Natural Abundance of Nitrogen‐15 in a Forest Soil

Abstract

Because of measurement difficulties, only a few studies on natural 15N abundance (δ15N) of inorganic N in forest soil have been pursued despite its importance for interpretations of plant δ15N signatures. To investigate stable N isotope ratios in inorganic N, the δ15N values and concentrations of total N, NH+4‐N, and NO−3‐N of forest mineral soils in four profiles were measured along a slope (altitude of 765–870 m) in a coniferous (Japanese red cedar, Cryptomeria japonica D. Don) forest in Japan. Generally, the δ15N values of total N, NH+4‐N, and NO−3‐N increased with increasing soil depth. The values of δ15N ranged from 1.0 to 6.8‰, 2.5 to 15.6‰, and −14.8 to 5.6‰ for total N, NH+4‐N, and NO−3‐N, respectively. Additionally, the δ15N values were different between NH+4‐N and NO−3‐N for each soil depth. Thus, it was concluded that the assumptions about inorganic N used in interpretation of plant δ15N values were valid. Moreover, on upper slope sites where soil inorganic N was predominantly NH+4‐N, the order of δ15N was generally total N > NH+4‐N > NO−3‐N for each depth, whereas the order of δ15N was NH+4‐N > total N > NO−3‐N on lower slope sites where NH+4‐N was less dominant as soil inorganic N and relatively high net nitrification rates were measured. Our results suggested that nitrification plays an important role in regulating δ15N in forest‐soil N.

Stable Nitrogen Isotopes in a Forested Watershed in Taiwan

“…Where studies have shown net rates of N mineralization and nitrification to vary among hillslope positions in forest soils, higher rates occurred at footslope locations (Hiroto and Haibara 1994;Koba et al 1998). In WS1, there is little evidence to suggest that net nitrification rates are different at the higher and lower elevation sites (Owen et al 2010).…”

“…The limited data available (Koba et al 1998; suggest that measurements of the isotopic composition in both soil extractable pools and soil solution NO 3 and NH 4 could contribute to understanding regional differences in rates and patterns in N cycling. Currently there are no measurements of the N stable isotope composition of atmospheric inputs, soil solution or denitrification products available for forests in Taiwan.…”

Ectomycorrhizal fungal taxa differing in response to nitrogen deposition also differ in pure culture organic nitrogen use and natural abundance of nitrogen isotopes

“…Discrimination that occurs against the heavier N isotope ( 15 N) during mineralization and nitrification of organic N compounds should lead to the following enrichment patterns in δ 15 N signatures in labile soil N pools: organic N ≥ NH 4 + > NO 3 -, at least under low N conditions (Nadelhoffer & Fry, 1994;Koba et al , 1998). However, the diversity of soil N pools and processes can lead to more complex patterns of pool labeling, for example high nitrification rates can enrich NH 4 + pools relative to soil organic matter (Koba et al , 1998;Hobbie et al , 2000).…”

“…However, the diversity of soil N pools and processes can lead to more complex patterns of pool labeling, for example high nitrification rates can enrich NH 4 + pools relative to soil organic matter (Koba et al , 1998;Hobbie et al , 2000).…”

Natural15N and13C abundance in Andisols influenced by long-term fertilization management in Japan

“…In temperate forest soil, 8 15 N values in soil largely depended on net nitrification (Koba et al 1998), while in highly weathered tropical soils, these values resulted from the selective losses of 15N-depleted nitrate associated with both nitrification and denitrification (Piccolo et al 1994).…”