Push‐pull incubation method reveals the importance of denitrification and dissimilatory nitrate reduction to ammonium in seagrass root zone

Abstract

In this study, we developed a push-pull incubation method to measure denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in subsurface sediments of subtidal seagrass meadows. This subtidal push-pull technique, adapted from a push-pull method developed for intertidal salt marshes, used mini-piezometers to directly sample pore water in the root zone of the seagrass during an in situ incubation. The pore water was amended with 15 NO3− and with Ar(g) as a tracer gas, and the denitrification products (28N2(g), 29N2(g), and 30N2(g)) were measured with membrane inlet mass spectrometry (MIMS), using the Ar tracer to correct for dilution and gas loss. Production of 15 NH4+ was also measured using hypobromite oxidation and MIMS to determine rates of DNRA. Using this new technique, subsurface rates of denitrification and DNRA were determined to be 17.5 μmol N m−2 h−1 and 14.7 μmol N m−2 h−1, respectively, for a restored Zostera meadow. Rates showed substantial spatial variability, likely due to both heterogeneous conditions in the root zone sediment and variable in situ conditions. When compared to traditional core and slurry incubations, push-pull rates were greater and more variable, suggesting that the push-pull technique more accurately captures heterogeneity and the natural range of denitrification and DNRA in subsurface sediments under field conditions. In vegetated systems with low water column nitrate concentrations, the majority of denitrification and DNRA occurs below the sediment surface, and the subtidal push-pull technique provides an effective method to assess these subsurface rates.