Carbon and water fluxes in two adjacent Australian semi-arid ecosystems

The southern hemisphere and especially Australian arid and semi-arid ecosystems played a significant role in the 2011 global land carbon sink anomaly. Arid and semi-arid regions occupy 70% of the Australian land surface, dominated by two biomes: Mulga woodlands and spinifex grasslands or savannas. We monitored carbon and water fluxes in two of these characteristic ecosystems: a Mulga woodland (2010–2017) and a Corymbia savanna dominated by spinifex grasses (2012–2017). The aims of this study were to compare net ecosystem productivity (NEP) and evapotranspiration (ET) of these two ecosystems and to identify precipitation thresholds at which these ecosystems switched from being a C source to a C sink. Annual NEP in the Mulga woodland ranged from −47 to 217 gC m⁻² y⁻¹ (2010–2017), with the second largest positive NEP observed during the global C sink anomaly (162 gC m⁻² y⁻¹, 2010–2011). By contrast in the Corymbia savanna, annual NEP ranged from −190 to 115 gC m⁻² y⁻¹, with frequent occurrences of negative NEP and larger ET rates than for the Mulga woodland. Precipitation thresholds were identified at 262 mm y⁻¹ and 507 mm y⁻¹ in the Mulga woodland and the Corymbia savanna, respectively. Soil water content (SWC), along with air temperature and vapour pressure deficit, was a significant driver for water fluxes in both ecosystems (SWC–ET correlation of 0.5–0.56) and for carbon fluxes in the woodland (SWC–NEP and SWC–GPP correlation of −0.51 and −0.41, respectively). Arid and semi-arid ecosystems have dominated the inter-annual variability of the global terrestrial C sink, thus identifying precipitation thresholds at which ecosystems switch from being a C source to a C sink is important for furthering our understanding of the global C and water budget and for modelling of future climate.