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Frontiers in Soil Science ·

Volume 6 - 2026 | https://doi.org/10.3389/fsoil.2026.1937202

Soil-specific effects of biobased fertilizers and struvite on phosphorus dynamics and wheat growth

  • Sana Boubehziz1*corresponding author
  • Lucía Guerrero-Gallardo1
  • Vidal Barrón1
  • María Carmen del Campillo1
  • María Angeles Martín Santos2*corresponding author
  • María Carmen Gutiérrez Martín2
  • Antonio Rafael Sánchez-Rodríguez1
  1. 1Department of Agronomy, (Departamento de Agronomía de la Universidad de Córdoba (DAUCO), ‘María de Maeztu’ Excellence Unit ETSIAM) Universidad de Córdoba (UCO), Córdoba, Spain
  2. 2Department of Inorganic Chemistry and Chemical Engineering, Area of Chemical Engineering, Instituto Químico para la Energía y el Medioambiente (IQUEMA), Campus de Excelencia Internacional Agroalimentario ceiA3, University of Cordoba, Córdoba, Spain
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Abstract

Biobased fertilizers are increasingly considered viable alternatives to mineral phosphorus (P) fertilizers, however their agronomic performance in calcareous soils remains insufficiently documented. This study aimed to assess the effectiveness of composted organic waste (olive mill pomace, OMP; solid urban residue, SUR, and sewage sludge, SWS), applied individually or in combination with struvite (STR) to increase soil fertility, enhance plant growth and promote crop biofortification. A pot experiment was conducted using wheat as a model crop in three Mediterranean soils (Entisol, Inceptisol, and Vertisol) differing in carbonate content and P phytoavailability. All treatments supplied 50 mg P per kg of soil, including the application of diammonium phosphate (DAP), while unamended soils received no P (CON). Soil type modulated the performance of the biobased fertilizers. SUR and SWS, applied alone in the Entisol and Vertisol or combined with STR in all soils, increased plant-available soil P (74-238%) relative to CON. SWS, applied alone or with STR, achieved wheat yields comparable to those of DAP in the Entisol and Vertisol and exceeded DAP yields in the Inceptisol. The combination of SUR or SWS with STR resulted in plant P use efficiencies comparable to or higher than those obtained with DAP. SUR and SUR+STR enhanced wheat biofortification in the Inceptisol. By contrast, OMP showed limited effectiveness as a biobased fertilizer due to insufficient stabilization during the composting process, although its performance improved when combined with STR. Overall, while SUR and SWS alone or in combination with STR, can substitute mineral P fertilizers, their effectiveness depends on soil type and compost stabilization.

Keywords

1 Introduction

Extreme climate events and inappropriate land management practices are major drivers of soil degradation in Mediterranean regions (1). This degradation has profound consequences for soil health and plant nutrient availability. Over the past six decades, a significant decline in soil nutrient availability for crops has been reported, negatively affecting both crop yields and nutritional quality (2). According to the European Union Mission ‘A Soil Deal for Europe’, 60-70% of European soils are currently considered unhealthy, largely due to low soil organic carbon (C) stocks and degradation associated with certain agricultural practices, including the inappropriate use and overuse of chemical fertilizers (3). Addressing soil degradation is therefore essential to restore soil health and fertility, particularly in semiarid agricultural regions such as southern Europe. Soils in these areas are typically characterized by low organic matter contents and limited availability of macronutrients, especially phosphorus (P), as well as micronutrients such as iron (Fe) and zinc (Zn).

Phosphorus is a critical element for plant growth and for maintaining key soil functions. When plant-available soil P is scarce, P deficiency disrupts the photosynthetic process, impairs plant metabolism, and significantly reduces biomass production (4). In soils, P is highly reactive and undergoes multiple processes, such as P adsorption, immobilization, and precipitation, which limit its availability to plants (5, 6). One of the major challenges associated with P in agriculture is its finite nature due to the increasing risk of depletion in the near future (7) driven by the rising consumption of primary P sources and inefficient management practices (8). This misuse and excessive mineral P fertilization have caused widespread contamination of water bodies, leading to eutrophication. Consequently, improved soil and P management strategies are required to enhance agricultural sustainability and reduce reliance on mineral fertilizers (9, 10).

Alternative P sources, such as struvite (STR) and nutrient-rich biobased fertilizers derived from composted organic waste [e.g., olive mill pomace (OMP), solid urban residue (SUR), and sewage sludge (SWS)] are considered promising options to overcome P limitations in agriculture. Reclaimed STR is a P source obtained from wastewater, mainly in the form of magnesium ammonium phosphate (NH4MgPO4·6H2O). It can supply P to soils and crops while aligning with circular economy principles through nutrient recovery, thereby reducing the need for synthetic inputs (11). STR is characterized by slow P release, lower solubility than conventional mineral fertilizers, and the potential to mitigate nutrient losses (12). However, its contribution to global agricultural P demand remains limited (13) due to low recovery rates from wastewater streams (14).

Biobased fertilizers, defined by the EU Fertilizing Products Regulation 2009/1009 as “fertilizers in which organic C and nutrients are solely of biological origin”, constitute an alternative source of P and organic matter for agricultural soils. Their use represents a strategy to enhance nutrient cycling within sustainable agricultural systems (15). These fertilizers stimulate microbial activity and enzyme production, by supplying labile C, thus enhancing nutrient cycling in soil and P phytoavailability (15, 16), while reducing reliance on synthetic chemical inputs (17, 18). Despite the constraints imposed by soil mineralogy and reactive mineral surfaces on P solubility and phytoavailability, P recovery from biobased fertilizers has received limited attention. This knowledge gap is especially relevant because most previous studies have focused primarily on C and nitrogen (N) dynamics (19, 20).

Calcareous soils, which are widespread in Mediterranean regions, pose specific challenges for plant nutrition. Their high carbonate content buffers soil pH around 8, limiting the solubility and availability of essential nutrients such as P, Fe, and Zn (21). Under these conditions, applied P rapidly reacts with calcium minerals, further reducing its availability to plants. Given the reactivity of P under alkaline conditions, controlled mechanistic experiments are required to disentangle the effects of STR and especially biobased fertilizers on soil P dynamics and plant uptake. Although organic matter can delay P absorption in soils (22), the effects of specific biobased fertilizers commonly used in Mediterranean regions (e.g., OMP, SUR, or SWS) on short-term P availability, as modulated by soil physicochemical properties, require further assessment to support their integration into more sustainable agricultural strategies. Previous studies have shown that biobased fertilizers modify plant P availability as a function of soil properties; consequently, their agronomic effectiveness should be evaluated across contrasting soil types (23).

The application of biobased fertilizers is often linked to concerns regarding the presence of heavy metals [e. g., cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), and Zn] that may enter the soil-plant system and food chain, posing a potential risk to human health (24). Beyond these concerns, such metal inputs may also interact with soil microbial communities and influence enzymatic processes involved in nutrient cycling. In this context, P availability following biobased fertilizer application is regulated by soil microbial activity and associated enzymes (25), particularly phosphatases. These enzymes control organic P mineralization and the release of inorganic P into the soil solution (26). Although the number of studies evaluating biobased fertilizers as nutrient sources is increasing, limited information is available on how biobased fertilizers interact with soil properties, such as carbonate content and nutrient availability, to shape microbial and enzymatic responses. These soil-fertilizer-microbial activity interactions can substantially influence microbial functioning and P dynamics, and are critical for predicting fertilizer performance across contrasting soil types.

