Impacts Of Drying And Rewetting On The Radiocarbon Signature Of Respired Co2 And Implications For Incubating Archived Soils

The radiocarbon signature of respired CO2 ( increment C-14-CO2) measured in laboratory soil incubations integrates contributions from soil carbon pools with a wide range of ages, making it a powerful model constraint. Incubating archived soils enriched by "bomb-C" from mid-20th century nuclear weapons testing would be even more powerful as it would enable us to trace this pulse over time. However, air-drying and subsequent rewetting of archived soils, as well as storage duration, may alter the relative contribution to respiration from soil carbon pools with different cycling rates. We designed three experiments to assess air-drying and rewetting effects on increment C-14-CO2 with constant storage duration (Experiment 1), without storage (Experiment 2), and with variable storage duration (Experiment 3). We found that air-drying and rewetting led to small but significant (alpha < 0.05) shifts in increment C-14-CO2 relative to undried controls in all experiments, with grassland soils responding more strongly than forest soils. Storage duration (4-14 y) did not have a substantial effect. Mean differences (95% CIs) for experiments 1, 2, and 3 were: 23.3 parts per thousand (+/- 6.6), 19.6 parts per thousand (+/- 10.3), and 29.3 parts per thousand (+/- 29.1) for grassland soils, versus -11.6 parts per thousand (+/- 4.1), 12.7 parts per thousand (+/- 8.5), and -24.2 parts per thousand (+/- 13.2) for forest soils. Our results indicate that air-drying and rewetting soils mobilizes a slightly older pool of carbon that would otherwise be inaccessible to microbes, an effect that persists throughout the incubation. However, as the bias in increment C-14-CO2 from air-drying and rewetting is small, measuring increment C-14-CO2 in incubations of archived soils appears to be a promising technique for constraining soil carbon models.