Table 1.
Summary of sampling dates, conditions, and 15N incubation experiments conducted.
Figure 1.
Comparison of incubation methods.
Incubations were carried out using either membrane inlet mass spectrometry (MIMS) which allowed for continuous online measurements or by discrete porewater sampling into exetainers and subsequent measurement on a GC-IRMS. The example shown is taken from a replicate during the spring incubations directly after the addition of 15NO3−.
Figure 2.
Rates of nitrogen cycling processes determined over 3 seasons.
Anaerobic dissimilatory nitrate reduction to ammonium (DNRA) and denitrification (Denit.) rates from the incubation amended with 15NO3−. Rates were determined from linear production slopes after oxygen had been consumed. Error bars are SD (n = 3)
Figure 3.
Seasonal patterns in DNRA rates mediated by the prokaryotic and eukaryotic community.
A) Eukaryotic DNRA rates B) Prokaryotic DNRA rates C) Overall DNRA rates. Total rates were determined from the incubation amended with 15NO3−. Prokaryotic rates were determined after addition of the eukaryote inhibitor cycloheximide. Eukaryotic rates were calculated by subtracting prokaryotic rates from total rates. Error bars are SD (n = 3)
Figure 4.
Exchange of 15NO3− and 14NO3− over time in summer incubations.
The dashed line indicates the initial amount of 15NO3− added to the incubation
Figure 5.
Respiration of stored intracellular nitrate by eukaryotes and prokaryotes.
A) Example of the appearance of 15N labeled compounds stored in the sediment during the secondary incubation to which no label was added (example from spring incubations) B) Comparison of the overall fraction of 15N recovered as either N2 or NH4+ in both the 15NO3− incubation and the subsequent incubation with no additional 15NO3− and the partitioning between eukaryotes and prokaryotes C) Eukaryotic denitrification rates E) prokaryotic denitrification rates D) eukaryotic DNRA rates F) prokaryotic DNRA rates. Error bars are SD (n = 3).
Figure 6.
Average percentage of 15N recovered in various pools.
Denitrification denotes 15N that was present as 29+30N2, DNRA denotes 15N that was present as 15NH4+ and intracellular storage represents 15N which was present as either 29+30N2 or 15NH4 at the termination of the secondary incubation to which no additional 15NO3 was added. The ‘missing label’ fraction refers to the discrepancy between added 15NO3−, measured 15NO3− and 15N2 production (shown in Fig. 4). Error bars are overall SD (n = 3)