Sustainable phosphorus (P) management in agriculture requires a circular economy approach through the use of so-called bio-based fertilizers (BBFs). The properties of BBFs vary widely depending on raw materials and production processes. However, it is still unknown how these properties, and particularly the dominant P compounds determine not only the efficiency of BBFs in supplying P to crops, but also their effects on soil functioning and crop quality. This study aimed to evaluate the efficiency of a representative set of BBFs, and relate this efficiency to their composition and dominant P compounds. To this end, 14 BBFs were studied: four from water purification (struvite, vivianite, and sewage sludge with and without composting), four composts (municipal solid waste (MSW), vineyard residues, and two using olive husks), three vermicomposts (two homemade and one commercial), fish meal, digestate, and a commercial organic fertilizer. Phosphorus forms in BBFs were determined using 31P nuclear magnetic resonance spectroscopy (P-NMR). The BBFs were compared to a single superphosphate (SSP) in a pot experiment growing wheat in two different alkaline soils, one rich in iron (Fe) oxides and one rich in carbonates. The effects on critical elements in grain magnesium, Fe, zinc (Zn), manganese, and copper and enzyme activities related to soil functioning and P cycling were also assessed. The dominant P compound in the BBFs was orthophosphate (73.8–89.5% of the total P in the NaOH–EDTA extracts). The MSW had the highest polyphosphate content (4.1%), a complex inorganic P compound. The organic P content ranged from 9.2% (fish meal) to 25.5% (Moge). Sewage sludge and composted sludge contributed high levels of phosphonates (4.1 and 5.6% of extracted P). The most abundant organic P compound class was inositol hexakisphosphates (IHPs), and myo-IHP (phytate) was the dominant IHP stereoisomer (1.2–6.4%) followed by D-chiro-IHP and scyllo-IHP. Plant dry matter and grain yield with most BBFs were not significantly different from that of SSP in both soils, likely due to the high concentrations of phosphate in relatively soluble forms in most of the BBFs. Vivianite and sewage sludge resulted in significantly higher grain yield than SSP (43% and 40%, respectively) in the carbonate-rich soil, likely due to progressive phosphate dissolution, which decreased the precipitation rate of insoluble calcium (Ca) phosphates. The highest P recoveries were obtained with horse manure vermicompost (65% and 15% higher than SSP in the Fe oxide-rich and in the carbonate-rich soil, respectively), partially attributed to the decreased precipitation rate of insoluble Ca phosphates with the added organic matter. Some BBFs increased micronutrient concentrations in grains and most decreased the P-to-Zn ratio relative to SSP. Overall, phosphatase and β-glucosidase activities increased with carbon-rich BBFs. Most of the studied BBFs could effectively replace fertilizers from non-renewable sources, in some cases with better crop P recoveries. Furthermore, some BBFs could provide additional benefits to grain quality, in terms of micronutrient supply for humans, and soil functioning.
Nunez-Romero et al. (Wed,) studied this question.