# Early pedogenesis of a young Technosol made from organic wastes

Baptiste Grard, Alexandre Barrier-Guillot, Claire Chenu

► To cite this version:

Baptiste Grard, Alexandre Barrier-Guillot, Claire Chenu. Early pedogenesis of a young Technosol made from organic wastes. Geoderma, 2025, 459, pp.117343. 10.1016/j.geoderma.2025.117343. hal-05097320

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Submitted on 4 Jun 2025

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# Early pedogenesis of a young Technosol made from organic wastes

B.J.P. Grard<sup>a,b</sup>, A. Barrier-Guillot<sup>a,c</sup>, C. Chenu<sup>a,*</sup>

## ARTICLE INFO

### Keywords:

Technosol

Pedogenesis

Urban agriculture

Urban waste

Compost

Crushed tiles and bricks

Spent mushroom substrate

## ABSTRACT

Constructed soils based on urban waste, i.e. Technosols appear as a promising solution to enhance circular economy while delivering ecosystem services. Their ability to provide such services depend on their constitution as well as on their temporal evolution and pedogenesis, which is poorly known. We studied during three years the changes in an isolatic Technosol made essentially of organic urban wastes.

The study took place on a Parisian rooftop with an experimental trial of 24 plots. We compared four Technosols with similar upper layers (a mix of biowaste compost and spent mushroom substrate), but with either a mineral (shredded tiles and bricks) or an organic lower layer (shredded wood). Both types were run with and without plants. Technosols were monitored over three years and samples were taken at different times to study pedogenetic processes.

We observed a rapid and sharp volume loss of the Technosols, responsible for an important mass loss, but not leading to any increase in bulk density. A clear physico-chemical differentiation of treatments from the parent materials appeared after one and three years. The Technosoil organic C stocks decreased over time. During three years, all Technosols were a net source of nitrogen and dissolved organic carbon, released in the drainage waters. Identified pedogenesis processes were the same as in natural soils, but the changes were very fast. The key equilibrium between mineralisation of organic matter and its stabilization that must be ensured when creating such Technosols. They show the necessity for long term research on the development of Technosols.

## 1. Introduction

Greening the cities depends on the presence of soils for the vegetation to grow and thrive. However, this is frequently limiting as urban soils can be heavily contaminated or sealed (Joimel et al., 2016; Morel et al., 2014). Constructing soils de novo then appears as a promising option for the reclamation of abandoned, contaminated or sealed city plots and for the development of vegetation (Barbillon et al., 2023; Fabbri et al., 2021), either for recreational purposes, for improving the water cycle, reducing the heat island effect or for producing food within the framework of urban agriculture.

Technosols are "soils whose characteristics and pedogenesis are dominated by their technical origin" (IUSS Working Group WRB, 2015). In practice, Technosols can be constructed from a diversity of parent materials that are wastes: building materials wastes (Nehls et al., 2012), industrial by-products such as paper mill sludge (Rokia et al., 2014; Séré et al., 2008), urban organic wastes such as green composts or tree pruning residues (Grard et al., 2020, 2018), agricultural wastes such as

coffee grounds (Grard et al., 2015) and excavated soils (Séré et al., 2008). Employing waste materials as parent materials for Technosols offers a dual benefit: it circumvents the depletion of finite natural resources like peat and curtails the energy expenditure and financial burdens associated with waste transportation and processing for landfill disposal (Dorr et al., 2021; Rodrigues et al., 2019). It hence contributes to a more sustainable and more circular economy, in particular when local wastes are used (Fabbri et al., 2021).

Many Technosols actually mimic natural soils with a structural layer, often made of coarse and mineral parent materials, above which lies a fertile layer, made of organic decomposable material (Grard et al., 2020, 2018; Séré et al., 2010; Vidal-Beaudet et al., 2012). The composition of such constructed Technosols is also contrasted depending on their location and associated constraints, e.g., rooftop constructed Technosols face issues of bearing capacity and are hence generally built with lightweight materials, such as bricks, tree prunnings and organic wastes.

Technosols can provide a variety of ecosystem services, the main expected one being to support vegetation, for aesthetic or recreational

<footnote><sup>a</sup> Université Paris-Saclay, INRAE, AgroParisTech, UMR ECOSYS, Palaiseau, France</footnote>

<footnote><sup>b</sup> Agroecology and Environment Research Unit, ISARA, Lyon, France</footnote>

<footnote><sup>c</sup> Fédération Régionale de l'Agriculture Biologique d'Auvergne Rhône-Alpes, Valence, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: claire.chenu@inrae.fr (C. Chenu).</footnote>
purposes (Ugolini et al., 2020), for food production (Grard et al., 2020; Orsini et al., 2014), for mitigating water runoff (Czemiel Berndtsson, 2010; Grard et al., 2018; Whittinghill et al., 2015) while maintaining the quality of percolating waters (Berndtsson et al., 2009; Whittinghill et al., 2016), for carbon sequestration (Allory et al., 2022), for microclimate regulation (González-Méndez and Chávez-García, 2020) and as a habitat for biodiversity (Hedde et al., 2018; Joimel et al., 2022). Their ability to provide such services depend on their constitution: parent materials and design (Grard et al., 2018; Rokia et al., 2014; Séré et al., 2008).

The pedogenesis of natural soils involves a number of processes of transformation, transport, re-organization, inputs and losses under the influence of climate and biological activity. Artificial soils are subjected to the same environmental factors and hence pedogenesis is expected to take place in constructed Technosols. However, the parent materials are different from those of natural soils. The few studies addressing the temporal changes of Technosols show processes that exist in natural soils, such as eluviation of fine particles (Bouzouidja et al., 2018), weathering of minerals (Howard and Olszewska, 2011) and lixiviation of elements (Séré et al., 2010), differentiation of horizons (Howard and Olszewska, 2011; Séré et al., 2010) and changes of soil structure from a particulate structure to aggregated ones (Jangorzo et al., 2013; Séré et al., 2010; Watteau and Villemin, 2011). However, some processes are characteristic of constructed Technosols, such as the rapid shrinkage and rearrangement of particles, resulting in increases in bulk density (Grosbellet et al., 2011; Séré et al., 2010), the weathering of technogenic materials such as gypsum or iron oxides associated with leaching (Howard and Olszewska, 2011; Huot et al., 2011; Séré et al., 2010). Still, most studies on the evolution of Technosols concern soils developed on the ground, on industrial materials or on natural soils (Allory et al., 2022; Howard and Olszewska, 2011; Huot et al., 2014; Séré et al., 2010), rather than on Technosols that are implemented on rooftops. The nature of the parent materials, essentially organic in the later case, is expected to influence the processes and the extent of pedogenesis, as well as the biological activity, including that of roots. Specifically, the biodegradation of organic materials is anticipated to significantly influence the fluxes of elements within and from the Technosol, as well as its physical characteristics (e.g., Grosbellet et al., 2011).

As the ability of constructed Technosols to provide ecosystem services depends on their composition and structure, changes related to pedogenesis might influence the provisioned services. For example, the rapid decrease of organic matter content in rooftop constructed Technosols, due to biodegradation and mineralization as well as to eluviation of the organic materials (Bouzouidja et al., 2018) causes changes in the hydric properties. Important leaching of organic carbon and mineral nitrogen (Grard et al., 2018) and rapid shrinkage of the soils (Grard et al., 2020) has indeed been quantified with constructed Technosols made of organic wastes. While it did not seem to compromise the productivity of the soils in this case and yields remained constant (Grard et al., 2020), this raises the question of the consequences of the pedogenesis of constructed Technosols on the provision of ecosystem services.

