Effects of sulfate amendments on mineralization and phosphorus release from South Florida (USA) wetland soils under anaerobic conditions

We investigated the potential effects of elevated water-column sulfate (SO₄) levels on heterotrophic microbial respiration and net phosphorus (P) release for soils collected from impacted and unimpacted Everglades wetlands in south Florida. Soils from three sites, ranging from low P and low SO₄ to high P and high SO₄ environments, were examined under controlled laboratory conditions. The soils were subjected to anaerobic incubations to evaluate net P release and organic matter decomposition in response to SO₄ amendments of 32 or 96 mg l⁻¹ (0.33 and 1.0 mM).Three processes have been described in the literature to explain why SO₄ enrichment may lead to P release from soils under anaerobic conditions. First, alkalinization can lead to a more favorable pH environment for decomposition. For the soils examined here, alkalinization due to the hydrogen ion-consuming reaction of SO₄ reduction was not a prominent mechanism. We found that pH decreased in the incubation vessels, and that increases in alkalinity were more likely attributable to calcium carbonate dissolution than SO₄ reduction. Moreover, all the soils exhibited near circum-neutral pH levels, with moderate to high concentrations of native alkalinity.Second, formation of iron sulfide (FeS_x) compounds has been shown to mobilize iron (Fe)-associated P. Soils from only one of the study sites had Fe concentrations that would be expected to be high enough to influence P mobility. Relatively high porewater Fe:soluble reactive P (SRP) ratios (>83:1) were observed at this site, which suggests that Fe could theoretically exert control over the release of P from the soil. However, soil P levels at this site were too low to measure any substantial influence of Fe on net P mobilization.Finally, availability of electron acceptors such as SO₄ is a major determinant of decomposition rate, and thus rate of organic P release. Amending the soils with SO₄ did not result in either more heterotrophic microbial respiration as measured by carbon dioxide (CO₂) and methane (CH₄) production, or increased net P mobilization. In two of the SO₄-amended soils where post-incubation total sulfide concentrations were as high as 23.4 mg l⁻¹, SO₄ addition reduced production of respiratory carbon end products, suggesting hydrogen sulfide inhibition. Moreover, limitations imposed by substrate quality and low P contributed to the lack of meaningful enhanced decomposition of organic matter with the addition of 32 or 96 mg SO₄ l⁻¹ to the oligotrophic wetland soils. Even though P release did occur under anaerobic conditions for the more enriched site, addition of SO₄ did not enhance P release.