1. Introduction
According to current estimates, the number of people in the world is increasing by more than 200,000 individuals every day [
1]. If growth tendencies continue, the world population will approach 9 billion by 2037 and reach 10 billion in 2057 [
1]. Since agriculture nowadays provides ~97.0% of the global food supply [
2], it is mainly this sector that will face the dilemma of feeding the world in the coming decades [
3]. The required corresponding increase in food production must come from increased crop productivity (higher yields, crop** intensities) [
4,
5] because the expansion of agricultural land is limited [
4]. An additional challenge is to increase agricultural productivity on a sustainable basis, i.e., without impacting environmental security, while conserving and enhancing biotic and abiotic resources [
3,
6]. Reducing food loss and waste should be an accompanying strategy [
7].
Increased crop productivity comes with the need for plants to take up more nutrients, mainly from soil resources. Soil nutrient abundance or nutrient availability often does not meet the nutritional requirements of high production potential crops, and replenishment of nutrients from external sources is necessary [
8]. Chemical/mineral fertilizers are fast-acting nutrient carriers that promote plant growth by rapidly increasing soil fertility [
9].
Current world agriculture consumes 107.7, 43.4, and 37.4 million tons of N, P
2O
5, and K
2O, respectively and, on average, per 1 hectare of cropland, 69.2 kg N, 27.9 kg P
2O
5 and 24.0 kg K
2O are applied in the mineral fertilizers [
2]. However, the global use of fertilizers is highly unbalanced and both overfertilization and fertilizer underutilization occur in different world parts. [
10]. Both fertilizer underuse and overuse cause specific problems in the regions of occurrence [
11,
12]; however, in recent years, more attention has been paid to the consequences of the latter [
13,
14,
15].
The adverse effects of mineral fertilizers on human health (including the formation of carcinogenic nitrosamines in the human body, methemoglobinemia, health risks of heavy metals, ‘hidden hunger’) and the environment (including soil acidification, toxic element pollution, eutrophication of aquatic ecosystems, global warming, alterations of biotic communities) have been addressed in numerous publications [
16,
17,
18,
19]. The production of mineral fertilizers requires high energy input [
20]. What is more, they are based on fossil fuels (N-fertilizers on Haber–Bosch process) or fossil ore deposits (phosphate rock), which are finite, non-renewable resources [
21].
In the past decade, the concept of the so-called circular economy (CE) has gained momentum [
22]. Regardless of its many definitions [
23], it generally implies closing material loops to preserve products, parts, and materials in the industrial system and extract their maximum utility [
24]. CE is seen as a promising strategy for supporting sustainable agriculture [
25].
One of the practical approaches towards CE is nutrient recovery from biological waste and their application in the form of biobased fertilizers [
26]. Some materials of biological origin (biomass) are traditionally used directly as organic/natural fertilizers or soil improvers, e.g., animal manure and crop residues. In addition to these so-called agricultural wastes, the organic fertilizer industry currently recycles organic by-products from various industries (value chain), including food and beverage, forestry, wood, paper and packaging, cosmetics and pharmaceuticals, environmental management, petroleum, and textiles [
27]. Among waste streams, animal manure, sewage waste and food chain waste, especially slaughterhouse waste, are considered the most promising substrates for fertilizer production [
28]. Most wastes require further processing for various reasons and, depending on the substrate, different technologies are recommended, including drying, composting, biological treatment, anaerobic digestion, incineration, liming, NP-precipitation, among others. As a result, various types of organic, organic-mineral, and inorganic fertilizers from renewable sources are developed [
27]. Despite occurring estimates, the empirical knowledge on the markets for waste and waste-derived fertilizers used in agriculture is still insufficient [
29]. The content of fertilizer components, however, estimated at about 22 million tons/year for nitrogen and 1.3 million tons/year for phosphorus, argues for the huge potential of biomass waste streams [
27].
The case of phosphorus (P) recycling and the real possibility of replacing or supplementing phosphate rock-based fertilizers with recycled ones show that CE principles work in practice [
30,
31]. Phosphate rock, the main raw material base for phosphate fertilizers, is a limited, non-renewable resource and, in addition, unevenly distributed around the globe [
32]. The scarcity of phosphate rock is a serious problem for Europe, making it dependent on importing virtually the entire raw material needed. In 2014, phosphate rock was placed by the European Union (EU) on the list of critical raw materials, to which P was also added in 2017 [
33,
34]. For these reasons, many European countries have intensified efforts to use renewable secondary P resources more efficiently [
35]. Abundant P-rich waste streams of biological origin flow from municipal and industrial wastewater treatment systems and slaughterhouses [
36], and this reservoir can be recovered through different technologies and be re-used in the form of new generation fertilizers [
27,
36]. In recent years, some countries have begun to develop legislative means to enforce P removal from wastewater and P recovery, and Switzerland was the first country to make P recovery from sewage sludge and slaughterhouse waste mandatory [
37]. From July 2022, a new regulation of fertilizer products will come into force in the EU to promote the use of fertilizers made from organic or recycled materials [
38].
