Limitations of methods for preserving ammonium in agricultural runoff samples

Ammonium concentrations in simulated and actual field runoff samples were not stable within 1 d under refrigeration due to microbial activity as shown by evolution of CO2. Changes in frozen samples after freezing and thawing appeared to be due to non-biological processes and this type of preservation should be avoided. A few drops of toluene added to 10 mL of simulated runoff sample stabilized ammonium concentrations at room temperature for 4 d and reduced CO2 evolution during that time substantially. Chemical preservation by adding sufficient HCl, LiCl and CuSO4 to have molarities of these chemicals up to 0.1 M strength was not acceptable because of limited reduction in microbial activity and chemical changes to the sample. Preservation effects of prestorage filtration were not consistent and this would not be a suitable procedure if total N measurements are to be included because a portion of the sample would be removed. It is apparent that microbial activity in runoff or similar types of water samples ...