Microbial Nitrogen Cycling Processes in a Sulfidic Coastal Marsh

Abstract

Sulfide distribution is a key controller of vegetation zonation in coastal ecosystems, but data are limited regarding its impact on the spatial distribution of important N cycling processes. We assessed vegetation distribution and density and, mineral N pool sizes, composition and transformations in a sulfidic coastal marsh in relation to distance from sulfur springs. We observed strong relationships between vegetation attributes (species and density) and mineral N status with greater total inorganic N, NO3− and denitrification enzyme activity (DEA) in sediment samples from areas populated by Crithmum maritimum (mid-way between S springs and sea shore) than in sediments from areas colonized by either Agropyron repens (closest to the S springs) or mangrove (Rhizophora mangleL., farthest from the springs). Our data also suggest close links between N cycling and SO4−2 reduction. The latter resulted in net release of NH4+ ranging from 0.9 mg N kg−1 in the low density C. maritimum to 3.2 mg N kg−1 in the high-density A. repens, during a 3-day incubation. We also tested for microbial adaptation to long-term high sulfide exposure by measuring DEA using the C2H2 block method (which has been found to be strongly affected by the presence of sulfide) and amendment of marsh sediment samples with NaMoO4 to suppress reduced S production. In sediments extracted from sites near the sulfur springs (A. repens and C. maritimum), the C2H2 blockage assay yielded similar results without and with NaMoO4 addition. However, in samples from a mangrove located further downstream from the springs, DEA was substantially lower (2.3 vs. 6.8 mg N2O-N kg−1 sediment d−1) when production of reduced S was not inhibited by NaMoO4. These results suggest that denitrifying microbes in the high sulfide areas may have adapted to the presence of sulfide, allowing for high rates of N and S cycling to occur simultaneously in these marshes.