Effect of Processing Solid Organic Municipal Wastes on Their Phosphorus Fertilizer Value

Abstract

In the circular economy framework, municipal wastes are seen as secondary raw materials that can be used to fertilize agricultural soils. This study assessed the effect of different biowaste and green waste treatment schemes on P fertilizer value to learn about the optimal valorization of these feedstocks. The wastes were used either fresh, after composting or anaerobic digestion, or as biochars produced at various pyrolysis conditions. The fertilizer value was determined from the change in soil concentration of plant-available P (P_CAL) in incubation experiments with different soils and the temporal dynamics of fertilizer-induced growth and P accumulation of ryegrass in a pot experiment with eight harvests. The mode of waste treatment significantly influenced the P fertilizer value in the incubation and in the pot experiment. In the incubation experiment, the amendment-induced P_CAL increase varied between 22% and 33% of applied P on low-P acidic soil and between 55% and 88% of applied P on high-P acidic soil, whereby the amendment effects were mainly determined by their effects on soil pH. In the pot experiment with low-P acidic soil, the apparent P recovery in the plant biomass (APR) varied between 2% of applied P for fresh green waste and 42% for fluid digestate. The amendment effects on APR were not related to soil pH but to the P_CAL supply with the amendments and amendment effects on soil P supply. Our data show great potential for increasing the P fertilizer value of organic municipal waste materials through appropriate processing prior to application.

1. Introduction

Phosphorus (P) is a macro-nutrient for plants that is needed in large amounts for agricultural plant production [1]. In the long term, the P withdrawn from agricultural soils through harvested products must be replaced by P fertilizers to sustain crop yields and to avoid a decline in soil fertility. Mineral P fertilizers are derived from phosphate rock [2]. Phosphate rock is a finite resource [3,4] which has been added to the list of critical raw materials of the European Union [5]. By far the most important use of phosphate rock is fertilizer. Therefore, it has been suggested to replace first-generation P fertilizers derived from phosphate rock with second-generation P fertilizers recovered from wastes [6,7,8].

Wastes with high potential for P recycling and recovery include livestock manures, wastes from food processing industries, domestic wastewater, and urban organic wastes [6,8,9]. It is estimated that more than two billion tonnes of municipal waste are generated globally per year, whereby more than 50% of this waste is food and garden waste [10,11]. The P recycling through use of wastes is associated with several problems, including contamination with potentially toxic mineral elements and persistent organic compounds, microplastics, antibiotic resistance genes and pathogens [12,13]. In addition, the P concentration in most waste materials is much lower than in mineral fertilizers, which makes transport, storage, and application more expensive. Another problem is the difference in the stochiometric nutrient ratios between the waste-derived fertilizers and the harvest products [14]. Some of the obstacles to using waste materials to fertilize agricultural land can be overcome by appropriate treatment prior to application.

The organic waste treatment technologies can be grouped into four categories: direct land application, biological treatment, e.g., through composting or anaerobic digestion, thermo-chemical treatment, e.g., through pyrolysis and physico-chemical treatment [15]. Composting and anaerobic digestion have been the most commonly used methods for treating biowaste and green waste [12,14,16]. In recent decades, the pyrolysis of waste materials for the production of biochar has increasingly become the focus of scientific research [17,18,19,20]. The properties of biochar and its effects on soil and plants largely depend on feedstock and pyrolysis conditions [21].

Mineral P fertilizers, such as triple superphosphate, add mainly phosphate to the soil and are used for direct P nutrition of the plants. The organic P fertilizers contain all the mineral nutrients required by the plants and additionally provide the soil with organic carbon. It is well documented that organic amendments can improve physical, chemical and biological soil characteristics that are relevant for soil fertility [22]. This has been shown for composts [23,24,25,26], digestates [27] and biochars [28,29,30]. Furthermore, organic soil amendments can support the remediation of polluted soils [31,32,33]. Biochars can reduce the emission of nitrous oxide from soil and nutrient leaching [34,35].

The acceptance of waste-derived fertilizers for agriculture depends largely on their direct effect on the mineral nutrition of plants and their ability to replace mineral fertilizers. Methods for the quantification of the fertilizer value of P recycling fertilizers can roughly be divided into “extraction methods” and “growth methods” or “bioassays”. In the former, the fertilizer value is assessed by measuring P solubility in different chemical extractants or P accumulation in artificial sinks that mimic plant roots. In the latter, the fertilizer value is assessed by the plant performance, e.g., growth or P uptake [36,37,38]. These methods usually compare the effect of the fertilized and unfertilized variants and indicate the effect, e.g., as the fertilizer-induced change in soil P, which is soluble in a specific extraction agent, or P accumulation in plants. The fertilizing effect of organic fertilizer is often compared to that of mineral fertilizer commonly used in agricultural practice and expressed as relative agronomic efficiency or mineral fertilizer equivalent [39,40]. There are many studies in which the P fertilizer value was determined for composts [37,41,42], digestates [43,44] and biochars [45,46,47]. The availability of P is dependent on soil pH [48] which, in turn, can be influenced by the organic soil amendments [25,33,49]. However, there is a lack of studies that examine how different treatment methods for a specific waste material affect its P fertilizer value.

The overall objective of this study was to contribute to an improved use of municipal organic waste materials for fertilization purposes. This study investigated how different treatments of organic municipal waste before application to the soil affect its P fertilizer value. The waste treatments included composting, anaerobic digestion and pyrolysis at different temperatures. In an incubation experiment with three soils, we addressed the questions: (1) How does the mode of treatment of green waste (GW) influence the soil concentration of plant available P? (2) Is the treatment effect only due to the treatment effect on the amendment-induced modification of the soil pH? In a pot experiment with ryegrass, we addressed the question: (3) How does the mode of treatment of GW and biowaste (BW) influence various indicators of fertilizer efficiency, including growth rate, P uptake rate and mineral fertilizer equivalent? The pot experiment was conducted over 231 days, and the plants were harvested eight times to capture the residual effects of the single fertilizer application at the start of the experiment.

