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

• Main Authors: S. K. E. Goulden, N. Ohkouchi, K. H. Freeman, Y. Chikaraishi, N. O. Ogawa, H. Suga, O. Chadwick, B. Z. Houlton
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-10-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/16/3869/2019/bg-16-3869-2019.pdf

<p>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 <span class="inline-formula">CO₂</span> 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 <span class="inline-formula"><i>δ</i>¹⁵N</span>. Specifically, we test the hypothesis that <span class="inline-formula"><i>δ</i>¹⁵N</span> of plant leaves (<span class="inline-formula"><i>δ</i>¹⁵N_leaf</span>) is recorded in <span class="inline-formula"><i>δ</i>¹⁵N</span> of pheophytin <span class="inline-formula"><i>a</i></span> (<span class="inline-formula"><i>δ</i>¹⁵N_pheo</span>) along the leaf–litter–soil continuum across an array of ecosystem climate conditions and plant functional types (C<span class="inline-formula">₃</span>, C<span class="inline-formula">₄</span>, legumes, and woody plants). The <span class="inline-formula"><i>δ</i>¹⁵N</span> 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 <span class="inline-formula"><i>a</i></span> present in amounts required for isotopic analysis (<span class="inline-formula">&gt;10</span>&thinsp;<span class="inline-formula">nmol</span>). <span class="inline-formula"><i>δ</i>¹⁵N_pheo</span> in leaves, litter, and soil track <span class="inline-formula"><i>δ</i>¹⁵N_leaf</span> of plant leaves. We find potential for <span class="inline-formula"><i>δ</i>¹⁵N_pheo</span> records from soil to provide proxy information on <span class="inline-formula"><i>δ</i>¹⁵N_leaf</span>.</p>