Sulphide addition favours respiratory ammonification (DNRA) over complete denitrification and alters the active microbial community in salt marsh sediments.

The balance between nitrate respiration pathways, denitrification and dissimilatory nitrate (NO3 - ) reduction to ammonium (DNRA), determines whether bioavailable nitrogen is removed as N2 gas or recycled as ammonium. Saltwater intrusion and organic matter enrichment may increase sulphate reduction leading to sulphide accumulation. We investigated the effects of sulphide on the partitioning of NO3 - between complete denitrification and DNRA and the microbial communities in salt marsh sediments. Complete denitrification significantly decreased with increasing sulphide, resulting in an increase in the contribution of DNRA to NO3 - respiration. Alternative fates of NO3 - became increasingly important at higher sulphide treatments, which could include N2 O production and/or transport into intracellular vacuoles. Higher 16S transcript diversity was observed in the high sulphide treatment, with clear shifts in composition. Generally, low and no sulphide, coupled with high NO3 - , favoured the activity of Campylobacterales, Oceanospirillales and Altermonadales, all of which include opportunistic denitrifiers. High ∑sulphide conditions promoted the activity of potential sulphide oxidizing nitrate reducers (Desulfobulbaceae, Acidiferrobacteraceae and Xanthomonadales) and sulphate reducers (Desulfomonadaceae, Desulfobacteraceae). Our study highlights the tight coupling between N and S cycling, and the implications of these dynamics on the fate of bioavailable N in coastal environments susceptible to intermittent saltwater inundation and organic matter enrichment.