Dynamics of the denitrification in soil from the Brimstone Farm experiment, UK

The effect of restricted drainage during winter on the dynamics of denitrification was investigated in clayey sod from the Brimstone Experimental Farm, Oxfordshire, UK. Sods taken in autumn 1995 and spring 1996 from the 0–20 and 20–50 cm layers of plots with restricted and unrestricted drainage were amended with or without C2H2 to inhibit reduction of N2O to N2 and with or without chloramphenicol to inhibit de novo synthesis of reduction enzymes. Samples were incubated anaerobically for 72 h at 25°C and the production of CO2 and N2O and the concentrations of NO3− and NO2− were monitored. Restricting drainage had no significant effect on the concentrations of NO3− and NO2− nor on the production of N2O. The production of N2O was very variable and less than 1.5 mg N2O–N kg−1 after 72 h in both plots. The N2O-to-CO2 ratio on both plots was only 0.02 and no significant amounts of N2 were produced. However, conditioning soil from the 0–20 cm of the plot with restricted drainage by keeping it under waterlogged conditions for 28 d at 25°C in the laboratory changed the dynan-dcs of the denitrification process substantially: the N2O-to-CO2 ratio increased to 0.56, 54% of the gaseous product of denitrification was N2 and the resulting N2O-to-N2 ratio was 1.84. The application of either 50 mg NO3−–N or 50 mg NO3−–N plus 100 mg glucose C decreased the production of N2; consequently the N2O-to-N2 ratio increased to 2.66 and 3.37, respectively. With a clay content of 55–60% and very slowly permeable subsoil, the Brimstone Farm sod was assumed to be well adapted to anaerobic conditions, but its capacity in 1995/1996 to reduce NO3− to N2O and N2 was very limited, even when it was incubated under strict anaerobic conditions and supplied with NO3−. However, waterlogging the sod in the laboratory caused the microbial community to change its functional characteristics by adapting to the anaerobic conditions. The inability to replicate this effect in the field by restricting winter drainage can be attributed to dry antecedent sod conditions resulting from the very low rainfall in summer 1995.