Transformations of added and indigenous nitrogen in gnotobiotic soil: A comment on the priming effect

When 15N-labelled fertilizer is added to soil, it frequently appears that more indigenous soil-N is mineralized-a phenomenon called the “priming effect”. We have made an experimental study of the interaction of added and indigenous N during laboratory incubations with well-defined N concentrations, microbial populations and incubation conditions. Populations of a single species each of bacteria and amoebae were inoculated into propylene-oxide sterilized soil and their growth, respiration and N-mineralization were monitored for 34 days. In one experiment, bacterial numbers doubled (from 17.2 to 36.7 × 10 8 g −1) and respired C nearly doubled (from 966 to 1892 μg g −1) as added N (ammonium sulfate) increased from 0 to 70 μg g −1. In a second experiment, mineral-N following incubation increased from 2.6 to 55.9 μg g −1 and mineral-N of soil origin increased from 2.6 to 16.3 μg g −1 as added N increased from 0 to 100 μg g −1. However, in the same experiment, the amount of unlabelled soil-N required to dilute the 15N-labelled ammonium to the observed atom% 15N decreased from 72.1 to 41.2 μg g −1. Mineralization of soil-N was enhanced (an apparent priming effect) in spite of decreased interaction with unlabelled-N. This “priming effect” resulted from increased net N-mineralization that accompanied increased N-fertilization so long as mineral-N concentrations remained low enough to limit soil microbial activity.