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The effect of water repellency on the short-term release of CO2 upon soil wetting
Geoderma, Volume: 375, Start page: 114481
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The high peak of carbon dioxide (CO2) observed after rewetting of dry soils, known as the ‘Birch effect’, can contribute substantially to total soil carbon (C) emissions, however, the exact mechanisms and timings underlying this sudden CO2 release remain unclear. The amount of applied water and dura...
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The high peak of carbon dioxide (CO2) observed after rewetting of dry soils, known as the ‘Birch effect’, can contribute substantially to total soil carbon (C) emissions, however, the exact mechanisms and timings underlying this sudden CO2 release remain unclear. The amount of applied water and duration of the previous dry period are considered the main factors affecting the magnitude of the CO2 peak, but the preceding change in soil wettability, triggered by low soil water content, could also be an important contributor.We investigated the effect of soil water repellency (SWR, assessed by water drop penetration time test) on the short-term release of CO2 upon wetting of dry soils with different water quantities. The experiments were conducted under laboratory conditions using homogeneous and autoclaved soil from two locations in South Wales (UK) in both wettable and extremely water-repellent states. The CO2 efflux was measured using chambers above and below the samples. Upon wetting, CO2 efflux was up to 10 times lower in water-repellent soils as a result of rapid percolation through preferential pathways, with only a small amount of water (up to 10%) retained in the soil. Total CO2 efflux was proportional to the water retained in the soil after infiltration, suggesting that the release of CO2 occurred only from limited pore-spaces of the soil. The quick CO2 release suggests that chemical or biochemical processes, rather than microbial respiration, is the main source of CO2 efflux in this study. Part of the CO2 released was transported to the bottom chamber, which under natural conditions could enhance the entrapment of gas in the subsoil. This study shows that alterations in the water-filled pore-space as a result of SWR significantly reduced the CO2 efflux upon wetting and suggests that SWR could be a key factor when investigating and predicting C fluxes.
HydrophobicityBirch effectCO2 pulseC emissionsSoil degassingRain pulses