ACETATE MINERALIZATION AS A MEASURE OF SPARTINA ALTERNIFLORA ROOT ENHANCEMENT OF MICROBIAL ACTIVITY. Rebecca Tirrell and Linda Blum. We have been examining the potential for live roots of Spartina alterniflora to enhance belowground decomposition in marsh sediments. Data from Phillips Creek and Great Sippewissett marshes, and qualitative observaations in North Carolina marshes suggest that the presence of live roots stimulates belowground decompostion. For example, Hackney and de la Cruz (1980) observed that decomposition in buried litter bags did not begin until living plant roots came into contact with the bag contents, while Hines et al. (1991) noted a correlation between live roots and acetate concentrations in sediment porewater. These observations suggest that organic materials excreted by live marsh plant roots might enhance microbial activity in the rhizosphere, and thus decay. We used a litter bag technique to measure decay of dead S. alterniflora roots. Three clipped (no live roots) and 3 vegetated (live roots) plots were established in both creekbank and interior sections of Phillips Creek marsh. One month later, litter bags containing dead Spartina alterniflora roots and rhizomes were buried in the marsh plots. Every 2 months, a litter bag was removed from each treatment plot and examined for decay and the presence of live roots. Samples were also collected for bacterial abundance and activity (as acetate mineralization). Over an 18-month period, little root decay was measured regardless of the location or the presence of live roots. However, significantly greater numbers of bacteria were evident in the vegetated plots of both creekbank and interior marsh locations. Rates of acetate mineralization were not correlated with bacterial abundance; rates were greater in creekbank than in interior sediments regardless of the presence or absence of live roots. Not surprisingly, acetate mineralization was greatest in the spring and summer, and was minimal during fall and winter. These results were presented at the last VCR-LTER All- Scientists meeting. Because spring and summer differences in acetate mineralization between creekbank and interior sediments could be related to the concentration of acetate in sediment pore water, we have been measuring actetate mineralization and pore-water concentrations, and bacterial abundance during the past year in both vegetated and clipped plots at the creekbank and interior locations. Bacterial abundance and acetate mineralization rates were similar to those measured during the decay experiment. Pore-water acetate concentrations are currently being measured using an enzymatic assay which couples the synthesis of acetyl-CoA (from acetate) to AMP production. AMP concentrations are measured by HPLC. Once sediment acetate concentrations have been obtained, acetate turn-over rate constants will be calculate for plots with and without live roots. These data will yield evidence to indicate the potential for live roots to enhance microbial activity in the rhizosphere of intermediate (creekbank) and short-form (interior) S. alterniflora. ------------------------------------------------------------------------------------------------------------------ THE EFFECT OF DOC SOURCES ON BACTERIAL CARBON CYCLING IN TIDAL CREEKS. Katherine MacMillin and Linda Blum. Bacteria from tidal creeks on the Chesapeake Bay and ocean sides of the lower Delmarva peninsula were examined for their response to the addition of DOC derived from Spartina alterniflora or phytoplankton. Water samples collected from Hungars Creek, a bayside creek, and from Phillips Creek, a seaside creek, were incubated with both types of DOC. The flasks were monitored for 48 hours and samples were collected to measure bacterial abundance, biovolume and productivity as well as DOC concentration. The response of the bacteria to the two sources of DOC was similar; however, DOC enrichment affected the seaside bacteria differently than the bayside bacteria. After 12 hours, the biomass (abundance x biovolume x a carbon conversion factor) of seaside bacteria increased over 4- fold. After 48 hours, the biomass of seaside bacteria returned to near the initial value. In contrast, the biomass of bayside bacteria increased only 35% after 12 hours and remained elevated through 48 hours. The distribution of seaside bacterial biovolumes also changed following enrichment. After 12 hours, the smaller size classes (<0.05del-m(cubed)) contained relatively fewer cells and large cells were more abundant. After 48 hours the biovolume distributions resembled the initial frequency distributions: smaller cells were present in large numbers and the larger cells were generally absent. The average biovolume and biovolume distributions of bayside bacteria did not change during the enrichment experiment. The changes in bacterial biomass were accompanied by increased bacterial productivity (as measured using tritiated thymidine incorporation) and a decrease in DOC in both seaside and bayside samples. These results are consistent with the hypothesis that seaside bacterial productivity may be substrate limited while grazing on bacterial cells may control the cell-size frequency distributions. This work was supported in part by NOAA Office of Sea Grant, U.S. Department of Commerce, under Grant No. NA90AA-D-SG045 to the Virginia Graduate Marine Science Consortium and Virginia Sea Grant College Program, and a Bannon Foundation Graduate Student Research Grant. Logistical support was provided by the Virginia Coast Reserve Long- Term Ecological Research Project. ----------------------------------------------------------------------------------------------------------------------- BELOW GROUND MARSH GRASS PRODUCTION AND DECAY ALONG A TIDAL/ELEVATIONAL GRADIENT. Linda Blum and Robert Christian. Organic matter accumulation in marsh sediments is dependent on the balance between production and decay of belowground materials which, in turn, are dependent on the plant species and the sediment properties. Plant species distribution and dynamics vary greatly along transects from the creekbank to the high marsh in response to differences in elevation and tidal inundation. The combination of plant species interspecific and intraspecific differences that occurs along an elevational/tidal gradient may result in differences in organic matter production above- and belowground and in decomposition of the organic matter. We used a litter bag technique to compare root and rhizome decay of Spartina alterniflora and Juncus roemerianus along an elevational/tidal gradient (Fig. 1) including low marsh (intermediate height- form S. alterniflora), mid marsh (short height-form S. alterniflora), and high marsh (J. roemerianus) locations. Experiments were carried out in the marsh surrounding the upper portion of Phillips Creek which drains into the coastal lagoon complex of the Virginia Coast Reserve. 