Changes in physical properties of growing media constituents according to biodegradation

The use and exploration of more sustainable materials for growing media, in response to the search for alternatives to peat, are expanding. Many locally available organic resources are currently under study for this purpose. However, their biological stability remains a major limitation. Understanding the effect of biodegradation on the physical properties of these materials, in order to assess their long-term suitability for ensuring water supply and root system respiration, remains largely unexplored. The objectives of this research were to evaluate the biological stability of various organic materials over a 6-week biodegradation process; to investigate how biodegradation alters key physical properties; and to determine whether these alterations stem from changes in particle size distribution. Initial and post-biodegradation physical properties and particle size distribution were analyzed. Biodegradation was measured via respirometry. The protocol employed aerobic conditions, fertilizer addition, and a temperature of 35°C to reach a substantial biodegradation rate. As a result, the methodology employed led to biodegradation rates comparable to, or exceeding, those reported in the literature. No significant changes were observed in particle size distribution after biodegradation, probably due to overall low biodegradation rates coupled with heterogeneous particle sizes. No significant changes were observed in total porosity or available water either. The significant changes observed were a decrease in air-filled porosity, resulting from a general increase in water retention properties at suctions of −10 cm and above. These observations therefore suggest the hypothesis of a change in the integrity of the particle surface, with the formation of fine pores, rather than fragmentation leading to changes in particle size and organization. These observations highlight the importance of assessing changes in physical properties during relatively long biodegradation processes for potentially biologically unstable raw materials, in order to adapt agronomic practices to the changing properties of the materials.

1 Introduction

Increasing sustainability in soilless cultivation requires the use of growing media based on low-carbon-footprint and widely available constituents while offering suitable agronomic performance. The use of peat, which is a staple in horticultural substrates, has been reconsidered in recent years for environmental reasons (Gruda, 2019). The need for plants (food, ornamental, forestry) from soilless production is increasing, and so is the need for materials used as growing media. However, in this context of transition to a more sustainable industry, the quantity of materials needed for the years to come is not being met (Blok et al., 2021).

Woody materials are traditionally used in the formulation of growing media on a local scale, due to the cost of importing peat (Pokorny, 1979; Lemaire et al., 1980, 1989; Rivière and Caron, 2001). Use of barks began in the 1960s, followed by wood fibers in the 1980s, and has grown significantly over the past 15 years and continues today. In recent years, these materials have been thoroughly studied and improved (Marble et al., 2012; Gruda, 2019; Durand et al., 2021). Their low cost, local availability, innocuity, and agronomic properties adapted to soilless cultivation allowed their democratization on a global scale, leading to the establishment of many production facilities close to consumer basins (Lemaire, 1997).

The use of non-woody lignocellulosic biomass, after being transformed (i.e., by composting, defibration, grinding, steaming), might also be a valuable source of raw material for the design of more sustainable horticultural substrates (Vandecasteele et al., 2018). In this context, alternative materials, mostly derived from local industries, have garnered attention as potential substitutes for peat in growing media (Abad et al., 2001). Many herbaceous materials, some considered waste or by-products, were assessed as potential growing media constituents, e.g., flax shives, reed straw, bamboo, and miscanthus (Zhong et al., 2018; Dittrich et al., 2021; Nguyen et al., 2022; Vandecasteele, 2023; Durand and Michel, 2025). However, the lack of biological stability of these non-woody plant materials is an obstacle to their use (Lemaire, 1997; Vandecasteele, 2023). An organic material with suitable biological stability has the ability to keep its original physicochemical properties over several months; this property is essential for soilless production (Lemaire, 1997). The gain in sustainability concerning growing media lies not only in finding sustainable alternatives available in sufficient quantity, but also in ensuring that the selected materials meet agronomic requirements, including biological stability criteria, which are essential for effective use as a growing medium (Vandecasteele, 2023). Thus, characterizing this phenomenon and its consequences is a prerequisite for improving the sustainability of soilless crops.