In this study, we aimed to describe and quantify the temporal changes of rooftop constructed Technosols to identify the involved pedogenic processes, analyze the influence of the nature of the parent materials and of the presence of growing plants on the pedogenesis. We hypothesized that: (i) similar pedogenic processes would occur in Technosols as in natural soil formation; (ii) rapid biodegradation and mineralization would alter the quantity and properties of organic components, leading to transfers within the constructed Technosol; and (iii) the presence of plants would modify particle arrangement and mitigate leaching through water uptake. We took advantage of an experimental rooftop farm in Paris where vegetables have been grown for three years on constructed Technosols made of constrained parent materials, with a structural layer made either of tree pruning or shredded tiles and bricks.

# 2. Materials and methods

## 2.1. Experimental site and set-up

The study took place on a Parisian rooftop of 861 m² (48°50'23.6"N 2°20'52.4"E) between November 2015 and November 2018. The climate of Paris is oceanic, with a mean rainfall during the experimental year of 620.6 mm, a mean temperature of 13.5 °C [min: -4.3 °C; max: 36.8 °C] and a mean solar radiation of 72.3 W.m⁻². The experimental trials consisted of 24 plots of 0.50 m² (inside perimeter: 71 cm * 71 cm) and 40 cm height in wooden boxes. Plots were positioned distant of at least 50 cm and placed above pallets, i.e. 37 cm above the roof ground (Fig. 1), with a randomized block design. Each treatment had six replicates. Yields were measured at the plot level.

## 2.2. Technosol parent materials

Four different urban wastes were used as parental materials (Fig. 2): (a) a biowaste compost made by Veolia under NF U 44-051;(b) a substrate made from spent coffee grounds after being used to grow mushrooms (Pleurotus Ostreatus) by the firm Laboiteàchampignons©. (c) shredded wood obtained from tree prunings and commercially produced by BioYvelinesServices©; (d) a mix of shredded tiles and bricks. The biowaste compost and the substrate made from spent coffee grounds were mixed (60/40 v/v) to constitute a specific substrate as described below. In order to gain information on their potential changes in the Technosol, we characterized the organic parent materials by quantifying their C and N contents by elemental analysis after dry combustion (NF ISO 10694), their biochemical composition to estimate their ISMO (XP U44-162; Lashermes et al., 2009) and their mineralisation potential by incubating them in the laboratory, at 20 °C and pF1.7 (XP U44-162:2009). The ISMO index was calculated as follow:

$$ ISMO = 44,5 + 0,5 * SOL - 0,2 * CEL + 0,7 * LIC - 2,3 * MinC3 \quad (1) $$

With:

SOL: biochemical soluble fraction, express in g.100 g⁻¹ of organic matter;

CEL: approximation of the cellulose fraction, express in g.100 g⁻¹ of organic matter;

LIC: approximation of lignin and cutin fraction, express in g.100 g⁻¹ of organic matter;

MinC3 corresponds to the percentage of organic carbon mineralized after 3 days of incubation

## 2.3. Plot design

The design of our experiment follows three main principles already used in other experiments (Grard et al., 2020, 2018, 2015):

1. To be transposable to citizens without specific agricultural skills and with limited economic resources, mimicking a rooftop community garden;

2. To use only local urban wastes as substrates to contribute to the urban metabolism as defined by Barles (2009);

3. To avoid the use of fertilizers and chemical pesticides in order to limit the consumption of fossil fuel and natural resources and the contamination of food products and the ecosystem.

The plots were divided in five layers: two main layers corresponding to our Technosol profiles and two technical layers plus one vegetation layer, as shown on (Fig. 3), from down to top:
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Left: Picture of the experimental trial on the rooftop AgroParisTech in 06-22-2016. © MairiedeParis. Right: Soil profile from experimental plot [OP] in the 7th of November 2016 (after one growing year). ©Baptiste Grard.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** The parent materials used as substrate in the experimentation: a) biowaste compost, b) mushroom growth substrate, c) shredded wood, d) shredded tiles and bricks and e) mixture of a and b materials.</figcaption></figure>

i. a drainage layer [technical layer] made of shredded tiles and bricks. This layer is designed to avoid stagnant water in the bottom part of the experimental boxes;

ii. a filter layer made of polyester geoxtextile (120 g.m⁻²) [technical layer] to avoid fine particles (>70 µm) to be washed from the Technosol;

iii. a Technosol skeleton layer of 15 cm height [soil profile] which has the function to store water but also allow the drainage of

water in excess to ensure aeration for roots. We compared two available materials: shredded tiles and bricks or shredded wood.

iv. a Technosol nutritive layer of 15 cm height [soil profile] which has the function to fulfill plant needs for nutrients and water. We used a Mix of biowaste compost with spent mushroom substrate (60/40 v/v).

The upper layers can be observed in Fig. 1b. Two types of experimental plots were built on October 14th 2015 (Fig. 4), either with an
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Structure of one experimental plot with the different layers. The skeleton layer, i.e. lower layer is either organic (shredded wood) or mineral (crushed tiles and bricks).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 4.** The four treatments implemented in the experimental trial.</figcaption></figure>

organic skeleton layer, i.e. shredded wood (treatments [O]) or with a mineral skeleton layer, i.e. shredded tiles and bricks (treatments [M]). Both treatments have an upper layer with the mix and exhibit one mobility with plants [OP or MP]. Fig. 4 summarizes the two treatments and modalities implemented. Those Technosols could be classified as garbic isolatic Technosol for O and OP and isolatic Technosol close to spolic Isolatic Technosol for M and MP (WRB, 2015).

Once filled, experimental plots were left one month to allow settlement. Plants were introduced in November 2015 11th. Two types of irrigation were installed: spray and drip irrigation. Both were filled with tap water and drip irrigation was mostly used (spray were turned on only during a short period of time for beans germination). Irrigation was used generally from May to October and irrigation time was manually set up. A possible lack of irrigation was observed during July and August 2016 through yellow and dried leaves of beans and cabbage.

## 2.4. Crop rotation

During the three years of the experimentation (corresponding to three growing seasons), 12 different vegetable species were grown (S1- Fig. 1). The crop rotation was designed according to the following principles: (i) maximizing the use of space regarding considering

vegetables with complementary development patterns and nutrients needs and (ii) growing only vegetables that could be manageable and profitable in a small surface area. Transplants were used for the following crops: basil, cabbage, chard, cucumber, lamb's lettuce, lettuce, spinach and tomatoes. Bean, Green beans, peas and radish were sowed. During the harvest, the edible parts of the vegetables were weighed. Root systems of the vegetables were left in place and before harvesting a superficial tillage (on the first 10 cm) was achieved.

## 2.5. Technosol sampling

Technosol samples were taken at different times: prior to experimentation for the analysis of parental material and after 1, 3, 6, 12 and 36 months of the experimentation. At each sampling date a surface area of 35 cm\*35 cm was defined to extract the Technosol. We divided the Technosol profile in two layers: an upper and a lower layer based on the initial vertical Technosol organization (see Fig. 3) and in-situ observation. During the sampling, the upper layer was defined by the first emergence of shredded wood or shredded tiles and bricks from the lower layer. First, an undistributed Technosol cylinder (10 cm\*12 cm) was sampled from middle of the upper layer. Then the rest of the upper layer was sampled separately for observation and analysis. The procedure was
repeated for the second layer. To minimize the Technosol disturbance, the first four sampling were achieved on three replicates of the six and the last one– at 36 months – was achieved on the 3 remaining replicates per treatment.

