Cryogenic vacuum distillation (CVD) is a widely used technique for extracting plant water from stems for isotopic analysis, but concerns about potential isotopic biases have emerged. Here, we leverage the Cavitron centrifugation technique to extract xylem water and compare its isotopic signature to that of CVD-extracted stem water as well as source water. Conducted under field conditions in tropical northern Australia, our study spans seven tree species naturally experiencing a range of water stress levels. Our findings reveal a significant deuterium bias in CVD-extracted bulk stem water when compared to xylem water (median bias \($-14.9\textperthousand$\)), whereas xylem water closely aligned with source water (median offset \($-1.9\textperthousand$\)). We find substantial variations in deuterium bias among the seven tree species (bias ranging from -19.3 to \($-9.1\textperthousand$\)), but intriguingly, CVD-induced biases were unrelated to environmental factors such as relative stem water content and pre-dawn leaf water potential. These results imply that inter-specific differences may be driven by anatomical traits rather than tree hydraulic functioning. Additionally, our data highlight the potential to use a site-specific deuterium offset, based on the isotopic signature of local source water, for correcting CVD-induced biases.

Coastal vegetated habitats, including mangroves, saltmarshes and seagrasses, mitigate climate change by storing atmospheric carbon. Previous blue carbon research has mainly focused on organic carbon stocks. However, recent studies suggest that lateral inorganic carbon export might be equally important. Lateral export is a long-term carbon sink if carbon is exported as alkalinity (TAlk) produced via sulfate reduction coupled to pyrite formation. This study evaluates drivers of pyrite formation in coastal vegetated habitats, compares pyrite production to TAlk outwelling rates, and estimates global pyrite stocks in mangroves. We quantified pyrite stocks in mangroves, saltmarshes and seagrasses along a latitudinal gradient on the Australian East Coast, including a mangrove dieback area, and in the Everglades (Florida, USA). Our results indicate that pyrite stocks were driven by a combination of biomass, tidal amplitude, sediment organic carbon, sedimentation rates, rainfall latitude, temperature, and iron availability. Pyrite stocks were three-times higher in mangroves (103 ± 61 Mg/ha) than in saltmarshes (30 ± 30 Mg/ha) and seagrasses (32 ± 1 Mg/ha). Mangrove pyrite stocks were linearly correlated to TAlk export at sites where sulfate reduction was the dominant TAlk producing process, however pyrite generation could not explain all TAlk production. We present the first global model predicting pyrite stocks in mangroves, which average 155 (range 128 – 182) Mg/ha. In mangroves, estimated global TAlk production coupled to pyrite formation (~3 mol/m2/y) is equal to ~24% of their global organic carbon burial rate, thus highlighting the importance of including TAlk export in future blue carbon budgets.