The objectives of this study were to assess the effectiveness of a selection of composted organic waste (OMP, SUR, and SWS), applied individually or in combination with STR to (i) improve soil fertility and plant growth, with particular emphasis on P availability, plant P uptake, and soil enzyme activities, (ii) achieve crop biofortification, and (iii) evaluate fertilizer performance across contrasting soil types because P bioavailability is strongly regulated by soil type and order (27). For that, we analyzed nutrient availability and microbial responses to soil-fertilizer interactions. To this end, a pot experiment was developed under controlled conditions using durum wheat as a model crop in three soils differing in pH, carbonate content, texture and nutrient availability (Entisol, Inceptisol, and Vertisol), collected from semiarid regions. We hypothesized that (i) the effects of the different biobased fertilizers and STR on plant development and plant-available soil P would depend on soil properties, particularly pH and carbonate content, given that nutrient availability tends to decline under alkaline conditions (27); (ii) soil enzyme activity would respond differently depending on the properties of biobased fertilizers; (iii) combining biobased fertilizers (rich in organic C) with STR (rich in P) would generate enhanced effects on plant growth, plant-available soil P, and P use efficiency due to their complementary composition and solubility; and (iv) certain biobased fertilizers could enhance plant uptake of micronutrients such as Fe and Zn, contributing to biofortification, while maintaining heavy metal concentrations below toxicity thresholds.

2 Materials and methods

2.1 Soil sampling and characterization

Three agricultural soils were selected to represent contrasting physical and chemical properties and levels of P availability. All soils had a documented history of agricultural use under rainfed cereal systems, typically rotated with leguminous or oilseed crops. The first soil was an Entisol, collected in Soria, Spain (41°50′26′′N, 2°27′16′′W), developed on alluvial parent material from the Tera River (Pleistocene sedimentary deposits). The second soil was an Inceptisol, sampled in León, Spain (42°19′47′′N, 5°25′14′′W), developed on calcareous clay and sandy sedimentary rocks of Miocene origin. The third soil was a Vertisol, collected in Córdoba, Spain (37°47′0′′N, 4°36′35′′W), developed on Miocene sedimentary materials including sandy marls and sandstones (see Supplementary Figure 1 for the location map).

The soil samples were collected from the arable layer (0–25 cm), air-dried for one week at room temperature (≈25 °C), and sieved to 1 cm to remove stones and roots before use in a pot experiment. Subsamples (1 kg) were further ground to pass through a 2-mm sieve for laboratory analysis. Soil clay content was determined using the Robinson pipette method (28). Oxidizable organic C was measured following the Walkley and Black method (29), while total N was determined using a TruSpec® analyzer (Leco Corp., St Joseph, MI), and the organic C/total N (C/N) ratio was calculated. Carbonate content was quantified by mass loss after treatment with 6 M HCl (30). Soil pH (pH1:2.5) and electrical conductivity (EC1:5) were measured using a calibrated pH meter (Crisol GLP 21) and a conductivity meter (Crison Micro cM 2200), after shaking a soil:deionized water solution (1:2.5 and 1:5, respectively) at 220 rpm for 30 min and centrifuging for 10 min (3000 rpm). Cation exchange capacity (CEC) was determined after extraction with 1 M ammonium acetate buffered at pH 7, and exchangeable cations were quantified by atomic absorption spectrometry (Ca and Mg) or flame photometry (Na and K). Plant-available soil P was estimated using the Olsen method (31) (POlsen), extracting 1.5 g of soil with 30 ml of 0.5 M sodium bicarbonate [(NaHCO3), buffered at pH 8.5], followed by shaking for 30 min, and centrifugation for 10 min at 3000 rpm. P concentration was then measured colorimetrically according to Murphy and Riley (32) using a Lambda 35 spectrophotometer (PerkinElmer) at 882 nm. Labile Fe, Cu, Mn, and Zn were extracted using the diethylenetriaminepentaacetic acid (DTPA) extraction method (33). Total soil contents of Ni, Cr, As, Cd and Pb were determined by X-ray fluorescence (Rigaku ZSX Primus IV spectrometer). This soil characterization is shown in Table 1.

Table 1. Physicochemical characteristics of the sampled Entisol, Inceptisol and Vertisol.

SoilClayOCTNC/NCaCO3pH1:2.5EC1:5CECCa+2Mg+2K+
g kg-1g kg-1g kg-1g kg-1µS cm-1cmol(+) kg-1cmol(+) kg-1cmol(+) kg-1cmol(+) kg-1
Entisol13819.4 ± 0.80.9 ± 0.021.2 ± 1.0236.75022 ± 120 ± 11.3 ± 00.2 ± 0.0
Inceptisol3903.9 ± 0.70.6 ± 0.05.8 ± 1.11838.711626 ± 123 ± 13.2 ± 00.6 ± 0.0
Vertisol43911.5 ± 2.00.9 ± 0.013.2 ± 2.53937.914047 ± 041 ± 04.7 ± 01.3 ± 0.1
SoilNa+POlsenFeDTPACuDTPAMnDTPAZnDTPANiCrAsCdPb
cmol(+) kg-1mg kg-1mg kg-1mg kg-1mg kg-1mg kg-1mg kg-1mg kg-1mg kg-1mg kg-1mg kg-1
Entisol0.00 ± 022.4 ± 0.927.0 ± 1.01.3 ± 0.134.5 ± 1.00.3 ± 0.049 ± 1385 ± 1828 ± 0<LOD<LOD
Inceptisol0.00 ± 04.1 ± 0.55.8 ± 0.10.4 ± 0.35.7 ± 0.10.2 ± 0.062 ± 25123 ± 528 ± 9<LOD<LOD
Vertisol0.18 ± 07.8 ± 0.53.1 ± 0.11.4 ± 0.12.0 ± 0.10.3 ± 0.063 ± 3695 ± 8<LOD<LOD<LOD

The values represent the mean ± SEM (n = 3), except for the clay content, carbonate content, pH and electrical conductivity, for which n = 2. The data are expressed on a dry weight basis where applicable.

OC, organic carbon; TN, total nitrogen; CaCO3, carbonate content; pH1, 2.5, pH of 1, 2.5 soil extract (soil, deionized water); EC1, 5, electrical conductivity in 1, 5 extract (soil, deionized water); CEC, cation exchange capacity; and POlsen, plant-available soil phosphorus. FeDTPA, ZnDTPA, CuDTPA, and MnDTPA: soil available Fe, Zn, Cu, and Mn, extracted with DTPA, respectively; total Ni, Cr, and As, determined with an X-ray fluorescence spectrometer. LOD, limit of detection.

