The role of mycorrhizal networks in the phosphorus nutrition of intensively-grown vegetable crops

Abstract

Vegetable production in the Bundaberg district of Queensland includes the practices of soil fumigation and the application of relatively large amounts of P fertiliser. Both activities are known to eliminate or severely depress vesicular-arbuscular mycorrhizal (VAM) colonisation of plant roots. The mycorrhizal symbiosis has been shown to greatly improve the efficiency of P utilisation by plants as well as conferring other benefits such as disease suppression and soil aggregate stability, all of which are relevant to the development of sustainable systems. In this thesis, an initial greenhouse study (Experiment 1) was conducted to assess the merit of incorporating a small quantity o f VAM inoculum into the potting mix of vegetable seedlings prior to field transplanting as an alternative to field inoculation; the methodology involved determining the effect of the addition of VAM inoculum (Glomus mosseae [Nicol. a Gerd.] Gerdemann a Trappe and Glomus etunicatum Becker a Gerdemann) on the growth of capsicum (Capsicum annuum L. cv. Target), sweet com (Zea mays L. cv. Snosweet) and tomato (Lycopersicon esculentum Mill. cv. Floradade) plants. Three further greenhouse studies (Experiments 2, 3 and 4) were carried out to gain an understanding of the functioning of an intact extraradical mycorrhizal network on plant growth; the response of capsicum, sweet com and tomato plants was assessed in the presence or absence of an extraradical mycorrhizal mycelium (comprising the same VAM species used in Experiment 1) at different times of the year. Finally, a trial was employed to address the dearth of information which is available on the effect of an extraradical mycorrhizal mycelium on plant response in the field environment; the effect of a live or killed extraradical mycorrhizal mycelium (comprising Acaulospora mellea Spain a Schenck, Gigaspora margarita Becker a Hall, Glomus darum Nicolson a Schenck, Glomus etunicatum Becker a Gerdemann, and Scutellospora pellucida (Nicol. a Schenck) Walker a Sanders) on capsicum production was assessed in the field and an economic analysis of the results was conducted to determine the feasibility of employing a mycorrhizal network as standard cultural practice in the Bundaberg district. In Experiment 1, germinated seeds were sown above a core of five or killed inoculum in a low P sandy loam (6 mg NaHC03-extractable P/ kg) to which 5 P rates (0, 10.3, 30.9, 92.7 or 278 mg P/ kg oven-dry soil, equivalent to 0, 20, 60, 180 or 540 kg P/ ha, respectively) were applied. For capsicum grown at 0 kg P/ ha, the loge transformed mean dry weight and P concentration in the 5 youngest mature leaves were greater for +VAM than for -VAM plants. At low P rates, the dry weight of +VAM and -VAM sweet com (0 and 20 kg P/ ha) and tomato (0 kg P/ ha) plants did not differ, despite inoculated plants having higher P concentrations in index tissues. For sweet com and tomato grown at intermediate P rates (20-60 kg/ ha), the dry weights of +VAM plants were generally lower than those of -VAM plants. For all 3 crops, a lack of VAM response at high P (g180 kg P/ ha) was related to low VAM colonisation. The likelihood of a significant field response to VAM inoculation of seedling capsicum (the most responsive species) was considered to be small, given that most currently-cropped agricultural soils in the Bundaberg district have adequate levels of NaHC03-extractable P.n n n