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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Nitrification and Denitrification near a Soil–Manure Interface Studied with a Nitrate-Nitrite Biosensor

Department of Microbial Ecology, Aarhus University, Bd. 540, Ny Munkegade, DK-8000 Aarhus, Denmark

* Corresponding author (biorlm{at}biology.au.dk)

A two-layer system of cattle (Bos taurus) manure and soil was used to study the development of nitrification and denitrification processes associated with the oxic–anoxic interface of soil and manure over a period of 18 d. By use of microsensors, depth profiles of O₂ and nitrate plus nitrite were measured. We observed an initial depletion of NO^-_x originally present in the soil, followed by an increase after 1 wk caused by the activity of nitrifying bacteria. Fluxes of NO^-_x from the soil into the manure were calculated from NO^-_x profiles and compared with measurements of N₂ production. Generally, NO^-_x fluxes and N₂ production were concordant. Denitrification (NO^-_x consumption) was closely coupled to nitrification until Day 18, when NO^-_x accumulated in the anoxic zone and therefore no longer seemed to limit denitrification. Maximum denitrification rates were measured at Day 9/10 and reached 732 and 497 nmol N m^-2 h^-1 when measured as NO^-_x flux and N₂ production, respectively. An experiment was set up to investigate the temporal change in NO^-_x profiles when nitrification was inhibited by acetylene. Profiles of NO^-_x were measured at 0, 2, 4, 8, 16, 24, and 48 h of incubation with 1% acetylene. Calculated NO^-_x fluxes were 39% of the original rates after 2 h and only 2% after 48 h. Thus, this experiment stresses the importance of very short incubation time when using the acetylene inhibition technique to measure denitrification rates.

Abbreviations: D_e, effective diffusion coefficient • D_s, diffusion coeeficient of the compound in a particular substrate • DW, dry weight • ESC, electrophoretic sensitivity control • NO^-_x, nitrate plus nitrite • PVC, polyvinyl Cl • SE, standard error