The Technosol height was directly measured placing a flat wood stick horizontaly in the diagonal of each plot then the height between the surface of the measured layer and the wood stick was measured. Total height was the average of 10 replicate measurement points per plot.

## 2.6. Technosol characterization

Once collected, Technosol samples were air dried for at least two weeks at 40 °C and samples were entirely shredded to pass at 2 mm. Sample analyses were conducted to determine: (i) the stocks of key elements (N, P, K, Fe, Mn) and their potential losses via leaching; (ii) the biodegradation of organic materials (C and N analyses); and (iii) the fertility status of the Technosols (pH, EC, CEC, nutrient element content). For this, the samples were sent for analysis to the soil laboratory of INRAE Arras to quantify CaCO₃, organic C, total nitrogen, P₂O₅, Fe, K and Mn contents, pH<sub>water</sub>, EC, and CEC). CaCO₃, was determined using NF ISO 10693, CEC – NF X 31–130; organic carbon content (dry combustion by heating at 1000 °C with O₂) – NF ISO 10694; electrical conductivity (1/25 m/v; 25 °C) – NF ISO 11265; Fe (EDTA 1/10 m/v) – NF X 31–120 using an ICP-AES; K – NF X 31–147, Mn (1/10, m/v) internal method DIV-1754); pHwater (ratio soil/solution = 1/5 v/v) – NF ISO 10390, P2O5 (1/20 m/v) – NF ISO 11263 and total nitrogen (dry combustion by heating at 1000 °C with O₂) – NF ISO 13878.

In parallel, bulk density was measured on the sampled cores according to NF EN 13041.

## 2.7. Measurement of water runoff and drainage water

Technosols, as natural soils are open systems and the fluxes of elements, within and out of the Technosol are expected to determine the changes of its composition. Consequently, throughout the three years of the experiment, water was collected four days at most after it had leached from the plot to the tank bottle (see Fig. 1). The volume of leachate was measured by weighing the tank and a sample of 50 ml was collected and stored at – 24 °C prior to analysis.

Concentrations of dissolved organic and inorganic carbon were determined using an elemental analysis. N-NH₄⁺ and N-NO₃⁻ were analyzed by colorimetry on a continuous flow (Skalar Analytical, Breda, the Netherlands). DOC was measured with a total organic carbon analyzer (Shimadzu TOC- 5000/5000A and TOC 5050/5050A) using catalytically aided combustion oxidation for TC and a pre-acidification for IC. Our samples were filtered at 70 µm, which implies that the dissolved organic and inorganic carbon analyzed correspond to particulate, colloids, and dissolved matter.

## 2.8. Data treatment

### 2.8.1. Calculations

#### SOC stocks

$$Q_{Corga} = \text{Technosollayerdryweight} * C_{Corga} \qquad (2)$$

With:

- $Q_{Corga}$ = stock of organic carbon in kg of C, per layer and per plot

- Technosol dry weight = measured (after drying at 80 °C) or estimated, per layer in kg

- $C_{Corga}$ = Organic carbon content of the Technosol layer in gC.kg⁻¹

Technosol layer dry weight being:

$$\text{Technosol dry weight} = BD * V_{\text{layer}} * 10^{-3} \qquad (3)$$

With:

- Technosol dry weight = measured per layer in kg

- BD = Bulk Density in g.cm⁻³

- $V_{\text{layer}}$ = Volume of the Technosol layer in cm⁻³

$$V_{\text{layer}} = H_{\text{layer}} * 71 * 71 \qquad (4)$$

With:

- $H_{\text{layer}}$ = Height of the Technosol layer in cm

- 71*71 = Internal dimension of the square plot (width*width) in cm

#### Waterflow regulation

**Rainwater retention:**

$$R_{\text{Rain}} = (1 - D/V_{\text{rain}}) * 100 \qquad (5)$$

With:

- $R_{\text{rain}}$ = Rainwater retention in mm.m⁻²

- D = Discharge, i.e. the volume of leached water per box expressed in mm.m⁻²

- $V_{\text{Rain}}$ = Volume of rainwater in mm.m⁻²

**Total water retention (rainwater + irrigation):**

$$R_{\text{Rain+irrig.}} = \left( 1 - \frac{D}{V_{\text{Rain}} + V_{\text{irrig.}}} \right) * 100 \qquad (6)$$

With:

- $R_{\text{rain+irrig.}}$ = Volume of total water input (rain + irrigation) received per box in mm.m⁻²

- Irrig. = Irrigation (mm.m⁻²)

- D = Discharge i.e. the volume of leached water per box expressed in mm.m⁻²

- $V_{\text{rain}}$ = Volume of rainwater in mm.m⁻²

- $V_{\text{irrig.}}$ = Volume of water used for irrigation per box expressed in mm.m⁻²

Total mass loss of specific mineral elements:

$$Qx = C_{xt1}/V_{t1} + C_{xt2}/V_{t2} + C_{xt3}/V_{t3} + \dots \qquad (7)$$

With:

- Qx (g): mass of the elements loss in drainage water

- $C_{xt1/2/3/...}$: concentration of the elements at the first drainage event (t1/2/3/...)

- $V_{t1/2/3/...}$: Volume of drainage water the first/second/third/etc. drainage event (t1/2/3/...)

**Conversion from plot to square meter:**

$$y = \frac{x}{0.71 * 0.71} \qquad (8)$$

With:

- y = Value converted in mm

- x = Volume in liter per plot

- 0.71*0.71 = Dimension of the plot in m².

### 2.8.2. Statistical analysis

Statistical analyses were performed using R software (R-4.4.1). Treatments with three replicates each were compared using an analysis
of variance after ensuring the normality of the data with a Shapiro test. A multiple comparison of means was determined by the post-hoc Tukey test. When the normality of data was not respected, a Kruskal Wallis test followed by a post-hoc Nemenyi test was applied. A significance level of p-value < 0.05 was used for each test.

# 3 Results

## 3.1. Technosol profile and volume

Technosol profiles were initially composed of two distinct layers that were still visible after three years for M and MP and clearly modified for O and OP (Fig. 5). After one year, a Technosol shrinkage was observed for O and OP with a mixture of shredded wood and compost within the organic lower layer. For M and MP, an intermediary horizon could be observed after three years. This thin intermediary horizon appeared as a mixture of compost and shredded tiles and bricks. Root development was observed in the whole Technosol profile, i.e. also in the lower layer of the Technosols (results not shown). However, the presence of roots did not modify the Technosol profiles.

During the first six months, all treatments followed a similar pattern, loosing ~ 10 % of their initial height (Fig. 6). After six months and significantly after three years a more pronounced shrinkage was observed for the organic treatments (O and OP) when compared to mineral ones (M and MP). The development of plants did not signifi- cantly impact this shrinkage as no significant differences were observed between O and OP nor between M and MP, but only a trend to lower shrinkage with plant for both treatments (O and M) after 3 years (Fig. 6). Most of the shrinkage for M and MP took place in the upper layer but occurred in both layers for O and OP. The presence of plants was asso- ciated with thicker upper layers after three years (Fig. 6).

