Nitrification, Nitrate Reduction, and Nitrogen Immobilization in a Tidal Freshwater Marsh Sediment

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The dominant microbial transformations of ammonium and nitrate in the sediments of a tidal freshwater marsh estimated from measured rates of isotope dilution of ammonium [15N] added to sediment and litter samples, reduction of nitrate[15N] to ammonium[15N] in fresh sediments, and net exchange of ammonium and nitrate between the marsh and river water in flumes built in the field. In two separate laboratory experiments, nitrification in mixed, fresh, surface sediments was 2.8 and 3.4 nmol°cm—3.h—1. In undisturbed field sediments, nitrification rates are probably ≤1 nmol°cm—3°h—1. Additions of nitrate[15N] to fresh sediments showed that nitrate produced from ammonium by nitrification was reduced very quickly. Less than 10% of this nitrate was reduced back to ammonium by dissimilation. The net transfer of nitrate in this marsh is to the sediments, and since nitrate does not accumulate in these sediments, the remaining nitrate produced by nitrification must be reduced by denitrification or by assimilatory nitrate reduction. Ammonium was removed quickly from filtered and aerated river water that was mixed with plant litter in laboratory incubations. Ammonium—N removal from experimental flasks (215 nmol°g—1°h—1) was dependent on the presence of litter and was biologically mediated, but was not due to nitrification since nitrate concentrations were constant in the same experiments. Under anaerobic conditions, ammonium—N was released from litter to river water (93 nmol°g—1°h—1). The estimated total rate of ammonium—N immobilization by microbes on aerobic litter (130 μmol°m—2°h—1) agrees well with measured losses of ammonium from river water in field flume studies (156 μmol°m—2°h—1). An aerobic litter layer may enable marshes to regulate the loss of ammonium that is regenerated in reduced, subsurface sediments and that would otherwise be lost by diffusion or advection. For example, during portions of the year, excess ammonium may be immobilized on plant litter in aerobic surface layers. Some of this litter is incorporated into reduced subsurface sediments where, later, microbial mineralization may regenerate the immobilized nitrogen. The importance of the nitrogen recycling process in a particular marsh will depend on the degree to which litter is mineralized in place or exported from the marsh before it is incorporated into the reduced sediment, as well as the extent to which plant roots can take advantage of the regenerated nitrogen. If conditions are favorable (low export fraction, low litter C:N ratio, and advantageous root distribution) then mineralization may supply a major portion of the annual plant requirement for nitrogen in excess of that stored in perennial tissues. Both internal recycling and translocation make the plant community less dependent on nitrogen imported from the river.