**1. Introduction**

Spring wheat (*Triticum aestivum* L.) is infected by several dozens of pathogenic fungi. Species such as *Mycosphaerella graminicola*, *Pyrenophora tritici-repentis*, *Tilletia caries*, and *Ustilago tritici*, as well as numerous species of the genus *Fusarium* are the most dangerous pathogens of wheat that are transmitted with grain [1]. Fungal species of the genera *Alternaria*, *Cladosporium*, and *Epiccocum* are regarded as weak pathogens or saprotrophs [2]. Research has demonstrated close interactions between plants and microbes [2]. Wheat grain is infected by fungal pathogens, but it is also colonized by non-pathogenic fungi which inhibit the proliferation of pathogens and promote the growth and development of wheat plants [3,4]. The interactions between these fungal groups determine grain health and improve the consumer value of grain by reducing its mycotoxin content [3,5,6]. Growing conditions and nutrient availability can exert both positive and negative effects on the occurrence of pathogenic and non-pathogenic fungi [5–7].

Phosphorus is essential for root growth, healthy development of stems and ears, a desirable growth rate, high yield and quality, and resistance to abiotic and biotic stress factors [8]. Among the latter, fungal pathogens deserve special attention. A high supply of plant-available phosphorus has been linked with increased levels of fungistatic components, such as phenolic compounds and flavonoids, in different plant parts [9]. The indirect effect of phosphorus on increased plant growth seems to outweigh the direct effect of fungi by increasing the synthesis of phenolic compounds which contribute to resistance against fungal pathogens [10]. Since the natural amount of available phosphorus in arable soils does not fully cater to the nutritional needs of plants [11], crops have to be fertilized [12]. Phosphate rock is the raw material for the production of phosphorus fertilizers [13] which are indispensable in modern agriculture [14].

Rational phosphorus management poses a contemporary global challenge [15–17]. Primary sources of phosphorus are being massively wasted in the production process and it is estimated that only 20–25% of mined phosphorus reaches the produced food [17]. The above raises significant concerns about the availability of phosphorus for agriculture in the future [16]. Global phosphorus resources have not yet reached critical levels [18], but they are undeniably limited and non-renewable. Phosphate rock is distributed unevenly around the world [18] and many countries are dependent on phosphorus imports [19]. This problem applies to the European Union, which has recently added phosphate rock to the list of 20 critical raw materials [20].

Recycled phosphorus provides an alternative to non-renewable phosphate rock deposits [14]. The most abundant secondary sources of phosphorus include sewage and sludge from municipal and industrial wastewater treatment plants [21,22] and waste products from the meat processing industry [23].

Unprocessed phosphorus compounds from both primary and secondary sources are characterized by low solubility [24]. Fertilizer efficiency can be improved through the use of phosphorus solubilizing microbes (PSMs) which transform insoluble phosphorus compounds (PO4 <sup>3</sup>−) into highly bioavailable forms (HPO4 <sup>2</sup><sup>−</sup> and H2PO4 −) [4,25,26]. PSMs are a natural component of the soil edaphon [11]. *Bacillus megaterium* is one of the most effective PSMs [27]. These bacteria solubilize phosphorus by producing weak organic acids (gluconic, lactic, acetic, and succinic) [28]. Through solubilization and other biological mechanisms, PSMs can also act as plant growth-promoting microorganisms (PGPMs) [25,29]. It could be expected that by solubilizing phosphorus from soil and fertilizers, PSMs could contribute to a reduction in the fertilizer rate. The production of phosphorus biofertilizers from cheap renewables resources by PSMs promotes sustainable phosphorus management [16] and contributes to a circular economy [30].

Research into the production of phosphorus biofertilizers has been conducted by a Polish scientific consortium established by the Wrocław University of Science and Technology, the New Chemical Syntheses Institute in Puławy, and the University of Warmia and Mazury in Olsztyn [31]. Innovative biofertilizers are expected to deliver similar yield-forming effects to chemical fertilizers and to guarantee the safety of the produced crops. One of the most recent research concepts postulates the use of sewage sludge ash, dried animal blood, and *B. megaterium* in the production of biofertilizers.

This research aimed to determine the effect of the fertilizers produced from sewage sludge ash and dried animal blood on the species composition and structure of fungal communities colonizing wheat grain. The recycled fertilizer (Rec) and biofertilizer (Bio), i.e., Rec activated by *B. megaterium*, were assessed against commercial superphosphate. Mycological analyses were conducted using culture-dependent methods based on fungal sporulation as well as next-generation sequencing in the Illumina MiSeq system.
