Responses of sugarcane, maize, and soybean to phosphorus and vesicular-arbuscular mycorrhizal fungi

Abstract

The presence of vesicular-arbuscular mycorrhizal (VAM) fungi in long-term cane-growing fields associated with yield decline led to the supposition that VAM fungi may be responsible for the poor yields. A glasshouse trial was established to test the effectiveness of a species of VAM fungi, Glomus clarum, extracted from one of these North Queensland fields on the growth of sugarcane (Saccharum interspecific hybrid), maize (Zea mays), and soybean (Glycine max) for 6 phosphorus (P) rates (0, 2.7, 8.2, 25, 74, 222 mg/kg). For maize and soybean plants that received VAM (+VAM), root colonisation was associated with enhanced P uptake, improved dry weight (DW) production, and higher index tissue-P concentrations than those without VAM (–VAM). By comparing DW responses of maize and soybean for different P rates, savings in fertiliser P of up to 160 and 213 kg/ha, respectively, were realised. Sugarcane plants were generally less responsive. Apart from a 30% DW increase with VAM when 2.7 mg P/kg was added, DW of +VAM plants was equivalent to, or worse than in the case of 222 mg P/kg, DW of –VAM plants. For all 3 host species, colonisation was least at the highest P application, presumably from excessive P within the plant tissue. Critical P concentrations for the 3 host species were below those reported elsewhere, and for soybean and sugarcane, the critical concentration for +VAM plants was lower than that of –VAM plants. There are 3 implications that arise from this study. First, VAM fungi present in cane-growing soils can promote the growth of maize and soybean, which are potential rotation crops, over a range of P levels. Second, the mycorrhizal strain taken from this site did not generally contribute to a yield decline in sugarcane plants. Third, application of P fertiliser is not necessary for sugarcane when acid-extractable P is <30 mg/kg if sufficient VAM propagules are present, or mp;lt;47 mg/kg if a mycorrhizal response is not anticipated.

Growth responses of sugarcane to mycorrhizal spore density and phosphorus rate

“…The isolate may be sensitive to these higher P rates. Similar observations were made by Kelly et al (2001) at the low-P rate. At 8.2 mg P/kg, dry weight did respond to 4 spores/g (Fig.…”

“…However, results here and elsewhere (Kelly et al 2001) indicate that Gl. clarum can lead to positive growth responses in sugarcane, and also in corn and soybean under low-medium P. Despite originating from long-term monocultured fields, there is little evidence from this study to suggest that this strain is an inferior symbiont.…”

“…Kelly et al (2001) found that dry matter responses (apart from one P treatment) were <10%. Pinchin (1986) found that responsiveness by sugarcane to AM was 12% or less.…”

“…The soil was a Dermosol (Isbell 1996) within the Tully-Coom series (Cannon et al 1992). A comprehensive list of the soil chemical properties has been presented by Kelly et al (2001). Briefly, the soil was found to be deficient in P, potassium (K), calcium (Ca), magnesium (Mg), and zinc (Zn) for adequate sugarcane growth (Table 1).…”

“…;Kelly 1999). A 10% reduction in the dry weight was reported by Kelly et al (2001) when young sugarcane plants, grown in soil amended with 240 kg/ha P equivalent, were inoculated with AM taken from long-term cane-growing soils. This slight loss in yield was unrelated to P insufficiency, and appeared to be due to a poorly symbiotic AM isolate.…”