Nitrogen dynamics in contrasting forest ecosystems exposed to enhanced atmospheric N deposition

Despite chronically enhanced nitrogen (N) deposition to forest ecosystems in Europe and NE America, considerable N retention by forests has been observed. It is still unclear which factors determine N retention in forest soils. However, this knowledge is crucial to assess the impact of changing anthropogenic N emissions on future N cycling and N loss of forests. For coniferous and deciduous forest stands at comparable sites, it is known that both N deposition to the forest floor as well as N loss by leaching below the rooting zone are significantly higher in coniferous stands (De Schrijver et al., 2007). In addition, the N loss in coniferous stands is often more enhanced than can be explained by the higher N input only, which suggests lower N retention by coniferous stands and may be related to differences in litter quality, microbial activity, and N uptake by plant roots. To test this hypothesis, we studied the effect of forest type on N retention. N dynamics were examined for two adjacent forest stands (pedunculate oak (Quercus robur L.) and Scots pine (Pinus sylvestris L.)) on a well-drained soil type and with a similar stand history, which are located in a region with high N deposition (Belgium). Firstly, input-output N budgets were established by quantifying atmospheric deposition and leaching, which confirmed the above finding of higher N deposition and disproportionately higher N loss by the pine stand than the oak stand. Secondly, the fate of inorganic N within the ecosystems was studied by spraying dissolved 15N onto the forest floor, both as ammonium (NH4+) and nitrate (NO3-). The 15N recovery over time in organic and mineral soil layers, tree roots, water leaching, ferns, foliage, and stem wood was compared between the two forest stands and N treatments. Thirdly, in situ gross N transformation rates in undisturbed mineral forest soils were determined via a 15N tracing approach (Müller et al., 2007). Meaningful differences between the two forest stands were found for the rates of mineralisation, heterotrophic and autotrophic nitrification, and NH4+ and NO3- immobilisation. Unexpectedly, dissimilatory NO3- reduction to NH4+ (DNRA) was detected in the oak soil. This process has mainly been described for unpolluted soils (e.g., Huygens et al., 2008), and to the best of our knowledge, this is the first report of DNRA under field conditions in a temperate forest soil under high N deposition.Despite chronically enhanced nitrogen (N) deposition to forest ecosystems in Europe and NE America, considerable N retention by forests has been observed. It is still unclear which factors determine N retention in forest soils. However, this knowledge is crucial to assess the impact of changing anthropogenic N emissions on future N cycling and N loss of forests. For coniferous and deciduous forest stands at comparable sites, it is known that both N deposition to the forest floor as well as N loss by leaching below the rooting zone are significantly higher in coniferous stands (De Schrijver et al., 2007). In addition, the N loss in coniferous stands is often more enhanced than can be explained by the higher N input only, which suggests lower N retention by coniferous stands and may be related to differences in litter quality, microbial activity, and N uptake by plant roots. To test this hypothesis, we studied the effect of forest type on N retention. N dynamics were examined for two adjacent forest stands (pedunculate oak (Quercus robur L.) and Scots pine (Pinus sylvestris L.)) on a well-drained soil type and with a similar stand history, which are located in a region with high N deposition (Belgium). Firstly, input-output N budgets were established by quantifying atmospheric deposition and leaching, which confirmed the above finding of higher N deposition and disproportionately higher N loss by the pine stand than the oak stand. Secondly, the fate of inorganic N within the ecosystems was studied by spraying dissolved 15N onto the forest floor, both as ammonium (NH4+) and nitrate (NO3-). The 15N recovery over time in organic and mineral soil layers, tree roots, water leaching, ferns, foliage, and stem wood was compared between the two forest stands and N treatments. Thirdly, in situ gross N transformation rates in undisturbed mineral forest soils were determined via a 15N tracing approach (Müller et al., 2007). Meaningful differences between the two forest stands were found for the rates of mineralisation, heterotrophic and autotrophic nitrification, and NH4+ and NO3- immobilisation. Unexpectedly, dissimilatory NO3- reduction to NH4+ (DNRA) was detected in the oak soil. This process has mainly been described for unpolluted soils (e.g., Huygens et al., 2008), and to the best of our knowledge, this is the first report of DNRA under field conditions in a temperate forest soil under high N deposition.C