EFFECTS OF PHOSPHORUS FERTILIZATION AND MYCORRHIZAL DEVELOPMENT ON PHOSPHORUS-NUTRITION AND CARBON BALANCE OF LOBLOLLY-PINE

The influence of ectomycorrhizal (ECM) development on the physiology of loblolly pine (Pinus taeda L.) seedlings was examined during seedling recovery from phosphorus (P) deficiency and for several weeks after seedlings had become P-sufficient. Seedlings inoculated with Pisolithus tinctorius (Pers.) Coker and Couch and non-inoculated seedlings responded similarly to an increase in P fertilization, except that nearly twice as much of the absorbed P in ECM seedlings remained in the fine-root system compared to the non-inoculated seedlings. Presumably, most of this P was associated with the ECM fugal hyphae. The presence of mycorrhizas initially caused a 1- to 2-wk delay in the growth response of seedlings to P fertilization, possibly due to a restrictively slow rate of P transfer from the ECM fungus to the root. However, once seedlings had sufficient P for maximum growth, even high levels of mycorrhizal development (greater than 85%) did not reduce the growth rate of ECM seedlings compared to non-mycorrhizal seedlings grown under the same high-nutrient conditions. Possible explanations are discussed as to why a growth reduction due to ECM development is found in some studies but not in others.