Role of fire, herbivory and forest clearing in 15N/14N balances across diverse terrestrial ecosystems

Background/Question/Methods

Nitrogen (N) is a crucial element for life and acts as a strong control on net primary productivity and global carbon storage. The nitrogen (N) cycle is exceptionally complex with fractionation of natural isotope composition affecting ecosystem ¹⁵N/¹⁴N pools widely. Past work has relied on global N isotopic patterns to constrain the steady-state transfer of N along gaseous vs. dissolved pathways, suggesting that ~30% of N losses occur via microbial denitrification. However, this approach assumes a uniformly small isotope effect of N leaching losses across terrestrial ecosystems and conditions.  Here, we examine the isotope effect associated with hydrologic leaching losses of N in different ecosystems – from grassland to forest – exposed to different agents of disturbance. We measured soil and foliar ¹⁵N/¹⁴N and performed a persulfate oxidation of TDN to determine ¹⁵N/¹⁴N in stream samples and deposition inputs across eight ecosystem sites, including oak woodland, chaparral, and coastal redwood forest, with and without different watershed-scale disturbances, including fire, herbivory, and forest harvesting.

Results/Conclusions

Consistent with past work, δ¹⁵N ((¹⁵N/¹⁴Nsample/¹⁵N/¹⁴Nair – 1) * 1000) of soil and foliage increased slightly with herbivory; decreased with clear cutting; and increased with fire in the foliage but not in soil. Foliage showed more dramatic changes than soil, likely due to the larger mass and residence time of soil N. Regardless of the disturbance regime, δ¹⁵N of TDN in stream water differed only slightly (~1 per mil) from that of bulk soil. Specifically, the N leaching isotope effect (i.e., soil δ¹⁵N-TDN δ¹⁵N stream) for oak woodland control was 0.5; for ungrazed/burned: 0.17; for grazed/burned: 0.92; and for grazed/unburned: 0.9. In the chaparral, this difference was 0.84 in undisturbed watershed and -0.29 in the burned site. At redwood sites, the N leaching isotope effect was 1.04 in the control uncut site, and 1.18 in a 20 year old clear cut site. Lower input δ¹⁵N (deposition ~0.5 per mil; N fixation ~0 per mil) compared to streamwater TDN δ¹⁵N pointed to gaseous loss via denitrification as the main driver of ecosystem N isotope enrichment. We conclude that N leaching does not represent a significant N isotope fractionating pathway, regardless of the disturbance or terrestrial ecosystem examined.