The property of biological stability depends on the ability of a material to be degraded, or not, by microbial activity (Baffi et al., 2007). Microorganisms under aerobic conditions consume organic carbon for their metabolism, consuming dioxygen and producing carbon dioxide (Calvet et al., 2015; Blok et al., 2019). Thus, microbial activity is traditionally measured by the respiration of microorganisms, which is reflected by oxygen consumption or carbon dioxide emission (Komilis and Kletsas, 2012). A large diversity of testing systems based on respirometry methods are available, providing valuable insights into the degree of biodegradation and stability (Vandecasteele, 2023). Recently, the indicator oxygen uptake rate (OUR, in mmol O₂/kg OM/h) has become widespread. This indicator has emerged following a methodology proposed by Veeken et al. (2003) and Grigatti et al. (2007), in which oxygen consumption is monitored indirectly via the measurement of pressure variations due to CO₂ trapped by sodium hydroxide. This method, which is suitable for assessing the stability of different natural organic matter sources (Blok et al., 2019; Pigoli et al., 2021), has been standardized (EN 16087-1, CEN, European Committee for Standardization, 2011a). This standardized methodology has the particular advantage of being carried out over a short time period (5–7 days). However, it has the drawback of being conducted under conditions that differ from material use, as pointed out by several authors (Geuijen and Verhagen, 2014; Vandecasteele, 2023; Newman et al., 2023), thereby limiting the scope of the results.

Other methods assessing material biodegradation have also been proposed in the literature. In particular, the index of biostability proposed by Lemaire (1997), and variants proposed by Domeño et al. (2011), are based on the mass variation of a sample during long-term biodegradation (over 6 months or more). The advantage of this method is that it provides an account of the long-term stability of materials, but a drawback is that it requires a long time to obtain results.

Finally, other approaches based on chemical or biochemical composition to estimate potential biostability have been investigated. The carbon-to-nitrogen ratio (C/N) is a determinant factor driving material mineralization; however, this approach is not sufficient to estimate biostability, as it is difficult to generalize to all sources of organic matter (Thomas et al., 1998; Calvet et al., 2015; Vandecasteele, 2023). The biochemical composition of plant tissues (e.g., biochemical stability index; Linères and Djakovitch, 1993) was also investigated as an indicator but failed to establish a distinct link between mineralization dynamics and a particular fraction (Montagne et al., 2015). Overall, these methods are not routinely used for materials employed as growing media, but they provide additional information concerning their biological stability potential (Blok et al., 2019).

The literature presents several studies assessing the biological stability of materials, covering a wide range of constituents used as growing media. It shows that the most commonly used materials (peat, bark, coir, and wood fibers) are often qualified as stable or highly stable (Lemaire, 1997; Domeño et al., 2011; Vandecasteele, 2023). In contrast, evaluations of non-woody plant fiber materials revealed much lower biological stability (Lemaire, 1997; Vandecasteele, 2023). Stability within the same type of material can vary significantly. For example, Lemaire (1997) observed, for peats, a variation in biostability index ranging from 93 to 85% (of remaining organic matter) after 6 months of controlled biodegradation, while Vandecasteele (2023) observed an OUR ranging from 1 to 3.8 mmol O₂/kg OM/h for a variety of wood fiber batches. Thus, specific evaluation of each material according to its origin and manufacturing process is necessary to obtain a precise assessment of its biological stability. This literature review does not report studies describing biodegradation kinetics of growing media materials, which prevents discussion of their mineralization dynamics during use or storage. Obtaining such kinetics would help identify periods of highest mineralization and thus support more informed and efficient material use.

