Dynamics of mycorrhizal association in corn (Zea mays L.) : influence of tillage and manure

Mycorrhizal fungi are a major component of agricultural systems and play a key role in plant nutrition. Little is known about the effects of tillage practices and manuring on arbuscular mycorrhizal fungi (AMF). The purpose of this study was to evaluate the effect of soil disturbance on winter survival, development and distribution of AMF in soil and on plant nutrient uptake and productivity. This research was conducted in long-term corn plots in two soils and under controlled conditions.A growth chamber study with field soil demonstrated that most of the fungal hyphae with mycorrhizal plants were mycorrhizal rather than saprophytic. This result was extrapolated to subsequent experiments. Soil disturbance reduced corn nutrient uptake and growth by disrupting the AMF hyphal network. Similarly, fallow periods reduced density of AMF hyphae, leading to reduced mineral nutrients uptake and plant growth. Soil disturbance was also found to severely reduce winter survival of AMF hyphae in agricultural soil. AM hyphae could survive the winter in soil, even when they were not attached to roots. Their survival however, was improved when they remained attached to roots.Under field conditions, indigenous AMF were more abundant in no-till soil, less abundant under reduced tillage and least abundant under conventional tillage. Under all tillage systems, most of AMF hyphae were located in the top 15 cm of the soil profile suggesting that deep plowing could result in dilution of AMF propagules in the seeding zone. There was a seasonal variation in the abundance of hyphae in soil. Soil hyphae and root colonization declined after the silking stage of corn. Hyphal abundance decreased further over the winter, to reach their lowest level in the spring.The spatial distribution of fungal hyphae in the field was not homogenous. Hyphal density was maximal directly under the corn rows and decreased linearly up to the mid-row. Marked seasonal variations in hyphal densities were observed on the row but fluctuations at mid-row were not significant suggesting that little AMF hyphae were ever present between the rows. Liquid dairy manure had little effect on the abundance of hyphae and spores