Near-tree soil footprints show higher organic carbon and lower bulk density with no detectable end-of-summer moisture penalty

Abstract. Scattered trees can create fine-scale surface soil enrichment, but it is unclear how far these patterns extend through the mineral soil profile, translate into carbon stocks, and influence moisture content. We quantified the soil functional footprint of 14 established trees from four contrasting cohorts in an arboretum–grassland system under a Mediterranean climate in South Australia. Soil was sampled at 1, 2, 4 and 8 m from each stem at depth intervals to 30 cm. Directly measured surface bulk density was lower at 1, 2 and 4 m than at 8 m, while soil organic carbon (SOC) concentration was 30% higher at 1 m and declined by 8% with each doubling of distance. Total nitrogen, nitrate-N and Colwell-extractable P also varied with distance, whereas gravimetric moisture did not decline towards the trees under the end-of-summer conditions sampled. Distance × tree cohort interactions were not significant for these principal responses. After accounting for depth and tree identity, distance explained 3% of joint soil property variation whereas depth explained 39%. On an equivalent-soil-mass basis, SOC stock at 1 m distance from the tree exceeded that at 8 m by 16.5 Mg C ha⁻¹, although the categorical fixed-depth 0–30 cm SOC-stock test was not significant. Within the radially-weighted 0-8 m footprint, we combined two carbon pools, model-assisted mineral-soil SOC and current projected whole-tree carbon. Together these amounted to 67.4–123.2 Mg C ha⁻¹, with mineral-soil SOC accounting for 59–89% of this combined soil tree total. These results identify a shared, property-specific radial soil footprint characterised by lower bulk density and higher carbon and nutrient concentrations without a detectable dry-season moisture penalty. They also demonstrate why depth resolution, soil mass correction and radial area weighting are essential when converting near-tree enrichment into spatially explicit carbon accounting and monitoring. Because the near-tree zone retained higher carbon and nutrients without an end-of-summer moisture penalty, established trees may support understorey forage production and its resilience under increasing aridity rather than imposing a dry-season water cost.