Soil sterilization methods to show VA-mycorrhizae aid P and Zn nutrition of wheat in vertisols

Abstract

Methods to partially sterilize soil to kill vesicular-arbusculur mycorrhizal (VAM) fungi and show the value of VAM to P and Zn nutrition of wheat in vertisols were assessed. At anthesis. mycorrhizal wheat in sterilized soil re-inoculated with VAM propagules had three times the P and Zn concentrations and uptake of non-mycorrhizal wheat. Mycorrhizal wheat yielded up to 37% more grain than non-mycorrhizal wheat. VAM improved recovery of Zn from both soil and fertilizer sources. Multiple regression analysis showed (i) plant P concentration was positively related to VAM colonization level and to P fertilizer application and (ii) plant Zn concentration was positively related to VAM colonization level and to Zn fertilizer, but negatively related to P fertilizer. In unsterilized soil, mycorrhizal colonization was decreased by P fertilizer at 50mg kg −1 but not by Zn at 15 mg kg −1. γ-radiation at 5, 7.5. 10 or 20 k Gy. aerated steam for 30 min at 60, 70 or 80 C, but not at 50 C, and methyl-bromide fumigation, effectively killed VAM propagules in the soil. Effective VAM colonization was re-established in all soil sterilization treatments by inoculating with VAM spores and colonized root pieces. The pathogenic fungus Fusarium graminearum survived γ-irradiation to 10 k Gy, possibly because ofits pink pigmentation. Methyl-bromide fumigation resulted in high Br concentrations in the wheat tissue and phytoxicity symptoms. Partial sterilization with γ-radiation of 5–20kGy, aerated steam of 60– 80° C and methyl-bromide fumigation, increased net mineralization (NH 4+NO 4) of organic N on incubation. Nitrification was inhibited by 20 k Gy of γ-radiation, aerated steam at 70 or 80° C and methyl-bromide. Soil pH was decreased by as much as 0.4 U where the additional mineral N nitrified to NO, than where it remained as NH 4 Extractable P after incubation was increased by 20 k Gy of γ-irradiation and by aerated steam at 60– 80° C. Respiration, assessed by CO 2 evolution, was decreased by γ-irradiation of 5–10k Gy, probably due to elimination of a fungal population capable of decomposing plant residues. Narrow C-to-N ratios (5– 6) of net mineralization during incubation indicated decomposition ofmicrobial biomass killed by all partial sterilization treatments, except aerated steam at 50° C. Comparison of VAM-inoculated and uninoculated plants in soil partially sterilized with aerated steam at 60 or 70 °C or γ-irradialion at 10 k Gy (for soil without F. graminearum) or 20 k Gy, is recommended for future nutritional studies with VAM in vertisols.