Functional stability of the nitrate-reducing community in grassland soils towards high nitrate supply

To study the effects of short-term fluctuation of nitrate concentrations on the nitrate-reducing community, repacked soil cores were amended with 0, 100 and 300μgNO3-‐Ng-1 soil and incubated for 3, 7 and 14 days, respectively. The nitrate reductase activity was determined in a laboratory-based enzyme assay. In parallel, the community structure of nitrate-reducing microorganisms was characterised by RFLP-PCR using the functional gene narG  , which encodes the catalytic site of the membrane-bound nitrate reductase. The community structure remained constant over the experimental period indicating that this functional community is characterised by a high resistance towards fluctuating nitrate concentrations. Decreases in nitrate concentration as well as increase in pH values indicated a very active nitrate-reducing community under nitrate addition. Surprisingly, inhibition of nitrite reductase by 2,4-dinitrophenol, which is a precondition for the measurement of nitrate reductase activity, could not be achieved in the 100μgNO3-‐N treatment despite increased concentrations of the inhibitor. However, comparison of the nitrate reductase in the control and the 300μgNO3-‐N treatment showed a significant increase in the latter at day 3. No further differences were observed at days 7 and 14, which suggests a high resilience of the nitrate reductase activity.