From waste to soil : Can we create functioning manufactured soils by recycling rock processing waste?

Rock mining industries do not only exploit and transform extensive areas of land, but also produce vast amounts of rock waste material that lacks an adequate utilization. Some of these rock wastes have the potential to provide nutrients to plants and can therefore have positive impacts on soil properties. Consequently, we tested their potential for valorization as components of manufactured soils for use in urban areas. We conducted a 10-week incubation experiment of soil mesocosms with sunflowers (Helianthus annuus L.) to evaluate the performance of manufactured soils with respect to plant growth and soil properties. We used three common rock materials (augite-porphyry, greywacke-hornfels, basalt), ground to powder and mixed into natural soils of either clayey or sandy texture. In order to test the performance under challenging environmental conditions, we applied a drought treatment in addition to a regular watering treatment. All manufactured soils were able to maintain plant growth, although the yield of aboveground biomass was significantly lower compared to the original soils. However, the effects of the water regime and the original soils on the overall plant growth were stronger than the effect of the rock powders, indicating that the manufactured soils were not hampering plant development more than challenging environmental conditions. The preparation of the manufactured soils altered the grain size distribution of the originally sandy and clayey soils. Since the rock powders contributed mainly to the silt-sized particles, their addition to soils may improve the physical properties of the soil, especially the plant's available water content. We used wet-sieving to isolate aggregate size fractions and thus analyse the formation of soil aggregates. The manufactured soils had a higher mass contribution of microaggregate-sized particles, although this was mostly attributed to the presence of silt-sized rock powder particles instead of aggregate formation. The total organic carbon (OC) content of the original soils was diluted in the manufactured soils, as the rock powders did not contain OC. However, the manufactured soils may have the potential for future OC storage due to the abundance of OC-free mineral surfaces, which can retain organic matter as well as capture CO2 through enhanced weathering of primary minerals. Based on this work, the tested rock materials have the potential to be utilized as components in manufactured soils in the urban context that provide soil-like functions, particularly in terms of sustaining plant growth. For improved results, additional measures to initiate a rapid development of soil structure are highly recommended, for example, by adding an easily decomposable source of organic matter.