2. Materials and Methods

2.1. Soils

Three soils were used in this study: (i) low-P acidic soil (sandy loam from a meadow at Hanhofen, Germany, provided as Standard soil 2.2 by LUFA Speyer, Speyer, Germany); (ii) high-P acidic soil (sandy loam from an arable field at Dahlem Experimental Station, Berlin, Germany); and (iii) high-P neutral soil (silty clay from an arable field near Samarkand, Uzbekistan 39°43′41.8″ N 67°10′04.0″ E). All soils were collected from the 0–0.2 m layer, mixed thoroughly, air dried and sieved (<2 mm). The P concentrations and pH markedly differed among the three soils (

Table 1. Properties of the soils used in the incubation experiment and the pot experiment (only low-P acidic soil is used in the pot experiment); means ± standard deviation, n = 3. P_total—total phosphorus, P_CAL—calcium acetate lactate soluble phosphorus.

Soils	Ptotal, g (kg Soil)−1	PCAL, g (kg Soil)−1	pH
Low-P acidic soil	0.56 ± 0.002	0.018 ± 0.001	5.26 ± 0.04
High-P acidic soil	0.91 ± 0.004	0.079 ± 0.004	5.64 ± 0.07
High-P neutral soil	1.90 ± 0.016	0.113 ± 0.010	7.32 ± 0.09

).

2.2. Organic Soil Amendments Derived from Municipal Waste

The organic soil amendments were derived from biowaste and from green waste (

Table 2. Overview of the biowaste (BW) and green waste (GW) treatments and abbreviations used in this study.

Treatment of Municipal Waste	Abbreviation
No further treatment	Fresh_BW	Fresh_GW
Composting	Com_BW	Com_GW
Pyrolysis at 350 °C	P350_BW	P350_GW
Pyrolysis at 450 °C	P450_BW	P450_GW
Pyrolysis at 700 °C	P700_BW	P700_GW
Pyrolysis at 700 °C, modified atmospheric conditions	P700a_BW	P700a_GW
Anaerobic digestion for biogas production, solid residue	FRsolid_BW
Anaerobic digestion for biogas production, fluid residue	FRfluid_BW

). The fresh and composted biowaste, as well as the solid and fluid fermentation residues from biogas production from biowaste stemmed from a waste treatment plant in Berlin (BSR, Ruhleben). The biowaste consisted of food waste and garden waste, mainly senescent leaves and twigs. Visible plastic materials were removed from the biowaste. The fresh and composted green waste stemmed from a composting facility near Berlin (GALAFA, Falkensee). The green waste contained green grass clippings and senescent tree leaves (25–30% of fresh mass), twigs (15–20%), and shredded log wood (50–55%). For the production of biochars, the biowaste and green waste were pyrolyzed at 350 °C (P350) or 450 °C (P450) for 0.5 h under 3 L min⁻¹ N₂ flow rate in a continuous screw reactor. Aliquots of the biochars produced at 450 °C were transferred to a semi-batch reactor and further pyrolyzed at 700 °C for 2 h under 10 L min⁻¹ flow rate of N₂ (P700) or a flow rate of 6 L min⁻¹ of N₂ + 2 L min⁻¹ H₂O + 2 L min⁻¹ CO₂ (P700a). Further details for the treatment of the municipal wastes and the chemical characteristics of the derived amendments are given in [33] and Table S1.

2.3. Experimental Procedure

The P fertilization effect of the organic amendments was tested in an incubation experiment without plants and a pot experiment with ryegrass (Figure S1).

2.3.1. Incubation Experiment

The incubation experiment included all three soils (Table 1). The soils were incubated either without amendment (control) or after mixing with 50 mg P_total (kg dry soil)⁻¹ in the form of triple super phosphate (TSP), fresh GW, composted GW, or biochars from GW produced under four different pyrolysis conditions (P350, P450, P700, P700a). On day 0, 120 g of dry soil/amendment mixture was weighed into plastic cups with a volume of 120 cm³. The soil water content was adjusted to 70% of the maximum water-holding capacity of the unamended soil with demineralized water. The plastic cups were placed in a climate chamber at 20 °C. Soil samples (approx. 30 g) were taken after 1, 7, 30 and 100 days of incubation. All treatments were replicated four times, resulting in a total of 96 plastic cups.

In another set of treatments with the two acidic soils (Table 1), which tested the effect of soil pH on the soil P_CAL concentration, the soil was amended with different amounts of 0.5 M KOH: KOH/soil mass ratios 0/100, 1/100, 3/100, 5/100 and 10/100 in the low-P acidic soil and 0/100, 1/100, 3/100 and 5/100 in the high-P acidic soil. The incubation conditions and number of repetitions were identical to those described above. Soil samples were taken after 1, 7, 30, and 100 days of incubation in the low-P acidic soil and after 30 days of incubation in the high-P acidic soil.

2.3.2. Pot Experiment

The pot experiment was conducted from 19 January 2021 to 8 September 2021. For the pot experiment, only the low-P acidic soil was used. The pots (19 cm height, 16 and 13 cm diameter at the top and bottom, respectively) were filled with 2.7 kg dry soil/amendment mixture at a density of 1.1 g cm⁻³. Ryegrass (Lolium perenne cv. Valerio, Deutsche Saatveredelung AG, Lippstadt, Germany) was sown at a density of about 100 seeds (0.3 g seeds) per pot. The soil was irrigated to 70% water-holding capacity of the unamended soil. The soil water content was held between 65 and 75% water-holding capacity throughout the experiment. Plants were cultivated for 231 days in a glasshouse at an average air temperature of 23 ± 2 °C and air humidity of 52 ± 7%. In winter and spring, the day length was extended to 12 h with additional lighting. The aboveground biomass was harvested three cm above the soil surface 36, 57, 77, 97, 126, 160, 196 and 231 days after sowing.

Before filling the soil/amendment mixture into the pots, 16 treatments were applied to the soil. In the control, no P was added. In the other 15 treatments, 25 mg of P_total (kg dry soil)⁻¹ was added in the form of TSP, fresh GW or BW, composted GW or BW, biochars from GW or BW produced under four different pyrolysis conditions (P350, P450, P700, P700a), or the solid and fluid fraction of the digestate of biogas production from BW. The experiment was performed with 5 repetitions per treatment, which were arranged in a fully randomized block design.

