The origin of carbon dioxide released from rewetted soils: A poisonous paradox

Background/Question/Methods

The act of re-wetting a dried soil typically causes a CO₂ pulse, the precise reasons for which remain unclear.  Several causes have been postulated including the metabolism of necromass accumulated during the dry phase, rapid release and subsequent metabolism of osmolytes accumulated by cells to protect against dehydration and increased availability of native organic matter due to the breaking of soil aggregates upon rewetting.  Samples were taken from 48 sites across England encompassing a wide variety of soil properties, land uses and landscape contexts, and subjected to series’ of four dry:wet cycles, CO₂ evolution was measured every six minutes for 96 hours after rewetting.  A sub-study was conducted based on 5 soils to attempt to disentangle the potential causes of the immediate flush of CO₂following rewetting, i.e.  physical (release of entrapped gas), chemical (dissolution of carbonates), biochemical (free enzymatic activity) or biological (derived from intact cells). Soils were air-dried and rehydrated with either sterile water (i.e. ‘standard’ control) or one of 3 metabolic inhibitors (15% TCA, 10% formaldehyde, and 1% silver nitrate solutions). Further treatments were autoclaving the soils prior to air-drying, or oven-drying at 200°C for 12 h, and rehydrating with sterile water in both instances.

Results/Conclusions

Results are consistent with those reported in the literature: a rapid flush of respiration on rewetting; an average of 4.5% of the total CO₂ efflux over 96 hours was apparent in the hour following rewetting, and circa 20% of this evolved in the first 6 minute interval.  Soils autoclaved prior to drying generated insignificant amounts of CO₂ upon rewetting, suggesting the CO₂ flush is predominantly biological.  All other treatments resulted in a rapid CO₂ flux, following the characteristic ‘rewetting pattern’. Total CO₂ evolved over 96 hours from non-autoclaved treatments were statistically similar including those from unsterilized controls. This leads to the paradox that air-dried soils, likely containing intact biological systems but rewetted to inhibit biological activity still generate equivalent amounts of CO₂ to those not thus inhibited.  Oven-dried soils, sterile at rewetting, similarly generate comparable amounts of CO₂. These latter modes suggest a cryptic origin for the initial CO₂ flush. We cannot readily reconcile this paradox, and raise it as a matter that needs further investigation across a wider range of soils with a more comprehensive range of treatment combinations to separate the potential contributing sources.