Edaphic legacy of phosphorus fertiliser in the restoration of the biodiverse Kwongan ecosystem in ultra-low P soils

Abstract

The kwongan ecosystem is a biodiversity hotspot on the coastal sandplain of southwest Western Australia. It harbours many rare and endemic plant species adapted to nutrient impoverished soils, particularly to low phosphorus (P). In some kwongan regions surface strip-mining has drastically disturbed these ecosystems, requiring restoration by soil profile reconstruction and re-establishment of the native vegetation, often assisted by mineral fertiliser applications. We examined soil monitoring data from several post-mining sites of restored kwongan vegetation (n = 127), categorised by age groups (10, 20, 30 and 40-year-old sites) and fertilisation history (type of fertiliser applied). Thirty two soil variables were compared to the surrounding native soils (n = 135) by multivariate and univariate statistical approaches to determine whether different restoration ages and fertilisers had long-term impacts on soil physicochemical characteristics. Our analyses showed that restored soils differed from native, with substantially lower carbon (−41 %) and nitrogen (−15 %). Sites restored within the last 10 years had higher sodicity and much lower porosity, while 40-year-old sites presented nearly ten times more Total P. Recent non-fertilised soils showed similar P concentrations to the reference sites, however, older restored soils had a high legacy P, which is a clear consequence of past superphosphate applications, and greatly decreased their N:P stoichiometric ratios compared to native soils. We show that, after decades, restored soils are still impacted by the past mining activities and fertiliser application. These changes will likely alter microbial and plant communities, impairing restoration trajectories towards the native kwongan vegetation, particularly for P-sensitive species. Despite benefits of P-fertilisation (e.g. improved plant cover), the low carbon stocks and N:P ratios indicate a divergent ecosystem from the native state. Such changes in ecosystem stoichiometry may affect plant species competitiveness, and alter the composition of other trophic levels. Adjusting future restoration practices to reduce P fertilisation is promising, but addressing long-term stoichiometric shifts and soil compaction remains crucial.