Sulfur is an essential plant nutrient but in the sulfide form it can be toxic to plants. Sulfide also mediates the availability to plants of iron-bound phosphorus. Given strong direct and indirect influences of sulfide on plant performance, we hypothesized that sulfide can act to control plant community composition in some freshwater wetland ecosystems. Unlike estuarine ecosystems where sulfur is ubiquitous, or upland ecosystems where sulfur is primarily in oxidized forms, freshwater wetlands typically exhibit a heterogenous pattern of sulfate supply and subsequent sulfide production. Variations in supply depend on both geological and anthropogenic factors, while the transformation to sulfide depends on microbial as well as abiotic factors. To address direct and indirect influences of a heterogeneous pattern of sulfide upon plant species composition, we established an intensive grid of 400 sampling locations in a 40-m by 40-m area within a calcareous fen where ground water is in contact to varying degrees with the underlying sulfur-rich gypsum substrate. At each location in early summer 2009, we measured sulfate, sulfide, and reduced iron in 10-cm deep porewater samples and phosphate availability from resin strips. We then related these chemistry variables to plant species composition in 10-cm by 10-cm sampling quadrats.

Results/Conclusions

As expected, sulfide in the study area was heterogenous (SD=1.23 mg/L) at the localized scale of sampling and was associated with shifts in vegetation. Summer sulfide (H₂S + HS^-) ranged across 3 orders of magnitude, from 0-12.7 mg/L (mean=0.63, median=0.22). Percent cover of monocots, including the dominant species, Eleocharis rostellata, correlated negatively with porewater sulfide and, contrary to expectations, was negatively correlated with phosphate availability as well. In contrast, percent cover of dicots correlated positively with porewater sulfide and did not correlate with phosphate availability. In addition to shifts in functional groups, species-level associations with spatial patterns of sulfide and phosphate identify the species most responsive to sulfide and phosphate mobilization. These findings lay the foundation for our controlled experimental studies addressing plant physiological mechanisms of sulfide tolerance or avoidance.