Push-pull methods for sediment processes
In sediments, biogeochemical processes occur in a complex environment. Steep redox gradients and uneven distribution of substrate affect metabolism, nutrient cycling, and other processes. In sediments colonized by seagrass shoots, seagrass presence can affect sediment biogeochemistry because the plant roots exude oxygen and labile carbon into the surrounding sediments. However, conventional methods, such as core incubations, don’t fully account for these belowground plant effects.
As part of my PhD research, I developed a new in situ method to measure nitrogen cycle processes in seagrass sediments. The method uses a “push-pull” approach; first, an isotope label is added to the porewater and “pushed” into the sediments. Then, the label is allowed to incubate in situ; during the incubation, the porewater and sediment are resampled (“pulled”). By conducting the incubation in the field, the push-pull method maintains the complex sediment matrix and does not disturb the environmental drivers of seagrass activity such as light and flow. The push-pull method therefore captures the belowground plant effects on sediment processes.
I have developed versions of the push-pull method to measure denitrification, dissimilatory nitrate reduction to ammonium (DNRA), and nitrogen fixation. For all three processes, I measured higher rates using the push-pull method than using conventional methods (cores and slurries). The push-pull rates were also more variable, likely due to the heterogeneous plant effects. Overall, the push-pull method shows the importance of in situ measurements in seagrass sediments.
A full description of the push-pull method is published in Limnology and Oceanography: Methods here. In-depth measurements of denitrification and DNRA using the push-pull method are published in Marine Ecology Progress Series here. An evaluation of nitrogen fixation rates measured using the push-pull method is published as a Feature Article in Marine Ecology Progress Series here.