Potential water repellency influenced CO₂ and CH₄ release from moist soils: Experiments on water-repellent Japanese...

Soil, the largest terrestrial carbon (C) pool on earth is an important source of greenhouse gasses. Soil water repellency (SWR), a moisture-dependent characteristic, is an important factor that affects microbial decomposition and gas release since it causes non-uniform moisture distributions in the soil matrix. It is not clear whether potential water repellency (PWR), measured on dried soils, can influence carbon dioxide (CO₂) and methane (CH₄) emissions under unsaturated moist conditions. This study aims to fill this knowledge gap using two water-repellent soils, and one non-repellent soil (NRS; control), under two temperature levels. Samples were collected from water-repellent Chamaecyparis obtusa (CYP) and Cryptomeria japonica (CED) forests, and the NRS was collected from Field Science Center in Ibaraki University, Ibaraki, Japan. Capillary-moistened samples were placed in closed chambers and separately exposed to 25 °C and 35 °C. Gas sampling was conducted in 1, 7, 14, 21, and 28 d. The PWR was measured with separate samples under the same conditions. CED soils showed significantly higher PWR compared with the NRS and CYP. The gas emission per g soil organic C (EG_C; mg g^-1 SOC h^-1) was significantly low in CED soils compared with NRS and CYP, while NRS showed the highest emission. EG_C of CO₂ and CH₄ showed significant strong negative correlations with PWR. The emission of CO₂ and CH₄ significantly increased with increasing temperature, and the Q₁₀ was significantly high for CH₄. Results indicated that the sensitivity to elevated PWR was higher for emission of CO₂, whereas the sensitivity to the elevated temperature was higher for the emission of CH₄. It was clear that PWR can influence moisture distribution in soil matrix in unsaturated moist soils and the processes that limit gaseous C emission. Further experiments are required on possible processes that enable PWR to influence gaseous C emission from moist soils.