Microbial Nitrogen Dynamics During Decomposition of Phragmites australis Compared to Typha angustifolia

Stuart E.G. Findlay
Institute of Ecosystem Studies

The overall goal of this project is to compare the nitrogen retention and release in microbial biomass during all phases of plant decomposition for the invasive common reed relative to cattail, a commonly displaced species. To quantify any differences in nitrogen dynamics we must measure total plant nitrogen and the nitrogen contained in bacterial and fungal biomass from the time of plant senescence throughout the decomposition sequence. The key points in the process are the time of maximum plant biomass (August), followed by translocation of nitrogen to belowground parts and initiation of the decomposition sequence. Plants will have fully senesced by the end of October and efforts to date have focused on 1) finalizing sampling and analytical techniques and 2) documenting the nitrogen retention within living plants at the time of peak live plant biomass. Here, Dr. Findlay reports results on peak biomass at several sampling sites, fungal biomass measured in last year’s standing dead Phragmites and the relationship between stem height and mass necessary for describing decomposition of standing dead plant material.

Using standard clip-plot techniques Dr. Findlay determined the peak above-ground plant standing stock for reed and cattail on August 18, 1998 at four sites within Tivoli North Bay. As expected, standing stocks of both plants are high, ranging from 1 to > 2 kg dry mass per m2. At two of the four sites Phragmites biomass is significantly greater than cattail biomass. These samples, once analyzed for total nitrogen content, represent the N retained in plant biomass. This N is gradually lost during translocation to belowground parts and eventual decomposition mediated by bacteria and fungi.

To determine the mass loss from standing dead plant material, the relationship between stem mass and height must be established. From this relationship the initial mass of a stem collected at any time during the decomposition sequence can be calculated. The need to assay decomposition of standing dead material is particularly important for plants such as common reed that undergo a large portion of the decay sequence as standing dead. For reed stems at time of peak living biomass there was a highly significant curvilinear relation between plant height and mass. Some of the curvilinearity was due to presence/absence of seed heads and some simply to the normal allometry of plant growth. As standing dead plants undergo decomposition, they will loose mass but not height and thus an individual stem will follow the downward arrow as shown on the figure. The trajectory of this arrow is the decay sequence. Decay of other litter components such as blades of Typha and leaves of reed will be assayed using standard litter bag techniques.

To finalize procedures for measuring fungal biomass (ergosterol content) the scientists sampled last year’s standing dead (collected 19 June 1998) to assess among-stand and among-plant variability. Four stems from each of four plots were collected and small sections removed from above and below the apparent high-tide inundation mark on each stem (roughly 40 cm above the marsh surface). The ergosterol concentrations were quite similar across three of the four sites but markedly lower for the northernmost site. Dr. Findlay plans to focus detailed measures at site five with less frequent sampling of other sites within Tivoli North Bay and other marshes to provide a regional context for results. Surprisingly, the fungal biomass was consistently greater on the portions of the plant above the high-tide mark although Dr. Findlay had expected fungi to be subject to desiccation on these upper portions.