Low-cost autonomous chambers enable high spatial and temporal resolution monitoring of soil CO₂ efflux across landscapes

Jonathan Gewirtzman , Ashley Keiser, Matthew A Nieland, Corey Palmer, Vatsal Patel, Kelly Caylor, Elizabeth Forbes

Soil CO₂ efflux is a central component of ecosystem carbon budgets, but the cost and logistics of measuring it often force a trade-off between spatial coverage and temporal resolution. Manual survey chambers can sample many locations but only occasionally, whereas automated chambers measure continuously but are costly and tethered to line power and a shared analyzer, which limits them to a few chambers at one site. Fluxbot 2.0 is a low-cost (about US$400 per unit), autonomous chamber designed for continuous, distributed soil CO₂ flux measurements without external power or infrastructure. To test whether distributed Fluxbot arrays reproduce reference measurements, we deployed two eight-unit arrays alongside an established automated chamber network in two forest stands at Harvard Forest, USA, for one month, processing fluxes from both systems with the same code. Over 305 compared hours, Fluxbot system means averaged 0.3% below autochamber means, a difference no larger than that between disjoint chamber subsets of the same system, as expected for systems sampling different collars. The chamber-adjusted difference was statistically equivalent within 20%. Daily means correlated at r = 0.85 across 15 qualifying days (hourly r = 0.72). Averaged across stands, the gap-filled October budget was about 2% lower for Fluxbots. Fluxbot fluxes were depressed during likely sensor wetting, which the units' humidity records could flag. Low-cost arrays thus gave reference-like mean fluxes in both stands and make dense replication affordable. Because each unit is self-contained, extending arrays across sites, land-cover types, environmental gradients, or treatments requires only more units.