Soil-water repellency characteristic curves for soil profiles with organic carbon gradients

Soil water repellency (SWR) of soils is a property with significant consequences for agricultural water management, water infiltration, contaminant transport, and for soil erosion. It is caused by the presence of hydrophobic agents on mineral grain surfaces. Soils were samples in different depths at three forest sites in Japan and three pasture sites in New Zealand, covering soil organic carbon (SOC) contents between 1 and 26%. The SWR was measured over a range of water contents by three common methods; the water drop penetration time (WDPT) test, the molarity of an ethanol droplet (MED) method, and the sessile drop method (SDM). The aim to (i) compare the methods, (ii) characterize the soil-water repellency characteristic curves (SWRCC) being SWR as a function of the volumetric soil-water content (θ) or matric potential (ψ), and (iii) find relationships between SWRCC parameters and SOC content. The WDPT, MED, and SDM generally agreed well in predicting the θ range where SWR occurred, and there was close agreement between SWR results determined by average MED and SDM at similar θ. Generally, SWR was only found within the top 20cm of the soil profiles. Six SWR parameters were introduced: (i) the area under the curve (S_WR(θ)); (ii) θ at the maximum SWR (θ_WR-max), (iii) θ where SWR ceased (θ_non-WR), (iv) the maximum SWR (CA_i-max), (v) pF at the maximum SWR (pF_WR-max) and (vi) pF where SWR ceased (pF_non-WR). The relationship between the first three parameters and SOC content were best described with Langmuir type equations (r² of 0.5-0.7), while the other three parameters changed linearly with SOC contents.