Dynamics of atmospheric N deposition in a temperate calcareous forest soil

In temperate forest ecosystems, soil acts as a major sink for atmospheric N deposition. A 15N labeling experiment in a hardwood forest on calcareous fluvisol was performed to study the processes involved. Low amounts of ammonium (15NH4 +) or nitrate (15NO3 −) were added to small plots. Soil samples were taken after periods ranging from 1 h to 1 yr. After 1 d, the litter layer retained approximately 28% of the 15NH4 + tracer and 19% of 15NO3 − The major fraction of deposited N went through the litter layer to reach the soil within the first hours following the tracer application. During the first day, a decrease in extractable 15N in the soil was observed (15NH4 +: 50 to 5%; 15NO3 −: 60 to 12%). During the same time, the amount of microbial 15N remained almost constant and the 15N immobilized in the soil (i.e., total 15N recovered in the bulk soil minus extractable 15N minus microbial 15N) also decreased. Such results can therefore be understood as a net loss of 15N from the soil. Such N loss is probably explained by NO3 − leaching, which is enhanced by the well-developed soil structure. We presume that the N immobilization mainly occurs as an incorporation of deposited N into the soil organic matter. One year after the 15N addition, recovery rates were similar and approximately three-quarters of the deposited N was recovered in the soil. We conclude that the processes relevant for the fate of atmospherically deposited N take place rapidly and that N recycling within the microbes–plants–soil organic matter (SOM) system prevents further losses in the long term.