Elucidating Mineral N Retention Pathways in an Old-Growth South Chilean Temperate Forest Using a 15N Tracing Model.
Dries Huygens1, Pascal Boeckx1, Oswald Van Cleemput1, Roberto Godoy2, and Christoph Müller3. (1) Laboratory of Applied Physical Chemistry - ISOFYS, Faculty of Bioscience Engineering, Ghent Univ, Coupure 653, Gent, 9000, Belgium, (2) Institute of Botany, Univ Austral de Chile, Casillas 657, Valdivia, Chile, (3) Dept of Plant Ecology, Justus-Liebig-Univ Giessen, Heinrich-Buff-Ring 26-32, Giessen, 35392, Germany
Nitrogen cycling studies in unpolluted forests can provide valuable information on kinetics and pathways of soil N transformations, and how N deposition might affect the N cycle. In a laboratory experiment, gross N consumption and production rates were determined in the mineral soil layer of an old-growth evergreen unpolluted forest in southern Chile, by a combination of 15N enrichment experiments and a dynamic 15N tracing model. Tracing models are powerful tools, allowing quantification of the extent of simultaneous N transforming fluxes. They provide significant surplus values compared to classical analytical solutions. The possibility to account for recycled N material and different kinetic parameter options are main advantages. The 15N tracing model included six different N pools (labile organic matter (Nmom), recalcitrant organic matter (NrSOM), NH4+, NO3-, and two dissolved organic N pools (NH4+-DON and NO3--DON)), and ten important gross transformation fluxes. The model was able to simulate the obtained data set, including three different labeling treatments of NH4+ and NO3-. Results indicated a fast turnover of the NH4+ and DON pool, contrary to slow N dynamics related to the NO3- pool. Neither biotic nor abiotic NO3- immobilization reactions towards insoluble soil organic matter (SOM) were observed in this forest. However, low gross nitrification fluxes act as a mechanism preventing accumulation of NO3- in the ecosystem, and retaining N within the soil profile. Immobilization processes having NH4+ as substrate include fluxes towards Nmom, NrSOM and DON pools. The DON immobilization was found to be most important flux in this soil, indicating the potential of NH4+ as substrate for DON formation. DON can be adsorbed at insoluble organic matter, but high DON leaching fluxes observed in this forest indicate likewise the potential of DON losses, originating from the NH4+ pool. N losses in gaseous form were negligible in this forest as NH4+ was shown to be the end product of NO3- reduction via dissimilatory nitrate reduction (DNRA). It was found that this forest ecosystem managed low inorganic N inputs with a high efficiency in order to avoid N losses from the ecosystem. However, the absence of a real NO3- immobilization mechanism indicates a potential risk for NO3- leaching losses, eventually under increased future N deposition scenarios, influencing forest vitality and water quality of the surrounding watersheds.