Denitrification in a soft bottom lake: evaluation of laboratory incubations

Annual benthic denitrification in a shallow soft bottom lake was estimated in laboratory-incubated sediment cores, using the isotope pairing technique . Rates were <30% of the estimate calculated from nitrogen mass balance of the lake. To investigate this discrepancy, in situ measurements were perfor med in flexible enclosures to which (NO3-)-N-15 was added. Accumulation of N-29(2) and N-30(2) in the sediment and water column was measured and the l oss to the atmosphere was estimated from depletion of argon, added in exces s to the water of the enclosures. The calculated denitrification activities in the enclosures were 6 to 26 times higher than the activity in the incub ated cores. We suggest that denitrification in situ was enhanced by wave fo rces, increasing the transport of oxygen and nitrate into these soft sedime nts. Such transport is not simulated by conventional stirring of the water column in laboratory incubations, which consequently may underestimate the in situ activity.