Soil CO2 flux data from Kopp et al. 2026 publication in the Soil Science Society of America Journal

Uneven and sparse high-resolution temporal coverage of soil CO2 efflux (RS) data limit our understanding of its variability across ecosystems, leaving fundamental questions with applied consequences for collecting and scaling site-level observations. Processes driving RS can operate on the scale of hours or days, but in forests, data are often limited to weekly morning observations based on long-standing assumptions of diel patterns and what controls them. Here, we re-examine these assumptions using a new database of hourly RS measurements from many forests, consider their implications for sampling, and then parse controls on diel RS at one experimental forest. Across 31 globally distributed forests mid-morning measurements could represent daily fluxes on average, but diel flux patterns at individual sites deviated substantially from this central tendency. For example, at our study site, monitoring only between 09:00-12:00 would underestimate annual RS ~12% (±1.4), but the error varied across ridge, midslope, and valley floor topographic positions. Co-located biotic and abiotic data (root respiration, turbulence, soil moisture, soil temperature, water table depth, and soil pCO2) suggest near surface soil temperature drove these diel patterns, but curiously, if temperature were a universal mechanism, we may have expected our global dataset to reveal more consistent results among sites. Exploring these controls furthers our fundamental understanding of what drives diel RS across forests and provides insights for improved collection and upscaling of local RS observations for global estimates.