VESICULAR-ARBUSCULAR MYCORRHIZA IN FIELD-GROWN CROPS: III. MYCORRHIZAL INFECTION AND RATES OF PHOSPHORUS INFLOW IN...

The importance of vesicular-arbuscular mycorrhiza (VAM) and P fertilizer for P nutrition and dry matter production in field peas (Pisum sativum L.) was studied in moderately P-deficient soil. Half of the experimental plots were fumigated to reduce the level of VAM infection. Shoots and 0 to 30 cm soil cores were sampled on three occasions. An extensive VAM infection was rapidly established in untreated soil, whereas infection levels were low in fumigated soil. Root growth responded to fumigation by increased root length and decreased root diameter. Fumigation reduced the P content in shoots considerably, and correspondingly the mean rates of P inflow per unit root length were 60% lower in fumigated than in untreated soil during flowering. These effects of fumigation were ascribed to the low levels of VAM infection in fumigated soil. The production of dry matter was not decreased accordingly in fumigated plots, although both it and P uptake were increased by adding P fertilizer. The possible reasons for this discrepancy are discussed. A supplementary survey on infection development at five other field sites showed that peas are extensively colonized by VAM fungi, even in soils where a standard procedure is being used. These results indicate that VAM is of major importance to P uptake by the field-grown pea.