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.

Seagrass restoration reestablishes the coastal nitrogen filter through enhanced burial

“…While the denitrification rates in this system were likely limited by the very low nitrate availability (undetectable in surface waters throughout the year) and relatively low sediment organic matter content (2.6%), it is not immediately clear why the rates differ so dramatically between locations. Factors such as residence time and benthic microalgae presence also influence denitrification rates (Cornwell et al ; Welsh et al ), and methodological differences likely contribute to the wide range of denitrification rates reported in the literature (see Eyre et al and Aoki and McGlathery for more discussion of how methodology affects denitrification measurements). Further investigation of denitrification in seagrass meadows is needed to clarify the range of rates in natural systems under different conditions and to establish the importance of denitrification relative to N burial.…”

“…Hourly denitrification rates in sediments were measured from June 2014 to July 2015 using an in situ, push‐pull, isotope pairing method; the method is described in detail in Aoki and McGlathery (). Using the push‐pull method, the hourly rate measurements were made under variable in situ daylight conditions; extrapolating from hourly to daily rates therefore required assumptions about sediment denitrification under dark conditions.…”

Application of the isotope pairing technique in sediments: Use, challenges, and new directions

“…Sediments colonized by vascular plants or other sediments where patchy NO 3 − reduction may occur below the NO 3 − ‐diffusion zone pose another challenge to the whole‐core IPT. A recent adaptation of the IPT is its combination with a push‐pull technique, which was developed for use in vegetated salt marsh (Koop‐Jakobsen and Giblin ) and subtidal seagrass sediments (Aoki and McGlathery ). An additional variation of the push‐pull and IPT method has also been applied to advection‐dominated carbonate sands in combination with flow‐through reactors (Erler et al ).…”

“…In vegetated sediments, the potential for the combination of IPT with other methods may go some way toward overcoming issues associated with using IPT alone. For example, applying the IPT in push‐pull methods (described further later in this article) may be an additional promising solution to investigate subsurface effects of the rooted plants on sediment N cycling (Koop‐Jakobsen and Giblin ; Aoki and McGlathery ).…”

“…A recent adaptation of the IPT is its combination with a pushpull technique, which was developed for use in vegetated salt marsh (Koop-Jakobsen and Giblin 2009) and subtidal seagrass sediments (Aoki and McGlathery 2017). An additional variation of the push-pull and IPT method has also been applied to advection-dominated carbonate sands in combination with flow-through reactors (Erler et al 2014).…”