2.2 Treatments, characterization of the products, experimental design, and pot experiment

Diammonium phosphate (DAP, (NH4)2HPO4; Fertiberia) was used together with a untreated soil (CON)as reference treatments. Reclaimed struvite (STR), consisting of P-magnesium (mg) ammonium phosphate crystals with low impurity levels, was obtained from a wastewater treatment facility in Getafe (Madrid, Spain). In addition, three types of composted organic waste were assessed: (i) composted olive mill pomace (OMP), obtained from an oil mill located in Cazorla (Jaén, Spain); (ii) composted solid urban residue (SUR), sourced from a municipal waste treatment plant (SADECO) in Córdoba (Spain); and (iii) composted sewage sludge (SWS), collected from a municipal wastewater treatment plant (La Golondrina, EMACSA) in Córdoba (Spain).

For each soil, the corresponding fertilizer (DAP, STR, OMP, SUR, or SWS) was homogeneously mixed with the soil prior to pot filling. Application rates were calculated based on the total P content of each fertilizer (Table 2) to supply 50 mg of P per kg of soil, a rate considered appropiate for plants growth in P-poor media in pot experiment (34). In addition, STR was combined with each biobased fertilizer (OMP+STR, SUR+STR, and SWS+STR, respectively), with STR contributing 50% of the total P input (i.e., 25 mg of P per kg of soil). In total, 27 soil × treatment combinations were established (3 soils × 8 fertilization treatments plus CON), with five replications per combination, resulting in a total of 135 pots (experimental units).

Table 2. Chemical composition of the applied fertilizer (DAP, diammonium phosphate) and potential biobased fertilizers (STR, struvite; OMP, composted olive mill pomace; SUR, composted solid urban residue; and SWS, compostedsewage sludge) used in the study on a dry basis.

Mineral and biobased fertilizerspH1:25CE1:25OCMSVSTNKPTotalZnTotalCuTotalPbTotalCrTotalNiTotalCdTotal
dS m-1g kg-1g kg-1g kg-1g kg-1g kg-1mg kg-1mg kg-1mg kg-1mg kg-1mg kg-1mg kg-1
DAP7.4 ± 0.0229.7 ± 0.10.8 ± 0.1516 ± 2484 ± 2195 ± 1.4200.0 ± 37397 ± 122 ± 18 ± 044 ± 027 ± 012 ± 0
STR9.0 ± 0.030.3 ± 0.010.6 ± 0.0750 ± 2250 ± 592.0 ± 5.4120.3 ± 7<LOD25 ± 237 ± 1<LOD14 ± 18 ± 0
OMP8.8 ± 0.011.0 ± 0.01474 ± 11250 ± 1750 ± 215.9 ± 8.30.2 ± 092 ± 085 ± 011 ± 032 ± 027 ± 02 ± 0
SUR8.1 ± 0.012.6 ± 0.10639 ± 19438 ± 1562 ± 232.5 ± 1.74.4 ± 0470 ± 1211 ± 173 ± 117 ± 032 ± 09 ± 0
SWS6.7 ± 0.012.7 ± 0.10456 ± 39552 ± 2448 ± 128.8 ± 1.117.1 ± 1589 ± 1538 ± 187 ± 116 ± 037 ± 05 ± 0

The values are the means ± SEMs (n = 3).

pH1, 25, pH of 1, 25 extract (fertilizer, deionized water); EC1, 5, electrical conductivity of 1, 25 extract (fertilizer, deionized water); OC, organic carbon; MS, mineral solids content; VS, volatile solids content; TNK, total nitrogen obtained by the Kjeldahl method; PTotal, ZnTotal, CuTotal, PbTotal, CrTotal, NiTotal, and CdTotal: total phosphorus and total heavy metal contents, i.e., Zn, Cu, Pb, Cr, Ni, and Cd, respectively, determined by acid digestion. LOD: limit of detection.

The chemical composition of the used fertilizers is summarized in Table 2. Briefly, pH and EC were determined in a 1:25 (w:v) fertilizer:deionized water suspension. Total N was measured using the Kjeldahl method (35), oxidizable organic C content by the Walkley and Black method (29), and total P following acid digestion (36). Total solids (TS) were determined after drying the samples at 105 °C for 24 h, and mineral solids (MS) were obtained by ignition at 550 °C for 3 h. Volatile solids (VS) were calculated as the difference between TS and MS, according to the equations shown below Equation 1:

(1) TS(mgkg)=(G3−G1)·1000000(G2−G1)

where G1 is the weight of the empty crucible, G2 the weight of the crucible containing the air-dried sample, and G3 the weight after drying at 105 °C for 24 h.

MS was calculated as according to Equation 2:

(2) MS (mgkg)=(G4−G1)·1000000(G2−G1)

where G4 is the weight after ignition at 550 °C.

VS was obtained as using Equation 3:

(3) VS (mgkg)=TS−MS

Heavy metal concentrations (Ni, Cd, Pb, Hg, Cr, and As) were determined by wavelength-dispersive X-ray fluorescence (Rigaku ZSX Primus IV). The stability of the potential biobased fertilizers was assessed by measuring the specific oxygen uptake rate (SOUR), following the method proposed by Chica et al. (37).

Pots (330 ml volume, 14.6 cm height, 5.5 cm diameter at the top, and 4.6 cm diameter at the base) were filled with 270 g of soil. Three seeds of durum wheat (Triticum durum cv. Kiko Nick; commonly used by local farmers and provided by a local farmers’ association, SAT Córdoba; www.satcordoba.es) were sown per pot. Pots were covered externally with aluminum foil to prevent light exposure, and a cellulose acetate filter was placed at the bottom to allow drainage. Seedlings were thinned to one plant per pot 20 days after sowing (DAS, experimental unit).

The experiment was conducted in a growth chamber under controlled environmental conditions: 23 °C (day), 18 °C (night), 14 h photoperiod (325 μmol m− 2 s− 1), and 60% relative humidity. Pots were watered daily with deionized water to maintain soil moisture at 75% of water-holding capacity and randomly repositioned weekly. To avoid nutrient limitations other than P, each pot received a weekly application of 10–15 ml (120 ml per pot over the experiment) of a P-free Hoagland solution containing: Ca(NO3)2 (5 mM), KNO3 (6 mM), MgSO4 (2 mM), Fe (EDDHA) (40 μM), KCl (100 μM), H3BO3 (50 μM), MnSO4 (4 μM), ZnSO4 (4 μM), CuSO4 (1 μM), and Na2MoO4 (6 μM). Although P treatments (DAP, STR, and biobased fertilizers) supplied nutrients other than P, this approach ensured a non-limiting supply of all essential nutrients except P, allowing plant responses to be primarily attributed to differences in P availability and treatment dynamics across soils.

