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

This field trial determined the importance of mycorrhizae [Acaulospora mellea Spain & Schenck, Gigaspora margarita Becker & Hall, Glomus clarum Nicolson & Schenck, Glomus etunicatum Becker & Gerdemann, and Scutellospora pellucida (Nicol. & Schenck) Walker & Sanders], established from an existing intact network of vesicular-arbuscular mycorrhizae (VAM) propagules, for production 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 mycorrhizal network was formed previously with sweetcorn (Zea mays L. cv. Snosweet) plants grown in distinct rows. A narrow band of milled superphosphate was 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 VAM+ than of VAM- plants; this finding confirmed the importance of VAM to the enhanced 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 unaffected by P application, indicating that addition of P may not reduce VAM colonisation of roots if the inoculum potential of the soil is high. An alternative hypothesis to account for the relatively undiminished mycorrhizal colonisation of VAM+ plants at high P rates may relate to a reduction in colonisation 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 production was surplus to the requirements of the plant and fungus for the irradiance encountered during the trial. The gross margin for VAM-plants was maximal at P-5 ($AU3340/ha), and this amount was similar to the margin for VAM+ plants grown at all treatments except pi, which had a lower gross margin. As a substitute for P, mycorrhizae have limited potential in intensive vegetable production systems since the cost of P is low compared with total costs. However, other benefits of VAM such as decreased susceptibility to disease and improved structure of the soil need to be fully assessed to determine the full benefit of mycorrhizae in such systems