## 3.2. Bulk density

Bulk density exhibited minor fluctuations over the experimental period. A statistically significant, but small decrease was observed in the upper soil layer of the OP (organic with plants) treatment after three years (Table 1). Conversely, a marginal increase in bulk density was noted in the lower Technosol layer for both the M and MP treatments after one year. The parent materials exhibited large porosities, with the shredded wood and the Mix exhibiting pores volume respectively 11.7 and 1.6 times higher than that of shredded tiles and bricks (Table 1). Most pores volume of shredded wood and shredded tiles and bricks corresponded to macro-porosity whereas macro and micro-porosity volumes were similar for the Mix. After one year, the total pores vol- ume did not significantly evolve in any layer, but the macro-porosity significantly increased at the expense of micro-porosity in the upper layer of both treatments (OP and MP) (S2-Table 1). Opposite changes were observed for the lower layer of shredded wood (OP) and no sig- nificant changes were observed for MP.

## 3.3. Changes of Technosol characteristics

When comparing Technosol samples between layers and treatments we could see a clear differentiation of the Technosols over time (Fig. 7). In the PCA, first and second axis explained circa 84 % of the observed variability, between treatment, layers and across time. The lower layer differentiated from the parent materials during the first and following years mainly linked to changes in pH, total nitrogen, CaCO₃, CEC and Fe contents. The lower layers of shredded tiles and bricks evolved less, except regarding their organic carbon content, CaCO₃ and Fe contents. In the upper layer, organic carbon content, CaCO₃ and Mn contents, and of CEC and electrical conductivity varied over time. While the upper layer was clearly differentiated from the parent material after one year, there was little further differentiation between the first and third years. The newly formed intermediary horizon in M and MP strongly differ- entiated from the initial parent material and from the lower layer even after 3 years in relation to its CaCO₃ content.

The OC content of the upper layer decreased compared to the parent material (the Mix) and then stabilised after 1 year, while the carbonate content increased compared to the parent material and remained stable between 1 and 3 years (Table 1). Compared to the parent material, the pH slightly decreased in the presence of plants but remained alkaline. The electrical conductivity decreased during the first year but remained stable thereafter. Changes depended on the nature of the parent material in the lower layers: in shredded tiles and bricks the OC content and carbonate content increased, but pH, CEC and electric conductivity of shredded wood did not vary. The intermediary layer exhibited inter- mediary characteristics, as shown with the PCA. In the lower shredded wood layer OC content decreased with time, carbonate content consis- tently increased, while pH increased then decreased again, CEC consistently increased and conductivity decreased only after 1 year.

## 3.4. Biodegradation

### 3.4.1. Biochemical characteristics of the parent materials

The analysis of the biochemical composition of the parent materials highlighted differences between the Mix and the shredded wood. Cel- lulose was predominant in the shredded wood (54 g.100 g⁻¹ of organic matter) followed by lignin and cutin and hemicellulose (Fig. 8). The soluble fraction was predominant in the Mix, representing 48 g.100 g⁻¹ of organic matter, followed by cellulose then lignin, cutin and hemi- cellulose. As a consequence, the Mix exhibited a higher ISMO (71.1 ± 1.5) than the shredded wood (50.2 ± 2.7) showing is lower biodegradability.

Incubations in the laboratory showed that both parent materials mineralized quickly in 239 days, losing respectively 24 % and 17 % of their initial carbon content (S3 -Fig. 2). The mix followed a classical pattern with a high initial rate of mineralization that decreased after 100 days, whereas the shredded wood followed a sigmoidal pattern. After 239 days, more than 24 % of the initial nitrogen content of the mix

<figure><img src="image_6.png" /><figcaption>Fig. 5. Technosol profiles after 1 month, 1 year and 3 years for OP (left) and MP (right) treatment.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Height evolution per layer for the different treatments. Results are expressed in cm. In the figure, bars represent standard deviation, same letters show the absence of statistical differences regarding the total height across time.</figcaption></figure>

had been mineralized compared to only 0.8 % of the nitrogen content in the shredded wood. In terms of degree-days, the 239 days of incubation corresponded to 4780 °C equivalent to ~ 1 year outdoor based on the daily temperature on the rooftop experiment.

### 3.4.2. C and N concentrations and stocks

The C/N ratio decreased in the Technosols upper layers from 12.1 to 9.8 in average after 1 year and 9.7 after 3 years (Table 1). There was no significant effect of the presence of plants. In the lower layers the initial C/N values were very high (105.1 ± 10.9 in tiles and bricks and 94.5 ± 11.7 in shredded wood) and they sharply decreased during the first year. They continued to decrease afterwards in the shredded wood, while they increased again in mineral lower layers (Table 1).

The overall mass of the Technosols decreased in three years (Table 2), the decrease being sharper for O and OP with a loss of respectively 42 and 39 % of their initial mass, whereas the total mass of M and MP decreased by 11 and 17 % respectively. For O and OP, the upper layer decreased by 50 % (O) and 40 % (OP) of their initial mass against 24 % (O) and 36 % (OP) for the lower layers. Regarding the total mass, MP was heavier after three years than M. A similar trend was observed for O and OP but not significant – except for the upper layer. The draining layer did not exhibit any significant mass change in three years for any treatment.

O and OP consistently exhibited higher organic carbon stocks than M and MP (Fig. 9). SOC stocks decreased by 40 to 52 % in all treatments over the three years, the largest reduction happening in the first year (data not shown). Plant presence significantly reduced this loss (MP vs M: 10 %; OP vs O: 3 %). In M and MP, the SOC loss was upper layer specific, while O and OP exhibited comparable losses in both layers SOC.

N stocks also decreased, to a lesser extent than SOC, with losses ranging 24 to 36 % of the initial values across treatments (Fig. 10). The presence of plants was associated with a smaller N loss, which was only significant between M and MP. Across all treatments, nitrogen stocks decreased in the upper layer while increasing in the lower layers (Fig. 10).

Potassium stocks exhibited a general similar trend to that of nitrogen (Fig. 11), decreasing across all treatments over the 3 years. The presence of plant did not affect K stocks for O, while for M treatments, K stock was higher for M than MP after three years. All layers lost Potassium but the decrease was more pronounced in upper layers.

## 3.5. Lixiviation and run-off

### 3.5.1. Water flow and retention

The inputs of irrigation water to the plots were very important for the second and third years and varied across years but very little across treatments (Table 3). In average the experiment was irrigated at 1329 ± 482 mm per year with a maximum value of 1960 mm.y⁻¹. As a consequence, the overflow was very important and likely to promote leaching.

In the first year, the overflow of water was greater for M and MP than O and OP (Table 3) but treatments exhibited similar values afterwards– except the third year where the overflow was larger in O than in M. The presence of plants, logically decreased the amount of overflow. With a similar trend, the number of discharge events was larger without plants (O, M) than with plants (OP, MP), with similar values for O & M as well as OP & MP (Table 3).

The proportion of incident water that was retained was larger in the presence of plants, but no significant differences were observed when comparing plots with organic vs mineral lower layers (Table 3). A negative percentage of rainwater retention was recorded in 2017 and 2018, corresponding to a discharge that was larger than the incipient rainwater received, due to a part of the irrigation water being drained.