2.3 Plant measurements and nutrient analysis

Plant height and the leaf chlorophyll index (LCI) were measured at 20, 41, 56, and 71 DAS using a SPAD-502 chlorophyll meter (Minolta Camera Co., Osaka, Japan) on the two youngest fully expanded leaves (five measurements per leaf). Plants were harvested at 106 DAS, cut 1.5 cm above the soil surface, dried at 65 °C for 72 h, and separated into grain and straw. Grain yield was determined gravimetrically and the number of grains counted. Plant tissues were ground and digested (0.2 g) with 3 ml of 65% nitric acid, and after 24 h, 1 ml of 60% perchloric acid was added. The mixture was heated at 180 °C until a completely colorless solution was obtained. P was colorimetrically measured as previously described. Plant P uptake was calculated as the product of tissue P concentration and dry biomass. Grain P use efficiency (PUE) was calculated as follows using Equation 4:

(4) PUE (%) = P, gt−P,gcP applied×100

where P, gt is the grain P uptake (grain yield multiplied by grain P concentration) of plants receiving each treatment (DAP, STR, OMP, SUR, or SWS and their combinations), P, gc is the mean grain P uptake of the control plants [grown on unamended soil (CON)], and P applied is the amount of P applied to the pots. PUE was calculated relative to the corresponding unamended control. Positive PUE values (> 0) indicate a net increase in grain P uptake compared with CON-treated plants, whereas negative PUE values (< 0) indicate lower grain P uptake than the CON. Agronomic efficiency (AE; mg kg-1 P) was calculated as the increase in grain yield relative to the CON per unit of P applied (38).

The concentration of macro- and micronutrients (Ca, Mg, Fe, Zn, Mn, and Cu) in grain and straw samples were quantified by atomic absorption spectrometry, while K and Na concentrations were measured by flame photometry. Heavy metals in grain (Ni, Cr, As, Cd, Pb, and Hg) were determined by X-ray fluorescence as described above.

2.4 Soil analysis after harvest

Fresh soil (50 g) was collected immediately after harvest from the central lower third of the pot, where root density was higher. Soil moisture was determined gravimetrically. Soil enzyme activities (acid and alkaline phosphatases and β-glucosidase) were measured in fresh soil following Eivazi & Tabatabai (39). Briefly, two replicates and a control (for the baseline) per pot were prepared, with 4 ml of 4-methylumbelliferone (MUB, 2 mM) solution. Two MUB solutions with different pH values were utilized to assess acid and alkaline phosphatase (pH 6 and 11, respectively). Subsequently, 4-nitrophenyl phosphate (pNPP) was added to the replicates (but not to the baseline control), and samples were incubated at 37 °C for 1 h. The p-nitrophenol (PNP) produced was quantified at 400 nm using a microplate spectrophotometer (Biotek Power Wave HT). The absorbance of the control was subtracted from the mean value of the two replicates for each soil sample. Soil β-glucosidase activity, related to the hydrolysis of glycosidic compounds in soil or p-nitrophenyl-β-D-glucopyranoside (pNPG) into p-nitrophenol (PNP), was also determined. The analysis was performed using two replicates and one control per sample, with the addition of 4 ml of MUB (2 mM). The amount of PNP produced in each soil sample was measured as described for the phosphatase activity and according to Eivazi and Tabatabai (40).

The remaining fresh soil not used for enzyme analyses was air-dried at room temperature (25 °C) for one week. Soil pH1:2.5, EC1:5 and POlsen were then determined as described above.

2.5 Statistical analysis

Data were tested for normality and homoscedasticity using the Shapiro-Wilk test and Levene’s test, respectively [implemented in the ‘car’ package, Version 3.1.1 (41)], prior to analysis of variance (ANOVA). Repeated measures ANOVA (time × soil × treatment) was applied to plant height and the LCI. Two-way ANOVA (soil × treatment) was performed for the remaining plant and soil variables. Significant soil × treatment interactions were frequently observed (p < 0.05), indicating that fertilizer behavior, nutrient dynamics, and plant responses differed substantially among soils. Therefore, treatment effects were evaluated within each soil using one-way ANOVA. Comparisons among soils were used only to contextualize plant performance and to identify soil-specific constraints, rather than to infer direct equivalence across heterogeneous soil matrices. When differences were significant (p < 0.05), Fisher’s least significant difference (LSD) post hoc test was applied to separate treatment means [R core stats package (42)]. Although this post hoc test is sensitive and may increase the risk of type I errors, results were interpreted conservatively, with emphasis on consistent patterns rather than isolated differences. A Principal Component Analysis (PCA) was performed to explore the soil × treatment interaction and relationships among variables, including pH1:2.5, POlsen, and soil enzyme activity (acid and alkaline phosphatase and β-glucosidase) at the end of the experiment, plant biomass, and plant nutrient uptake (P, Zn and Fe). All statistical analyses were performed using RStudio (version 2025.05.1 + 513) (43) with the packages stats, plyr, agricolae, tidyverse, emmeans, dplyr, factoextra, and FactoMineR. Figures were produced using the “ggplot2” package (version 3.3.6) (44).

3 Results

3.1 Plant development

3.1.1 Plant height and leaf chlorophyll index during the pot experiment

Significant interactions among time, soil type, and treatment were detected for both plant height and the LCI (Supplementary Table 1). Additionally, two-way ANOVA revealed a significant soil × treatment interaction for LCI at harvest (71 DAS; F(2, 430) = 43.06, p < 0.001; Supplementary Table 2), whereas no such interaction was observed for plant height.

Plants grown in the Entisol were taller than those grown in the Vertisol (Figures 1A, E). Although plant height did not differ significantly between the Entisol and the Inceptisol, plants grown in the Inceptisol were, on average, 10% shorter (Figures 1A, C). In the Entisol, the SWS treatment produced the tallest plants, which were 9.3% and 30% taller than those under the CON and OMP treatments, respectively (Figure 1A). In the Inceptisol, DAP, STR, SWS and SUR+STR significantly increased plant height relative to CON (Figure 1C). In the Vertisol, DAP and STR resulted in the tallest plants, followed by SWS and SWS+STR, whereas OMP resulted in the shortest plants (Figure 1E).

Time evolution of plant height **(A, C, E)** and leaf chlorophyll index (LCI, SPAD values; **B, D, F**) in three soils (Entisol, Inceptisol and Vertisol) under different treatments (CON, control or unamended soil; DAP, diammonium phosphate; STR, struvite; OMP, composted olive mill pomace; SUR, composted solid urban residue; SWS, composted sewage sludge).
Figure 1. Time evolution of plant height (A, C, E) and leaf chlorophyll index (LCI, SPAD values; B, D, F) in three soils (Entisol, Inceptisol and Vertisol) under different treatments (CON, control or unamended soil; DAP, diammonium phosphate; STR, struvite; OMP, composted olive mill pomace; SUR, composted solid urban residue; SWS, composted sewage sludge).

Plant height and the leaf chlorophyll index values followed a similar soil-dependent pattern. The highest mean LCI values were observed in plants grown in the Entisol, whereas the lowest values occurred in the Vertisol (Figure 1). Across soils, OMP application, applied alone or in combination with STR, generally reduced LCI relative to the other treatments, except in the Vertisol, where CON resulted in the lowest values. At 71 DAS, DAP produced the highest LCI in the Entisol, followed by SWS and SWS+STR. In the Inceptisol, SWS+STR and STR produced the highest LCI values, whereas in the Vertisol, DAP produced the highest LCI and CON the lowest.

