Influence of 15NH4+-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 15NH4+ is applied homogeneously to soil. Since the labeling is applied to the product of the mineralization, the application of 15NH4+ should in theory not alter the mineralization rate. However, recent studies have indicated inverse relation between the amounts of 15NH4+ applied and the determined gross N mineralization rates, due to overestimated rates when ‘low’ amounts of 15NH4+ were added, as a result of preferential 15NH4+ 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 15NH4+ were added, as a result of preferential 14NH4+ consumption, when the applied 15NH4+ was incomplete distribution in the soil.