Material biodegradation can affect growing media properties and lead to the emergence of certain phenomena, including variation of pH, electrical conductivity (EC), cation exchange capacity (CEC), nitrogen immobilization, oxygen deficiency, volume shrinkage, and particle size alteration (Verhagen, 2009; Jackson et al., 2009; Domeño et al., 2011; Blok et al., 2019; Nerlich and Dannehl, 2021). Concerning traditionally measured physical properties, few studies have focused on modifications attributable solely to biodegradation. Indeed, many works in the literature present results on physical property modifications under cultivation conditions (Domeño et al., 2010; Michel and Kerloch, 2017), where several factors (e.g., watering/drying cycles), other than microbial activity, could have induced these modifications. Jackson et al. (2009), studying bark- and wood-based substrates over 70 weeks of biodegradation with and without plants, observed constant total porosity, an increase in water-holding capacity, and a decrease in air-filled porosity associated with a decrease in particle size distribution. In contrast, Lemaire et al. (1980), studying bark, observed a particle size distribution shifted toward larger particle size fractions after 8 months of biodegradation, although variations in physical properties were not assessed.

Faced with the need to meet the growing demand for growing media and the need for peat alternatives that are sustainable and agronomically efficient, this study seeks:

1. to propose a new experimental design for assessing the biological stability of organic materials already on the market and/or of other potentially usable organic resources, that is more representative of in situ conditions and provides accurate kinetics of their biodegradation,

2. to compare the results obtained with those from other existing methodologies or experimental designs,

3. to analyze the consequences of biodegradation in terms of particle size distribution of materials and of changes in their physical properties (water retention, etc).

2 Materials and methods

2.1 Rationale

To meet the objectives, the physical properties and particle size distribution were analyzed prior to and post-biodegradation. Environmental parameters for material biodegradation were chosen to maximize biodegradation under realistic conditions. To assess the rate of biodegradation of materials, the device BPC Blue Aerobic (BPC Instruments) was used, allowing continuous monitoring and precise quantification of biodegradation under aerobic conditions over a long-term process, thanks to oxygen renewal during the process. Additionally, the sample volume allowed sufficient biodegraded material to be obtained for subsequent measurement of physical properties.

The use of dynamic image analysis (QicPic device, Sympatec GmbH) for particle size characterization (Durand et al., 2024a) and the evaporative method (Hyprop device, Meter) to assess the water retention curve (Fields et al., 2016) is relatively novel compared to standardized methods used for characterizing physical properties of growing media (EN 13041, CEN, European Committee for Standardization, 2012c). However, these methods have already been described in detail and their robustness has been demonstrated for this type of material; above all, they offer greater precision.

2.2 Materials selected for analysis

The selection of materials was guided by a review of the scientific literature on their stability (Lemaire, 1997; Pigoli et al., 2021; Vandecasteele, 2023), as well as by their current and expected future share in the volume of raw materials used worldwide as growing media (Blok et al., 2021). The raw materials were supplied without any prior addition of lime or fertilizer. The list of materials is given in Table 1.

Table 1

Table 1. Main descriptive information about the materials and measured values of interest for their preparation for the biodegradation procedure.

2.3 Sample preparation prior to biodegradation

The dry bulk density of the materials was determined using the standard procedure EN 13041 (CEN, European Committee for Standardization, 2012c), for which four replicates were carried out. Then, the moisture content of the material was adjusted according to its moisture content at −50 cm matric potential (FD U44-163, CEN, European Committee for Standardization, 2018). To reach this matric potential, the material was first saturated with water in a PVC cylinder (h: 20 cm, r: 5 cm) and then placed on a suction table for 48 h to reach a matric potential of −50 cm. Finally, the moisture content (g g^-¹) was measured using an Ohaus MB90 to calculate the moist mass of material corresponding to 300 cm³. The dry bulk density and the moisture content (% by mass) are given in Table 1.

The fertilizer Start&Gro (ICL) 14–16–18 (NPK, + micronutrients) was added to the material to enhance biodegradation, at a concentration of 4 g L^-¹ (Vandecasteele, 2023), based on the dry bulk density of the material. The moist materials mixed with fertilizer were then packed according to their dry bulk density (Table 1) in 2 L clear glass bottles. The quantity of material introduced corresponded to a volume of 300 cm³, resulting in a relatively low ratio between the volume of solids and the volume of the bottle, in order to promote biodegradation (Komilis and Kletsas, 2012).

