Denitrification in an intertidal seagrass meadow, a comparison of 15 N-isotope and acetylene-block techniques: dissimilatory nitrate reduction to ammonia as a source of N 2 O?

Abstract

Tests of the isotope-pairing technique over a range (15–200 µM) of 15N-nitrate additions to the water column of intact or defoliated cores of the seagrass Zostera noltii demonstrated that whilst overall rates of denitrification were highly dependent on the nitrate concentration in the water column, the calculated in situ rates, D 14, D W and D N, were not significantly affected. Rates of coupled nitrification–denitrification determined following additions of 15N-ammonium to the sediment porewater were within the range of D N values determined by isotope pairing. Thus, this technique appears to accurately measure denitrification rates in these seagrass-colonised sediments, probably due to the effective limitation of coupled nitrification–denitrification to the surficial sediments, which ensures the homogenous mixing within the denitrification zone of the added 15N-nitrate tracer and the 14N-nitrate generated by nitrification in the sediment. Denitrification rates of nitrate diffusing from the water column were compared using the isotope-pairing and acetylene-block techniques. Whilst rates were similar during dark incubations, during light incubations, rates of N2O accumulation during acetylene blocking were consistently twofold greater than denitrification rates measured by isotope pairing. We propose that this excess N2O could have been generated during dissimilatory nitrate reduction to ammonium by fermentative and/or sulphate-reducing bacteria.