3.1.2 Plant biomass and yield at the end of the pot experiment

A significant soil × treatment interaction was detected for plant biomass at harvest (F(15, 108) = 4.76, p < 0.001; Supplementary Table 2). Plants grown in the Entisol produced higher biomass compared with those grown in the Inceptisol and the Vertisol (Figure 2). Across soils, DAP and STR significantly increased wheat biomass relative to CON. In addition, SWS and SWS+STR produced biomass values comparable to those obtained with DAP in the Inceptisol and with STR in the Entisol and Vertisol (Figures 2A, C, E). In contrast, OMP significantly reduced biomass in all three soils.

Wheat biomass **(A, C, E)** and grain yield **(B, D, F)** at harvest in the three soils (Entisol, Inceptisol, and Vertisol), under different P treatments (CON, control or unamended soil; DAP, diammonium phosphate; STR, struvite; OMP, composted olive mill pomace; SUR, composted solid urban residue; SWS, composted sewage sludge). Different letters indicate significant differences between treatments within each soil type (Entisol, Inceptisol, and Vertisol), as determined by one-way ANOVA followed by Fisher’s LSD, (*p* < 0.05).
Figure 2. Wheat biomass (A, C, E) and grain yield (B, D, F) at harvest in the three soils (Entisol, Inceptisol, and Vertisol), under different P treatments (CON, control or unamended soil; DAP, diammonium phosphate; STR, struvite; OMP, composted olive mill pomace; SUR, composted solid urban residue; SWS, composted sewage sludge). Different letters indicate significant differences between treatments within each soil type (Entisol, Inceptisol, and Vertisol), as determined by one-way ANOVA followed by Fisher’s LSD, (p < 0.05).

Plant yield was also significantly affected by the soil × treatment interaction (F(15, 108) = 5.03, p < 0.001; Supplementary Table 2). Plants grown in the Entisol yielded 72% and 79% more grain than those grown in the Inceptisol and Vertisol. In the Entisol, DAP, STR, SUR+STR, SWS and SWS+STR significantly increased yield relative to CON (Figure 2B). In the Vertisol, the highest yields were obtained with DAP, STR, SWS and SWS+STR (Figure 2F). In the Inceptisol, SWS produced the highest yield, whereas DAP produced the lowest yield (Figure 2D). Across soils, OMP consistently reduced yield, although this negative effect was partially alleviated when OMP was combined with STR, particularly in the Entisol.

3.2 Soil analysis after the pot experiment

3.2.1 Soil pH, electrical conductivity, and plant-available soil phosphorus

Significant soil × treatment interactions were detected for pH1:2.5, EC1:5, and POlsen at the end of the pot experiment (Supplementary Table 3), indicating that soil properties modulated treatment effects. Soil pH1:2.5 responded differently in noncalcareous and calcareous soils depending on the treatment (Figure 3). In the Entisol, DAP significantly reduced pH1:2.5 relative to CON (Figure 3A). In contrast, several treatments increased pH1:2.5 compared with CON; these increases were significant for OMP, OMP+STR, SUR, and SUR+STR in the Entisol, SUR and SUR+STR in the Inceptisol; and all the treatments except SUR in the Vertisol. Treatment effect on EC1:5 varied among soils (Supplementary Figure 2). POlsen was significantly affected by treatments in all three soils (Figures 3B, D, F). Overall, DAP and STR resulted in the highest POlsen values. In the Entisol, and Vertisol, the combination of biobased fertilizers with STR significantly increased POlsen relative to the individual biobased fertilizers. In contrast, in the Inceptisol, SUR and SWS applied alone did not significantly increase POlsen relative to CON.

Soil pH **(A, C, E)**, and plant-available soil phosphorus (P<sub>Olsen</sub>; **B, D, F**) at harvest in the three soils under different P treatments (CON: control or unamended soil; DAP: diammonium phosphate; STR: struvite; OMP: composted olive mill pomace; SUR: composted solid urban residue; SWS: composted sewage sludge). Different letters indicate significant differences between treatments within each soil type (Entisol, Inceptisol, and Vertisol), as determined by one-way ANOVA followed by Fisher’s LSD, (*p* < 0.05).
Figure 3. Soil pH (A, C, E), and plant-available soil phosphorus (POlsen; B, D, F) at harvest in the three soils under different P treatments (CON: control or unamended soil; DAP: diammonium phosphate; STR: struvite; OMP: composted olive mill pomace; SUR: composted solid urban residue; SWS: composted sewage sludge). Different letters indicate significant differences between treatments within each soil type (Entisol, Inceptisol, and Vertisol), as determined by one-way ANOVA followed by Fisher’s LSD, (p < 0.05).

3.2.2 Soil enzyme activity

Significant soil × treatment interactions were observed for acid phosphatase activity (F(16, 108) = 5.44, p < 0.001), and β-glucosidase activity (F(16, 108) = 1.94, p = 0.002), but not for alkaline phosphatase activity (F(16, 108) = 1.29, p = 0.220; Supplementary Table 3). Overall, enzyme activities were generally higher in the Entisol than in the other two soils, except for alkaline phosphatase activity. This enzyme activity was higher in the Vertisol (Figure 4). Acid phosphatase activity responded markedly to the treatment and soil type (Figures 4A, D, G). In both the Entisol and the Inceptisol, DAP resulted in the highest acid phosphatase activity, whereas OMP produced the lowest values in the Entisol. In the Vertisol, treatment effects on this enzyme activity were less pronounced. No significant treatment effects were detected for alkaline phosphatase activity in either the Entisol or Inceptisol (Figures 4B, E). In contrast, in the Vertisol, SUR and SUR+STR significantly increased alkaline phosphatase activity relative to CON, while DAP significantly reduced this activity (Figure 4H). β-glucosidase activity showed a distinct soil-dependent response pattern. No significant differences among treatments were observed in the Entisol (Figure 4C). However, in the Inceptisol, β-glucosidase activity increased significantly following the application of SUR, SUR+STR and DAP, in that order (Figure 4F), whereas in the Vertisol, significant increases were observed with DAP and SUR+STR compared with CON (Figure 4I).

Acid **(A, D, G)** and alkaline **(B, E, H)** phosphatase and β-glucosidase **(C, F, I)** enzyme activities at harvest in three soils under different P treatments (CON, control or unamended soil; DAP, diammonium phosphate; STR, struvite; OMP, composted olive mill pomace; SUR, composted solid urban residue; SWS, composted sewage sludge). The values are the means ± SEMs (*n* = 5). Different letters indicate significant differences between treatments within each soil type (Entisol, Inceptisol, and Vertisol), as determined by one-way ANOVA followed by Fisher’s LSD, (*p* < 0.05).
Figure 4. Acid (A, D, G) and alkaline (B, E, H) phosphatase and β-glucosidase (C, F, I) enzyme activities at harvest in three soils under different P treatments (CON, control or unamended soil; DAP, diammonium phosphate; STR, struvite; OMP, composted olive mill pomace; SUR, composted solid urban residue; SWS, composted sewage sludge). The values are the means ± SEMs (n = 5). Different letters indicate significant differences between treatments within each soil type (Entisol, Inceptisol, and Vertisol), as determined by one-way ANOVA followed by Fisher’s LSD, (p < 0.05).