### 3.5.2. Drainage water quality and elements losses

Percolated solutions were characterized by brownish colours, and in the first year had an opalescent character, showing the presence of particles in suspension. They had high electrical conductivity values, which sharply decreased with time (Table 4). pH remained basic across treatments and years with only slight variations. Dissolved organic C was leached with irregular patterns with time but concentrations and
<caption>**Table 1**</caption>

Physical and chemical characteristics evolution of Technosols over the three years. BD = Bulk Density; EC = Electrical conductivity; CEC = Exchangeable Cation Capacity. Similar letters correspond to the absence of statistical differences among treatments, modalities and parental materials (corresponding to the Mix) per parameter. *: correspond to the lower layer sample of OP at three years for which two replicates were lost. Only one analysis was achieved for this sample.

<table><thead><tr><th>Height</th><th>Treatment</th><th>Time</th><th>BD (g.cm<sup>-3</sup>)</th><th>Organic carbon (g.kg<sup>-1</sup>)</th><th>Corga/Ntot</th><th>CEC metson (cmol<sup>-1</sup>.kg<sup>-1</sup>)</th><th>EC (mS.cm<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="7">Upper layer</td><td>Mix</td><td>T0</td><td>0.37 ± 0.02 (d)</td><td>306.7 ± 5.8 (d)</td><td>12.1 ± 0.4 (gh)</td><td>33.5 ± 5.6 (e)</td><td>1.5 ± 0.03 (a)</td></tr><tr><td>MP</td><td>1 year</td><td>0.36 ± 0.05 (d)</td><td>263.7 ± 1.5 (ef)</td><td>9.96 ± 0.2 (h)</td><td>56 ± 7.3 (bc)</td><td>0.3 ± 0.05 (def)</td></tr><tr><td></td><td>3 years</td><td>0.32 ± 0.04 (de)</td><td>264.4 ± 18 (ef)</td><td>9.8 ± 0.4 (h)</td><td>61.7 ± 3.4 (ab)</td><td>0.3 ± 0.03 (fgh)</td></tr><tr><td>M</td><td>1 year</td><td>0.33 ± 0.01 (de)</td><td>241.7 ± 9.3 (g)</td><td>9.4 ± 0.2 (h)</td><td>49.5 ± 6.1 (d)</td><td>0.3 ± 0.03 (defg)</td></tr><tr><td></td><td>3 years</td><td>0.35 ± 0.02 (de)</td><td>241.5 ± 7.5 (g)</td><td>9.3 ± 0.2 (h)</td><td>62.1 ± 2.5 (ab)</td><td>0.3 ± 0.03 (efgh)</td></tr><tr><td>OP</td><td>1 year</td><td>0.32 ± 0.01 (de)</td><td>268 ± 7 (e)</td><td>9.98 ± 0.3 (h)</td><td>53 ± 7.2 (cd)</td><td>0.3 ± 0.07 (de)</td></tr><tr><td></td><td>3 years</td><td>0.3 ± 0.02 (e)</td><td>269.6 ± 4.8 (e)</td><td>10.3 ± 0.5 (h)</td><td>64.5 ± 2.7 (a)</td><td>0.2 ± 0.03 (h)</td></tr><tr><td rowspan="3">Lower layer</td><td>Shredded tiles and bricks</td><td>T0</td><td>0.32 ± 0.02 (de)</td><td>249.7 ± 8 (fg)</td><td>9.7 ± 0.2 (h)</td><td>54.5 ± 7.2 (cd)</td><td>0.3 ± 0.03 (efgh)</td></tr><tr><td>MP</td><td>1 year</td><td>0.34 ± 0.02 (de)</td><td>248.9 ± 4.7 (fg)</td><td>9.5 ± 0.1 (h)</td><td>61.2 ± 1.4 (ab)</td><td>0.3 ± 0.01 (gh)</td></tr><tr><td></td><td>3 years</td><td>1.06 ± 0.03 (c)</td><td>4.6 ± 0.9 (i)</td><td>105.1 ± 10.9 (a)</td><td>1.96 ± 0.9 (g)</td><td>0.06 ± 0.01 (i)</td></tr><tr><td rowspan="4">Intermediary layer</td><td>MP</td><td>T0</td><td>1.13 ± 0.02 (b)</td><td>7.3 ± 0.5 (i)</td><td>21.3 ± 1.4 (fg)</td><td>1.4 ± 0.2 (g)</td><td>0.09 ± 0.03 (i)</td></tr><tr><td></td><td>3 years</td><td>1.09 ± 0.01 (bc)</td><td>5.2 ± 0.2 (i)</td><td>35.3 ± 5 (de)</td><td>1.4 ± 0.04 (g)</td><td>0.07 ± 0 (i)</td></tr><tr><td>M</td><td>1 year</td><td>1.15 ± 0.05 (b)</td><td>6.2 ± 0.7 (i)</td><td>21.2 ± 1.9 (fg)</td><td>1 ± 0.2 (g)</td><td>0.07 ± 0.01 (i)</td></tr><tr><td></td><td>3 years</td><td>1.14 ± 0.09 (b)</td><td>6.4 ± 1.5 (i)</td><td>30.9 ± 11.7 (ef)</td><td>1.6 ± 0.4 (g)</td><td>0.07 ± 0 (i)</td></tr><tr><td rowspan="3">Lower layer</td><td>Shredded wood</td><td>T0</td><td>0.17 ± 0.03 (fg)</td><td>48.4 ± 16.1 (h)</td><td>10.3 ± 0.6 (h)</td><td>13.5 ± 7.4 (f)</td><td>0.1 ± 0.02 (i)</td></tr><tr><td>OP</td><td>1 year</td><td>0.14 ± 0.02 (fg)</td><td>48.9 ± 8.3 (h)</td><td>9.8 ± 0.2 (h)</td><td>11.9 ± 1.6 (f)</td><td>0.1 ± 0 (i)</td></tr><tr><td></td><td>3 years</td><td>0.18 ± 0.03 (fg)</td><td>470 ± 6 (a)</td><td>94.5 ± 11.7 (b)</td><td>17.4 ± 1.7 (f)</td><td>0.4 ± 0.07 (c)</td></tr><tr><td rowspan="6">Drain</td><td>Shredded tiles and bricks</td><td>T0</td><td>0.13 ± 0.03 (b)</td><td>433 ± 18.4 (b)</td><td>36.8 ± 5.6 (cde)</td><td>37.2 ± 2.2 (e)</td><td>0.5 ± 0.16 (b)</td></tr><tr><td>MP</td><td>1 year</td><td>0.19 ± 0.02 (f)</td><td>355 (c)*</td><td>19.6 (fgh)*</td><td>66.6 (a)*</td><td>0.2 (h)*</td></tr><tr><td></td><td>3 years</td><td>1.06 ± 0.03 (c)</td><td>438 ± 8.5 (b)</td><td>40 ± 6.2 (cde)</td><td>38.2 ± 2.3 (e)</td><td>0.4 ± 0.03 (cd)</td></tr><tr><td>OP</td><td>1 year</td><td>0.106 ± 0.03 (c)</td><td>360.7 ± 30.2 (c)</td><td>19.2 ± 7.2 (gh)</td><td>58.7 ± 5.4 (abc)</td><td>0.3 ± 0.03 (gh)</td></tr><tr><td>OP</td><td>3 years</td><td>0.106 ± 0.03 (c)</td><td>4.8 ± 0.5 (i)</td><td>43.7 ± 7.7 (cd)</td><td></td><td>0.06 ± 0 (i)</td></tr><tr><td>OP</td><td>3 years</td><td>0.106 ± 0.03 (c)</td><td>5.6 ± 0.4 (i)</td><td>45.3 ± 5.8 (c)</td><td></td><td>0.06 ± 0 (i)</td></tr><tr><td rowspan="2">Drain</td><td>Shredded tiles and bricks</td><td>T0</td><td>0.3 ± 0.02 (e)</td><td>0.02 ± 0.003 (f)</td><td>0.9 ± 0.03 (g)</td><td>9.9 ± 0.7 (h)</td><td>998 ± 169.7 (h)</td></tr><tr><td>MP</td><td>1 year</td><td>0.2 ± 0.02 (e)</td><td>0.02 ± 0.003 (f)</td><td>0.9 ± 0.02 (fg)</td><td>8.5 ± 0.1 (hi)</td><td>733.5 ± 29.4 (ij)</td></tr><tr><td rowspan="3">Intermediary layer</td><td>MP</td><td>1 year</td><td>0.3 ± 0.01 (e)</td><td>0.03 ± 0.003 (f)</td><td>0.9 ± 0.02 (g)</td><td>8.6 ± 0.6 (hi)</td><td>789.7 ± 46.6 (ij)</td></tr><tr><td></td><td>3 years</td><td>0.24 ± 0.12 (e)</td><td></td><td>0.9 ± 0.02 (fg)</td><td>8.6 ± 0.4 (hi)</td><td>738.6 ± 70.9 (ij)</td></tr><tr><td></td><td>3 years</td><td>4.7 ± 1.3 (d)</td><td></td><td>1.1 ± 0.1 (e)</td><td>14.5 ± 1.6 (g)</td><td>1110.1 ± 66.8 (h)</td></tr><tr><td rowspan="3">Lower layer</td><td>Shredded wood</td><td>T0</td><td>5 ± 0.9 (d)</td><td>0.4 ± 0.03 (e)</td><td>0.6 ± 0.1 (i)</td><td>17.1 ± 1.6 (f)</td><td>1265.7 ± 107.3 (g)</td></tr><tr><td>OP</td><td>1 year</td><td>11.9 ± 1.4 (c)</td><td>0.6 ± 0.05 (d)</td><td>0.7 ± 0.2 (h)</td><td>22.7 ± 1.6 (e)</td><td>665.3 ± 132.8 (j)</td></tr><tr><td></td><td>3 years</td><td>18.1 (b)*</td><td></td><td>0.7 (hi)*</td><td>40.8 (bc)*</td><td>1454.4 (efg)*</td></tr><tr><td rowspan="3">Drain</td><td>Shredded tiles and bricks</td><td>T0</td><td>11.1 ± 1.4 (c)</td><td>0.5 ± 0.07 (d)</td><td>0.7 ± 0.06 (hi)</td><td>22.5 ± 1.7 (e)</td><td>673 ± 69.1 (j)</td></tr><tr><td>MP</td><td>1 year</td><td>20 ± 4.9 (b)</td><td></td><td>0.7 ± 0.2 (hi)</td><td>39.4 ± 3.3 (bc)</td><td>1523.7 ± 18.9 (def)</td></tr><tr><td></td><td>3 years</td><td>0.1 ± 0.03 (e)</td><td></td><td>0.9 ± 0.03 (g)</td><td>7.5 ± 0.3 (i)</td><td>687.9 ± 45.9 (ij)</td></tr><tr><td rowspan="3">Drain</td><td>OP</td><td>1 year</td><td>0.1 ± 0.02 (e)</td><td></td><td>0.9 ± 0 (g)</td><td>8.2 ± 0.3 (hi)</td><td>822.3 ± 86.4 (i)</td></tr><tr><td>OP</td><td>3 years</td><td>0.1 ± 0.02 (e)</td><td></td><td>0.9 ± 0.02 (g)</td><td>8.1 ± 0.8 (hi)</td><td>729.6 ± 73.9 (ij)</td></tr><tr><td></td><td>3 years</td><td>0.1 ± 0.05 (e)</td><td></td><td>0.9 ± 0.03 (g)</td><td>7.3 ± 0.7 (i)</td><td>699.6 ± 19.2 (ij)</td></tr></tbody></table>

