Patterns of δ¹⁵N in forest soils and tree foliage and rings between climate zones in relation to atmospheric nitrogen deposition: A review

The stable nitrogen (N) isotope ratio (δ¹⁵N) of forest samples (soils, tree foliage, and tree rings) has been used as a powerful indicator to explore the responses of forest N cycling to atmospheric N deposition. This review investigated the patterns of δ¹⁵N in forest samples between climate zones in relation to N deposition. Forest samples exhibited distinctive δ¹⁵N patterns between climate zones due to differences in site conditions (i.e., N availability and retention capacity) and the atmospheric N deposition characteristics (i.e., N deposition rate, N species, and δ¹⁵N of deposited N). For example, the δ¹⁵N of soil and foliage was higher for tropical forests than for other forests by >1.2 ‰ and 4 ‰, respectively due to the site conditions favoring N losses coupled with relatively low N deposition for tropical forests. This was further supported by the unchanged or increased δ¹⁵N of tree rings in tropical forests, which contrasts with other climate zones that exhibited a decreased wood δ¹⁵N since the 1920s. Subtropical forests under a high deposition of reduced N (NH_y) had a lower δ¹⁵N by 2–5 ‰ in the organic layer compared with the other forests, reflecting high retention of ¹⁵N-depleted NH_y deposition. At severely polluted sites in East Asia, the decreased δ¹⁵N in wood also reflected the consistent deposition of ¹⁵N-depleted NH_y. Though our data analysis represents only a subset of global forest sites where atmospheric N deposition is of interest, the results suggest that the direction and magnitude of the changes in the δ¹⁵N of forest samples are related to both atmospheric N and site conditions particularly for tropical vs. subtropical forests. Site-specific information on the atmospheric N deposition characteristics would allow more accurate assessment of the variations in the δ¹⁵N of forest samples in relation to N deposition.