Where is the missing CO2? A regional multi-species approach to trace the fate of atmospheric CO2 in Fiordland National Park, New Zealand.

A key challenge in estimating carbon dioxide (CO2) budgets is the discrepancy of bottom-up and top-down techniques. CarbonWatch-NZ is a research programme to quantify Aotearoa New Zealand’s CO2 budget using a CO2 inversion. This top down approach suggests that Aotearoa New Zealand’s native forests absorbs much more CO2 than previously thought, especially in Fiordland National Park, located at the south-western tip of the South Island. To test this finding, we designed an air sampling programme to measure a range of carbon cycle tracers in flask samples. Air samples were taken before and after the air has passed over approximately 40 km of Fiordland’s forest. Measured tracers include CO2 mole fractions, stable and radiocarbon isotopes in CO2, as well as carbonyl sulfide (COS). Most recently, we installed two in situ CO2 stations in close proximity to the flask sampling sites. We present our first results and elaborate on the process-based insights. Using stable carbon isotopes in CO2, we identify terrestrial vegetation as the primary CO2 sink. This finding is supported by shipboard observations of ocean-atmosphere CO2 exchange, precluding that the large CO2 uptake as suggested by the CO2 inversion is caused by surface water uptake within the Fiordland region. Our multi-tracer approach provides information on CO2 respiration and photosynthetic CO2 uptake, enabling us to calculate snapshot CO2 budgets. We discuss future work to compare these snapshot CO2 budgets to the outputs of different modelling techniques used in bottom-up and top-down approaches.