LONG-TERM N-15-NITROGEN RETENTION IN THE VEGETATED SEDIMENTS OF A NEW-ENGLAND SALT-MARSH

N-15 was used in a 7-yr field study and a laboratory investigation of a single growing season to quantify the amount, timing, and mechanisms of annual N retention and loss in the plant-sediment system of a short Spartina alterniflora marsh. There was an initial rapid loss of similar to 25% of the added (NH4+)-N-15 through nitrification-denitrification at a rate of 25.2 mg N m(-2) d(-1), with the remaining label being incorporated into plant tissues. Label losses decreased throughout the study as N-15 was increasingly sequestered in the dead organic N pool. About 40% of the injected label remained after seven growing seasons. Total annual N losses were 7.3-7.6 g N m(-2) yr(-1) based on N-15 losses and estimates of the actively cycling N pool. Export accounted for 26-44% and denitrification for 54-77% of the total N loss. Burial of N in dead belowground organic matter was 3.7-4.1 g N m(-2) yr(-1), similar to estimates determined from accretion and total sediment N data. Recycling of N through translocation from aboveground to belowground biomass and remineralization of dead belowground biomass was the major pathway in the sediment N cycle, equivalent to 67-79% of the annual plant N demand. Annual N losses were balanced by inputs, primarily N, fixation. Long-term N retention appears to be controlled primarily by the competition for DIN between the plants and bacterial nitrifiers-denitrifiers and secondarily by the relative incorporation of N into aboveground vs. belowground biomass.