During set-up, the soil was fertilized with 100 mg of N, 150 mg of K, 90 mg of Mg and 180 mg of S (kg dry soil)⁻¹, supplied as NH₄NO₃ and mineral N in the organic amendments and as K₂SO₄ and MgSO₄. During the first three weeks of the experiment, soils amended with GW or BW received an additional fertilization with 109 and 190 mg of N (kg dry soil)⁻¹, respectively, supplied as NH₄NO₃ in three applications to compensate for N immobilization induced by rapid C mineralization and the wide C/N-ratio of the amendments. After each harvest, the soil was fertilized with 100 to 125 (after harvest 6) mg N and 150 to 200 (after harvest 6 and 7) mg of K (kg dry soil)⁻¹, supplied as NH₄NO₃, Ca(NO₃)₂ (after harvest 3 and 4) and K₂SO₄. Nutrients were added within five to seven days after harvest, except after the third harvest, where nutrients were added 15 days after harvest, when plants in all treatments developed symptoms of N deficiency.

2.4. Measurements and Chemical Analyses

The pH of soils and amendments was measured in a 0.0125 M CaCl₂ solution with a soil: solution ratio of 1:2.5 (mass) for soils and 1:8 (mass) for amendments [50]. The total P concentration in the soil and the amendments was determined in the aqua regia extract with inductively coupled plasma-optical emission spectrometry (ICP-OES, iCAP 6300 Duo MFC, Thermo, Waltham, MA, USA). As an indicator of plant available P, the concentration of calcium acetate lactate soluble P (P_CAL) in soils and amendments was measured according to the studies of [51,52].

Plant dry mass was recorded after drying to a constant weight at 70 °C in a forced-air oven. The dried plant material was ground to particles (<0.5 mm) using a centrifugal mill (ZM1; Retsch, Haan, Germany). The P concentration in the plant dry mass was measured with a spectral photometer (Specord 40, Analytik Jena GmbH, Jena, Germany) after dry ashing and dissolution in 2 N HCl and phosphate staining with the ammonium–vanadomolybdate reagent [53].

2.5. Calculation of Indicators for the P Fertilization Effect

In the pot experiment, the following indicators for the P fertilization effect were calculated. The shoot growth rate (SGR) was calculated by dividing the shoot dry mass (SDM) increment between two consecutive harvests by the number of days between the harvests according to Equation (1):

$$\mathrm{S}\mathrm{G}\mathrm{R}=(\mathrm{S}\mathrm{D}\mathrm{M}2-\mathrm{S}\mathrm{D}\mathrm{M}1)/(\mathrm{t}2-\mathrm{t}1)$$

(1)

where SDM1 and SDM2 are the shoot dry mass at time 1 and time 2.

The P uptake of the plants (PU) was calculated by multiplying the shoot dry mass by the shoot P concentration (mg (g DM)⁻¹) according to Equation (2):

$$\mathrm{P}\mathrm{U}=\mathrm{S}\mathrm{D}\mathrm{M}\times \mathrm{s}\mathrm{h}\mathrm{o}\mathrm{o}\mathrm{t}\mathrm{P}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}$$

(2)

The P uptake rate (PUR) was calculated by dividing PU between two consecutive harvests by the number of days between the harvests and the soil mass per pot.

$$\mathrm{P}\mathrm{U}\mathrm{R}=\left[\right(\mathrm{P}\mathrm{U}2-\mathrm{P}\mathrm{U}1\left)\right]/(\mathrm{t}2-\mathrm{t}1)/\mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{p}\mathrm{o}\mathrm{t}$$

(3)

The apparent amendment-P recovery in the plants (APR) was calculated from the difference in PU in treatments with (PU_amend) and without amendment (PU_control) divided by the total amendment P added (P_add) and multiplied by 100.

$$\mathrm{A}\mathrm{P}\mathrm{R}=({\mathrm{P}\mathrm{U}}_{\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{d}}-{\mathrm{P}\mathrm{U}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}})/{\mathrm{P}}_{\mathrm{a}\mathrm{d}\mathrm{d}}\times 100$$

(4)

The mineral fertilizer equivalent (MFE) was calculated from the quotient of APR with organic amendment (APR_amend to APR) with triple superphosphate (APR_TSP) times 100.

$$\mathrm{M}\mathrm{F}\mathrm{E}=({\mathrm{A}\mathrm{P}\mathrm{R}}_{\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{d}}{/\mathrm{A}\mathrm{P}\mathrm{R}}_{\mathrm{T}\mathrm{S}\mathrm{P}})\times 100$$

(5)

In the incubation study, the amendment-induced increase in the soil P_CAL concentration (API) was calculated according to Equation (6):

$$\mathrm{A}\mathrm{P}\mathrm{I}\left(\mathrm{\%}\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{d}\mathrm{P}\right)=({\mathrm{P}\mathrm{C}\mathrm{A}\mathrm{L}}_{\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{d}}-{\mathrm{P}\mathrm{C}\mathrm{A}\mathrm{L}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}})/{\mathrm{P}}_{\mathrm{a}\mathrm{p}\mathrm{p}}\times 100$$

(6)

whereby PCAL_amend and PCAL_control are the soil concentrations of CAL-soluble P with and without amendment (mg P (kg soil dry)⁻¹), and P_app is the amount of total P applied to the soil with the amendment (mg P (kg dry soil)⁻¹).

In the incubation experiment, the application of organic soil amendments resulted in an increase in soil pH. Therefore, API can be due to (i) the API that is induced by the change in soil pH (API_pHrel.) and (ii) the API that is not related to the change in soil pH (API_pHadj.):

$${\mathrm{A}\mathrm{P}\mathrm{I}=\mathrm{A}\mathrm{P}\mathrm{I}}_{\mathrm{p}\mathrm{H}\mathrm{r}\mathrm{e}\mathrm{l}.}+{\mathrm{A}\mathrm{P}\mathrm{I}}_{\mathrm{p}\mathrm{H}\mathrm{a}\mathrm{d}\mathrm{j}.}$$

(7)

The pH effects on soil P_CAL were derived from the treatments, which increased the soil pH by adding KOH (see Section 2.3.1). For both acidic soils, P_CAL was closely related to soil pH. The relationships differed between the soils:

low-P acidic soil: P_CALpHrel. = 6.35 pH–17.9 (R² = 0.87)

high-P acidic soil: P_CALpHrel. = 28.1 pH–95.1 (R² = 0.96)

whereby P_CALpHrel. is the soil P_CAL concentration related to the pH effect of the amendment, that is, the soil P_CAL concentration expected if the amendment effect were solely due to pH effects. The amendment effect on P_CAL that is not related to the pH effect, i.e., the amendment effect adjusted for the pH effect (P_CALpHadj.), was calculated from the difference between P_CAL and P_CALpHadj. of the amended soil. The API_pHrel. and API_pHadj. were calculated according to eq. 6 with the exception that P_CALamend was replaced by P_CALpHrel. and P_CALpHadj. respectively.

