In search of a soil-based, compound-specific proxy for plant leaf nitrogen isotope ratios

Background/Question/Methods

Nitrogen (N) is the nutrient which most commonly limits terrestrial primary production, and has an important regulatory role on the structure and functioning of ecosystems. Reconstructing past terrestrial N cycles would inform our understanding of the behavior of this nutrient in response to future changes in climate, atmospheric CO₂ levels, and plant community composition. Natural abundance stable isotopes of plant leaf N (δ¹⁵N_f) are useful integrators of N cycle processes in modern terrestrial systems; few tools have been available, however, for reconstructing past plant δ¹⁵N_f. δ¹⁵N of bulk materials is subject to alteration during decomposition and diagenesis. Compound-specific sources of δ¹⁵N help avert these concerns, but are a tool with particular challenges in the soil environment. We here explore the potential for a soil-based compound-specific proxy for δ¹⁵N_f. Specifically, we target the nitrogenous plant compound, pheophytin a (pheo a), a degredation product of the chlorophyll molecule long used as a paleo-proxy in subaqueous sediments. We here explore nitrogen isotope offsets across the leaf-litter-soil continuum, to evaluate whether plant δ¹⁵N_f may be quantitatively retained in pheo a δ¹⁵N.

Results/Conclusions

We find potential for pheo a in soil to provide a window into past plant δ¹⁵N_f. The compound is present at depth in soil, though concentration falls off dramatically down-profile. We here report plant pheo a δ¹⁵N (δ¹⁵N_phe-leaf) and bulk leaf δ¹⁵N_f of six species across a rainfall gradient in the Kohala Mountains of Hawaii, and find that δ¹⁵N_phe-leaf tracks δ¹⁵N_f over a wide range of environmental conditions (210mm – 2500mm annual precipitation and 23°C – 17°C mean annual temperature). Isotopic fidelity is better than in the aquatic environment where the proxy was originally developed. Comparison of bulk soil and pheo  a-specific δ¹⁵N in soil depth profiles across the same rainfall gradient reveals that the soil pheo  δ¹⁵N (δ¹⁵N_phe-soil) record is different from the bulk soil δ¹⁵N (δ¹⁵N_s) record. δ¹⁵N_phe-soil does not follow δ¹⁵N_s, nor does it track δ¹⁵N_phe-leaf of modern plants at the same sites, therefore δ¹⁵N_phe-soil may reflect past plant δ¹⁵N_f.