Automated Daily Measurements of Soil Pore Space CO ₂ and O ₂ : Signals Revealing Soil Physical and Biogeochemical Processes

Abstract

There is a need to understand and monitor active soil processes in the field; however, it is challenging to observe and separate co‐occurring belowground processes. High precision monitoring of soil pore space gases is a potentially powerful approach, complicated by the variety of biological and physical processes that impact gas. To evaluate processes controlling soil CO ₂ and O ₂ concentrations across weekly to seasonal timescales, we installed a custom‐built automated gas sampling system outside of Bastrop, Texas that enabled field‐based, high‐precision measurements of soil CO ₂ and O ₂ concentrations for over a year. We focus on the ratio of soil‐atmosphere differences in CO ₂ and O ₂ concentrations, referred to as the apparent respiratory quotient (ARQ). Rainfall events physically modified soil gas signals by driving short‐term (2–5 day) transient decreases in ARQ values throughout the soil profile. Filtering out data impacted by transient effects allows seasonal soil gas signals to emerge, illustrating the dominance of root respiration and microbial respiration of plant derived carbohydrates during the growing season. Systematically higher ARQ values were observed during the dry summer and fall, whereas lower ARQ values predominated during the wet and cold winter months. We consider that a shift in microbial carbon‐use efficiency is the most likely driver of high ARQ values during the dry season as autotrophic respiration slows. The low winter ARQ signals could be driven by a number of different processes such as respiration of more reduced substrates, increased nitrification rates, or metal oxidation.