Given the growing problems associated with sewage sludge disposal (growing production [
39], emerging new chemical and biological contaminants [
40], legal restrictions for agricultural use [
41]), thermal sludge conversion is thought one of the most promising methods of sludge management [
42]. Raw sewage sludge ash (SSA), while classified as waste [
43], is also seen as a potentially valuable source of P for fertilizer production [
44,
45]. According to common recent estimates, the annual global production of SSA is about 1.7 million tones and is expected to increase in the future [
46]. The P content in dry matter of SSA ranges from less than 10% to less than 20% [
47], which is comparable to the content of this element in commercial phosphate rock (10.9–16.13% P) [
48]. Apart from P, SSA is also a carrier of other macro-and micronutrients [
49]. Although the potential of P recovery from SSA is high, the P bioavailability in SSA is low and more than half of the ashes cannot be used as fertilizers due to their high potentially toxic element (PTE) content [
44]. In recent years, several new technologies have been developed with the potential to convert SSA into marketable fertilizer products after further treatment [
44,
45].
Animal blood is considered one of the main by-products of slaughterhouses, which, after being dried and powdered [
50], is widely used as blood meal, an environment-friendly fertilizer [
51]. Although blood meal is not rich in P (only 0.22% according to [
52]), it contains a high proportion of N (about 12% N [
52]) in the N-NO
3 form, which is readily available for plants [
53], and trace elements [
52]. In addition, animal blood is a good binder that can be used in fertilizer production [
52].
A new, innovative approach in waste-based fertilizer production is manufacturing these agrochemicals using a microbial solubilization process [
54]. Many naturally occurring microbial organisms, including bacteria, fungi, actinomycetes and algae, exhibit P solubilization ability. Among all microorganisms in the soil, phosphorus-solubilizing microorganisms (PSM) can constitute up to 50% [
55]. They are mostly associated with the plant rhizosphere. The most powerful PSM are strains from the bacterial genera
Pseudomonas,
Bacillus,
Rhizobium and
Enterobacter along with
Penicillium and
Aspergillus fungi, and
Bacillus megaterium is reported as one of the most important strains [
55]. PSM solubilize inorganic P compounds via the release of organic and inorganic acids and phosphatase enzyme [
56]. In addition to P solubilization, other biological mechanisms involving PSM (e.g., nitrogen fixation, potassium solubilization, phytohormone excretion, the release of antibiotics and antifungal metabolites, guarding plants from abiotic and biotic stresses and pollutant detoxification) function in soil, allowing them to promote plant growth directly or indirectly [
57,
58]. PSM have already been used in agriculture: alone, to activate ‘legacy P’ [
59], or with P substrate of low plant availability [
60]. Any substance containing live microorganisms that exhibit beneficial properties for plant growth and development is named a biofertilizer [
61]. Recently, PSM have also been shown to effectively solubilize P from waste materials [
62]. These findings provided the basis for the development of a technology to produce biofertilizers from secondary raw materials in a formulation in which living PSM cultures were incorporated [
54].
The recommendation of all waste biomass-based agrochemicals as substitutes for conventional fertilizers should be based on the results of their agronomic evaluation conducted under real (field) conditions. This evaluation should include not only a yield-forming efficiency analysis but also agricultural product quality and environmental impact. Although many papers on modern P recovery technologies and recycled fertilizer production have been published in recent years [
44,
45,
63,
64,
65,
66,
67], field test reports of such agrochemicals are not very common [
68,
69].
This paper presents the major results of the field evaluation of two biomass-based agrochemicals produced from SSA and dried animal blood, i.e., fertilizer (AF) and biofertilizer (BF, i.e., AF containing B. megaterium cells). The aim of the study was to assess the effect of these chemicals on wheat (test crop) productivity, i.e., grain yield and yield structure components, and on selected properties of the soil environment under the test crop, i.e., pH, available P content, the content of PTE (As, Cd, Cr, Ni and Pb) and the abundance of heterotrophic bacteria and fungi. It was hypothesized that the new biomass-based fertilizers would not show inferior yield-forming efficiency versus a commercial P fertilizer, superphosphate (SP), and their impact on the soil environment should be the same or more beneficial than that of SP.