Gross rates of ammonium production and consumption by microbes were estimated from isotope dilution of extractable 15NH4+ in surface sediment samples from a tidal freshwater marsh in Massachusetts [USA]. At 13.degree. C gross ammonium production was 11.6 .+-. 2.9 nmol N .cntdot. (cm3 fresh sediment)-1 .cntdot. h-1. This rate equaled or exceeded gross ammonium consumption in the absence of plant root uptake [6.4 .+-. 4.0 nmol N .cntdot. (cm3 fresh sediment)-1 .cntdot. h-1] so that ammonium accumulated linearly for 50-70 h in the sediment samples [5.2 .+-. 1.6 nmol N .cntdot. (cm3 fresh sediment)-1 .cntdot. h-1]. In the field, this excess ammonium is presumably available for plant uptake. Ammonium production and consumption rates were highest in surface sediments and fell rapidly with depth. Temperature has a strong influence on both rates (Q10 = 2.6). Substrate quality and availability, both of which generally decrease with sediment age or depth, are probably more important than end-product inhibition as rate-controlling factors in these sediments. In particular, the relationship between ammonium production and consumption rates at different depths and sediment lignin content is clearer than between these rates and dry weight or ash-free dry weight sediment. The annual, net rate of ammonium production in the top 10 cm of unmixed, surface sediments was estimated from short term ammonium accumulation rates to be about 1.7 mol N/m2. Net ammonium production severely overestimated the true turnover time of ammonium as estimated by isotope dilution (5 days). Most important, while isotope dilution of ammonium is a good indicator of ammonium turnover rates, it may be a poor indicator of total N turnover in sediments (and so of total microbial activity) because it is insensitive to the N that microbes assimilate directly from the solid substrates to which they adhere.
