Citation: Carroll-MacDonald, T.-A.; Rayburg, S.; Neave, M. Spatial Variability of Topsoil Properties on a Semi-Arid Floodplain. Soil Syst. 2023, 7, 42. https://doi.org/10.3390/ soilsystems7020042 Academic Editor: Adriano Sofo Received: 14 October 2022 Revised: 5 April 2023 Accepted: 12 April 2023 Published: 23 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Spatial Variability of Topsoil Properties on a Semi-Arid Floodplain Tiffany-Anne Carroll-MacDonald 1 , Scott Rayburg 2, * and Melissa Neave 3 1 School of the Environment, University of Technology Sydney, 15 Broadway, Ultimo, NSW 2007, Australia 2 Department of Civil and Construction Engineering, Swinburne University of Technology, Hawthorn, VIC 3122, Australia 3 Centre for Urban Research, School of Global, Urban and Social Studies, Royal Melbourne Institute of Technology University, Melbourne, VIC 3001, Australia * Correspondence: srayburg@swin.edu.au; Tel.: +61-(3)-9214-4944 Abstract: This study relates the spatial heterogeneity (or patterning) of geochemical elements in the topsoil of a semi-arid floodplain/hillslope system in north-eastern Australia to vegetation distribution and rates of flood inundation. A total of 540 topsoil samples were collected from six flood frequency zones, ranging from a frequently flooded area (RI = 1:1–2 yrs) to two zones that have not flooded in living memory (RI > 50 yrs). Within each zone, topsoil samples were collected from both vegetated and non-vegetated surfaces, and each sample was analysed for 26 parameters. A combination of multi- and univariate analyses reveals that vegetation is an important contributor to topsoil heterogeneity. In zones subject to relatively frequent flooding, the spatial distribution of parameters in the topsoil is greatly influenced by the movement of water, with vegetation acting as a sink rather than a source. However, as floods become increasingly rare, distinct resource-rich units become evident in the topsoil beneath the vegetation. These findings indicate that topsoils in semi-arid floodplains are altered when their natural flooding regimes are reduced, beginning to approximate hillslopes when flood frequencies exceed 1-in-7 to 10 years. This points to the need for frequent flood (overbank) releases that are able to cover the 1-in-20-year floodplain to maintain the character of the soils and support vegetation growth in these environments. Keywords: ecosystems; geochemistry; resilience; resource islands; soil nutrients; soil heterogeneity 1. Introduction Semi-arid floodplains often contain highly productive ecological systems that represent biological hotspots in what are otherwise nutrient-poor landscapes [1–4]. The floodplains of the semi-arid Murray–Darling River system in eastern Australia, for example, are critical environments for a range of aquatic and terrestrial organisms that are not found on the dry landscapes surrounding them, including waterbirds (both migratory and non-migratory), aquatic and terrestrial plants, fish, frogs and invertebrates [5–10]. Not surprisingly, research into the impacts of overbank events reveals that floods drive process functioning in semi-arid floodplain systems. In addition to transporting water, which is directly used by organisms for their biochemical functioning and influ- ences soil processes such as nitrogen and carbon mineralization [11,12], floods deliver sediments and nutrients (both in particulate and dissolved form) to the floodplain surfaces [13–15]. Indeed, the presence of nutrients in semi-arid soils is often closely linked to water availability, largely because water releases them from the soil particles to which they are bound [16]. As such, the spatial distribution of materials such as organic carbon, nitrogen and phosphorous across semi-arid floodplains is linked to flood inundation [17]. In addition, the materials transported by floods (particularly water and nutrients) support biological growth and, therefore, the distribution (both Soil Syst. 2023, 7, 42. https://doi.org/10.3390/soilsystems7020042 https://www.mdpi.com/journal/soilsystems