Influence of 15NH+4-application on gross N turnover rates in soil

Determination of gross N mineralization rate in soil, by use of the isotopic pool dilution approach implies that (NH4+)-N-15 is applied homogeneously to soil. Since the labeling is applied to the product of the mineralization, the application of (NH4+)-N-15 should in theory not alter the mineralization rate. However, recent studies have indicated inverse relation between the amounts of (NH4+)-N-15 applied and the determined gross N mineralization rates, due to overestimated rates when 'low' amounts of (NH4+)-N-15 were added, as a result of preferential (NH4+)-N-15 consumption. We present here results from a similar study. We observed no effect from the amount of applied NH4+ on the measured gross N mineralization rates. Our results indicate, that the inverse relation as described earlier, probably was due to underestimated rates when 'high' amounts of (NH4+)-N-15 were added, as a result of preferential (NH4+)-N-14 consumption, when the applied (NH4+)-N-15 was incomplete distribution in the soil. (C) 2003 Elsevier Science Ltd. All rights reserved.