Electrical conductivity and pH of the samples (mixed with fertilizer) were measured according to the standard protocols EN 13038 and EN 13037, respectively (CEN, European Committee for Standardization, 2012b, 2012), involving an extraction ratio of dry material (based on dry bulk density) to distilled water of 1:5 (by vol.). Three replicates per material were carried out. These variables were measured to provide descriptive information on the biodegradation conditions of the materials (Table 1).

2.4 Biodegradation procedure and carbon mineralization assessment

Biodegradation of the materials was investigated using a BPC Blue Aerobic (BPC Instruments). This device operates as a closed chamber composed of several elements. Glass bottles containing the materials were placed in a thermostatic water bath at 35°C to maximize biodegradation (Komilis and Kletsas, 2012; Calvet et al., 2015; Newman et al., 2023). Each bottle was topped with a screw-top container filled with 80 mL of sodium hydroxide solution (NaOH, 3 mol L^-¹). Each bottle and its top container were connected with PVC tubes to an individual flow cell, the latter being connected to a common O₂ gas bag.

During aerobic biodegradation, the CO₂ produced reacted with the NaOH solution (Equation 1), creating a vacuum effect in the bottle. The volume of CO₂ that reacted with the NaOH was proportionally replaced by O₂ supplied from the gas bag into the reactor via the flow cell. The flow cell consists of a box filled almost completely with water, with a trapdoor at its base; the trapdoor opens by the force of buoyancy when a gas volume of 2.07 mL has accumulated. Each opening of the trapdoor was recorded by the device. After the trapdoor opened, O₂ flowed into the bottle.

$\begin{array}{ll}\begin{array}{l}2 NaO{H}_{(l)}+ C{O}_{2_{(ɡ)}}\to N{a}_{2}C{O}_{3_{(l)}}+H_2{O}_{(l)}\hfill \\ N{a}_{2}C{O}_{3_{(l)}}+C{O}_{2_{ɡ}}+H_2{O}_{(l)}\to 2NaHC{O}_{3_{(l)}}\hfill \end{array}& (1)\end{array}$

Letters “l” and “g” in brackets indicate the state of the matter respectively liquid or gas.

Thus, the aerobic biodegradation rate of the materials was quantified by measuring the volume of O₂ entering each bottle to proportionally replace CO₂ (according to the ideal gas law). The amount of gas involved in the process was automatically converted to standard temperature and pressure conditions (STP; T: 273 K, P: 1013.25 hPa) by the device, which continuously registered temperature and pressure. In this work, to illustrate biodegradation rate, it was assumed that O₂ consumption reflected an equivalent consumption of organic carbon. In practice, this assumption is not strictly accurate, as other biochemical processes unrelated to organic carbon biodegradation can also consume O₂, such as nitrification. For this reason, some studies use nitrification-inhibiting compounds (Vandecasteele, 2023). In those cases, measurements are carried out under conditions where the substrate is suspended in a liquid containing an inhibitor.

Biodegradation was performed over 6 weeks, in order to reach a significant level of material degradation (>1%) that would allow changes in material properties to be detected. Biodegradation of the materials was expressed as the ratio (in %) between the mass of carbon mineralized into CO₂ and the initial mass of organic matter (Equation 2). Organic matter content (Table 1) was determined using the standard protocol EN 13039 (CEN, European Committee for Standardization, 2011b), which involves burning the material for 7 h at 450°C in a muffle furnace. Biodegradation was also expressed in terms of oxygen uptake rate (OUR; mmol O₂/kg OM/h) to allow comparison with other studies using this index. However, as the biodegradation procedure lasted much longer than the 5 or 7 days proposed in the EN 16087–1 standard (CEN, European Committee for Standardization, 2011a) or other methodologies (Vandecasteele, 2023), the OUR indicator was calculated over various time intervals.

