Fast nitrate immobilization in N saturated temperate forest soils

Recent application of 15N pool dilution techniques has suggested that gross nitrate immobilization rates in temperate forest soils with low N deposition may be significantly greater than previously thought. In contrast, there are some data that suggest forest soils, which have received high N-deposition, may not immobilize NO3−. Such studies do not include quantification of the relative importance of fast and slow immobilization of NO3−. We have examined the kinetics of NO3− immobilization in two temperate forest soils exposed to a range of experimental N deposition. We found that the greatest potential for NO3− immobilization in these soils was by a fast process of immobilization and only the hardwood forest with low (ambient) N deposition showed any significant slow NO3− immobilization (typically equated with microbial immobilization). High N additions have resulted in the loss of the slow immobilization process and a reduction in the amount of fast immobilization. The patterns of NO3− immobilization we report are important for two reasons. First, they demonstrate that immobilization of N, which exhibits rapid kinetics, may play an important part in regulating the N retention capacity of forests in response to N deposition. Second, they suggest that current models which include only the slower phase of N immobilization may be inaccurate representations of N immobilization processes in soils.