3.3 Plant nutrient uptake

3.3.1 Phosphorus plant uptake, phosphorus use efficiency, and agronomic efficiency

Also, significant interactions were detected for plant P uptake (F(16, 108) = 7.06, p < 0.001), grain PUE (F(14, 93) = 9.7, p < 0.001) and AE (F(14, 94) = 7.64, p < 0.001; see Supplementary Table 2). As shown in Figure 5, plants grown in the Entisol exhibited the greatest P uptake. A similar pattern was observed for grain PUE, whereas the highest AE values were recorded for plants grown in the Vertisol. The application of DAP and STR significantly increased plant P uptake, resulting in the highest uptake values (Figures 5A, D, G). P uptake in plants treated with SUR+STR and SWS in the Inceptisol and with SWS+STR in the Vertisol was comparable to that observed with DAP and STR (Figures 5D, G). In contrast, CON and OMP treatments resulted in the lowest plant P uptake.

Total plant P uptake **(A, D, G)**, phosphorus use efficiency in grain (PUE; **B, E, H**), and agronomic efficiency (AE; **C, F, I)** at harvest in three soils under different P treatments (CON, control or unamended soil; DAP, diammonium phosphate; STR, struvite; OMP, composted olive mill pomace; SUR, composted solid urban residue; SWS, composted sewage sludge). The values are the means ± SEMs (*n* = 5). Different letters indicate significant differences between treatments within each soil type (Entisol, Inceptisol, and Vertisol), as determined by one-way ANOVA followed by Fisher’s LSD, (*p* < 0.05).
Figure 5. Total plant P uptake (A, D, G), phosphorus use efficiency in grain (PUE; B, E, H), and agronomic efficiency (AE; C, F, I) at harvest in three soils under different P treatments (CON, control or unamended soil; DAP, diammonium phosphate; STR, struvite; OMP, composted olive mill pomace; SUR, composted solid urban residue; SWS, composted sewage sludge). The values are the means ± SEMs (n = 5). Different letters indicate significant differences between treatments within each soil type (Entisol, Inceptisol, and Vertisol), as determined by one-way ANOVA followed by Fisher’s LSD, (p < 0.05).

In the Entisol, the highest grain PUE was obtained with SUR+STR (10%), followed by STR, DAP, SWS and SWS+STR (Figure 5B). In the Inceptisol, the application of SWS, SWS+STR, or SUR+STR resulted in the highest grain PUE (Figure 5E), whereas DAP-treated plants exhibited the lowest values. In the Vertisol, grain PUE increased significantly with STR and DAP, followed by SWS+STR (Figure 5H). OMP resulted in the lowest grain PUE, although values increased slightly when OMP was combined with STR. A similar pattern was observed for AE (Figures 5C, F, I). The highest AE values were generally obtained with SWS or SWS+STR, and these values were comparable to or greater than those observed when DAP or STR were applied.

3.3.2 Plant biofortification

Significant soil × treatment interactions were observed for plant Fe and Zn uptake, as well as for grain Fe and Zn concentrations (Supplementary Table 2). In general, plants grown in the Entisol exhibited the highest Fe and Zn uptake. The results of the one-way ANOVA conducted separately for each soil are shown in Table 3, where significant differences were detected in all cases. In the Entisol, most biobased fertilizers resulted in reduced plant Fe uptake relative to CON, with the exception of SWS+STR that produced the highest Fe uptake values. In the Inceptisol, only DAP significantly increased plant Fe uptake compared with CON. In contrast, in the Vertisol, DAP, STR, and SWS+STR significantly enhanced plant Fe uptake (Table 3). Plant Zn uptake increased significantly with the application of SUR, SUR+STR, and SWS across the three soils relative to CON. Conversely, OMP and OMP+STR had a negative effect on Zn uptake in the Entisol only.

Table 3. Iron (Fe) and zinc (Zn) plant uptake and grain concentration at harvest in the three soils (Entisol, Inceptisol and Vertisol) under different treatments (CON, control; DAP, diammonium phosphate; STR, struvite; OMP, composted olive mill pomace; SUR, composted solid urban residue; and SWS, composted sewage sludge).

Plant uptakePlant uptakeGrain concentrationGrain concentration
SoilTreatmentFeZnFeZn
µg plant-1µg plant-1mg kg-1mg kg-1
EntisolCON364 ± 28 ab53 ± 3 b43 ± 2 b31 ± 2 bc
DAP291 ± 21 bc51 ± 3 b42 ± 2 b24 ± 2 d
STR359 ± 42 ab48 ± 4 b40 ± 1 b24 ± 2 d
OMP109 ± 9 e25 ± 1 c53 ± 2 a38 ± 3 a
OMP+STR146 ± 8 de31 ± 1 c48 ± 2 a33 ± 2 bc
SUR218 ± 31 cd73 ± 4 a42 ± 2 b41 ± 3a
SUR+STR221 ± 9 cd69 ± 2 a42 ± 2 b36 ± 1 ab
SWS246 ± 20 cd71 ± 3 a41 ± 1 b36 ± 2 ab
SWS+STR436 ± 67 a52 ± 2 b42 ± 1 b28 ± 1 cd
p value<0.001<0.001<0.001<0.001
InceptisolCON164 ± 14 bc24 ± 1 c23 ± 2 ab12 ± 2 d
DAP303 ± 82 a17 ± 1 c16 ± 5 bc4 ± 1 e
STR196 ± 28 b19 ± 1 c28 ± 2 a9 ± 0 d
OMP81 ± 13 cd26 ± 4 c13 ± 1 c12 ± 2 d
OMP+STR175 ± 17 bc19 ± 2 c16 ± 1 bc12 ± 1 d
SUR170 ± 19 bc86 ± 8 a29 ± 2 a34 ± 2 a
SUR+STR142 ± 19 bc61 ± 4 b28 ± 3 a28 ± 1 b
SWS212 ± 19 ab56 ± 4 b29 ± 2 a28 ± 2 b
SWS+STR209 ± 34 ab27 ± 4 c27 ± 2 a18 ± 1 c
p value<0.01<0.001<0.001<0.001
VertisolCON113 ± 18 cd21 ± 4 c77 ± 19 a41 ± 4 c
DAP324 ± 68 a27 ± 4 c41 ± 2 bc13 ± 1 g
STR245 ± 42 ab29 ± 2 c28 ± 2 b18 ± 1 fg
OMP40 ± 7 d26 ± 7 c55 ± 10 c53 ± 4 b
OMP+STR72 ± 9 d24 ± 2 c32 ± 3 bc26 ± 1 ef
SUR127 ± 42 cd69 ± 24 ab56 ± 12 ab64 ± 5 a
SUR+STR130 ± 18 cd56 ± 6 ab35 ± 3 bc37 ± 4 cd
SWS194 ± 15 bc72 ± 4 a29 ± 2 c36 ± 2 cd
SWS+STR241 ± 37 ab45 ± 3 bc32 ± 4 bc29 ± 2 de
p value<0.001<0.0010.002<0.001

The values are the means ± SEMs (n = 5). Different letters indicate significant differences between treatments within each soil type (Entisol, Inceptisol, and Vertisol), as determined by one-way ANOVA followed by Fisher’s LSD, (p < 0.05).

