The where, when and what of phosphorus fertilisation for seedling establishment in a biodiverse jarrah forest restoration after bauxite mining in Western Australia

Fertiliser application to restore nutrients lost in the mining process and facilitate early plant establishment and growth is a key step in the restoration of sites disturbed by mining. However, few studies have investigated the effects of different fertiliser types and application methods on mine restoration outcomes, especially in highly biodiverse ecosystems such as the jarrah forest. This forest is a unique, floristically diverse landscape with species adapted to growth on a highly weathered phosphorus impoverished Ferralsol. In this study we investigated the effect of fertiliser type (rock phosphate, single superphosphate, and an NPK fertiliser), application method (top-dressed versus incorporated), and the timing of application (winter vs. summer) on the trajectory of jarrah (Eucalyptus marginata) forest restoration following bauxite mining compared to an unfertilised control. All fertilised soil had elevated Colwell-P concentrations (bar rock phosphate) and had considerably less N than found in the native forest, even after N fertilisation. Fertiliser incorporation resulted in a more even distribution of P down the soil profile and increased overall plant growth (as assessed by percentage cover) compared with either top-dressed fertiliser application and no fertiliser, potentially offering better erosion control. In contrast, native species richness was highest in the zero fertiliser and NPK treatments and lowest in the phosphorus incorporation treatments. On average, unfertilised plots had 10 more native species per plot than those fertilised with P only. Fertiliser application also reduced the abundance and cover of Bossiaea ornata and Lomandra spp., both of which are small slow-growing understorey taxa. In contrast, the legume Acacia celastrifolia exhibited a vigorous growth response to fertiliser, with growth being greatest when P (either rock phosphate or SSP) was incorporated. These data suggest that P fertiliser incorporation is a potential strategy to both maximise early plant growth and cover and increase the efficiency of P application. However, if the goal of restoration is to maximise diversity then moderation in P application and using fertilisers that also contain N and K may be appropriate