amounts decreased consistently over years (S3-Fig. 2). Leached C amounts were larger in plots with an organic lower layer (O and OP) compared to those with a mineral lower layer (M and MP), and higher without plants, except in 2017 (Table 4). Mineral N leachates were essentially nitrates (84 % of mineral N leached the first year and 99.3 % thereafter, results not shown) and nitrates concentrations of leachates were also very variable over time and often higher than 50 mg.l⁻¹ (S4-Fig. 3). Plots with organic lower layers leached more mineral N during the first year but less afterwards. Plots with plants consistently leached less mineral N (Table 4).

## 4. Discussion

Pedogenesis refers to the evolution of soils from the profile scale to

the landscape one that comprises significant changes in the physical, chemical and biological properties of soils. Five factors of soil formation are identified: climate, parent material, organisms, relief, time, to which human activities can be added (Jenny, 1941). As highlighted by other authors, the knowledge on pedogenesis of Technosol is still scarce (Leguédois et al., 2016). We aimed to study an original combination of parent materials for productive Technosols that had little been studied so far.

Technosols, defined by the presence of technogenic materials, or artefacts (IUSS Working Group WRB, 2015) often exhibit unusual configuration and composition compared to natural soils (Huot et al., 2014; Leguédois et al., 2016). While relief and climate were constant, the studied Technosols differed in their parent materials and biota. Published studies addressing the pedogenesis of Technosols concerned
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** PCA of Technosols according to their layers, treatment and years of experimentation. Parameters considered in the PCA: CaCO₃, CEC, Corg, Electrical conductivity (called ‘conducti’), pH, K, Fe, Mn and Ntot.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Biochemical composition of the organic parent materials. Same letters correspond to not significantly different values between materials for a given fraction. NDF= Neutral Detergent Solution.</figcaption></figure>

**Table 2**

Dry weight evolution (kg) based on direct measurements at the beginning and at the end of the experiment. Similar letters correspond to the absence of statistical differences among treatments, modalities and regarding time per height considered.

