Soil fertilization with microalgae biomass from municipal wastewater treatment causes no additional leaching of dissolved macronutrients and trace elements in a column experiment

Microalgae are a promising bio-fertilizer that can be cultivated in municipal wastewater, where the organisms perform water purification by incorporation of nutrients and contaminants. Before bio-fertilization with wastewater-grown microalgae can be put into practice, its impact on the leaching of macronutrients and trace elements needs to be evaluated. Here, we studied the leaching behavior of a microalgae-fertilized soil against a control in column percolation setup. Microalgae were grown in real municipal wastewater supplemented with bromide for the analysis of within-cell Br⁻ accumulation by time-of-flight secondary ion mass spectrometry. Dry biomass (45.0 g N kg⁻¹ and 28.9 g P kg⁻¹) was added to the topmost layer of the fertilized column at a level of 3 g biomass kg⁻¹ on a whole soil basis. Column irrigation was equivalent to 3 years of precipitation in central Germany. The leaching of macronutrients and trace elements from the fertilized and control columns was largely identical. Except for P, depth profiles confirmed very low vertical translocation within the soil. This is held for total element contents as well as for operationally defined pools, suggesting that microalgae cultivated in municipal wastewater provide a slow-release fertilizer largely resistant to leaching. Mass spectrometric imaging gave clear evidence for bromide uptake by the microalgae, and pure cultures of the genus Scenedesmus showed that it was preferentially located in the cell membrane. Therefore, bromide could potentially be employed as a mineralization tracer in future studies on the use of microalgae as a bio-fertilizer.