The effect on decomposition of 4 different levels of nitrogen in aerial tissue ofSpartina alterniflora, collected at the end of its growing season litter, was studied in laboratory percolators for 56 days at 20‡C. The CO2 evolution and the release of organic nitrogen and organic carbon were monitored. From these data, the ash-free dry weight (AFDW), nitrogen (N) content, and carbon?nitrogen (C/N) ratio were calculated at various times during decomposition. Fungal biomass, bacterial biomass, and the relative autoradiographic activity of bacteria were measured at the end of the study. Decomposition was significantly affected by the nitrogen content of the litter. A 55% increase in plant N increased overall weight loss and k by 50% and 40%, respectively. Furthermore, k (calculated from time course weight loss data) responded linearly to the 4 different levels of nitrogen inSpartina tissue. Fungi appear to dominate the microbial community. At the end of the experiment, fungal biomass was between 2.23 and 3.08% of the AFDW, and was calculated to contain 12 to 22% of the nitrogen in the litter. Bacterial biomass was 1/10 of the fungal biomass, and 12-17% of the bacteria were active. The total microbial biomass was not affected by increased plant nitrogen. In the course of decomposition, the organic nitrogen and carbon were highest in the effluent water in all treatments during the first 8 days. The respiration rate (CO2 evolution) first increased to a maximum at day 18 and then decreased to a constant rate (1-2 mg C/day/g detritus). Respiration was highest in the high N litter. The C/N ratio in all treatments increased from the start to day 8, then decreased to day 20. In low N litter, C/N then increased again as a result of increased total organic nitrogen (TON) loss relative to carbon mineralization. In the high N, this was reversed.