Potential carbon mineralization assays are confounded by different soil drying temperatures

Abstract

Measuring carbon dioxide (CO2) produced after re-wetting previously dried soil is an increasingly popular soil health assay, but there is disagreement on the optimal soil drying temperature. We tested whether soil drying temperature impacts water-extractable organic carbon (WEOC) and soil CO2 emissions following rewetting. Soils were collected from corn/soybean croplands and adjacent perennial vegetation at four sites in Iowa, USA. Soil replicates were dried at 22 °C, 35 °C, 55 °C, 85 °C, and rewetted for incubation at 22 °C. Soil WEOC and CO2 emissions after re-wetting increased nonlinearly with drying temperature. Effects on CO2 were largest after four days of incubation, but cumulative differences persisted even after 42 days. Responses of CO2 and its stable isotope ratio (δ13C) to increased drying temperature varied among sites and vegetation types, indicating shifts in C sources. Soil health assays performed with different soil drying temperatures may not be directly comparable, effects of drying temperature may vary idiosyncratically among samples, and drying at 22 °C or 35 °C as opposed to higher temperatures may be preferable to avoid increasing C availability.