THE EFFECT OF HIGH SOLUTE CONCENTRATIONS ON NITRIFICATION RATES IN SOIL

A short term nitrification assay (< 18 h) was used to assess the effect of high concentrations of different solutes on the rate of nitrate production. High solute concentrations were found to inhibit nitrification and the degree of inhibition was related both to the osmotic pressure of the soil solution and the osmoticum used. Ammonium chloride, ammonium sulphate and sorbitol were used as sources of osmotic pressure. The results showed that, with ammonium salts, no inhibition followed the order ammonium chloride > ammonium sulphate > sorbitol up to the maximum osmotic pressure studies (25 atm). With ammonium chloride, a pressure of 3.5 atm. was sufficient to cause a 90% inhibition of nitrification rate. The inhibition produced by mixtures of ammonium chloride and sorbitol, each mixture generating an osmotic pressure of 5 atm. in the assay, was also investigated. The results suggest that inhibition by Cl-ion is disproportionate to its contribution to the osmotic pressure of the soil solution. The recovery of the nitrification rate, following exposure to high osmotic pressure solutions, was also investigated. It was found that the recovery of the nitrification rate was only partial, with the extent of the recovery diminishing as the severity of the initial osmotic stress applied increased. These results suggest that both reversible and irreversible mechanisms are involved in the inhibition of nitrification.