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

A robust understanding of CO₂ fluxes is vital for accurate estimates of future atmospheric carbon dioxide (CO₂) levels. However, current estimation techniques show large disagreement on terrestrial CO₂ fluxes. In particular, atmospheric data suggests a larger net CO₂ sink than previously thought for New Zealand's Fiordland region, one of our largest indigenous, temperate rainforests with an area of 12,600 km² and a mean annual precipitation of up to 15 m.

We will test the hypothesis that Fiordland's forest is more productive than previously thought with flask samples in air parcels before and after the air has passed over Fiordland's forests. We will use a multi-tracer ensemble to determine biogeochemical trace-gas exchange on spatial scales of 50-100 km (Figure 1A). While measurements of CO₂ mole fractions indicate CO₂ level changes directly, measurements of δ¹³C-CO₂ distinguish the forest's net CO₂ uptake due to the characteristic isotope fractionation during photosynthesis. A modified Keeling-Plot-Analysis of existing CO₂ mixing and isotope ratio data from a coastal site (Baring Head) and inland site (Lauder) suggests this approach is suitable to resolve atmospheric gradients on 100 km scales (Figure 1B). Moreover, we will use COS (carbonyl sulfide) as an independent tracer for photosynthetic CO₂ uptake, and Δ¹⁴C-CO₂ to examine ecosystem respiration and residence times. We will also analyse δ¹⁸O, Δ¹⁷O and clumped isotopes in CO₂ and assess the suitability of these tracers to further constrain Fiordland's CO₂ fluxes. Our measurement results will be interpreted using state-of-the-art bottom-up and top-down models.

This study will help to better understand regional CO₂ fluxes on process levels and assess the most suitable tracers for regional carbon cycle observations in comparable ecosystems.

Figure caption: A) schematic of observation strategy and selected tracers, B) modified Keeling-Plot-Analysis using CO₂ mixing and isotope ratios from Baring Head and Lauder during strong Southerly events.