<table><thead><tr><th rowspan="2">Time</th><th rowspan="2">Treatment</th><th colspan="4">Dry Technosol weight (kg)</th><th rowspan="2">Draining layer</th></tr><tr><th>Total</th><th>Upper</th><th>Intermediate</th><th>Lower</th></tr></thead><tbody><tr><td rowspan="3">T0</td><td>M - MP</td><td>145.0 (a)</td><td>38.3 (c)</td><td></td><td>106.8 (a)</td><td>21.2 (ef)</td></tr><tr><td>M</td><td>128.5 ± 1 (b)</td><td>19.5 ± 2.1 (ef)</td><td>19.6 ± 7 (ef)</td><td>92.8 ± 4.6 (b)</td><td>21.1 ± 0.3 (ef)</td></tr><tr><td>MP</td><td>120.9 ± 10.9 (c)</td><td>24.2 ± 0.8 (d)</td><td>10.2 ± 7.6 (h)</td><td>89 ± 5.6 (b)</td><td>21.4 ± 0.1 (ef)</td></tr><tr><td rowspan="3">T3years</td><td>O - OP</td><td>54.6 (d)</td><td>38.3 (c)</td><td></td><td>16.3 (fg)</td><td>21.2 (ef)</td></tr><tr><td>O</td><td>31.7 ± 0.6 (e)</td><td>19.2 ± 2.2 (ef)</td><td></td><td>12.5 ± 1.8 (gh)</td><td>20.2 ± 0.5 (ef)</td></tr><tr><td>OP</td><td>33.6 ± 2.9 (e)</td><td>23.1 ± 2.5 (d)</td><td></td><td>10.4 ± 0.8 (gh)</td><td>21.7 ± 1 (ef)</td></tr></tbody></table>

soils formation on industrial landfils (e.g. Huot et al. 2014; Howard and Olszewska, 2011), and to a lesser extent Technosols made out of mineral materials (e.g. high temperature decontaminated soil, pozzolan) combined with organic amendments (Bouzouidja et al., 2018; Cannavo et al., 2014; Grosbellet et al., 2011; Rees et al., 2019; Séré et al., 2010; Vidal-Beaudet et al., 2016, 2012). We are not aware of studies on purely

organic Technosols like our O and OP treatments, while these are installed on rooftops by some farmers and gardeners. Classical pedogenesis processes comprise additions (of vegetation litter), transformations (weathering of minerals, biodegradation and mineralisation of organic matter, neoformations of minerals), transfers through the profile of colloids and solutes, losses (leaching of solutes, mineralisation
<figure><img src="image_10.png" /><figcaption>Fig. 9. SOC stocks evolution per plot in the different Technosols treatment. Different letters correspond to statistically different values across dates, treatments and modalities.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 10. Total soil nitrogen stocks evolution per plot in the different Technosols treatment. Different letters correspond to statistically different values across dates, treatments and modalities.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 11.** Total soil potassium stocks evolution in the different Technosol treatments. Different letters correspond to statistically different values across dates, treatments and modalities.</figcaption></figure>

of organic compounds), and reorganisations such as soil structure formation (e.g. Bockheim and Gennadiyev, 2000). Here, we observed early changes of Technosol characteristics and a differentiation of its layers into horizons that reflected different pedogenetic processes, i.e. biodegradation, structural reorganization, leaching, lixiviation and particles transfer.

## 4.1. Biodegradation

We had hypothesized that rapid biodegradation and mineralization would alter the quantity and properties of organic components, leading to transfers within the constructed Technosol. Indeed, laboratory incubations of the parent materials (S5-Fig. 4) showed that they were easily mineralizable. During our three years experiment, the organic C stocks decreased, between 40 and 52 % of the initial SOC stocks, with a consequent volume loss in all organic layers (Fig. 9) that can be explained by a rapid biodegradation and subsequent mineralization of the Mix and shredded wood. The large amount of C and N lost by lixiviation likely originated from the organic soluble fraction of the parent materials but also from their biodegradation products. As commonly observed during biodegradation, the C/N ratio of organic layers decreased over time (Table 1). The CEC of the organic layers increased with time (Table 1), which can be explained by the fragmentation and biodegradation of coarse plant residues into smaller and more reactive organic particles.

Similar dynamics were observed by other authors: studying constructed soil made from papermill sludge, thermally-treated industrial soil and green-waste compost, Rees et al. (2019) reported a decrease of 43 % of the SOC stock in the upper horizon within three years. Vidal-Beaudet et al. (2012), reported a 27 to a 48 % loss of initial C content in 60 months for a sandy loam soil mixed with 20 to 40 % of either a co-compost of sewage sludge and wood chips or a green waste compost. In our experiment, the observation of a lower SOC stock loss in the planted plots, OP and MP can be explained both by less leaching in these plots and by the input of root biomass to the Technosols organic matter.

The SOC stocks of the studied Technosols were 240–380 t C·ha⁻¹ initially and 140 to 190 tC·ha⁻¹ after three years. These values are much higher than those of Paris urban soils, i.e. 99 tC·ha⁻¹ in average (Cambou et al., 2018) and even those of European grasslands, i.e. 96 tC·ha⁻¹ (Smith et al., 2005). These stocks were also larger than those of two constructed Technosols studied by Rees et al. (2019), with a lower layer of decontaminated industrial soil and an upper organic layer of a mixture of compost and paper mill waste, i.e. 120–200 tC·ha⁻¹ for the first 30 cm. They are also larger than the mean of Technosols reported by Allory et al. (2022) in their meta-analysis with 953 observations on Technosols the majority being Spolic Technosol of diverse origins i.e. mean of 73.2 tC·ha⁻¹. The very large SOC stocks we measured are explained by the composition of the garbic isolatic Technosols (O/OP) and isolatic Technosols close to spolic isolatic Technosol (M/MP) studied here (IUSS Working Group WRB, 2015). While this study tracked the first three years of Technosol development, the trajectory of their evolution overs decadal timescale is currently unknown. Rees et al. (2019) and Vidal-Beaudet et al. (2012) evidenced two phases in the Technosols SOC stock changes: after a sharp decrease during the first years, the SOC stock stabilized and even increased in the lower horizons.

