Present-day rates of mid-Atlantic marsh surface elevation change appear to be in balance iwth contemporary sea-level rise. However, these marshes are highly susceptible to submergence particularly along the Virginia coast which is exposed to the most rapid rates of sea-level rise along the Atlantic coast. Predicting the risk faced by coastal wetlands from global climate change requires a more detailed understanding of the processes controlling marsh elevation change, especially the relative role of sediment supply and deposition vs. organic material production and accumulation.

Along the lower Delmarva Peninsula "new" sediment is in short supply because of the relatively small watersheds and low amount of surface water input to the Virginia coastal bays. As a result, the importance of organic matter contribution to marsh surface accretion is critical. Organic matter accumulation in marsh sediments is dependent on the balance between the production and decay of below ground materials which in turn are dependent on the plant species present and the marsh sediment properties. Plant species distribution and dynamics vary greater along transects from the creekbank to the high marsh in response to difference in elevation and tidal inundation. The combination of plant species interspecific and intraspecific differences that occurs along the elevational gradient in salt marshes may result in differences in organic matter production above and below ground and in decomposition of the organic matter. Furthermore, the differences in sediment characteristics and type of organic matter (ie. Spartina vs. Juncus) may also affect decay processes mediated by microorganism.

The objectives of this on-going project are to:

1. compare below ground decomposition and production of two common salt marsh plants, Spartina alterniflora and Juncus roemerianus along an elevational gradient, and

2. determine if these differences result in differential rates of organic matter accumulation.

The experiments were carried out in the marsh surrounding the upper portion of Phillips Creek which drains into the coastal bays. The marsh is located behind a relict sand ridge, and the surrounding uplands are either farm lands or pine-forest wood lots. The distribution of plants in Phillips Creek Marsh (PCM) is very patchy. Spartina alterniflora is the dominant plant species near the regularly flooded low marsh and in the occasionally flooded mid marsh. The height form in the low marsh is intermediate and is short in the mid marsh. Juncus roemerianus is dominant in the high marsh.

Three sites were established along a transect from the low marsh to the high marsh. At each site, litter bags containing either dead roots of S. alterniflora or Juncus roemerianus were inserted vertically into the marsh sediments at 30 cm intervals along level contours. Bothe organic matter types were incubated at each of the three sites. Over 1200 litter bags were placed in the marsh on February 6, 1991. For the first 2 years of the experiment, bag were sampled every two months. After 2 months in the marsh, the bags were difficult to find and have become increasingly difficult to locate as so many plant roots have grown into the bags. Plant roots have no difficulty in penetrating the 1 mm openings of the litter bags. In addition, tension lysimeters were installed at 5 and 15 cm depths in the sediments. Sediment pore water sampled from the lysimeters was used to characterize pore water concentrations of ammonium, phosphate, hydrogen sulfide, pH, and salinity during the first summer of the experiment (Fig. 1).

For the first 2 years, 3 bags of each root type were sampled from the low, mid, and high marsh sites. The bags were returned to the lab where live roots and rhizomes were sorted from the dead material. After sorting, the live and dead root materials were handled separately. The materials were dried and ashed. In following years, bags have been sampled annually on February 6.

In an earlier VCR LTER research report, we described the first two years of root production and decay data (those data are not shown here). Briefly, those results showed that among site differences in decay were not evident for either plants species over the first year of decomposition. However, regardless of location Juncus decayed more rapidly than Spartina. This unexpected difference in decay was consistent with C:N ratios of Juncus and Spartina belowground materials we measured of 37:1 for Juncus roots and 47:1 for Spartina roots. It is important to note that literature values for above ground materials are just the opposite. When decay constants were calculated assuming an exponential model of decay, Juncus constants were 2 to 3 times those for Spartina and even though the fit of the data to the exponential model was often poor, the differences between the decay constants are significant.

When the litter bags are sampled on February 6 each year, we rarely observe any live roots in the bag suggesting that it is reasonable to consider the amount of organic matter in the litter bags to be representative of the balance between organic matter production and decay for the locations. Thus, decay is discussed only for the first year of the experiment because root growth into the litter bags and subsequent death of those roots was substantial (data shown in previous reports) and were considered to be have confounded measurement of decay.

When the total amount of organic material in the litter bags on February 6 of each year of the experiment is compared, it is clear that organic matter is accumulating in the bags (Fig. 2 & 3). By the end of the 4th year of the experiment, bags incubated in the high marsh Juncus stand had accumulated over 3 times the amount organic matter as they started with and by the end of the 5th year bags in the Spartina stand had increased in weight by over 150%. The low marsh site is the only site with bags still remaining in the sediment. It is unfortunate that no bags remained in the mid marsh after the first year and in the high marsh after the 4th year. However, there are sufficient bags to continue with annual sampling for another 4 years at the low marsh site. It will be interesting to see if these bags continue to accumulate organic matter.

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