Strong correspondence between nitrogen isotope composition of foliage and chlorin across a rainfall gradient: Implications for paleo-reconstruction of the nitrogen cycle

Nitrogen (N) availability influences patterns of terrestrial productivity and global carbon cycling, imparting strong but poorly resolved feedbacks on Earth's climate system. Central questions concern the timescale of N cycle response to elevated CO₂ concentration in the atmosphere and whether availability of this limiting nutrient increases or decreases with climate change. Nitrogen isotopic composition of bulk plant leaves provides information on large-scale patterns of N availability in the modern environment. Here we examine the utility of chlorins, degradation products of chlorophyll, hypothesized to persist in soil subsequent to plant decay, as proxies for reconstructing past plant δ¹⁵N. Specifically, we test the hypothesis that δ¹⁵N of plant leaves (δ¹⁵Nleaf) is recorded in δ¹⁵N of pheophytin a (δ¹⁵N_pheo) along the leaf-litter-soil continuum across an array of ecosystem climate conditions and plant functional types (C₃, C₄, legumes, and woody plants). The δ¹⁵N of live foliage and bulk soil display marked declines with increasing rainfall, consistent with past studies in Hawaii and patterns worldwide.We find measurable chlorin concentrations along soil-depth profiles at all sites, with pheophytin a present in amounts required for isotopic analysis (> 10 nmol). δ¹⁵N_pheo in leaves, litter, and soil track δ¹⁵N_leaf of plant leaves.