The terrestrial carbon cycle in transition: tracking changes using novel tracers on multiple timescales

Soils store more carbon than the atmosphere and terrestrial vegetation combined, therefore, changes in soil carbon storage can affect the global carbon budget. One of the forms in which carbon is exported from soil is as dissolved organic carbon (DOC). Upon release from soil, components of DOC can be oxidised to CO₂, thus potentially contributing to the global climate crisis. In freshwater systems, DOC is considered a pollutant because of its detrimental impact on freshwater ecology and biodiversity (by blocking light and reducing photosynthesis and altering thermal-mixing regimes in lakes), increasing freshwater acidity, transporting nutrients (particularly N and P) and trace metal pollutants. Further, during the water treatment process (for human consumption), DOC can form toxic by-products. This project aimed to shed new light on terrestrial carbon dynamics by testing the hypothesis that climate can drive increases in soil DOC export. This research focused on past soil DOC exports in New Zealand, an archipelago located in the mid-latitudes of the Southern Hemisphere. New Zealand’s palaeo-environmental archives (e.g. speleothems and lake sediments) provide a unique opportunity to reconstruct the concentrations and characteristics of DOC under changing climate conditions over centennial/millennial timescales. New Zealand was uninhabited by humans until the 13th century. Palaeo-environmental records that extend beyond the 13th century, therefore, exclude interference from anthropogenic factors, thus enabling the climate hypothesis to be assessed. Using lake sediment archives to reconstruct DOC concentrations and characteristics through the past 14,000 years This study explored the use of 3D EEM (excitation-emission matrix spectroscopy) fluorescence of water extractable dissolved organic matter (WEDOM) from lake sediments in reconstructing both past soil export of DOM and past trophic status. DOM and water quality are linked because DOM can contain nutrients (N, and P), provide an energy source to heterotrophs and algae, can reduce light penetration, and lead to reduced dissolved oxygen concentrations. Mean trophic level index (TLI) monitoring data of the lake water columns were compared against protein-like fluorescence from contemporary sedimentary WEDOM from ten lakes, producing strong positive correlations with total phosphorous (R²= 0.81), and TLI scores (R²= 0.74 ), and weak positive correlations with total nitrogen (R²= 0.5) and chlorophyll a (R²= 0.44). The equation produced from the correlation between TLI scores and protein-like fluorescence of WEDOM was used to reconstruct TLI scores through the past 13,700 years at Adelaide Tarn (a climatically sensitive, sub-alpine lake located in the north west of the South Island), indicating predominantly oligotrophic conditions throughout the record. Humic-like DOM fluxes in Adelaide Tarn responded to known climate shifts over the course of the Holocene, indicating a climatic control on soil DOM production and/or export. The results indicate that WEDOM…