2.6. Statistical Analysis

The data were analyzed with separate one-way ANOVAs for each feedstock to test the effect of processing and with a two-way ANOVA to test the effects of feedstock and processing. For the two-way ANOVA, the fermentation residues were excluded from the analysis because data were available only for biowaste (see Table 2 for factor levels). Model assumptions were checked using quantile-quantile plots (normality of errors) and plots of standardized residuals against fitted values (homoscedasticity of errors) [54]. For pair-wise comparisons, Tukey’s honest significant differences (HSDs) were calculated for p ≤ 0.05. Statistical analyses were calculated in R (Version R 4.5.0, Core Team, 2025).

3. Results

3.1. Effect of Processing on P_total, P_CAL and pH of the Amendments

The different modes of processing significantly influenced the concentrations of P_total and P_CAL, the proportion of P_CAL in P_total and the pH of the amendments derived from green waste (GW) (

Table 3. Effect of processing on total P (P_total) and calcium acetate lactate soluble P (P_CAL) and pH of the amendments derived from green waste (A) and biowaste (B); means ± standard deviation, n = 3. Different lowercase letters indicate significant differences (p < 0.05) between amendments derived from one feedstock.

Processing	Ptotal	PCAL	PCAL	pH
	g (kg DM)−1	g (kg DM)−1	% of Total P
(A) Green waste-derived amendments
Fresh	1.0 ± 0.1 c	0.43 ± 0.01 c	41 ± 3 a	5.4 ± 0.02 f
Com	2.1 ± 0.3 b	0.57 ± 0.02 b	27 ± 3 b	7.1 ± 0.01 e
P350	2.1 ± 0.1 b	0.86 ± 0.02 a	42 ± 3 a	7.7 ± 0.04 d
P450	2.0 ± 0.1 b	0.88 ± 0.01 a	44 ± 2 a	8.8 ± 0.06 c
P700	2.7 ± 0.1 a	0.46 ± 0.03 c	17 ± 1 c	12.4 ± 0.01 a
P700a	3.1 ± 0.2 a	0.45 ± 0.02 c	15 ± 1 c	11.4 ± 0.01 b
(B) Biowaste-derived amendments
Fresh	2.2 ± 0.1 c	1.02 ± 0.04 c	46 ± 3 ab	4.9 ± 0.01 h
Com	3.3 ± 0.3 bc	0. 98 ± 0.06 c	30 ± 5 bc	7.5 ± 0.06 e
P350	4.0 ± 0.3 abc	1.07 ± 0.03 bc	27 ± 2 cd	8.6 ± 0.02 d
P450	4.9 ± 0.2 ab	1.27 ± 0.02 b	26 ± 2 cd	9.1 ± 0.02 c
P700	4.5 ± 0.4 ab	0.32 ± 0.01 d	7 ± 1 e	12.2 ± 0.02 a
P700a	4.9 ± 0.1 ab	0.53 ± 0.04 d	11 ± 1 de	11.2 ± 0.07 b
Fluid	5.0 ± 0.8 a	3.22 ± 0.17 a	62 ± 3 a	7.2 ± 0.11 f
Solid	3.1 ± 1.4 bc	1.17 ± 0.02 bc	40 ± 1 bc	7.0 ± 0.06 g

A) and biowaste (BW) (Table 3B). The P_total concentrations varied between 1.0 (Fresh) and 3.1 g (kg DM)⁻¹ (P700a) for the GW-derived and from 2.2 (Fresh) to 5.0 g (kg DM)⁻¹ (Fluid) for the BW-derived amendments. The P_CAL concentrations varied between 0.43 (Fresh) and 0.88 g (kg DM)⁻¹ (P450) for the GW-derived and from 0.32 (P700) to 3.22 (kg DM)⁻¹ (Fluid) for the BW-derived amendments. The effects of processing on P_CAL concentrations were different from the processing effects on P_total concentrations. Therefore, the proportion of P_CAL in P_total significantly differed among the treatments. The proportion of P_CAL in P_total varied from 15 (P700) to 44% (P450) in the GW-derived amendments and from 7 (P700) to 62% (Fluid) in the BW-derived amendments. For biochars from both feedstocks, the proportion of P_CAL in P_total was much lower for the biochars produced at 700 °C than for the biochars produced at 350 and 450 °C. The pH of the amendments varied from 5.4 (Fresh) to 12.4 (P700) for the GW-derived, and from 4.9 (Fresh) to 12.2 (P700) for the BW-derived amendments. For the biochars from both feedstocks, the pH increased with increasing pyrolysis temperature. For biochars produced at 700 °C, pH was lower for P700a than for P700.

3.2. Effect of Green Waste Processing on the Soil P_CAL Concentration

In the incubation experiment, soil samples were taken after 1, 7, 30 and 100 days of incubation. In all three soils, the P_CAL concentrations did not change significantly during the incubation period (Figure S2). Therefore, the soil P_CAL concentration was calculated as the mean value from all four sampling dates. In the two acidic soils, the P_CAL concentration with amendments was significantly higher than without amendments (Figure 1). In the high-P neutral soil, the P_CAL concentration with amendments was not consistently higher than without amendments. In the two acid soils, the P_CAL concentration was significantly influenced by the fertilizer variant (

Table 4. p-values for the effects of the fertilizer variant on soil P_CAL concentration (g kg⁻¹ soil), amendment-induced P_CAL increase (% applied P), soil pH after 100 incubation days and pH-adjusted P_CAL increase (% applied P) in three soils.

Source of Variance	PCAL Concentration	Amendment-Ind. PCAL Increase (API)	Soil pH	pH-Adjusted PCAL Increase (APIpHadj.)
Low-P acidic soil
Fertilizer variant	<0.001	<0.001	<0.001	<0.001
High-P acidic soil
Fertilizer variant	<0.001	<0.001	<0.001	<0.001
High-P neutral soil
Fertilizer variant	0.079	0.046	<0.001

). In the amended soils, the P_CAL concentration was highest with TSP and lowest with fresh GW for the low-P acidic soil (Figure 1A), whereas, in the high-P acidic soil, the P_CAL concentration was highest with P700a and lowest with TSP (Figure 1B).

