New approach for measuring denitrification in the rhizosphere of vegetated marsh sediments

Ketil Koop-Jakobsen, Anne E. Giblin

Limnol. Oceanogr. Methods 7:626-637 (2009) | DOI: 10.4319/lom.2009.7.626

ABSTRACT: This study presents a new approach for measuring denitrification at depth in the marsh rhizosphere (below 5 cm). The method combines the push-pull technique and the 15N-isotope pairing technique, here referred to as the PPIPT. The PPIPT allows ambient denitrification rates to be measured in situ at depth in the marsh rhizosphere with minimal damage to roots and rhizomes and minimal disturbance of the ambient pore water composition. In this method, pore water is extracted from the sediment using micropiezometers without contact with the atmosphere. The porewater is amended with 15NO to trace denitrification, and Ar(g) is added to trace potential gas loss of denitrification end products. The “spiked” pore water is injected back into the sediment for incubation. Subsequently, samples are sequentially extracted from the sediment for analysis of 28N2, 29N2, 30N2, and Ar. Denitrification rates are calculated using the isotope paring technique and corrected for dilution and gaseous losses using the argon tracer. After several trials in the laboratory and in the field to optimize the method, the PPIPT was applied in the field to measure denitrification in the rhizosphere of various vegetation zones in two New England marshes in northeastern USA. The field applications verified the PPIPT method as a useful tool for measuring ambient rates of denitrification at depth in salt marsh rhizospheres. Generally, the rates measured were low (0–12 µmol m–2 h–1) and showed a high spatial variability in the marsh rhizosphere among different vegetation zones.