Sibylle Otto

Is the type of vegetation in a freshwater tidal marsh important to surface water quality? To answer this question, I measured available inorganic N, soil microbial biomass and activity, and plant N content and plant height in fertilized (urea, 2x5gN.m-2) and control plots in stands of Lythrum salicaria, Phragmites australis and Typha angustifolia in Tivoli North Bay, Dutchess County, New York. Fertilized soils had higher (P < .01) available inorganic N concentrations than controls between 1 and 2 days after each fertilization. After five days the available N in fertilized soils had decreased to within the range of the controls. I measured some moderate increases in plant and microbial N uptake and N transformation in response to the increased soil N availability. Denitrification rates in fertilized plots in stands of T. angustifolia were higher (P < .05) than controls and leaf N content of fertilized T. angustifolia was somewhat higher (P < .1) than controls. Denitrification enzyme activity in fertilized plots in stands of P. australis was greater (P < .05) than controls. Although microbial biomass did not differ significantly (P > .1) between fertilized and control plots, soils from stands of T. angustifolia had higher (P < .025) microbial biomass N content than soils from stands of L. salicaria. Leaf N content varied (P < .0005) among vegetation types: P. australis > L. salicaria > T. angustifolia. P. australis was taller (P < .0005) than either L. salicaria or T. angustifolia. This study indicates that the nature and amount of nitrogen retained in living biomass varies with vegetation type. The data also suggest that this marsh may be unable to function as a N filter. Approximately 85% of the N added appears to have leached from the marsh into adjacent surface waters.