Contribution of Plant Species and Carbon Pools to the Severity of Soil Water Repellency.

Soil water repellency (SWR) is a major soil management issue across farmland in southern Australia. Although its incidence and severity has been anecdotally related to the previous native vegetation little is known about the specific organic compounds that may be derived from this vegetation. In many regions, trees with glaucous leaves accumulate waxy substances in the surrounding soil, however the specific distribution of these compounds (e.g. as coatings or in interstices between soil particles) and their contribution to SWR is unknown. This study examined the contribution of different species and soil carbon pools to the incidence of SWR.

Soil samples were collected from under the canopies of major native plant species (Eucalyptus marginata, Banksia menziesii, E. wandoo, Allocasuarina fraseriana). All soils contained <5% clay. A sonication method was used for the extraction of samples using IPA/NH₃(70:30) as an organic solvent and samples were run through GC-MS to characterize the compounds. The severity of repellency of the collected samples was measured using the MED (molarity of ethanol droplet) test. Physical disruption processes namely, winnowing, washing and dilution of the samples with AWS (acid washed sand) were conducted to determine the contribution of different carbon pools to the severity of repellency.

Mean MED values in untreated soils were 2.46, 2.87, 2.53 and 1.47 M, for E. marginata, B. menziesii, E. wandoo and A. fraseriana, respectively. Winnowing removed almost all the interstitial matter, and SWR reduced to 2.13, 0.87, 1.93 and 0.93 M. After washing SWR reduced to 0.73, 0.73, 0.87 and 0.33 M, respectively and this may be because of washing out of polar and other amphiphilic compounds. At 75% dilution with AWS, SWR was removed. 

Different plant species and different soil carbon pools make different contributions to SWR. The types of compounds, the seasonal variation of production and the fate of these compounds in the soil awaits further investigation.