Evaluation of an undisturbed-soil incubation method as an index of soil nitrogen availability in a limed and non-limed soil
Nitrogen mineralization potentials were determined using an undisturbed-soil incubation procedure. The purpose of this study was to evaluate the above-mentioned method as an index of soil nitrogen availability. Undisturbed soil samples (0-150 and 150-300 mm) of a limed (pH 7.3) and an equivalent non-limed (pH 4.9) soil were incubated at 30°C and field water capacity. After every consecutive 10-day period, four samples of each treatment/depth combination were removed, dried and analysed for mineral N content. This was conducted over a period of 60 days. Distinction was made between changes in NH4+ and NO3- contents of the incubated soil, assuming the NO3- release represented N release from the soil due to high nitrification rates. The release of NO3- was depicted by a straight-line regression, where the slope was equal to the rate constant of mineralization (k). The mineralization potential (N0) was assumed to be the maximum mineral N content obtained in the soil during incubation. Comparing the amount of CO2 that evolved per unit NO3- released, it was found that the mineralizing microbial activity was lower, but more efficient in releasing N at pH 7.3 than at pH 4.9. Both ammonification and nitrification proved to be pH-sensitive processes. Although unsatisfactory for making fertilizer recommendations, the total N content of the soil was found to be a good indication of its N mineralization potential. The method used in this study gave no indication of the fraction of N0 that is released during the different seasons. The use of undisturbed samples makes this method cumbersome because of the large number of samples needed to accommodate a large spatial variability component.