Effect of vegetation cover on carbon dioxide fluxes and dissolved δ13C-CO2 composition in headwater streams, Auckland Region, New Zealand

Abstract:

Streams and rivers contribute disproportionately to Earth’s carbon cycle. However, the amount of carbon emitted from streams and the underlying processes driving stream CO₂ dynamics are still poorly understood, in particular for flashy headwater catchments. The objective of this study were to: (1) quantify CO₂ emissions from streams and (2) determine the sources (biogenic, geogenic, terrestrial, aquatic) of dissolved CO₂ in the streams of two headwater catchments differing in vegetation cover. The study was conducted in a native forest (16.5 ha) and a grassland (10.6 ha) catchment at Te Muri Regional Park, Auckland Region, New Zealand. Samples were taken during austral summer (November 2019 -February 2020). Rainfall during this period was less than 125 mm. Dissolved CO₂ was extracted using a headspace equilibration technique. The δ¹³C-CO₂ composition was determined using a Picarro G2131-i. Carbon dioxide emissions from the stream in the native forest catchment ranged from 0.1 to 0.6 g C m^-2 h^-1 (mean: 0.3 g C m^-2 h^-1). Significantly higher stream CO₂ emissions were measured in the grassland catchment (0.05-2.5 g C g C m^-2 h^-1; mean: 0.5 g C m^-2 h^-1). The dissolved δ¹³C-CO₂ composition in the native forest stream (-21.9 to -29.7 ‰; mean: -26.1±2.7 ‰) was more negative than the grassland stream (-7.5 to –25.6; mean: -19.5±6.3 ‰) (p=0.0021). The isotopic composition of dissolved CO₂ suggests that CO₂ in both streams mainly originated from biological sources and reflect the isotopic composition of the vegetation cover. The vegetation in the grassland catchment is dominated by Kikuyu (Cenchrus clandestinus), a C4 grass. Soil respiration in the grassland and native forest catchment (dominated by C3 trees) had a mean δ¹³CO₂ signature of -13.6 ‰ and -25.8 ‰, respectively. Our findings also suggest diverging patterns in dissolved δ¹³C-CO₂ values between catchments across the austral summer, highlighting that seasonal changes in micrometeorological conditions as well as source contributions need to be considered to assess the sources and sinks of dissolved CO₂ in headwater catchments.