Effects of a network of mycorrhizae on capsicum (Capsicum annuum L.) grownin the field with five rates of applied phosphorus

This field trial determined the importance of mycorrhizae [Acaulospora mell ea Spain & Schenck, Gigaspora margarita Becker & Hall, Glomus clarum Nicols on & Schenck, Glomus etunicatum Becker & Gerdemann, and Scutellospora pellu cida (Nicol. & Schenck) Walker & Sanders], established from an existing int act network of vesicular-arbuscular mycorrhizae (VAM) propagules, for produ ction of capsicum (Capsicum annuum L. cv. Target) in a low phosphorus (less than or equal to 14 mg NaHCO3-extractable P/kg) Typic Paleudalf. The mycor rhizal network was formed previously with sweetcorn (Zea mays L. cv. Snoswe et) plants grown in distinct rows. A narrow band of milled superphosphate w as applied at 0 (P-1), 5 (P-2), 15 (P-3), 45 (P-4), or 135 (P-5) kg P/ha to plots that were either fumigated (VAM-) or not fumigated (VAM+). The higher (P < 0.05) concentrations of P in the youngest mature leaf(blade plus petiole) of VAM+ than those of VAM- capsicum plants at P-1, P-2, and P-3 coincided with a greater (P < 0.05) weight of marketable fruit of VAMthan of VAM- plants; this finding confirmed the importance of VAM to the en hanced P nutrition of capsicum at low P levels. In the non-fumigated soil ( 1061 non-dormant infective VAM propagules/g air-dry soil), the inverse sine transformed means of percentage colonisation of roots were generally unaff ected by P application, indicating that addition of P may not reduce VAM co lonisation of roots if the inoculum potential of the soil is high. An alter native hypothesis to account for the relatively undiminished mycorrhizal co lonisation of VAM+ plants at high P rates may relate to a reduction in colo nisation for roots growing within the narrow band of applied P, but not for roots growing in the much larger volume of soil outside of this localised P zone. The absence of reduced yields and of lower starch concentrations of roots of VAM+ relative to VAM- plants suggested that photosynthate product ion was surplus to the requirements of the plant and fungus for the irradia nce encountered during the trial. The gross margin for VAM-plants was maxim al at P-5 ($AU3340/ha), and this amount was similar to the margin for VAMplants grown at all treatments except pi, which had a lower gross margin. A s a substitute for P, mycorrhizae have limited potential in intensive veget able production systems since the cost of P is low compared with total cost s. However, other benefits of VAM such as decreased susceptibility to disea se and improved structure of the soil need to be fully assessed to determin e the full benefit of mycorrhizae in such systems.