BIOGEOCHEMICAL INSIGHTS INTO CONTROLS ON LACUSTRINE STABLE NITROGEN ISOTOPE RATIOS

Stable isotope ratios of nitrogen (d¹⁵N) in bulk lacustrine organic matter are often interpreted in the context of past changes in primary production, which may be ultimately linked to past changes in climate or anthropogenic watershed alteration. However, the fundamental controls on these values are often uncertain, which, unlike d¹³C, are less frequently assessed using compound-specific biomarker techniques. Here we pair d¹⁵N values of organic matter in lake sediment cores and surface lake sediment samples from the central tropical Pacific island of Kiritimati (2°N, 157°W) with microbial taxonomic data obtained from 16S rRNA gene sequence libraries to investigate microbial controls on past changes d¹⁵N, and lacustrine productivity over the past 1500 years. Across a suite of 25 surface sediment samples, we find a significant, positive correlation between d¹⁵N values and the relative abundance of the Nitrospirae and Gemmatimonadetes phyla, which are associated with nitrification and nitrous oxide reduction, respectively. Thus, higher d¹⁵N values in this environment are likely due to the fractionation of nitrogen isotopes associated with key components of the nitrogen cycle, namely, aerobic nitrous oxide reduction and nitritification. Additionally, samples with high d¹⁵N, Nitrospirae, and Gemmatimonadetes abundance are also coincident with lower dissolved oxygen content in the lake water, suggesting this relationship is strong at lower levels of dissolved oxygen content, despite their aerobic nature. A period of high d¹⁵N values in one Kiritimati sediment record from 1200 to 1400 CE co-occurs with high d¹³C, %C, and low %N values, and suggest stronger nitrogen cycling and nitrate production in this lake following an arid, high salinity period during the Medieval Climate Anomaly ultimately contributed to higher primary production.