For the assessment of the amendment effects on soil P_CAL, the amendment-induced soil P_CAL increase (API) was calculated from the difference in soil P_CAL concentrations in the soil with amendment and without amendment. This difference is given as a percentage of the P applied with the amendments. The API was larger in the high-P acidic soil than in the low-P acidic soil and the high-P neutral soil (Figure S3). The API in all three soils was significantly influenced by the fertilizer variant (Table 4; Figure S3). In the low-P acidic soil, the API varied between 22 (Fresh) and 34% (TSP) of the applied P (Figure S3A). The API with TSP, P350 and P450 was significantly higher than with fresh and composted GW (Figure S3A). Among the biochars, the API was significantly higher with P450 than P700. This indicates that, for biochars, the pyrolysis conditions had an effect on the API in the low-P acidic soil. In the high-P acidic soil, the API varied between 41 (TSP) and 88% (P700a) of the applied P (Figure S3B). Thus, in contrast to the low-P acidic soil, in the high-P acidic soil, the API was lowest with TSP. The API was higher with biochars than with fresh and composted GW. The pyrolysis conditions had no significant effect on the API in the high-P acidic soil. In the high-P neutral soil, the API varied between 2 (Fresh) and 36% (TSP) of the applied P (Figure S3C). However, the large HSD value indicates a high degree of uncertainty for the API values in the neutral soil.

After 100 incubation days, soil pH in all three soils was significantly influenced by the fertilizer variant (Table 4). In the low-P acidic soil, the pH ranged from 5.3 (Control, TSP) to 6.9 (BC700a; Table S2). In the high-P acidic soil, the pH ranged from 5.6 (Control, TSP) to 7.0 (BC700). In the high-P neutral soil, the pH was only little influenced by the amendments and ranged from 7.3 (Control, TSP) to 7.5 (BC700). For the low-P and high-P acidic soils, we tested the effect of soil pH on soil P_CAL concentration in a set of treatments, in which the soil pH was modified by adding different amounts of KOH. In both soils, the P_CAL concentration increased linearly with increasing soil pH (Figure 2).

The relationships between soil pH and P_CAL concentration were used to calculate the pH-related P_CAL concentration (P_CALpHrel.), i.e., the P_CAL concentration that would be expected if the effect of the amendment was only due to its effect on soil pH (see Section 2.5). In both soils, for all organic amendments, the measured P_CAL concentration increase (total column in Figure 3) was larger than the P_CALpHrel. increase. This indicates that the organic amendments increased the soil P_CAL concentrations not only by increasing soil pH. The difference between the measured P_CAL concentration increase and the P_CALpHrel. increase is referred to as the “pH-adjusted P_CAL concentration increase” (P_CALpHadj.). The P_CALpHadj. increase due to amending the soil with GW-derived organic fertilizers was larger in high-P acidic than in low-P acidic soil, except for the biochars P350 and P450 (Figure 3). In both soils, P_CALpHadj was significantly affected by the fertilizer variant (Table 4; Figure 3). In the low-P acidic soil, the P_CALpHadj. increase for the organic amendments varied between 4 (P700) and 16% (compost; Figure 3). The increase was significantly higher for compost than for P700, P700a and Fresh. Furthermore, the increase was higher for P350 and P450 than for P700 and Fresh. In the high-P acidic soil, the P_CALpHadj. increase in the organic variants varied between 9 (P700) and 35% (compost; Figure 3). In both soils, the increase was significantly higher for TSP than for the organic amendments, except for compost. Among the organic amendments, the increase was significantly larger for compost than for the other organic amendments.

3.3. Effect of Amendments on Shoot Growth of Ryegrass

The shoot growth rates (SGRs) were generally higher in soils with TSP than without P addition, except for the fourth growth period (Figure 4). In the fourth period, the growth rates of all fertilized variants transiently decreased. This decrease was presumably due to transient N deficiency as a consequence of a delay in fertilizer application. The difference between the non-fertilized and the TSP variant decreased towards the end of the experiment, although the apparent P uptake was only about 50% of the TSP-P applied. In comparison to the non-fertilized control, the SGR were higher for all the variants fertilized with differently treated municipal wastes with the exception of fresh GW (Figure 4). Towards the end of the experiment, the differences between the control and the organic fertilizer variants decreased, especially in the BW variants. In comparison to the variant fertilized with TSP, the SGR of the organic fertilizer variants was lower only in the first three growth periods (Figure 4). The mode of municipal waste treatment had significant effects on the SGR in all growth periods except the 4th growth period for green waste (

Table 5. p-values for the effects of processing and feedstock on shoot growth rate (mg day⁻¹). For the two-way ANOVA, fermentation residues are excluded because data are available for biowaste only.

Source	Growth Periods
	1	2	3	4	5	6	7	8
(a) Green waste (one-way ANOVA)
Processing	<0.001	0.018	0.019	0.259	<0.001	<0.001	<0.001	0.016
(b) Biowaste (one-way ANOVA)
Processing	<0.001	0.042	0.075	0.013	0.006	0.030	0.734	0.778
(c) Green waste and biowaste (two-way ANOVA)
Feedstock (FS)	0.156	0.838	0.642	0.520	0.072	0.009	0.009	0.265
Processing	<0.001	0.076	0.013	0.020	<0.001	<0.001	<0.001	0.007
FS × Processing	0.939	0.068	0.326	0.360	<0.001	0.0149	0.002	0.085

; Figure 4A) and in the growth periods 1, 2, 4, 5 and 6 for biowaste (Table 5; Figure 4B).

3.4. Effect of the Amendments on P Uptake

The shoot P concentrations varied between 1.1 and 2.4 mg P (g DM)⁻¹ over the entire experimental period and across all fertilizer variants (Figure S4). The P concentrations were thus below the range required for optimum growth in all fertilizer variants [55]. There were no consistent changes in P concentrations during the experimental period (Figure S4).

For all fertilization variants, the P uptake rates (PURs) increased continuously in the first three growth periods and then decreased again (Figure 5). The initial increase was probably due to the progressive rooting of the soil and thus the better spatial exploitation of the soil and fertilizer P by the roots. The decrease in later growth periods was presumably related to the decline in chemical P availability of soil and fertilizer P.

