Nitrate turnover in a peat soil under drained and rewetted conditions: results from a [15N]nitrate-bromide double-tracer study

Under natural conditions, peatlands are generally nitrate-limited. However, recent concerns about an additional N input into peatlands by atmospheric N deposition have highlighted the risk of an increased denitrification activity and hence the likelihood of a rise of emissions of the greenhouse gas nitrous oxide. Therefore, the aim of the present study was to investigate the turnover of added nitrate in a drained and a rewetted peatland using a [N-15]nitrate-bromide double-tracer method. The double-tracer method allows a separation between physical effects (dilution, dispersion and dislocation) and microbial and chemical nitrate transformation by comparing with the conservative Br- tracer. In the drained peat site, low NO3- consumption rates have been observed. In contrast, NO3- consumption at the rewetted peat site rises rapidly to about 100% within 4 days after tracer application. Concomitantly, the N-15 abundances of nitrite and ammonium in soil water increased and lead to the conclusion that, besides commonly known NO3- reduction to nitrite (i.e. denitrification), a dissimilatory nitrate reduction to ammonium has simultaneously taken place. The present study reveals that increasing NO3- inputs into rewetted peatlands via atmospheric deposition results in a rapid NO3- consumption, which could lead to an increase in N2O emissions into the atmosphere.