Factors influencing dissolution of phosphate rock and oxidation of elemental sulphur in some New Zealand soils

Abstract

Forty-eight Waikato soils representing six major New Zealand soil groups (yellow-brown earths, yellow-brown loams, yellow-brown pumice soils, brown-granular loams, gley, and organic soils) were incubated with Jordan phosphate rock (PR) with and without elemental sulphur (S) at 25°C for 10 weeks. S was determined by a recently developed high performance liquid chromatography procedure. Inorganic phosphorus (P) in bicarbonate extracts (Olsen P_i) increased significantly in PR and PR + S treatments, indicating that some Jordan PR was dissolved. The amount of organic P in bicarbonate extracts (Olsen P₀) from the PR + S treatment was significantly larger than control and PR treatments. There was no significant increase in numbers of PR-dissolving bacteria in PR and PR + S treatments after 10 weeks' incubation. Soil pH and Olsen P_i were useful predictors of the relative dissolution of PR among the soil groups. S was oxidised in all soils. Among the soil groups from the S treatment, the gley soils were the least effective S oxidisers, oxidising 44% of added S whereas the other groups oxidised 67–90%. A larger amount of S was oxidised in the PR + S treatment than in the S treatment; the gley soils were particularly affected. Soil pH decrease and the amount of NaOH required to neutralise the acid produced by oxidation were useful but not quantitative measures of the oxidation. The decreases in soil pH and volumes of NaOH used were less in PR + S treated than S treated soils because some of the acid produced reacted with PR. A detailed study of 11 soils from the gley (3), organic (4), and yellow-brown earth (4) groups indicated that S was oxidised rapidly between Weeks 0 and 6 after which the oxidation rate decreased. Acidophilic thiobacilli numbers increased very rapidly to reach 10⁷–10⁸/g soil after 2 weeks' incubation. The decrease in amounts of S in soil was associated with the decrease in soil pH and increase in the amount of NaOH required to neutralise the acid produced. A small decrease in the numbers of S-oxidising heterotrophic bacteria occurred in the S treatment but numbers did not significantly change in the PR + S treatment. The role of heterotrophic S oxidisers is unclear as they may be actively metabolising S even though their numbers decrease or remain unchanged. Soil groups and the numbers of S-oxidising heterotrophic bacteria in soils before incubation were the best predictors of S oxidation but accounted for only 31% of variance.

Keywords:

• phosphate rock dissolution
• elemental sulphur oxidation
• Jordan phosphate rock
• Waikato soils
• incubation
• soil conditions
• phosphate rock-dissolving bacteria
• thiobacilli
• sulphur-oxidising heterotrophs