The PURs were significantly influenced by the type of amendment and, for organic amendments, by the feedstock and the mode of feedstock treatment (

Table 6. p-values for the effects of processing and feedstock on the P uptake rate in different growth periods and the mean P uptake rate for the 8 growth periods. For the two-way ANOVA, fermentation residues are excluded because data are available for biowaste only.

Source	Growth Period
	1	2	3	4	5	6	7	8	Mean
(a) Green waste (one-way ANOVA)
Processing	<0.001	0.001	<0.001	<0.001	<0.001	<0.001	<0.001	0.013	<0.001
(b) Biowaste (one-way ANOVA)
Processing	<0.001	<0.001	0.005	<0.001	<0.001	0.589	0.777	0.513	<0.001
(c) Green waste and biowaste (two-way ANOVA)
Feedstock	0.101	0.842	0.006	0.227	0.574	0.093	0.124	0.188	0.647
Processing	<0.001	0.112	<0.001	<0.001	<0.001	<0.001	0.002	0.004	<0.001
FS × Processing	0.121	<0.001	0.008	<0.001	<0.001	0.018	0.001	0.066	<0.001

; Figure 5). The PUR was higher for TSP than for the unfertilized control throughout the entire experiment, although the differences between these two variants were only small and no longer significant in the last two growth periods. The PUR of the organic fertilizer variants depended on the mode of feedstock treatment. For the organic amendments derived from GW, P uptake rates were lowest for fresh GW and highest for biochars produced at 350 °C and 450 °C throughout the experiment.

The PURs for compost and the two biochars produced at 700 °C were in between. For the organic amendments derived from BW, the PURs were influenced by the mode of treatment mainly in the initial growth periods, whereas in the last three growth periods the mode of treatment had no significant effect on the PUR. In the first five growth periods, the PURs were highest for FR_fluid.

The apparent P recovery (APR) increased with increasing experimental duration for all fertilizer variants except fresh GW (Figure S5). For TSP, the APR increased from 12% at the first to 50% of the applied P at the 8th harvest. The APR of the organic amendments derived from GW and BW was significantly affected by the mode of treatment (Table S3). For GW, the APR at the first harvest ranged from −2 (Fresh) to 4% of the applied P (P350) and from 1 (Fresh) to 47% of the applied P (P350) at the 8th harvest. The APR of the organic amendments derived from BW at the first harvest ranged from 0 (Fresh) to 11% of the applied P (FR_fluid) and at the eighth harvest from 23 (Com) to 42% of the applied P (FR_fluid).

The mineral fertilizer equivalent (MFE) at the first harvest ranged from −22% (Fresh) to 31% (P350) for the GW-derived amendments (Figure 6A) and from 1% (Fresh) to 84% (Fluid) for the BW-derived amendments (Figure 6B). With increasing experimental duration, the MFE continuously increased for all amendments with the exception of fresh GW and fluid fermentation residue from BW. At the end of the experiment, the MFE varied between 1% (Fresh) and 93% (P350) for the GW-derived amendments (Figure 6A) and between 50% (Fresh) and 97% (Fluid) for the BW amendments (Figure 6B).

4. Discussion

In this study the P fertilizer value of organic fertilizers derived from organic municipal wastes after different treatments was assessed in an incubation study with three different soils and a pot experiment with ryegrass. With both experimental approaches, the P fertilizer value was significantly affected by the mode of waste treatment.

4.1. Effects of the Mode of Treatment on the P Concentrations in the Amendments

In comparison to non-treated fresh green waste and biowaste, the P_total concentrations in composts, biochars and fermentation residues were significantly higher (Table 3). This is in agreement with findings from the literature for composts [56], biochars [57] and fermentation residues [43]. Composting, pyrolysis and anaerobic fermentation decrease the organic mass, while most P is retained. The P_total was higher for biochars produced at 700 °C than at 350 °C (Table 3). An increase in P_total with increasing pyrolysis temperature up to 700 °C is in agreement with findings from other studies [57,58,59]. This is presumably due to increasing losses of organic mass with increasing temperatures, whereas P is retained up to the threshold temperature for P volatilization of 700 °C [60].

Treatment of organic material by composting [56,61,62], pyrolysis [63,64] and fermentation [43] leads to transformation of the different P species contained in the material and may lead to modification of the P availability. In our study the treatment effects on P_CAL differed from effects on P_total (Table 3). Therefore, the proportion of available P_CAL in P_total significantly differed among the treatments (Table 3). The proportion of P_CAL was significantly lower in compost and biochars produced at 700 °C than in the fresh material and the biochars produced at 350 and 450 °C (Table 3). Pyrolysis can convert plant-available organic P into inorganic P that is less available [65]. The opposite has also been observed, with pyrolysis increasing plant-available P [66]. Lower concentrations of available P in biochars produced at high temperatures in comparison to the feedstock used for biochar production have also been found in other studies [57,59,67,68,69]. However, the effect of pyrolysis temperature on the concentration of available P in biochar is not consistent. In some studies, higher concentrations of available P were found with increasing temperatures, and, in other studies, lower concentrations were found [58,64]. Biochar P availability can be increased by selecting low Ca feedstocks or doping the feedstock with K, which leads to preferential binding of P with K instead of Ca, Mg or Fe and, thus, the formation of highly soluble salts [65]. In the fermentation residues, the proportion of P_CAL was higher in the fluid than in the solid fraction (Table 3). Higher proportions of available P in the fluid than in the solid fraction of fermentation residues have also been reported in other studies [70,71,72].

The pH in compost and biochars was significantly higher than in the non-treated fresh green waste, and, for biochars, the pH increased with increasing pyrolysis temperature (Table 3). This is in accordance with findings from the literature for composts [73,74] and biochars [58].

