Water repellency reduces soil CO₂ efflux upon rewetting.

Area of Science:

• Soil Science
• Ecology
• Environmental Science

Background:

• Soil carbon (C) efflux is a major ecosystem flux, with rainfall pulses significantly contributing to carbon dioxide (CO2) emissions.
• A common assumption is that drier soils produce larger CO2 pulses, but this has not been tested for water-repellent soils (SWR).
• Soil water repellency is prevalent in many soils, particularly after dry spells or fires, impacting soil hydrology and biogeochemical cycles.

Purpose of the Study:

• To investigate the effect of soil water repellency (SWR) on the carbon dioxide (CO2) pulse observed upon rewetting.
• To determine if SWR alters the widely accepted relationship between soil dryness and CO2 efflux magnitude.
• To assess the impact of SWR on CO2 dynamics in recently burned forest soils.

Main Methods:

• Conducted laboratory and in situ experiments measuring CO2 efflux in response to simulated wetting.
• Utilized severely and extremely water-repellent soils from recently burned forest sites.
• Compared CO2 efflux in water-repellent conditions versus a wettable scenario (using a wetting agent).

Main Results:

• CO2 efflux upon rewetting was significantly lower in water-repellent soil scenarios compared to wettable conditions.
• Laboratory experiments showed CO2 pulses up to four times lower in water-repellent soils due to limited water infiltration.
• Approximately 70% of applied water drained rapidly through preferential flow paths in repellent soils, leaving much of the soil volume unsaturated.

Conclusions:

• Soil water repellency (SWR) substantially reduces the magnitude of the CO2 pulse following rewetting.
• The reduced CO2 efflux is attributed to limited water infiltration and preferential flow paths in repellent soils.
• SWR must be considered in the measurement and prediction of soil CO2 fluxes, especially in post-fire or drought-affected ecosystems, due to its significant impact on C dynamics.