CARBON AND NITROGEN DYNAMICS IN A GRASSLAND SOIL WITH VARYING PH - EFFECT OF PH ON THE DENITRIFICATION POTENTIAL AND DYNAMICS OF THE REDUCTION ENZYMES

Soil pH is known to affect most soil biological processes, including d enitrification. There are conflicting results as to the effect of pH o n the denitrification process and the ability of denitrifiers to adapt to acid soil conditions. The denitrification potential and dynamics o f the reduction enzymes involved were investigated for four soils of d ifferent pH values from the Park Grass Experiment at Rothamsted Experi mental Station. Soils were incubated aerobically or anaerobically afte r being amended with 500 mg NO3--N kg(-1). Anaerobic treatments were w ith or without C2H2 Or With Or without chloramphenicol (known to inhib it protein synthesis). All samples were incubated for 48 h, while CO2 and N2O production and NO3-, NO2- and NH4+ concentrations were monitor ed. CO2 and N2O production decreased with decreasing pH under both aer obic and anaerobic conditions. The addition of chloramphenicol under a naerobic conditions appeared to have no effect on CO2 production, but decreased N2O production in soils of pHCaCl(2) greater than or equal t o 5, but not in soils of pHCaCl(2). less than or equal to 4, where N2O production was generally slow. Investigations of the enzyme dynamics involved in denitrification using the DETRAN model, suggested that the initial concentrations of N reductase enzymes were small in all soils and the biomass of Park Grass was not adapted to anaerobiosis. Data a lso suggested that soils of pHCaCl(2), greater than or equal to 4.9, N O2- could accumulate to high concentrations (15-30 mg N kg(-1)) during anaerobic periods of 6-36 h. In the aerobic incubations nitrification was observed in all soils even at pHCaCl(3) less than or equal to 3.9 . Antecedent water regime of the soil appeared to be important in cont rolling denitrification activity and the dynamics of the reduction enz ymes involved in the process. There were significant effects of pH on the number of microorganisms, the time to de novo enzyme synthesis and on the kinetics of the reduction enzymes involved in denitrification. (C) 1998 Elsevier Science Ltd. All rights reserved.