Capturing temporal heterogeneity in soil nitrous oxide fluxes with a robust and low-cost automated chamber apparatus

Abstract

Soils play an important role in Earth's climate system through their regulation of trace greenhouse gases. Despite decades of soil gas flux measurements using manual chamber methods, limited temporal coverage has led to high uncertainty in flux magnitude and variability, particularly during peak emission events. Automated chamber measurement systems can collect high-frequency (subdaily) measurements across various spatial scales but may be prohibitively expensive or incompatible with field conditions. Here we describe the construction and operational details for a robust, relatively inexpensive, and adaptable automated dynamic (steady-state) chamber measurement system modified from previously published methods, using relatively low cost analyzers to measure nitrous oxide (N2O) and carbon dioxide (CO2). The system was robust to intermittent flooding of chambers, long tubing runs (>100 m), and operational temperature extremes (−12 to 39 ∘C) and was entirely powered by solar energy. Using data collected between 2017 and 2019 we tested the underlying principles of chamber operation and examined N2O diel variation and rain-pulse timing that would be difficult to characterize using infrequent manual measurements. Stable steady-state flux dynamics were achieved during 29 min chamber closure periods at a relatively low flow rate (2 L min−1). Instrument performance and calculated fluxes were minimally impacted by variation in air temperature and water vapor. Measurements between 08:00 and 12:00 LT were closest to the daily mean N2O and CO2 emission. Afternoon fluxes (12:00–16:00 LT) were 28 % higher than the daily mean for N2O (4.04 vs. 3.15 nmol m−2 s−1) and were 22 % higher for CO2 (4.38 vs. 3.60 µmolm-2s-1). High rates of N2O emission are frequently observed after precipitation. Following four discrete rainfall events, we found a 12–26 h delay before peak N2O flux, which would be difficult to capture with manual measurements. Our observation of substantial and variable diel trends and rapid but variable onset of high N2O emissions following rainfall supports the need for high-frequency measurements.