer to the surface (0-15 cm) than the monocultural fields (respectively 3-12%, 0.6 g L-1 soil and 88-97%). Greater numbers of spores in the deeper soil layers may be explained by greater amounts of roots and may be an indicator of greater incidence of mycorrhiza in agroforestry than in monocultural coffee systems. Greater mycorrhizal incidence at deeper soil layers in the agroforestry system may change the dynamics of phosphorus cycling in soil, making this nutrient more available to plants.