Effects of spatial variations on soil nitrogen transformations in Chamaecyparis obtusa forest through in situ 15N tracing method

Soil inorganic nitrogen (N), i.e., NH₄⁺ and NO₃^-, are essential resources for tree growth in forest ecosystems. Input of these inorganic resources are regulated by ammonification and nitrification and output are consumed either by abiotic or biotic (soil microbes, tree fine roots) processes. To the best of our knowledge, although previous studies have discussed net uptake rates of fine roots among different soil groups or vegetation types, variations within a local scale (same soil group and vegetation) remain an enigma. Moreover, since widely-used experiment set-ups in soil N transformation studies exclude live fine roots, uptake strategy of trees and their competition for inorganic nutrients with microbes are unknown. Therefore, this study aims to clarify 1) net uptake rates of fine roots; and 2) uptake strategy of the same tree species and the ratio of microbial/plant assimilation along a hillslope.

An in situ incubation combining ¹⁵N tracing method with virtual soil cores was performed in Chamaecyparis obtusa (Japanese cypress) forests at up and down sites of a hillslope. ¹⁵N-labeled moieties (2.2 mg ¹⁵NH₄NO₃ g^-1 soil and 0.2 mg NH₄¹⁵NO₃g^-1 soil, both are 98+ atom% ¹⁵N) were injected evenly through the cores located around the trees with five replicates. A-horizon soil was sampled after 0.25 h and 24 h. Soil and fine roots were sent for further analyses after sieving. The soil-environmental factors were determined.

Differences in the soil-environmental variables were observed at up and down slopes, including soil pH, NH₄⁺, NO₃^-, total dissolved N, microbial biomass carbon (C) and N, total C and C/N ratio. Fine root biomass was higher at up slope (1.5 kg m^-3) than down slope (0.8 kg m^-3) while net uptake rates of fine roots in both ¹⁵NH₄NO₃ (11 and 20 μg ¹⁵N g^-1 roots d.w. d^-1) and NH₄¹⁵NO₃ (4.7 and 8.3 μg ¹⁵N g^-1 roots d.w. d^-1 at up and down slope, respectively) were higher at down slope. Ammonification and nitrification rates were also higher at down slope (0.6 and 3.2 mg N kg^-1 d^-1 for ammonification and 0.1 and 1.1 mg N kg^-1 d^-1 for nitrification at up and down slope, respectively), where soil pH was higher and C/N ratio was lower. However, net uptake amount per core did not demonstrate any trend at both slopes. Such results suggest that fine roots may try to ensure the supply of nutrients to trees by increasing their biomass when less soil inorganic N can be produced. Soil pH and C/N ratio could play a key role in determining uptake strategy of trees through affecting soil N transformation rates.

In addition, the ratio of microbial/plant assimilation of N was higher at up slope for ¹⁵NH₄⁺ and for ¹⁵NO₃^- to a lesser extent. Since soil moisture content was higher at down slope, such results might suggest a higher mass transfer of N nutrients induced by a higher mass flow of water. Therefore, higher N concentration could be maintained onto root surfaces at down slope, decreasing acquisition rates by microbes and increasing net uptake rates by fine roots.