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 is very patchy: Spartina alterniflora is the dominant plant species near the regularly flooded low marsh (intermediate form) and in the occasionally flooded mid marsh (short form) while large patches of Juncus roemerianus are dominant in the infrequently flooded high marsh areas. Transects were established along level contours in the low, mid, and high marsh areas. Litter bags containing dead roots and rhizomes of either S. alterniflora or J. roemerianus were inserted vertically into the sediments at 30 cm intervals along the transects. Over a 1000 litter bags were placed in the marsh on February 6, 1991. Nylon litter bags (1 x 2 mm opening) were filled in 10-cm sections with 12 g (wet weight) of dead roots and rhizomes per section. After filling, each section was sewn closed to separate it from the adjacent section so that the vertical distribution of decomposition and root growth could be examined. Three bags of each root litter type were randomly selected for sampling from each marsh area. Bags were collected approximately every two months and returned to the laboratory where live roots and rhizomes were sorted from the dead material. All organic materials were dried at 80 degrees C. Live and dead roots were handled separately. The rate of litter decay was measured as ash-free dry weight (AFDW) loss over time. Root productivity was calculated as the difference between the minimum and maximum amount of live root material in the bags during a growing season. Pore water sippers installed at 5 and 15 cm depths in the sediments were used to sample sediment pore water. Salinity and pH, and the concentrations of NH4+, PO43- , and H2S of the pore water were also monitored in the low, mid, and high marsh areas. Little difference in weight loss was observed between the mid marsh and high marsh locations, but weight loss at the low marsh location may be more rapid than at the two interior locations. Decay constants (k) were calculated using an arithmetic and an exponential model for decay (Table 1). The significantly greater values of k for Juncus are consistent with the difference in the starting C/N ratios for the two plant materials (37:1 and 47:1; Juncus and Spartina respectively). Root growth was highly variable, especially for the low andhigh marsh locations where the total amount of live roots in the litter bags did not exceed 300 g AFDW m-2. Root growth was much greater and less variable in the mid marsh location than near the creek or in the high marsh (900 g AFDW m-2). Aboveground plant standing stocks were greatest in the low and high marsh areas as compared to the mid marsh location. However, total plant biomass (above- and belowground) was significantly greater in the mid marsh because short-form Spartina root growth was significantly greater than root growth of either intermediate height-form Spartina or Juncus. Sediment pore water chemical properties were very different among marsh locations: mid marsh pore water salinities were lower on average than those for the low and high marsh while NH4+ and H2S concentrations were consistently greater than those for the low and high marsh. Based on one year of data from an ongoing 5-year experiment we can conclude that: 1. Regardless of the marsh location, the initial decay of Juncus roots occurs more rapidly than that of Spartina. Differences in sediment chemistry had no measurable affect on decay constants or on the amount of organic material remaining in the litter bags after one year. 2. Root growth is significantly different between marsh locations. This may be related to differences in plant type or growth form or to differences in sediment pore water chemistry. 3. The differences in organic matter accumulation in these marsh sediments are related to the type of plant and its ability to produce roots. 4. The mid marsh location may represent a unique transitional area in salt marshes as a result of its topographic position and tidal inundation characteristics. Table 1. Decay constants and percent AFDW remaining after one year of decay. ----------------------------------------------------------------------- Arithmetic Exponential _______________ ______________ Root Type Depth % REMAIN. in Location (cm) k r2 k r2 1 YEAR ---------------------------------------------------------------------- Sa in Low 0-10 -0.10092 0.77 -0.00128 0.77 59.0 10-20 -0.09689 0.65 -0.00122 0.65 60.8 Jr in Low 0-10 -0.09383 0.66 -0.00168 0.71 59.4 10-20 -0.12589 0.83 -0.00196 0.85 59.5 Sa in Mid 0-10 -0.09781 0.89 -0.00117 0.88 68.6 10-20 -0.09003 0.81 -0.00107 0.81 69.7 Jr in Mid 0-10 -0.1256 0.75 -0.00164 0.71 64.7 10-20 -0.1442 0.99 -0.00194 0.99 53.2 Sa in High 0-10 -0.07944 0.83 -0.00112 0.82 71.3 10-20 -0.07411 0.91 -0.00103 0.67 74.3 Jr in High 0-10 -0.05760 0.18 -0.00118 0.32 67.5 10-20 -0.06970 0.34 -0.00119 0.31 65.1 Sa in Pruned 0-10 -0.06996 0.76 -0.00082 0.78 70.9 Low 10-20 -0.08514 0.72 -0.00101 0.70 67.5 Jr in Pruned 0-10 -0.13630 0.96 -0.00248 0.96 45.9 High 10-20 -0.15103 0.71 -0.00208 0.85 52.9 Dried Sa in 0-10 -0.11172 0.85 -0.00145 0.82 60.1 Low 10-20 -0.11382 0.86 -0.00141 0.85 62.2 Dried Jr in 0-10 -0.14630 0.76 -0.00194 0.65 53.7 High 10-20 -0.11870 0.72 -0.00200 0.78 56.6 Sa in Low 0-10 -0.12906 0.90 -0.00163 0.88 59.7 Perched Marsh 10-20 -0.10744 0.89 -0.00135 0.91 63.2 Sa in Low 0-10 -0.06656 0.26 -0.00081 0.20 78.8 Island Marsh 10-20 -0.11973 0.72 -0.00162 0.68 50.1 ----------------------------------------------------------------------