VARIABLE CARBON CONTENT IN SHALLOW SALT MARSH SOILS, PLUM ISLAND ESTUARY, MASSACHUSETTS

We analyzed soil properties in 35 cores from two salt marsh study plots in the Plum Island Estuary, MA. The first study area, along Sweeney Creek, underwent 13 years of elevated nitrate conditions as part of the TIDE nutrient enrichment experiment. The West Creek marsh served as a control, or reference, site in the TIDE experiment. We obtained 18 cores from the N-enriched plot and 17 cores from the reference site. Bulk density and organic matter (OM) content was analyzed every 2cm down to 90cm depth at each core site. We also measured C:N and stable carbon isotopic composition in a subset of cores. All cores in both areas were located in high marsh dominated by S. patens. In order to compare marsh soils that experience similar tidal inundation, cores from a narrow elevation zone (1.39±0.06 m NAVD88) were collected at similar distances (15-20m) from the nearest tidal creek bank in each of the two study areas.

In cores from both the fertilized Sweeney Creek and the reference West Creek sites, OM values increase up-core in the depth interval from 40cm to 20cm, from 25±5 to 38±5 %, respectively. Age models based on ²¹⁰Pb, ¹³⁷Cs and pollution chronohorizons suggest the 40 and 20cm depths roughly correspond to 1840 and 1940 CE, respectively. The observed increase in soil OM may have resulted from marsh ecogeomorphological responses to warmer post-Little Ice Age conditions and the acceleration of relative sea-level rise ca. 1900. Above 20cm, mean OM values at the reference site are relatively stable, whereas in the N-enriched area mean OM values decline to 32±2% at 8.5cm depth. We hypothesize that the lower OM values in shallow soils of the N-enriched site result from enhanced anaerobic decomposition rates wherein the elevated nitrate provided additional electron acceptors. Closely spaced (every 1cm) analyses in several cores reveal 2% lower carbon concentrations in the 4 – 14cm interval of cores from the N-enriched site, compared with the same depth interval in cores at the reference site. A pattern of slightly lower average δ¹³C values (-16.5±0.3‰ VPDB) in the 4 – 14cm interval of the N-enriched site compared with those of the reference site (-16.0±0.5‰ VPDB) suggests enhanced decomposition of S. patens hemicellulose relative to lignin in soils at the experimental nitrate enrichment site.