$\begin{array}{ll}\%C_m=\left(\frac{(\frac{V_{o_2}}{V_{m_{ɡ}}})\times M_{C{O}_2}\times \frac{M_C}{M_{C{O}_2}}}{M_{OM}}\right)\times 100& (2)\end{array}$

With V_O₂, the volume of O₂ (in STP) consumed during the process; V_MG, the molar volume of gas at STP (i.e., 22.4 L/mol); M_CO₂, the molar mass of CO₂ (i.e., 44.01 g/mol); M_C, the molar mass of carbon (i.e., 12.01 g/mol); and M_OM, the mass of organic matter.

Each sample was analyzed in four replicates. To assess the repeatability of the biodegradation procedure, measurements on white peat were performed in triplicate over three different series of biodegradation procedures (i.e., nine individuals were measured). The distribution range of results, standard deviation, and coefficient of variation (ratio of standard deviation to the mean) were calculated to analyze result variability.

2.5 Analysis of materials before and after degradation

Following the 6-week biodegradation process, the materials were recovered in plastic zip bags and placed in a cold room at 4°C to limit biological activity prior to post-biodegradation analysis. Samples previously divided into four replicates for the biodegradation procedure were then re-aggregated to obtain a single substantial volume of material, required for physical property assessment.

2.5.1 Physical properties characterization of materials

Physical properties were assessed using Hyprop (Meter) devices. This device consists of a measurement base equipped with two tensiometers (at different heights: 1.25 and 3.75 cm), on which the measurement cylinder is placed, all positioned on a scale. Measurements were performed in 249 cm³ cylinders (h: 5 cm, r: 4 cm, considering the volume occupied by the tensiometers).

Sample preparation for measurement consisted of saturating materials (previously equilibrated at −50 cm) in a double (superimposed) cylinder for 24 h. After water had drained, the upper cylinder and its content were removed, and the lower cylinder was retained for measurement (Durand et al., 2024a). The cylinder was then placed on the Hyprop measurement base, and data acquisition was initiated, consisting of continuous measurement of water content and water potential during the free evaporation process, until the collapse of one tensiometer. After completion of the measurement process, sample dry mass was determined after 48 h of oven drying at 105°C.

Key values—total porosity, air-filled porosity, water-holding capacity, and available water—were calculated as defined by De Boodt and Verdonck (1972).

2.5.2 Particle size characterization

Particle size was assessed using a dynamic image analysis tool, the QicPic (Sympatec GmbH), and its associated software Paqxos. The measurement procedure consisted of dispersing materials in water (15 L) by agitation (Viscojet, Heidolph Instruments). Particles were then passed in front of the QicPic camera, controlled by a peristaltic pump, with the frame rate set to 80 frames per second (Durand et al., 2023a).

Twelve replicates were carried out, each corresponding to 0.25 g of dry material. The projected particle area was used as the weighting factor. Maximum Feret diameter (Feret _MAX) was selected as the particle length descriptor (Nguyen et al., 2022; Durand et al., 2023b, 2024). Particle size analysis results were expressed in terms of interpolated sizes at the 10^th, 50^th, and 90^th percentiles of the cumulative size distribution, as well as mean arithmetic size.

2.6 Statistical analysis

Statistical analysis was performed using RStudio software. A Pearson correlation matrix was established to test correlations between absolute differences in measured physical properties before and after biodegradation as a function of material biodegradation rate. Mean comparison tests, including the Shapiro–Wilk test (normality), Levene’s test (variance equality), and Student’s t-test (mean equality), were performed to test differences between mean particle sizes before and after biodegradation.

