Phosphorus (P) recovery from specific waste streams is necessary to develop environmentally sustainable and efficient fertilizers, achieving maximum productivity with minimum losses. A promising example of a P-recovery product is struvite (MgNH₄PO₄⋅6H₂O). Phosphorus availability from struvite is profoundly influenced by soil pH and/or processes in the rhizosphere. Root exudates (e.g., organic anions) and root morphology affect fertilizer bioavailability. The overall objective of our study was to identify root morphological and physiological traits of the narrow-leaf lupine (Lupinus angustifolius L. subsp. angustifolius, cultivar: blau “Boregine”) involved in the acquisition of P from struvite, compared with KH₂PO₄ as a soluble P source. The study included different pH conditions, as soil pH is one of the main factors affecting P availability.

Synthetic fertilizer production is associated with a high environmental footprint, as compounds typically dissolve rapidly leaching emissions to the atmosphere or surface waters. We tested two recovered nutrients with slower release patterns, as promising alternatives for synthetic fertilizers: struvite and a commercially available organic fertilizer. Using these fertilizers as nitrogen source, we conducted a rhizotron experiment to test their effect on plant performance and nutrient recovery in juvenile tomato plants. Plant performance was significantly improved when organic fertilizer was provided, promoting higher shoot biomass. Since the microbial community influences plant nitrogen availability, we characterized the root-associated microbial community structure and functionality. Analyses revealed distinct root microbial community structure when different fertilizers were supplied. However, plant presence significantly increased the similarity of the microbial community over time, regardless of fertilization. Additionally, the presence of the plant significantly reduced the potential ammonia oxidation rates, implying a possible role of the rhizosheath microbiome or nitrification inhibition by the plant. Our results indicate that nitrifying community members are impacted by the type of fertilizer used, while tomato plants influenced the potential ammonia-oxidizing activity of nitrogen-related rhizospheric microbial communities. These novel insights on interactions between recovered fertilizers, plant and associated microbes can contribute to develop sustainable crop production systems.

Struvite (St) recovered from wastewaters is a sustainable option for phosphorus (P) recovery and fertilization, whose solubility is low in water and high in environments characterized by a low pH, such as acidic soils. To broaden the use of struvite in the field, its application as granules is recommended, and thus the way of application should be optimized to control the solubility. In this study struvite slow-release fertilizers were designed by dispersing St particles (25, 50, and 75 wt%) in a biodegradable and hydrophilic matrix of thermoplastic starch (TPS). It was shown that, in citric acid solution (pH = 2), TPS promoted a steadier P-release from St compared to the pure St pattern. In a pH neutral sand, P-diffusion from St-TPS fertilizers was slower than from the positive control of triple superphosphate (TSP). Nevertheless, St-TPS featured comparable maize growth (i.e. plant height, leaf area, and biomass) and similar available P as TSP in sand after 42 days of cultivation. These results indicated that St-TPS slow P release could provide enough P for maize in sand, achieving a desirable agronomic efficiency while also reducing P runoff losses in highly permeable soils.

Biochars have long been associated with elevating plant productivity. An increasing number of studies, however, report that char application might also impair plant nutrient availability and reduce yields. In particular, char accompanying compounds as well as a hypothesized immobilization of nitrogen have been identified as playing a significant role in possibly diminishing plant productivity following char application. Herein, we tested the fertilizing effects of modified biochars in order to derive knowledge required to develop tailor-made chars, which predictably affect plant nutrition. Slow-pyrolysis maize cob biochar was modified by washing with either ethanol or hydrochloric acid to remove ash and organic compounds or by loading it with nutrient-rich residues in the form of digestate from the bioenergy sector. Maize plants were grown for 35 days on biochar-amended sand. We analyzed both substrate properties (pH, total carbon, and nitrogen, available magnesium and potassium) and plant functional traits (biomass, leaf area, root to shoot ratio, specific leaf area). Our results suggest that total plant biomass production remained unaffected by the application of biochar and its washed forms. Contrastingly, nutrient-loaded biochar induced a significant increase in productivity at similar nutrient levels due to improved plant nutrient uptake. Further research is required to understand the role of biochar modifications that facilitated improvements in plant productivity.