The following text partially duplicates the Methods section from a manuscript in review in Geoderma. We sampled soils in July 2022 from four sites with differing management histories and texture in north-central IA, USA (42.1 °N, 93.5 °W). Each site included a crop field in long-term rotation of corn (Zea mays, a C4 plant) and soybean (Glycine max, a C3 plant) and adjacent perennial vegetation (mixed forest, shrubs, and herbaceous plants, mostly C3 species) established on previously cropped soils for 14 y or 25 y prior to sampling. Because soils had differing C inputs from C3 and C4 plants, the stable isotope ratios (delta 13C values) of CO2 might address whether drying temperature impacted C sources of respiration.
The four sites differed in texture class, including loam, silt loam, sandy clay loam and clay loam. For each site and vegetation type, three soil cores of 15 cm depth were collected within 50 cm of each other using a 6 cm diameter steel tube and passed through a 2 mm sieve. On the date of soil collection, 500 g replicates of each sample were placed in aluminum trays and exposed to each of four temperature treatments (22 °C, 35 °C, 55 °C, 85 °C). Samples in these treatments reached constant mass after approximately 7 d, 4 d, 2 d, and 1 d, respectively, and initial soil moisture was 0.04-0.12 g g-1 soil. The 22 °C samples were dried on a lab benchtop and the others were dried in convection ovens (temperature was verified with a thermocouple). Then, 40 g of each dried sample was placed in a perforated cup in a glass jar (970 mL) and wet to field capacity via capillary action (Haney and Haney, 2010). Immediately after adding water, the headspace was sealed with a gas-tight lid with two septa. One septum was vented with a needle and the other was attached to a tank of CO2-free air, and the jar was flushed with > 10 L to remove ambient CO2. After 24 h, the headspace CO2 concentration and its delta 13C value (relative to Vienna Pee Dee Belemnite) was measured by injecting 5 mL into a tunable diode laser (Hall et al., 2017). After each measurement, the jar was flushed with CO2-free air and remained sealed until the next gas sample was collected (experiment days 1, 4, 7, 14, 28, and 42).
Hall, S.J., Huang, W., Hammel, K.E., 2017. An optical method for carbon dioxide isotopes and mole fractions in small gas samples: Tracing microbial respiration from soil, litter, and lignin. Rapid Commun. Mass Spectrom. 31, 1938–1946. https://doi.org/10.1002/rcm.7973
Haney, R.L., Haney, E.B., 2010. Simple and rapid laboratory method for rewetting dry soil for incubations. Commun. Soil Sci. Plant Anal. 41, 1493–1501. https://doi.org/10.1080/00103624.2010.482171
