The role of gross and net N transformation processes and NH4+ and NO3- immobilization in controlling the mineral N pool of a temperate mixed deciduous forest soil

In many forests of Europe and north-eastern North America elevated N deposition has opened the forest N cycle, resulting in NO3- leaching. On the other hand, despite this elevated N deposition, the dominant fate NO3- and NH4+ in some of these forests is biotic or abiotic immobilization in the soil organic matter pool, preventing N losses. The environmental properties controlling mineral N immobilization and the variation and extent of mineral N immobilization in forest soils are not yet fully understood. In this study we investigated a temperate mixed deciduous forest, which is subjected to an average N deposition of 36.5 kg N ha(-1) yr(-1), but at the same time shows low NO3- concentrations in the groundwater. The aim of this study was to investigate whether the turnover rate of the mineral N pool could explain these low N leaching losses. A laboratory N-15 pool dilution experiment was conducted to study gross and net N mineralization and nitrification and mineral N immobilization in the organic and uppermost (0-10 cm) mineral layer of the forest soil. Two locations, one at the forest edge (GE) and another one 145 in inside the forest (GF1), were selected. In the organic layers of GE and GF1, the gross N mineralization averaged 10.9 and 11.1 mg N kg(-1) d(-1), the net N mineralization averaged 6.1 and 6.8 mg N kg(-1) d(-1) and NH4+ immobilization rates averaged 3.8 and 3.6 mg N kg(-1) d(-1). In the organic layer of GE and GF1, the average gross nitrification was 3.8 and 4.6 mg N kg(-1) d(-1), the average net nitrification was -25.2 and -31.3 mg N kg(-1) d(-1) and the NO3- immobilization rates averaged 29.0 and 35.9 mg N kg(-1) d(-1). For the mineral (0-10 cm) layer the same trend could be observed, but the N transformation rates were much lower for the NH4+ pool and not significantly different from zero for the NO3- pool. Except for the turnover of the NH4+ pool in the mineral layer, no significant differences were observed between location GE and GF1. The ratio of NH4+ immobilization to gross N mineralization, gross N mineralization to gross nitrification, and NO3- immobilisation to gross nitrification led to the following observations. The NH4+ pool of the forest soil was controlled by N mineralization and NO3- immobilization was importantly controlling the forest NO3- pool. Therefore it was concluded that this process is most probably responsible for the limited NO3- leaching from the forest ecosystem, despite the chronically high N deposition rates.