Understanding the contribution of different Australian plant species to soil water repellency

Thi Thanh Mai Dao

Soil water repellency (WR) is known to be induced by hydrophobic organic compounds released from plants and soil micro-organism. Associations of various plant species and the degree of soil WR are often reported, however, in the field these relationships are affected by numerous factors such as soil properties, contributions from multiple plant species and compounds produced during decomposition of organic matter and fungal bio-products. It is thus hard to isolate the effects of a specific plant species on soil WR. This research examined the direct contribution of different native plant species from Western Australia including Banksia menziesii (BM), Eucalyptus marginata (EM), Xanthorrhoea preissii (XP) to soil WR. Powder of leaves, litter, decomposing litter, roots of these species was mixed with acid-washed sand (AWS) and measured for WR. Soil samples under BM, EM and XP in the Chelodina Reserve were collected to measure for WR. The chemical composition of organic materials and soils were characterised on Soxhlet-extracts via Gas Chromatography – Mass Spectrometry. Mixtures of BM leaf powder and AWS consistently exhibited the highest WR, followed by EM, while XP mixtures were wettable at all concentrations. The severity of WR in BM and EM leaf mixtures was associated with the presence and concentration of long chain alkanes, fatty acids and alcohols. In contrast, wettable XP mixtures had only C16 fatty acid and C16 alcohol at low concentrations. The severity of WR induced from leaf powder was significantly related to the total concentration of fatty acids (r = 0.94), alcohols (r = 0.95) and alkanes (r = 0.69). For the roots at 10% organic matter added, the severity of WR was in the order BM > XP > EM. The total concentration of fatty acids and steroids were significantly related to WR, both with correlations of r = 0.85 with p < 0.05 for fatty acids and p < 0.01 for steroids. For partially decomposed litter (OL1), the WR induced was in the sequence BM > EM > XP with WR from XP OL1 > 0. While the composition of BM and EM litter at initial stage of decomposition was mostly the same as leaves, the XP litter had transformed to a series of alkanes and fatty acids, which might be associated with the substantial increases in WR compared to the XP leaves. In this case, the severity of WR induced from OL1 materials was significantly related to total concentrations of alcohols (r = 0.82) and alkanes (r = 0.65); and had a weak correlation with the total concentration of fatty acids (r = 0.42). For more decomposing litter and soil samples under BM, EM and XP, the presence and concentrations of organic compound class in those samples did not reflect the severity of soil water repellency. Heating temperatures from 30–120 C increased in WR of all the AWS/ leaf powder mixtures from leaves of all species. This may have implications for the development of WR in Mediterranean climates, where soil surface temperatures may reach 70 C in summer, and for roots explain the induction of soil WR at depth after wildfire. The increase in WR may occur because of the release and re-distribution of organic compounds in the powder mixtures. In conclusion, there were substantial increases in WR with increasing organic matter concentration between species. The differences in WR were related to the chemical composition including the long chain fatty acids, alcohols and/or alkanes for leaves and litter at initial stage of decomposition. However, these relationships were not consistent for the roots, more decomposing litter and soils where the differences in the severity of WR between species were small and WR was weakly correlated with the total concentrations of organic compound classes. In addition, the approach of adding plant powders to AWS plus heating techniques provides a method for isolating the contribution of the factors that are likely to influence WR.

Understanding the contribution of different Australian plant species to soil water repellency

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