Abstract

Human activities have significantly increased the biological availability of Nitrogen (N) on a global scale since the industrial revolution. Yet, human activities have perturbed the natural N cycle at local or regional scales since the development of agriculture in many regions of the world. Key modifiers include: 1) land-use changes, 2) the addition of N to improve crop yields and 3) slash and burn practices. We use biogeochemical evidence from a lacustrine record from Lago Vichuquén in coastal central Chile to examine how local human activities have changed N input into the lake the SC. Two (2 m) sediment cores were recovered (Lemal-Vic11-2A-1g-1) in 2011 and 2015 (Vic15-2A-1G). Cores were sampled every 1 cm for bulk sediment analyses of δ15N, δ13C, Total Nitrogen (TN) and Total Carbon (TC) and every 4 mm for XRF analyses. AMS 14C dating shows that the cores span the last XX cal yr BP. The main results show rapid increases in TC and C/N ratios during the last centuries, resulting from an increase in allocthonous organic matter in the catchment. The concomitant decrease in δ13C and δ15N values (from -26.5 to -30.0‰ and +9 to +4.1‰ between 1700 and 1960 AD, respectively), however, suggests changes in N availability and enhanced primary productivity. We propose that the decrease of δ15N sediment values is due to significant anthropogenic influence in the last 150 years, when large-scale agricultural development and deforestation conspired to change N input into the local watershed. These trends continue to the present.