Thermal and Seasonal Controls on Organic Matter Decomposition in Headwater Streams in Western Patagonia

Headwater streams in western Patagonia represent a unique opportunity for understanding how hydrological and chemical drivers modulate decomposition and carbon cycling. This study aimed to identify drivers and patterns of stream decomposition in four groups of small catchments representing distinct climate and land use conditions. Standardized cotton‐strip assays were deployed bimonthly over an annual cycle (May 2017–July 2018) and autumn 2024, in streams representing dry deciduous and humid evergreen forested catchments, paired by reference and early intervention conditions. Exponential decay rates ( k ), developed using hierarchical Bayesian inference, were significantly lower than global means for temperate zone streams in autumn, while winter rates were well beyond the lowest reported range for the cellulose strip assay. Decay rates exhibited strong seasonal variation (summer > spring > autumn > winter), with moderate spatial differences among sites. k was positively associated with temperature and inorganic nitrogen, negatively related to discharge, but with no significant effect of land use. Thermal sensitivity based on an Arrhenius model was context‐dependent, with apparent activation energies in the range 0.33–1.24 eV, and evidence of flow‐driven suppression at one dry temperate deciduous site. Taken together, the summer peak of decomposition suggested an important role of leaf phenology (e.g., deciduous vs. evergreen), winter carbon retention, and the potential importance of shifting nutrient regimes and climate change scenarios. Meanwhile land‐use effects may not be detectable for early‐ stage intervention in Patagonia. The study highlights the importance of systematic characterization of local and regional variation for adequate regional representation within global assessments.