The effect of increased tidal inundation and the presence of wrack on denitrification and nitrification in a salt marsh

James H. Taylor1

1 East Carolina University, Dept. of Biology

The experiment was designed to determine if increased tidal inundation and/or the deposition of wrack affected denitrification and nitrification rates in a salt marsh dominated by Juncus roemerianus. Each of the 3 blocks at the Virginia Coastal Reserve's Brownsville Marsh Site contains three plots: (1) Flooded, F, plots flooded at each high tide; (2) Border Control, BC, to control for the border that keeps the water in the flooded plots; and (3) Control, C, the undisturbed control plots lacking a border. Each plot was divided in half; one half vegetated with Juncus roemerianus, while the Juncus roemerianus in the other half was covered with a layer of wrack.

Fifteen cores were taken from each half plot to see how increased tidal inundation, wrack, and/or their interaction affected denitrification and nitrification. The acetylene inhibition technique was used to estimate denitrification and nitrification. Denitrification rates were obtained from the change in N2O concentrations after a one hour incubation. Nitrification rates were obtained three ways from the changes in NH4+, NO3-, or NO2- concentrations after a six hour incubation period with and without acetylene. The background variation in NH4+ concentration precluded observable rates using the NH4+ concentrations.

In August and September, the rates of denitrification were analyzed together, to observe a marsh average, and, if an there was a block interaction, independently. The analyses indicate that many interactions between the treatments affect denitrification rates. Because of these interactions, the only treatment that significantly affects denitrification rates during both August and September was the presence of wrack. Denitrification rates were significantly higher in the wrack covered areas than in the vegetated areas. Any other effect observed to affect denitrification rates must be described as an interaction. Denitrification was affected by an interaction between flooding and vegetation--areas covered with wrack where the ponding of water occurs (F and BC plots) have significantly higher denitrification rates than areas without the ponding of water that are covered in wrack (C plot) or in any vegetated area.

Analyzing the blocks independently shows that, in August, denitrification was affected by an interaction between flooding and vegetation in Block 3. This interaction describes how areas with ponding water (F and BC plots) that are covered with wrack showed statistically higher denitrification rates than other areas (the C plots and the vegetated areas in all plots). Block 1 exhibited an interaction between increased flooding and vegetative cover--sections of the F plots that were covered with wrack had significantly higher rates than the wracl covered sections in the BC and C plots. Block 2 differs in that denitrification was not affected by any treatment.

The interaction between vegetation and flooding regime continued to occur in September, but with different results--denitrification rates in the wrack covered F and C plots were significantly higher than in the wrack covered BC plot. Denitrification rates in Block 1 were only affected by the presence of a wrack (wrack covered areas were higher than the vegetated areas). No difference in denitrification rates was observed in Block 2. The rate of denitrification in Block 3 was affected by the flooding treatment--the C plots had significantly higher denitrification rates than the F plots, but the BC plots were not significantly different from either the C or the F plots.

Nitrification rates for August and September were higher in the vegetated areas than in the wrack areas, but the rates were not significantly different, and no consistent pattern to nitrification was observed over both August' and September's samplings. The only significant effect on nitrification rates occurred when, using the NO3- data, an interaction between flooding and vegetation was observed. The interaction indicates that the nitrification occurs more rapidly in vegetated areas that are not subject to increased flooding.

The differences in the results are being analyzed in relation to elevation--to see if some areas were flooded more than others regardless of their flooding treatment (i.e. if microtopography might explain the differences). In summary, denitrification rates were significantly higher in areas covered by wrack. Denitrification exhibits a trend to be higher in the F plots, lower in BC plots, and lowest in the C plots. Since water tended to pool in the plots surrounded by a border, the ponding of water appears to affect the rate of denitrification by aiding in maintaining the anoxic conditions needed for denitrification to occur. Nitrification rates were only affected significantly during August, a dry month, where nitrification occurred more rapidly in vegetated areas not subject to pooling water. However, a trend of higher nitrification rates in vegetated areas was observed.