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

Abstract

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 of 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 ¹⁵N 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 m 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.