The role of gross and net N transformation processes and $NH_4^ + $ and $NO_3^ - $ immobilization in controlling the mineral N pool of a temperate mixed deciduous forest soil

• Published in: Plant and soil 2004-07, Vol.264 (1/2), p.349-357
• Main Authors: Vervaet, H., Boeckx, P., Boko, A.M.C., Van Cleemput, O., Hofman, G.
• Format: Article
• Language: English
• Subjects: Agricultural soils; Agronomy. Soil science and plant productions; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Chemical, physicochemical, biochemical and biological properties; Deciduous forests; Forest soils; Forestry; Fundamental and applied biological sciences. Psychology; General forest ecology; Generalities. Production, biomass. Quality of wood and forest products. General forest ecology; Grassland soils; Minerals; Nitrification; Nitrogen; Organic matter; Organic soils; Physics, chemistry, biochemistry and biology of agricultural and forest soils; Soil analysis; Soil biochemistry; Soil science; Synecology; Terrestrial ecosystems
• ISSN: 0032-079X; 1573-5036
• DOI: 10.1023/B:PLSO.0000047766.16919.5e

In many forests of Europe and north-eastern North America elevated N deposition has opened the forest N cycle, resulting in $NO_3^ - $ leaching. On the other hand, despite this elevated N deposition, the dominant fate of $NO_3^ - $ and $NH_4^ + $ 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 $NO_3^ - $ 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 NHj 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⁻¹d⁻¹ and the $NO_3^ - $ 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 $NH_4^ + $ pool and not significantly different from zero for the $NO_3^ - $ pool. Except for the turnover of the $NH_4^ + $ pool in the mineral layer, no significant differences were observed between location GE and GF1. The ratio of NH^ immobilization to gross N mineralization, gross N mineralization to gross nitrification, and $NO_3^ - $ immobilisation to gross nitrification led to the following observations, The $NH_4^ + $ pool of the forest soil was controlled by N mineralization and $NO_3^ - $ immobilization was importantly controlling the forest $NO_3^ - $ pool. Therefore it was concluded that this process is most probably responsible for the limited $NO_3^ - $ leaching from the forest ecosystem, de