Sequential Soil Pore Water Extractions with Tempe Cell Soil Water Extractors: A Novel Method to Measure Pore-Scale Oxygen Content

Soils are an important carbon (C) sink, and as such, increasing soil C storage while minimizing the microbial respiration of soil C is a promising strategy to decrease global carbon dioxide (CO ₂ ) emissions. Although studies have demonstrated that oxygen limitations slow upland soil respiration, current global C models largely assume that upland soils are entirely aerobic. Little is known about oxygen distribution in upland soils, mostly because traditional methods of soil sampling often alter or destroy native soil structure, thereby changing soil oxygen concentrations. Here, we use a novel method to examine soil oxygen content in surface agricultural soils. Intact soil cores were collected from an experimental farm and adjusted to 75% saturation with a glucose amended synthetic rainwater solution. Cores were incubated in glass jars at 25 °C for 7 and 14 days, and headspace samples were collected on days 1, 3, 7, and 14 of the incubation to examine CO ₂ , methane, and nitrous oxide efflux. Immediately following the incubation, soil cores were placed in modified Tempe cell soil water extractors, and pore waters bound by 100, 300, and 500 kPa matric potentials were collected by sequential extractions. Extracted pore waters were immediately measured with an optical oxygen sensor to determine pore water oxygen concentrations. This methodology provides a way to measure oxygen distributions in upland soils, while minimizing disturbances to native soil structure, thereby linking soil oxygen content to CO ₂ emissions.