Grain Fe concentration was significantly higher in the Entisol with OMP and OMP+STR than with the other treatments. In contrast, OMP application resulted in lower grain Fe concentrations in the Inceptisol, while all treatments except SUR significantly reduced grain Fe concentration in the Vertisol, relative to CON (Table 3). Grain Zn concentration increased significantly with SUR and OMP in the Entisol; with SUR, SUR+STR, SWS, and SWS+STR in the Inceptisol; and with SUR and OMP in the Vertisol, relative to CON. On the other hand, DAP in the three soils, STR in the Entisol and Vertisol, and OMP+STR and SWS+STR in the Vertisol significantly reduced grain Zn concentration compared with CON (Table 3).

3.4 Principal component analysis

Supplementary Figure 3 shows the PCA. The first two principal components (PCs) explained 69.1% of the total variance, with PC1 and PC2 accounting for 54.4% and 14.7%, respectively. A clear separation between the soil types was found and even between the means of the treatments, mainly along PC1. Entisol samples tended to cluster in proximity to most of the measured variables (P uptake, acid phosphatase, plant biomass, POlsen, β-glucosidase, PUE, Fe uptake, Zn uptake) except pH1:2.5 and alkaline phosphatase activity, indicating stronger associations between these variables.

3.5 Concentration of heavy metals in grain

Among the analyzed heavy metals, only Zn (reported above), Cu, Ni and Cr were detected in wheat grains at harvest. Significant soil × treatment interactions were observed for grain Cr and Cu concentrations (Supplementary Table 2). Significant treatment effects relative to CON for Ni and Cu concentrations were observed only in the Inceptisol. In this soil, grain Ni concentration increased significantly following the application of OMP+STR, with progressively lower values observed for STR and SWS (Supplementary Figure 4E). Similarly, grain Cu concentration in the Inceptisol increased significantly under the OMP+STR treatment compared with CON (Supplementary Figure 4F).

4 Discussion

Soil type played a key role in the performance of the evaluated biobased fertilizers as P suppliers. This effect was consistently reflected in plant responses (plant height, LCI, biomass, nutrient concentrations, and nutrient uptake), as well as in soil properties (POlsen, phosphatase activity, and β-glucosidase activity). The presence of carbonates in two of the three soils (the Inceptisol and, particularly, the Vertisol) constrained plant growth and soil nutrient availability. Carbonates buffer soil pH at values close to 8, reducing the solubility and phytoavailability of P, Fe, and Zn, thereby negatively affecting plant development (plant height, LCI, and biomass production) (45, 46). The organic C content, VS (proxy of organic matter), and nutrients supplied by SUR and SWS likely contributed to the positive effects observed on plant height, biomass, and grain yield at harvest. These results are consistent with previous studies (47–49) showing that biobased fertilizers can enhance soil fertility, improve soil structure and water retention, and ultimately promote plant growth and yield. However, our results clearly demonstrate that the effectiveness of these biobased fertilizers was soil-dependent (in line with our first hypothesis), as reflected by differences in grain yield and plant-available soil P among soils, with certain products performing better under specific edaphic conditions. Most P treatments significantly increased plant-available soil P, confirming their capacity to increase P phytoavailability. These results partially align with field studies reporting an initial increase in P solubilization under field conditions (50, 51) and with previous research in which soil orders were found to be key in terms of P bioavailability (27).

Diammonium phosphate and STR are well-recognized P sources in agricultural systems (52, 53), and produced the greatest increases in plant-available soil P in the conditions evaluated in this study. The superior performance of DAP in the Entisol and Vertisol compared with STR can be attributed to its higher solubility, which promotes rapid P release into the soil solution. In contrast, STR exhibits minimal solubility at soil pH values of approximately 8.5 or higher (54), which likely explains its reduced effectiveness in the Vertisol, where the highest soil pH values were recorded following STR application. Under such alkaline conditions, the low solubility of STR restricts P availability and limits its agronomic performance. The highest POlsen values observed following DAP application in the Entisol and Vertisol reflect a considerable P supply, that may also increase the risk of P losses through precipitation with Ca (55) and/or adsorption onto Fe and Al oxides and clay minerals (56). Excessive plant-available soil P can negatively affect the translocation of micronutrients such as Zn and Fe to the grain due to nutrient antagonism, potentially reducing crop yield (57). This mechanism may partially explain the reduced grain yield and PUE observed in wheat grown in the Inceptisol following DAP application. By contrast, the lower solubility of STR and the slower nutrient release from biobased fertilizers, compared with DAP, may result in better synchronization between nutrient supply and plant demand (58). This improved synchronization likely explains why STR and several biobased fertilizers achieved comparable or even higher plant P uptake, PUE, and AE than DAP, depending on the soil × treatment combination.

The application of OMP failed to improve plant growth, P uptake, PUE, or AE. This poor performance is likely linked to the insufficient stabilization of OMP, as indicated by its high SOUR values (> 1 mg O2 g-1 VS h-1). Incompletely composted OMP may contain phenolic compounds and fatty acids that disrupt soil microbial activity and limit organic matter mineralization and nutrient release (59). This issue is common when composting is conducted directly at olive oil production facilities, where space limitations and shortened composting periods often result in poorly stabilized products. Nevertheless, combining OMP with STR partially improved its performance, likely due to the complementary nutrient supply and enhanced P availability provided by STR.

In addition to P inputs, the organic matter supplied by biobased fertilizers influences plant and microbial P acquisition strategies (60, 61), including soil enzyme activity (62). In this study, biobased fertilizers not only supplied nutrients but also stimulated microbial activity, even under calcareous conditions where nutrient availability is inherently constrained by high soil pH. The contrasting effects of DAP on acid and alkaline phosphatase activities between calcareous and noncalcareous soils emphasize differential responses to mineral fertilization depending on soil type. Moreover, the increase in β-glucosidase activity observed in the calcareous soils (Inceptisol and Vertisol) may be due to an imbalance in soil C:P ratios (63, 64) following DAP application (higher POlsen) or stimulation of microbial activity by labile organic compounds supplied by SUR, such as carbohydrates and amino sugars (65). The additional N supplied by DAP and by SUR or SUR+STR may have further stimulated soil microbial activity, increasing the demand for C substrates and resulting in higher β-glucosidase activity. This supports our second hypothesis and is consistent with previous studies conducted in P-deficient calcareous soils (66). This is in line with enhanced organic matter decomposition and nutrient release (64, 67), ultimately contributing to increased plant P uptake (68).

