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

The effect of restricted drainage during winter on the dynamics of denitrif ication was investigated in clayey soil from the Brimstone Experimental Far m, Oxfordshire, UK. Soil taken in autumn 1995 and spring 1996 from the 0-20 and 20-50 cm layers of plots with restricted and unrestricted drainage wer e amended with or without C2H2 to inhibit reduction of N2O to N-2 and with or without chloramphenicol to inhibit de novo synthesis of reduction enzyme s. Samples were incubated anaerobically for 72 h at 25 degrees C and the pr oduction of CO2 and N2O and the concentrations of NO3- and NO2- were monito red. Restricting drainage had no significant effect on the concentrations o f NO3- and NO2- nor on the production of N2O. The production of N2O was ver y 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 N-2 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 degrees C in the laboratory changed the dynamics of the denitrifica tion process substantially: the N2O-to-CO2 ratio increased to 0.56, 54% of the gaseous product of denitrification was N-2 and the resulting N2O-to-N-2 ratio was 1.84. The application of either 50 mg NO3--N or 50 mg NO3--N plu s 100 mg glucose C decreased the production of N-2; consequently the N2O-to -N-2 ratio increased to 2.66 and 3.37, respectively. With a clay content of 55-60% and very slowly permeable subsoil, the Brimstone Farm soil was assu med to be well adapted to anaerobic conditions, but its capacity in 1995/19 96 to reduce NO3- to N2O and N-2 was very limited, even when it was incubat ed under strict anaerobic conditions and supplied with NO3-. However, water logging the soil in the laboratory caused the microbial community to change its functional characteristics by adapting to the anaerobic conditions. Th e inability to replicate this effect in the field by restricting winter dra inage can be attributed to dry antecedent soil conditions resulting from th e very low rainfall in summer 1995. (C) 1999 Elsevier Science Ltd. All righ ts reserved.