VARIABILITY IN LATE HOLOCENE TO MODERN ORGANIC MATTER CONTENT BENEATH SPARTINA PATENS SALT MARSHES, PLUM ISLAND ESTUARY, MASSACHUSETTS

We analyzed soil properties in 35 cores from two salt marsh areas in the Plum Island Estuary (PIE), MA. The first marsh study area, along Sweeney Creek, underwent 13 years of elevated nitrate conditions as part of the TIDE nutrient enrichment experiment. The marsh surrounding West Creek served as a control site in the TIDE experiment. We obtained 18 cores along Sweeney Creek (N-enriched) and 17 cores from the West Creek (control) area. Bulk density and organic matter (OM) content was analyzed down to 90cm depth at each core site. All core sites in both areas were in Spartina patens-dominated high marsh. In order to compare marsh soils that experience similar tidal inundation conditions, cores were collected parallel to, but set back 15-20m from, the natural creek banks. Leveling of the core tops shows close agreement of the high-marsh surface elevations between the two study areas: Sweeney Creek mean elevation 1.39±0.03 m NAVD88; West Creek: 1.39±0.05 m NAVD88.

Sedimentation rates in the upper parts of the cores were estimated by measuring ²¹⁰Pb and ¹³⁷Cs activities and Hg concentrations in a subset of the cores to provide age control. Results show the depth-averaged organic matter content from 20-90cm at West Creek is 3-8% lower than that of Sweeney Creek in the same depth interval. Despite the offset in %OM values, the cores show similar trends: OM content is relatively constant from 90-45cm depth, and then increases by ~15% from 45cm up to 22cm depth. The age model suggests that the 45 and 22cm depths roughly correspond to the years 1890 and 1945, respectively. Above 20cm, West Creek mean organic content is relatively stable at 38±5 %OM. In contrast, the Sweeney Creek mean OM values drop from 41% at 22.5cm to 32% at 8.5cm depth.

One hypothesis to explain the reduced OM values at shallow depths (<20cm) in the Sweeney Creek cores is a recent reduction in Spartina patens root growth in response to N enrichment. The N-enrichment occurred from 2004-2016, whereas the reduced OM values are observed from 22.5-8.5cm, i.e., 1952-1992, well before the experiment began, casting doubt on the reduced root-growth hypothesis. An alternate interpretation, that accounts for patterns observed in cores from both areas, is that extensive ditch construction throughout the PIE marshes prior to WW II ended the trend of increasing soil OM at both study areas.