Coupled nitrification-denitrification measured in situ in a Spartina alterniflora marsh with a 15NH4+ tracer

Measurements of N losses by denitrification from saltmarsh sediments have proved difficult because of the importance of plant metabolism and tidal cycles to sediment N cycling. In vitro approaches often do not measure the dominant coupled nitrification-denitrification pathway and/or alter in situ plant growth and redox conditions. We developed an in situ (NH4+)-N-15 tracer approach to measure coupled nitrification-denitrification fluxes in an undisturbed New England Spartina alterniflora saltmarsh. The tracer was line-injected into sediments underlying natural S. alterniflora stands and in similar areas receiving long-term N amendment (up to 11.2 mol organic N m(-2) yr(-1) for 16 to 23 yr), and N-15 retention and loss routes were followed for 1 to 5 d. Denitrification losses in unfertilized grass stands ranged from 0.4 to 11.9 mmol N m(-2) d(-1) (0.77 +/- 0.18 mol N m(-2) yr(-1)). Denitrification in unfertilized sediments remained low until late summer, but underwent a ca. 4-fold increase in August and September, although sediment temperatures and respiration rates were high throughout the summer. Plant N uptake may limit the availability of N to support denitrification during the early summer, and denitrification may be released from competition with plant uptake in late summer, when plant growth slows. Denitrification rates in fertilized areas ranged from 22 to 77 mmol N m(-2) d(-1) (10.5 +/- 4.9 mot N m(-2) yr(-1)), and denitrification was likely controlled by the availability of fertilizer N rather than by competition with plants, since N was added in excess of plant demand. Our results emphasize the importance of in situ measurements of denitrification in understanding the dynamics of saltmarsh N cycling.