| North American Atlantic coast salt marshes are dominated by the highly-productive foundation species, Spartina alterniflora . Dissolved organic nitrogen (DON) has been suggested to be an important source of nitrogen (N) for S. alterniflora, however, little is known about the availability or utilization of this N pool. The dissertation objectives were to: (1) investigate latitudinal differences in the availability and utilization of DON, (2) investigate seasonal variation in DON availability, (3) determine the effects of DON fertilization and, (4) determine the importance of shoot N uptake by S. alterniflora.;DON availability was quantified in eight field sites from Maine to Florida. DON concentrations decreased with decreasing latitude, which I attribute to latitudinal differences in microbial activity. Due to greater DON availability, and similar measured rates of DON and NH4+ uptake in mesocosm experiments, I suggest that high-latitude S. alterniflora ecotypes rely more upon DON as a N source than mid- or low-latitude plants.;In Virginia Coast salt marshes, DON availability did not vary significantly among sites, but concentrations were lower in May than September. Nitrogen availability was spatially heterogeneous, and microbial activity is suggested to control the availability of DON at regional scales.;DON and NH4+ fertilization increased plant N content and productivity, resulting in selective grazing upon N-rich plants. I suggest eutrophication will not universally affect grazing in all S. alterniflora growth forms equally, and that N-fertilization does not increase grazing in short-form S. alterniflora in mid-Atlantic salt marshes.;Shoot uptake by S. alterniflora was found to supply 24% of seasonal plant N demand in mesotrophic salt marshes, with 30% from DON. The importance of shoot uptake is largely dependant upon relative elevation and water column nutrient concentrations.;In conclusion, my research shows that the relative importance of DON is largely dependant upon its availability, which my data suggest is greater in high-latitude salt marshes due to lower rates of microbial activity. Shoot uptake, in addition to root uptake, is an important mechanism of DON and DIN assimilation for S. alterniflora in N-limited salt marshes. |