3 Results

3.1 Biodegradation quantification and kinetic

After 42 days of controlled biodegradation, peat showed the lowest biodegradation rate, with an average of 1.06% mineralized OM (Figure 1, Table 1). Acrotelm and coir then showed higher rates, with 1.89% and 2.67%, respectively. The six wood fiber samples were grouped and were easily distinguished from the other materials. Small differences were observed among them, with biodegradation rates ranging from 3.66% to 4.43%. Finally, miscanthus was the material with the highest biodegradation rate, at 8.41%.

Figure 1

Figure 1. Biodegradation kinetics of materials over the 42 days, expressed as a ratio of mineralized carbon to initial dry mass of material. WP, white peat; COI, coir; ACR, acrotelm; MIS, miscanthus; WF-A, -B, -C, -D, -E, -F: wood fibers.

Regarding biodegradation kinetics, for most materials, after a peak of mineralization during the first few days, mineralization gradually declined and stabilized (Figure 1). This pattern was particularly noticeable for wood fibers. In the case of peat, acrotelm, coir, and miscanthus, the biodegradation process showed rather linear kinetics.

3.2 Assessment of method reproducibility

To test the reproducibility of the biodegradation method, three series (over time) of three replicates (i.e., nine individuals) of white peat were measured. With a mean of 1.06% mineralized OM (Table 2), the largest absolute deviation calculated between two individuals after 42 days of biodegradation was 0.11%. The standard deviation around the mean was 0.05, and the coefficient of variation was 4%.

Table 2

Table 2. Biodegradation rate, oxygen uptake rates during a 42-day biodegradation process, and oxygen uptake rates measured between day 2 and day 5.

For the other materials, for which only four replicates were measured, the coefficient of variation was generally below 10%, except for samples WF-A (13%), WF-C (11%), and WF-F (17%).

3.3 Modification of particles size distribution

Measurement of particle size before and after biodegradation did not indicate any clear trend in the evolution of this variable following the process (Table 3). Indeed, values were sometimes higher and sometimes lower after biodegradation. Statistical analyses were carried out by pairwise comparison of means (before vs. after biodegradation) of the data sets of arithmetic mean size (12 replicates per material). For all materials except coir and peat, no significant differences in mean particle size were observed before and after biodegradation.

Table 3

Table 3. Information related to the particle size distribution of materials measured by dynamic image analysis.

Despite these statistically significant differences, the changes in mean particle size were small: for peat, a reduction of 90 µm; for coir, an increase of 96 µm (Table 3). The span between D₁₀ and D₉₀ attests to non-uniform particle size distributions, with factors ranging from 18 (coir) to 183 (WF-E). The standard deviation of mean particle size indicates variability among the mean sizes measured across the 12 replicates, with coefficients of variation ranging from 7% (WF-C) to 56% (WF-E) of the mean value.

3.4 Effect of biodegradation on physical properties

Regardless of the material considered, total porosity (Figure 2, Table 4) did not change after biodegradation (± 1%), indicating that this property was not modified by the process. Statistical analysis using the Pearson correlation coefficient showed no significant correlation between the absolute difference in total porosity and biodegradation rate (r = 0.07; p = 0.85).

Figure 2

Figure 2. Water retention curves before (n=3) and after biodegradation (n=3) of (a) white peat, (b) coir, (c) acrotelm, (d) miscanthus, (e) wood fiber ‘a’, (f) wood fiber ‘b’, (g) wood fiber ‘c’, (h) wood fiber ‘d’, (i) wood fiber ‘e’, (j) wood fiber ‘f’.

Table 4

Table 4. Main physical properties of the samples before and after biodegradation, measured using the evaporative method with the Hyprop system (mean over three replicates; standard deviation given below).

Air- and water-filled volumetric contents (%) at −10 cm (AFP and WHC) were modified as a result of the biodegradation process: AFP decreased, whereas WHC increased. Pearson correlation coefficients calculated between biodegradation rate and the absolute differences in AFP (r = −0.70; p = 0.02) and WHC (r = 0.71; p = 0.02) before and after biodegradation showed significant correlations (p< 0.05). For peat, coir, acrotelm, and WF-F wood fiber, these changes were null or very slight (<2% vol.). For the other wood fibers and miscanthus, changes were more marked, ranging from 5% to 7% vol.

