Substrate-Driven Phosphorus Bioavailability Dynamics of Novel Inorganic and Organic Fertilizing Products Recovered from Municipal Wastewater—Tests with Ryegrass

Municipal wastewater is a valuable source of phosphorus (P) for the production of fertilizing products, such as microalgae (MA), crab carapace material (CCM), P salt produced by chemical leaching of sludge (P salt CL), and sewage sludge ash produced by pyrolysis and the incineration of sludge (SSA PI). This study compares the P availability of these fertilizing products in three planting substrates (S1, S2, and S3) during a four-month growth period of perennial ryegrass. The unfertilized control in substrate S3 had a high and available P that masked the effect of the added fertilizing products. The P salt CL fertilizer exhibited the lowest shoot dry matter in the alkaline S2 substrate. Still, it can be used as a good source of P in both acidic and alkaline substrates, given that its shoot P content was among the highest in all substrates tested. The organic-rich fertilizing products, MA and CCM, are better suited for acidic substrates since a pronounced reduction in the shoot yield and P content was seen in the alkaline S2 substrate. In contrast, for the SSA PI fertilizer, the very small differences in shoot dry matter and P content in S1 compared to S2 indicated that it is suitable for both acidic and alkaline substrates. Four months were needed to observe the maximum shoot yields treated with these P fertilizing products. Considering that the substrate solution P (using Rhizons) for the P salt CL and MA fertilizers proved to be similar to shoot P uptake, Rhizon extraction could be used for assessing P bioavailability. The chemical composition of novel products indicated their potential to deliver not only P, but also other nutrients to plants. However, concentrations of inorganic contaminants in all products, except CCM, pointed out a possibility to pollute the environment by applying these fertilizers.