Enhanced effects (responses greater than those expected from individual application of STR or biobased fertilizers) were observed for plant P uptake when STR was combined with OMP or SUR, and for PUE and AE in SUR+STR-treated plants in the Entisol and Inceptisol. Enhanced effects were also found for plant-available soil P in the Vertisol, where combinations of STR with biobased fertilizers (OMP, SUR, or SWS) resulted in POlsen values comparable to those obtained with STR applied alone. This could also be explained by the additional supply of Mg and N from STR. These findings partially confirm our third hypothesis and demonstrate the importance of combining materials with contrasting chemical properties, while emphasizing the central role of soil properties in regulating biobased fertilizer performance. All three soils exhibited low ZnDTPA contents [< 0.8 mg kg-1 (33)], indicating a potential risk of Zn deficiency. However, no visible Zn deficiency symptoms were observed in the wheat plants. Notably, SUR application, either alone in all soils or combined with STR in the Inceptisol, resulted in wheat grain Zn concentrations equal to or exceeding the levels considered sufficient for human nutrition [> 35 mg kg-1; (69, 70)]. This represents an extra benefit of certain biobased fertilizers over mineral fertilizers and supports the first part of our fourth hypothesis. The strong Zn biofortification effect observed with SUR coincided with high β-glucosidase activity in the two calcareous soils (Inceptisol and Vertisol). This finding reinforces the interpretation that organic matter mineralization and nutrient release are enhanced when SUR is applied to calcareous soils, thereby improving grain Zn accumulation (71). Nevertheless, DAP and STR reduced the grain Zn concentrations, probably due to the well-known antagonistic interaction between P and Zn under conditions of high plant-available soil P (57). In addition, dilution effects associated with higher biomass and yield may have contributed to lower Zn concentrations in the grain of DAP- and STR-treated plants grown in the Entisol and Vertisol compared with those receiving SUR alone.

Finally, none of the evaluated biobased fertilizers, applied alone or in combination with STR, resulted in increases in grain heavy metal concentrations. Although OMP application increased the grain Ni and Cu concentrations, these values remained below critical thresholds [<100 mg kg-1 for Ni and < 50 mg kg-1 for Cu (58)]. Therefore, the second part of our fourth hypothesis, concerning potential heavy metal risks associated with biobased fertilizer use, was not supported by our results.

While certain limitations of our findings must be acknowledged, this study provides a robust foundation for the advancement and practical implementation of biobased fertilizers in Mediterranean agroecosystems. The experiment was conducted under controlled conditions using wheat as a model crop and over a single growing season, which limits direct extrapolation to field conditions. In addition, the relatively small volume of soil used in the pot experiment constrained root development and microbial activity and may have amplified treatment effects. Another limitation is that soil enzyme activities were assessed only at the end of the experiment; future studies should include multiple sampling times to better capture their temporal dynamics. Moreover, the nutrient composition of the fertilizers was not fully balanced across treatments. In particular, STR supplied additional N and Mg; therefore, treatment effects cannot be attributed exclusively to differences in P source, and some degree of confounding between P effects and co-supplied nutrients is expected. Then, although the use of insufficiently stabilized OMP negatively affected some soil and plant responses, this reflects a realistic agricultural practice in the study region. Our findings thus provide valuable insights into short-term soil-plant responses to biobased fertilizers, evidencing both their limitations and their potential for optimization within sustainable agricultural systems. Future research should focus on residual and long-term effects under field conditions.

5 Conclusion

This study demonstrates that the agronomic effectiveness of the tested biobased fertilizers depended on their chemical composition and soil type. The effects of the biobased fertilizers should be interpreted within each soil type, given the strong physicochemical differences between soils. Among the evaluated materials, composted solid urban residue and composted sewage sludge, applied alone or in combination with struvite, emerged as the most effective biobased fertilizers. These treatments increased plant-available soil P, total plant P uptake, P use efficiency, and agronomic efficiency, reaching values comparable to or, in some cases, exceeding, those obtained with the mineral P fertilizer (diammonium phosphate) within individual soils, including calcareous and noncalcareous soils. When combined with all biobased fertilizers, struvite systematically enhanced their performance as P suppliers, underscoring its potential role as a complementary P source that improves nutrient synchronization. In addition to improving P nutrition, composted solid urban residue contributed to wheat grain biofortification by increasing grain Zn concentration in plants grown in the Inceptisol, while simultaneously stimulating soil biological activity, as reflected by increased β-glucosidase activity in the two calcareous soils. However, the magnitude of these effects was soil-dependent, highlighting the importance of the soil × treatment interaction. Composted olive mill pomace did not improve plant or soil responses in any of the soils, most likely due to insufficient stabilization during composting. In general, our findings support the use of selected biobased fertilizers as alternative P sources within specific soil contexts, capable of reducing reliance on mineral fertilizers and promoting sustainable nutrient cycling in agricultural systems of semiarid regions. Future research should focus on the residual and long-term effects of these materials on soil properties and crop performance under both controlled and field conditions. Also, evaluating their performance across contrasting pedoclimatic conditions will be essential to optimize P management strategies in Mediterranean agroecosystems.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fsoil.2026.1937202/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: José María De La Rosa, Spanish National Research Council (CSIC), Spain

Reviewed by: Tamer A. Elbana, National Research Centre, Egypt

Dinesh Phuyal, University of Florida, United States

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author ARS-R declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.

Funding

The author(s) declared that financial support was received for this work and/or its publication. Grants PID2020-118503RB-C22 (‘FerPhOS’) and PID2023-149247OB-C22 (‘SoPhOS’) funded by MICIU/AEI/10.13039/501100011033 and, by “ERDF/EU”, European Union. Additionally, it was partially funded by the Spanish National Research Agency through the Severo Ochoa and María de Maeztu Program for Centers and Units of Excellence in R&D (reference: CEX2019-000968-M), a project of the Andalusian Government (GOPO-CO-23-0006), and by Grant PID2024-156093OB-I00 funded by MICIU/AEI/10.13039/501100011033 and, by “ERDF/EU”, European Union. SB is funded by the Spanish Government with a Research Staff Training Fellowship (Grant FPI-2020, reference: PRE2020-092577) MCIN/AEI/10.13039/501100011033 and LG-G is funded by the Spanish Ministry of Science, Innovation and Universities (FPU contract, Ref. FPU22/01371).

Data availability

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

SB: Writing – review & editing, Writing – original draft, Investigation, Software, Formal analysis, Data curation, Methodology, Conceptualization. LG-G: Writing – review & editing, Investigation, Formal analysis, Methodology, Data curation. VB: Project administration, Visualization, Resources, Data curation, Validation, Methodology, Funding acquisition, Writing – review & editing, Conceptualization. MCC: Validation, Data curation, Methodology, Project administration, Conceptualization, Writing – review & editing, Visualization, Funding acquisition. MAM: Formal analysis, Visualization, Data curation, Methodology, Writing – review & editing, Conceptualization, Validation. MCG: Data curation, Visualization, Conceptualization, Validation, Writing – review & editing, Formal analysis. ARS-R: Data curation, Methodology, Project administration, Conceptualization, Validation, Visualization, Writing – review & editing, Funding acquisition, Supervision, Investigation, Resources.

Acknowledgements

The authors thank Prof. Davey Jones from Bangor University for revising the English language and style of the manuscript.

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Citation

Boubehziz, S., et al. (2026). Soil-specific effects of biobased fertilizers and struvite on phosphorus dynamics and wheat growth. Frontiers in Soil Science, Volume 6. https://doi.org/
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Copyright © 2026 Boubehziz, Guerrero-Gallardo, Barrón, del Campillo, Martín Santos, Gutiérrez Martín and Sánchez-Rodríguez.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Sana Boubehziz; María Angeles Martín Santos

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