Low effective surface area explains slow oxidation of co-granulated elemental sulfur

Abstract

Elemental S (ES) is commonly used as a S fertilizer but must be oxidized to sulfate for plant uptake. Oxidation of ES in co-granulated fertilizers has been shown to be slower than that of powdered ES mixed with soil. We hypothesized that the slow oxidation is due to limited access of S oxidizers to the interior of the granule. To test this hypothesis, an experiment was conducted to compare chemical properties and bacterial populations in soil in the vicinity of a granule (diammonium phosphate [DAP] + ES [10%]) and in soil mixed with powdered DAP + ES. Copy numbers of 16S ribosomal ribonucleic acid (rRNA) genes and of a functional gene soxB were quantified to indicate the abundance of total and S-oxidizing bacteria, respectively, and the total bacterial community composition was analyzed by terminal restriction fragment length polymorphism (TRFLP). After 20 wk of incubation, 42% of the cogranulated ES was oxidized compared to 95% for the powdered ES. The soil pH dropped by 1.0 and 1.7 units for the granular and powdered treatments, respectively. We observed significant changes in bacterial abundance and community composition during incubation. However, none of the alterations in chemical properties and bacterial populations could explain the adverse effects of granulation on ES oxidation. The difference in ES oxidation rate between the co-granulated and powdered ES corresponded to the difference in surface area of the granule and of the individual ES particles, quantitatively corroborating that the available surface area exposed to S-oxidizing microorganisms is the limiting factor for co-granulated ES oxidation.