4.2. Effects of the Mode of Treatment on the Soil P Concentration After Application of the Amendments

In the two acidic soils, the concentrations of plant available P_CAL were significantly increased by the amendments (Figure 1). Higher P availability is in agreement with findings for soils amended with biochars [21,46,47,60,75], compost [42] and fermentation residues [43]. The amendment-induced increase in the soil P_CAL concentration (API) was significantly influenced by the soil and the treatment method for green waste (Figure S3 and Figure 3). The API was much larger in the high-P acidic soil than in the low-P acidic soil and the high-P neutral soil. This was possibly due to lower saturation of soil sites, which strongly bind P_CAL in the low-P acidic soil, and lower capacity for P sorption in the high-P neutral soil [76,77]. The API may be due to the direct supply of P_CAL with the amendments and/or amendment effects on soil P dynamics. In the high-P acidic soil, the amount of amendment-induced P_CAL increase (in mg P_CAL/100 g soil) for all amendments was substantially higher than the amount of P_CAL supply with the amendments, whereas, in the low-P acidic soil and the high-P neutral soil, the amendment-induced P_CAL increase was similar to or lower than the P_CAL supply (Table S4). This indicates that, in the high-P acidic soil, the amendments led to a transfer of soil P into the plant available P_CAL pool. A transfer of soil P into the P_CAL pool can be mediated by pH-related mechanisms [48] and mechanisms that act independently of the pH [42,60].

In the two non-fertilized acidic soils, the increase in the soil pH by adding KOH solution increased the P_CAL concentration (Figure 2). This shows that an elevation of the soil pH led to a transfer of soil P into the plant available P_CAL pool. In acidic soils, P availability is low due to phosphate interactions with Al- and Fe-ions, which lead to highly insoluble Al- and Fe-phosphates. Furthermore, low pH increases phosphate sorption to Fe- and Al-Oxides [48]. Under these conditions, increasing soil pH enhances P availability, with maximum P availability occurring at near neutral pH [48].

In our study, the soil pH without fertilization was 5.3 in the low-P and 5.6 in the high-P acidic soil. The fertilization with organic amendments increased soil pH, whereby the pH increase was significantly influenced by the method of green waste treatment (Table 4 and Table S1). The amendment-induced pH increase was lowest with compost, higher with fresh GW and largest with biochars. For biochar-amended soil, the pH increased with increasing pyrolysis temperature. The amendment-induced increase in soil pH is in agreement with findings for composts [25,78] and biochars [60,75,79]. It was found in many studies that the effect of organic amendments on P availability is dependent on soil pH [70,80]. The pH-related increase in the soil P_CAL concentration (API_pHrel.) varied between 36% of API for compost amended soil, about 50% of API in soils amended with P350 or P450 and about 80% of API for the soils amended with fresh green waste, P700 or P700a (Figure 3). Conversely, this means that the compost effect on the P_CAL concentration was mainly due to pH-independent mechanisms, while only 20% of the effect of fresh green waste and biochar produced at 700 °C was pH-independent. It is expected that the pH-dependent effects of organic amendments on soil P availability will be most important on acidic soils, while the pH-adjusted effects should also be relevant on neutral soils. This is ddsdsdin agreement with the finding in other studies that biochars increase P availability mainly in acidic soils [81,82].

The pH-adjusted increase in soil P_CAL content (mg P_CAL-P/100 g soil) in the low-P acidic soil was generally smaller than the P_CAL supply with the amendments (Table S4). This indicates that in this soil a portion of the P_CAL in the amendments and/or a portion of the soil P_CAL pool was transferred into the soil P pool that is not CAL-soluble. In the high-P acidic soil, the pH-adjusted increase in soil P_CAL content was smaller than the P_CAL supply for fresh green waste and the biochars with the exception of P700a (Table S4). With P700a and, in particular, with compost, the pH-adjusted increase in soil P_CAL content was larger than the P_CAL supply. This shows that these two amendments led to a transfer of soil P to the plant-available P_CAL pool that was independent of the pH effects. The supply of organic carbon to the soil can influence soil P dynamics directly by decreasing P sorption [9,42] and/or indirectly by modifying the activity of soil microorganisms [16,60,75,83,84].

4.3. Effect of the Mode of Treatment on P Uptake

The effect of the fertilization variants on plant growth rate (Figure 4) was broadly similar to the effect on PUR (Figure 5). The P uptake is more sensitive to the P availability than plant growth [36,66,82] because the growth responses to P availability may be buffered through an increase or decrease in plant internal P reserves. The discussion therefore focuses on the effects of the different fertilizer variants on P uptake. We used the P uptake rate (PUR), apparent P recovery (APR) and mineral fertilizer equivalent (MFE) to quantify the P fertilizer efficiency of the organic amendments. All three indicators were significantly influenced by the mode of treatment of the organic amendments, whereby the treatment effect changed with increasing experiment duration (Figure 5, Figure 6 and Figure S5).

There was no consistent effect of the fertilizer variant on the soil pH at the end of the pot experiment (Table S5). This indicates that the effects of the organic amendments on PUR, APR, and MFE were not due to changes in soil pH. This is in contrast to the incubation experiment in which the amendments had strong effects on soil pH (Table S2). The difference in the amendment-induced modification of soil pH between the pot and the incubation experiment was presumably caused by the lower amendment addition in the pot experiment (25 mg P kg⁻¹ soil) than in the incubation experiment (50 mg P kg⁻¹ soil). Furthermore, in the pot experiment, the soil pH was presumably influenced by the roots. It is well documented that nitrogen nutrition and the P nutritional status of plants may strongly influence soil pH in the rhizosphere [1].

Organic amendments can enhance plant P uptake by increasing root growth [85] and soil water content [86,87] and thus the spatial availability of soil P. In our pot experiment, the soil water content was maintained in an optimal range through regular irrigation. Due to the small pot volume, the root density was high at least at the end of the experiment. Therefore, we assume that the positive effects, which organic amendments can have on the spatial availability of soil P under field conditions, were of minor importance in our pot experiment.

All organic amendments delivered plant-available P_CAL to the soil (Table 3). For the four biochars produced at 700 °C, APR substantially surpassed the supply of P_CAL. This indicates that these biochars increased the uptake of soil P. Organic amendments can increase uptake of soil P by increasing the chemical availability of soil P through various mechanisms, including modification of the activity of soil enzymes associated with P mineralization and of microorganisms associated with P solubilization [60].

Fresh GW was the only organic amendment that had no positive P fertilizer effect throughout the pot experiment. This was possibly due to the very high C/P ratio of fresh GW (C/P = 369), which may have led to immobilization of soil P [88,89]. Immobilization of soil P was also found after application of manure by Brod et al. [90]. The low-P fertilizer effect of fresh GW in the pot experiment is in agreement with the very low pH-adjusted API in the incubation study (Figure 3).

