Effects of topography and fine roots on soil nitrogen transformations in acidic coniferous forest soils

Topographical variation shapes soil organic matter (SOM) accumulation, influencing soil nitrogen (N) flows, including fine root uptake. In this study, we quantified fine root uptake of inorganic N (NH₄⁺ and NO₃⁻) and its contribution to gross consumption in surface soils (0–2.5 cm) using in situ incubation on upslope and downslope positions in an acidic coniferous forest in Japan. We also examined differences in specific N transformation rates under incubations with severed roots (conventional soil cores (CSCs)) and those maintaining intact structures (virtual soil cores (VSCs)). Our results showed that fine roots in upslope positions had lower net NH₄⁺ uptake (0.13 mg N·m⁻²·day⁻¹) and contributed marginally (approximately 0.1%) to gross NH₄⁺ consumption, whereas downslope positions exhibited notably higher contributions from fine root uptake and nitrification (approximately 30%). Microbial immobilization appeared to be the dominant pathway of NH₄⁺ consumption on upslope positions, likely associated with the accumulation of SOM. Contrarily, variation in NO₃⁻ consumption pathways between slope positions was limited. Slope position exerted a pronounced effect on gross NH₄⁺ consumption rates (F = 37.0; P < 0.001), with enhanced immobilization upslope. Gross nitrification rates in VSC were higher downslope. Additionally, they were significantly influenced by core type (F = 15.3; P < 0.01) and were elevated in the absence of intact fine roots on upslope positions, which was unlikely due to reduced root NH₄⁺ uptake. Overall, these findings provide new field-based insights into the role of fine roots in ecosystem N strategies.