The increase in volumetric water content (%) at −100 cm observed post-biodegradation, ranging from +1% to +7% vol., was also significantly correlated with biodegradation rate (r = 0.65; p = 0.04).

The overall increase in water content observed from −10 cm and higher water suctions did not lead to significant changes in available water content (r = 0.40; p = 0.26).

4 Discussion

4.1 Material biodegradation and procedure

After 42 days of experimentation, white peat showed the lowest biodegradation rate; acrotelm and coir then showed biodegradation rates that were 1.8 and 2.5 times higher, respectively. The orders of magnitude of OUR₂–₅ observed are in line with the literature (Pigoli et al., 2021; Vandecasteele, 2023), although no data are available for acrotelm. According to the material stability classification system proposed by Veeken et al. (2003), these three materials were considered very stable (OUR< 5 mmol O₂/kg OM/h).

Miscanthus showed the highest biodegradation rate, and its biodegradation kinetics still appeared linear after 6 weeks. Oxygen uptake rate₂–₅ was higher than that observed using the standard methodology (EN 16087) by Vandecasteele (2023). According to the classification proposed by Veeken et al. (2003), this material was considered stable. However, in practice, this level of stability is not sufficient to consider miscanthus agronomically relevant, due to issues associated with biodegradation, such as nitrogen immobilization (Nguyen et al., 2022).

Oxygen uptake rate₂–₅ (mmol O₂/kg OM/h) values for the six wood fibers tested were higher than those reported in the literature, ranging from 5.32 to 10.59, compared with 4.10 in Pigoli et al. (2021) and values ranging from 1 to 3.8 measured across eight samples in Vandecasteele (2023). Measured OUR₂–₅ values for wood fiber samples varied from single to double, an order of magnitude smaller than that observed by Vandecasteele (2023). However, when considering OUR values calculated over 42 days, the wood fibers showed much lower variability, indicating that over the long term the samples may behave similarly.

Based on the material stability classification proposed by Veeken et al. (2003), wood fiber samples belong to the stable class, as does miscanthus. However, when considering the 42-day time step (OUR₄₂), these materials appear very stable. As biodegradation is not linear over time for most materials, measuring OUR over different time steps results in different interpretations of biostability, as already pointed out by Newman et al. (2023). Indeed, the biodegradation kinetics of wood fibers showed a much more intense respiration flux at the start of the measurement period (approximately the first 10 days) than toward the end of the experiment, unlike miscanthus, which showed a linear process. This phenomenon complicates comparisons between methodologies based on a few days and those based on several weeks, as observed in the work of Vandecasteele (2023).

Several methodological factors can affect results (e.g., temperature, sample size, water content, fertilizer quantity, oxygen supply) and therefore limit direct comparisons. In this work, the aim of the biodegradation procedure was to maximize biodegradation rates. Comparison with the literature indicates that the experimental parameters chosen enabled higher biodegradation rates to be achieved over a short period.

Finally, regarding reproducibility, assessed using nine replicates of white peat, statistical analysis showed that the procedure was highly reproducible. However, as observed for some wood fiber samples, standard deviations highlighted that these materials themselves are subject to intrinsic variability.

4.2 Particle size evolution assessment

The results obtained did not allow a clear conclusion regarding the effect of the experimental biodegradation process on particle size. For the majority of samples tested, statistical analysis showed that the experimental process had no effect on particle size.

The mineralization process, resulting in carbon loss in the form of CO₂, implies that material is removed from particles, and thus their morphology would not be expected to remain entirely intact. Assuming that particle morphology (here characterized by particle length) does change, several methodological limitations may explain why the applied approach did not detect particle size variation. First, the overall biodegradation rates were low, and consequently the magnitude of particle size modification may have been too small to detect. Indeed, only studies performing much longer biodegradation processes—8 months (Lemaire et al., 1980) and 16 months (Jackson et al., 2009)—reported particle size modifications. Second, the inherent heterogeneity of particle sizes in growing media constituents (Durand et al., 2023b) makes precise characterization of particle size challenging.

