Stable isotope trends in gilgaied Vertosols – variability between mounds and depressions and implications for sampling

Abstract

Stable isotopes (2H and 18O) are widely used in ecohydrological studies in Australia but their trends in Vertosols with microtopography (gilgai) is unknown. An understanding of short-distance variations in stable isotopes is important for designing cost-effective, sound sampling strategies in ecohydrological studies, but the knowledge can also further inform our understanding of infiltration and drainage processes in these soils. A comparison of mounds and depressions in sites with large and small gilgai revealed surprisingly little variation in stable isotope profiles between gilgai components or in relation to gilgai size. Variations in cracking, surface conditions, surface cover, solar radiation and wooded vegetation patterns could have potentially contributed to large variations in stable isotope profiles throughout the sites but the influence of these appears to have been minor and constrained to the upper 0.3m. Despite cracks being present to depths up to 1.5m, few samples are needed below 0.6m depth to characterise the isotope signature of the subsoil. Comparison of isotope profiles in closely spaced cores suggested that one core can sufficiently capture the profile trend, although bulking of multiple cores is recommended to minimise the likelihood of sampling error.