Water repellency reduces soil CO2 efflux upon rewetting

Carmen Sánchez-García; Bruna R F Oliveira; Jan Jacob Keizer; Stefan H Doerr; Emilia Urbanek

doi:10.1016/j.scitotenv.2019.135014

Water repellency reduces soil CO₂ efflux upon rewetting

Sci Total Environ. 2020 Mar 15:708:135014. doi: 10.1016/j.scitotenv.2019.135014. Epub 2019 Nov 4.

Authors

Carmen Sánchez-García¹, Bruna R F Oliveira², Jan Jacob Keizer², Stefan H Doerr³, Emilia Urbanek³

Affiliations

¹ Swansea University, Department of Geography, Swansea, United Kingdom. Electronic address: csgarcia002@gmail.com.
² Earth Surface Processes Team, Centre for Environmental and Marine Studies (CESAM), Department of Environment and Planning, University of Aveiro, Aveiro, Portugal.
³ Swansea University, Department of Geography, Swansea, United Kingdom.

PMID: 31759705
DOI: 10.1016/j.scitotenv.2019.135014

Abstract

Carbon dioxide (CO₂) efflux from soil represents one of the biggest ecosystem carbon (C) fluxes and high-magnitude pulses caused by rainfall make a substantial contribution to the overall C emissions. It is widely accepted that the drier the soil, the larger the CO₂ pulses will be, but this notion has never been tested for water-repellent soils. Soil water repellency (SWR) is a common feature of many soils and is especially prominent after dry periods or fires. An important unanswered question is to what degree SWR affects common assumptions about soil CO₂ dynamics. To address this, our study investigates, for the first time, the effect of SWR on the CO₂ pulse upon wetting for water-repellent soils from recently burned forest sites. CO₂ efflux measurements in response to simulated wetting were conducted both under laboratory and in situ conditions. Experiments were conducted on severely and extremely water-repellent soils, with a wettable scenario simulated by adding a wetting agent to the water. CO₂ efflux upon rewetting was significantly lower in the water-repellent scenarios. Under laboratory conditions, CO₂ pulse was up to four times lower under the water-repellent scenario as a result of limited wetting, with 70% of applied water draining rapidly via preferential flow paths, leaving much of the soil dry. We suggest that the predominant cause of the lower CO₂ pulse in water-repellent soils was the smaller volume of pores in which the CO₂ was replaced by infiltrating water, compared to wettable soil. This study shows that SWR should be considered as an important factor when measuring or predicting the CO₂ flush upon rewetting of dry soils. Although this study focused mainly on short-term effects of rewetting on CO₂ fluxes, the overall implications of SWR on physical changes in soil conditions can be long lasting, with overall larger consequences for C dynamics.

Keywords: Birch effect; Climate change; Hydrophobicity; Rain pulses; Wetting; Wildfire.