4.3 Evolution of physical properties

4.3.1 Hypothesis on particle surface modification

The physical properties of growing media are strongly related to particle size and to the structural arrangement of particles (Durand et al., 2024b). As these properties were modified without any detectable change in particle size, other mechanisms must explain the observed modifications. One hypothesis is that physical changes occurred at the scale of the particle surface and internal structure.

The results indicated a higher water content at −100 cm after biodegradation. Water retained at this matric potential corresponds to water stored in pores with an equivalent diameter of 30 µm or less, which is smaller than at least 90% of the particles composing the materials. An increase in pore volume of this size would therefore correspond to the formation of pores within the particle structure.

4.3.2 Consequences of physical properties modification

Changes in the properties of growing media during use are generally undesirable, as stability of properties is an asset for users. Such changes can be problematic, particularly during long storage periods and/or in long-cycle cultures. The short-term modifications observed in this study suggest that more pronounced changes may occur over the long term, potentially amplified by other factors.

Jackson et al. (2009), Qi et al. (2011), and Michel and Kerloch (2017) observed significant decreases in air-filled porosity and increases in water-holding capacity for a variety of growing media following root development and/or drying–irrigation cycles (including plant-free trials). The trends observed in the present work are consistent with those reported by these authors. As a result of biodegradation, in moderately aerated growing media, the decrease in air-filled porosity can be detrimental to plants by limiting adequate root respiration. In addition, changes in hydric properties imply that irrigation regimes should be adjusted in line with evolving water retention characteristics, in order to optimize water supply and avoid over-irrigation, which may lead to root asphyxia.

5 Conclusion

The proposed experimental procedure for biodegradation allowed the acquisition of results in terms of oxygen uptake rate (OUR) that were comparable to, or even higher than, those reported in the literature. This methodology had the advantage of enabling long-term measurements, allowing sufficiently high biodegradation rates to be reached in order to assess modifications in physical properties. Obtaining biodegradation kinetics over a longer time period than current standards highlighted the non-constancy of OUR as a function of time, thereby providing a more global characterization of material biological stability.

The tests conducted did not reveal any changes in particle size distribution resulting from the biodegradation process. However, an increase in water retention was observed at suctions of −10 cm and above, despite no change in total porosity or bulk density. This observation suggests the creation of new fine pores due to biodegradation of particle surfaces. From an agronomic point of view, biodegradation reduces air-filled porosity and increases water retention capacity. This indicates that irrigation practices should be adapted to the changing properties of biodegrading materials in order to maintain an adequate air–water balance in the root environment. As changes in physical properties driven by biological activity can occur as early as the first weeks following material preparation, this raises questions regarding the effects of longer-term experiments on particle size changes and the potential amplification of the modifications already observed in pore functional properties.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

Author contributions

SD: Conceptualization, Data curation, Formal Analysis, Investigation, Methodology, Project administration, Resources, Software, Validation, Visualization, Writing – original draft, Writing – review & editing. J-CM: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Validation, Writing – review & editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. The authors thank the private companies: Floragard, Kekkilä-BvB, Klasmann-Deilmann, and Premier Tech; as well as Angers Loire Métropole for providing funds to conduct this research.

Acknowledgments

We would like to thank the project’s partner companies for providing materials and for their financial contribution. We would also like to thank Angers Loire Métropole for its financial support. Finally, we would like to thank the members of the EPHor unit, and in particular Lucas Gilberton, for their participation in data collection.

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 J-CM 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.

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Abbreviations

AFP, air-filled porosity; OM, organic matter; OUR, oxygen uptake rate; WF, wood fiber; WHC, water holding capacity.

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