For both feedstocks the P fertilizer effect was higher for composts than for the fresh material. This is in accordance with the data from the incubation study in which the pH-adjusted API was substantially larger for GW-derived compost than for fresh GW (Figure 3). Cumulated over all 8 harvests, the APR was 24% for the GW-derived and 33% for the BW-derived compost, and the MFE was 48% for GW- and 66% for BW-derived compost. The APR values are in the range of a study with four BW-derived composts, which found APR values cumulated over six harvests between 18 and 50% [91]. In a study with three composts, the APR cumulated over four harvests ranged between 3% for a municipal waste compost and 13% for a compost from fermentation residues [92]. The MFE values were slightly higher than those found in a study with two types of compost made from a mixture of household/kitchen/park waste [37].

In our study all biochars had a positive P fertilizer effect with APR values ranging between 25% for BW-derived biochar produced at 700 °C and 47% for GW-derived biochar produced at 350 °C, and MFE values ranging between 54% and 93%. The positive P fertilizer effect of biochars is in line with a meta-study, which found that the biochar amendment increased plant P uptake by 55% on average across all studies [47]. In a study of [83], a P-rich biochar derived from willow woodchips produced at 550 °C increased P uptake of Lotus pedunculatus by about 50%, whereas two P-poor biochars derived from pine wood produced at 450 or 550 °C did not increase P uptake from a low-P acidic soil. The positive effect of the P-rich biochar was attributed to its high liming equivalence and provision of additional plant-available P [83]. In recent studies with ryegrass, the MFE value for biochar from poultry manure produced at 460 °C was 58% [81], and, for biochar from dairy, processing sludge was 31% at the lower rate and 46% at the higher rate of P supply [82].

The APR and MFE values for the GW-derived biochars produced at 350 °C and 450 °C were higher than for the biochars produced at 700 °C (Figure S5 and Figure 6). The lower P fertilizer efficiency for the GW-derived biochars produced at 700 °C is in agreement with the data for the pH-adjusted API from the incubation study (Figure 3). In the meta-study of [47], the pyrolysis temperature had no influence on the biochar-induced increase in P uptake.

In our study the fluid fermentation residue had higher APR (42%) and MFE (97%) values than the solid fermentation residue (APR 27%, MFE 64%). This is in agreement with a study with ryegrass, which found an APR value of 40% for fluid and of 30% for a solid digestate [93], and an MFE value of 75% for fluid and 66% for solid digestate [70]. In two non-separated digestates [94], MFE values of 85% and 80% were found at the first harvest, and 67% and 64% for two harvests. In the study of [44], the effect of fluid–solid separation of digestate on the digestate-induced P was dependent on the digestate and differed depending on the plant species used for the pot experiment.

The temporal course of the P fertilization effect differed substantially between the different fertilizer variants. This is particularly evident in the change in the MFE with increasing experiment duration (Figure 6). The fertilizing effect of the organic amendments, with the exception of FR_fluid., was slower than the fertilizing effect of TSP. This is in agreement with other findings that organic fertilizers often act more slowly than mineral fertilizers [49]. For crops with a short vegetation period, the slow action of the organic amendments can prevent sufficient P supply on P-poor soils in which plant P supply is mainly directly derived from the fertilizer.

The P fertilization in the form of TSP occurs by adding highly water-soluble P to the soil, and it immediately enters the plant-accessible and highly plant-accessible soil P [95]. With increasing time after fertilizer application, a portion of the TSP-P can be transferred to a soil pool with low or very low P accessibility for the plants [95]. In accordance with this concept, the PUR of the TSP variant at the end of the trial did not differ from the PUR of the non-fertilized variant (Figure 5), although only about 50% of the TSP-P had been taken up by the plants (Figure S5). The P fertilization in the form of organic amendments is adding P in inorganic and organic forms. It is possible that the slower P effect of the organic amendments compared to TSP was because some of the P contained in the fertilizers first had to be converted into forms that are easily accessible to plants. For example, organic P in the amendments has to be mineralized by soil microorganisms before it can enter the plant accessible soil pool. It is also possible that the organic amendments gradually led to an increase in the plant availability of soil P and uptake of soil P [96]. Amendments containing organic C increase soil microbial activity [96]. Depending on the C/P-ratio in the amendments, increased microbial activity may decrease uptake of soil P due to transient P immobilization into microbial biomass [97] or increase uptake of soil P due to mineralization of organic P and release of P mobilizing compounds by the microorganisms [42,83,84,98].

In the incubation experiment, the P_CAL concentration in the soil did not change systematically during the 100-day incubation period. This was not only true for the TSP variant but also for the organic fertilizer variants (Figure S2). This suggests that in the pot experiment the continuous increase in the P fertilizer value of the organic amendments relative to TSP (Figure 6) was at least in part caused by interactions of plant roots and the organic amendments. This suggestion is supported by comparison of the pH-adjusted increase in the soil P_CAL concentration (Figure 3) and the cumulative APR (Figure S5). For the GW-derived biochars, the P fertilizer value was markedly lower in the incubation experiment (API 4% to 14%) than in the pot experiment (APR 31% to 47%), where interactions between plant roots and biochars occurred. Plant roots may increase the P uptake, e.g., by the exudation of soluble organic compounds, which enhance the P availability directly or indirectly via stimulation of P-solubilizing microorganisms [99,100]. The ability of plants for acquisition of P from soils and fertilizers is dependent on plant species [101], whereby plant species differ in their P-mining strategies and, thus, in their ability for acquisition of specific P forms [102].

5. Conclusions

Our data show great potential for increasing the P fertilization effect of organic municipal waste materials through appropriate processing prior to application. The effect of the processing method on the P fertilization effect differed between the two waste materials. This indicates that the optimal processing method is dependent on the composition of the waste and presumably also on the soil to which the waste is applied. When choosing the processing method, it should also be taken into account that organic municipal waste materials also contain organic carbon and other nutrients and can be used as soil additives to improve heavy metal-contaminated soils. The choice of processing method for optimal valorization should therefore be based not only on the specific waste material but also on the primary use of the organic residues. From an agronomic perspective, the variability of the short-term P fertilization effect in particular is an obstacle to replacing conventional P commercial fertilizers with processed waste materials. The low short-term P fertilization effect can lead to suboptimal P supply to plants for crops with a short growing season on P-deficient soils.