Two laboratory-scale constructed wetlands planted with Juncus effusus were used to investigate the effect of nitrate load on the sulfur transformation dynamics related to the removal of organic carbon and ammonium. Organic carbon was removed at a constant high mean level of 80% (inflow of 25 mg/L total organic carbon), irrespective of the nitrate load. In contrast, sulfate reduction responded immediately to the presence or absence of nitrate in an opposing manner, indicating a high abundance of sulfate-reducing bacteria along with an inhibitory effect of nitrate on sulfur reduction. In general, sulfate reduction as well as nitrate reduction were at relatively high mean levels of 70% (inflow of 10 mg/L SO₄^2--S) and 87% (inflow of 15 mg/L NO₃^--N), respectively. In addition, the presence of elemental sulfur in the range of 30% of the decreased sulfate sulfur simultaneously indicated a re-oxidation of sulfide. The initially relatively high removal of approximately 80% (33.5 mg/d) of ammonium decreased to nearly 10% (18.2 mg/d) after enhancing the sulfate reduction and decreasing the redox potential by stopping the nitrate load. The results strongly suggest a highly sensitive correlation between the N and S cycles in the root-surrounding zones of constructed wetlands.