## 4.2. Structural re-organisation

The sharp volume loss of the Technosols, mainly during the first six months could be explained by two phenomena. During the first weeks, a physical re-arrangement of the coarse and finer elements likely took place and was accentuated by the first rain events as it has been described for horticulture materials (Caron et al., 2015). The continued volume loss after six months across treatments at a lower rate could be ascribed to biodegradation. Indeed, and contrary to other authors, we did not observe any compaction nor loss of porosity across treatments (Table 1 & S2-Table 1). In their study, Rees et al. (2019) quantified both a volume loss and a bulk density increase of 30 to 50 % for the upper horizon in five years. Similar observation was made by Cannavo et al. (2014) for a Technosol made of green-waste compost or co-compost from sewage sludge and wood chips mixed with sandy loam soil (40 % v/v). In these Technosols the mineralization of the parent material on
<table><caption>Table 3
Water input to the experimental plots, water overflow and retention. Different letters correspond to statistically different values across dates, treatments and modalities.</caption><thead><tr><th rowspan="2">Year</th><th rowspan="2">Rainwater (mm)</th><th colspan="3">Irrigation (mm)</th><th colspan="3">Overflow of water (l.m<sup>-2</sup>)</th><th colspan="3">Number of discharge events</th></tr><tr><th>M</th><th>MP</th><th>O</th><th>OP</th><th>M</th><th>MP</th><th>O</th><th>MP</th><th>O</th><th>OP</th></tr></thead><tbody><tr><td>2015</td><td>90.4</td><td>-</td><td>-</td><td>-</td><td>-</td><td>2.9 ± 1.3 (f)</td><td>2.1 ± 2.5 (f)</td><td>6.7 ± 5.4 (f)</td><td>3.4 ± 2.7 (f)</td><td>359.6 ± 45.4 (e)</td></tr><tr><td>2016</td><td>810.3</td><td>762</td><td>800</td><td>745</td><td>776</td><td>760.2 ± 58 (d)</td><td>360.2 ± 21.1 (e)</td><td>702.4 ± 31.7 (d)</td><td>772.9 ± 33.6 (d)</td><td>444.6 ± 19.5 (e)</td></tr><tr><td>2017</td><td>881.3</td><td>1920</td><td>1859</td><td>1853</td><td>1960</td><td>1458.1 ± 74.4 (a)</td><td>769.2 ± 69.3 (d)</td><td>1473.3 ± 105.6 (a)</td><td>437.3 ± 19.1 (e)</td><td>1106.6 ± 84.3 (b)</td></tr><tr><td>2018</td><td>731.8</td><td>1308</td><td>1320</td><td>1287</td><td>1355</td><td>1106.6 ± 84.3 (b)</td><td>437.3 ± 19.1 (e)</td><td>925.4 ± 243.8 (c)</td><td></td><td></td></tr><tr><th rowspan="2">Year</th><th colspan="3">Water retention (rain)</th><th colspan="3">Total water retention (rain + irrigation)</th><th colspan="3">Number of discharge events</th></tr><tr><th>M</th><th>MP</th><th>O</th><th>OP</th><th>M</th><th>MP</th><th>O</th><th>M</th><th>MP</th><th>O</th><th>OP</th></tr><tr><td>2015</td><td>97% ± 1% (a)</td><td>98% ± 3% (a)</td><td>93% ± 6% (a)</td><td>96% ± 3% (a)</td><td>97% ± 1% (a)</td><td>98% ± 3% (a)</td><td>93% ± 6% (ab)</td><td>97% ± 3% (a)</td><td>2.3 ± 0.6 (g)</td><td>3.3 ± 0.6 (g)</td><td>2.7 ± 0.6 (g)</td></tr><tr><td>2016</td><td>6% ± 7% (c)</td><td>56% ± 3% (b)</td><td>13% ± 4% (c)</td><td>56% ± 6% (b)</td><td>52% ± 4% (e)</td><td>78% ± 1% (cd)</td><td>55% ± 2% (e)</td><td>85% ± 14% (bc)</td><td>59 ± 2 (a)</td><td>40 ± 4.4 (d)</td><td>57.3 ± 0.6 (a)</td></tr><tr><td>2017</td><td>-65% ± 8% (e)</td><td>13% ± 8% (c)</td><td>-67% ± 12% (e)</td><td>12% ± 4% (c)</td><td>48% ± 3% (e)</td><td>72% ± 3% (d)</td><td>46% ± 4% (e)</td><td>82% ± 16% (bcd)</td><td>54 ± 0 (b)</td><td>44 ± 2.6 (c)</td><td>53.3 ± 0.6 (b)</td></tr><tr><td>2018</td><td>-51% ± 12% (e)</td><td>40% ± 3% (b)</td><td>-26% ± 33% (d)</td><td>39% ± 3% (b)</td><td>46% ± 4% (e)</td><td>79% ± 1% (cd)</td><td>54% ± 12% (e)</td><td>86% ± 12% (abc)</td><td>34 ± 0 (e)</td><td>27.3 ± 1.5 (f)</td><td>33.7 ± 0.6 (e)</td></tr></tbody></table>

<table><caption>Table 4
Evolution of pH, electrical conductivity, mineral nitrogen (N-mineral) and total carbon (TC) of drainage water. Letters show significant statistical differences between treatments, modalities and year per parameter.</caption><thead><tr><th rowspan="2">Year</th><th colspan="3">pH</th><th colspan="3">Electrical conductivity (µS/cm)</th><th colspan="3">Total Carbon of drainage water (g per plot)</th></tr><tr><th>M</th><th>MP</th><th>O</th><th>OP</th><th>M</th><th>MP</th><th>O</th><th>MP</th><th>O</th><th>OP</th></tr></thead><tbody><tr><td>2015</td><td>8.2 ± 0.1 (h)</td><td>8.5 ± 0.2 (fg)</td><td>8.5 ± 0.1 (fg)</td><td>8.4 ± 0.2 (gh)</td><td>9000 ± 5565.8 (a)</td><td>8363.3 ± 3320 (a)</td><td>8901.7 ± 4903.1 (a)</td><td>9588.3 ± 2956.6 (a)</td><td>9588.3 ± 2956.6 (a)</td><td>9588.3 ± 2956.6 (a)</td></tr><tr><td>2016</td><td>8.6 ± 0.1 (def)</td><td>8.7 ± 0.1 (bcd)</td><td>8.8 ± 0.04 (abc)</td><td>8.8 ± 0.1 (abc)</td><td>3890.4 ± 1049.6 (b)</td><td>3351.4 ± 391.3 (b)</td><td>2974 ± 730.8 (b)</td><td>3766.4 ± 370.1 (b)</td><td>3766.4 ± 370.1 (b)</td><td>3766.4 ± 370.1 (b)</td></tr><tr><td>2017</td><td>8.9 ± 0.2 (abc)</td><td>8.8 ± 0.1 (abcd)</td><td>8.9 ± 0.04 (a)</td><td>8.8 ± 0.03 (abc)</td><td>666.6 ± 41.2 (b)</td><td>702.7 ± 125.8 (b)</td><td>646.6 ± 49.4 (b)</td><td>765.9 ± 101 (b)</td><td>765.9 ± 101 (b)</td><td>765.9 ± 101 (b)</td></tr><tr><td>2018</td><td>8.5 ± 0.02 (efg)</td><td>8.7 ± 0.1 (bcd)</td><td>8.7 ± 0.1 (cde)</td><td>8.9 ± 0.1 (ab)</td><td>551.4 ± 83.3 (b)</td><td>412.2 ± 7.7 (b)</td><td>461.7 ± 78.4 (b)</td><td>437.6 ± 26.9 (b)</td><td>437.6 ± 26.9 (b)</td><td>437.6 ± 26.9 (b)</td></tr><tr><th rowspan="2">Year</th><th colspan="3">Mineral nitrogen of drainage water (g per plot)</th><th colspan="3">Total Carbon of drainage water (g per plot)</th><th colspan="3">Mineral nitrogen of drainage water (g per plot)</th></tr><tr><th>M</th><th>MP</th><th>O</th><th>OP</th><th>M</th><th>MP</th><th>O</th><th>MP</th><th>O</th><th>OP</th></tr><tr><td>2015</td><td>0.2 ± 0.4 (f)</td><td>0.2 ± 0.2 (f)</td><td>0.7 ± 0.6 (f)</td><td>0.5 ± 0.5 (f)</td><td>5.4 ± 4.6 (g)</td><td>5.2 ± 7.4 (g)</td><td>12.4 ± 14 (g)</td><td>7 ± 5.6 (g)</td><td>393.6 ± 37.8 (b)</td><td>393.6 ± 37.8 (b)</td></tr><tr><td>2016</td><td>80.7 ± 5.4 (b)</td><td>48.4 ± 18.4 (bcd)</td><td>42.8 ± 19 (bcd)</td><td>25 ± 23.5 (def)</td><td>413.7 ± 32.1 (b)</td><td>284.7 ± 3.4 (c)</td><td>474 ± 53.5 (a)</td><td>122.9 ± 21.4 (de)</td><td>153.6 ± 33.9 (d)</td><td>153.6 ± 33.9 (d)</td></tr><tr><td>2017</td><td>76.2 ± 44.2 (b)</td><td>24.9 ± 2.5 (def)</td><td>49.2 ± 34.8 (bcd)</td><td>7.7 ± 1.7 (ef)</td><td>102.6 ± 12.6 (e)</td><td>118.3 ± 29 (de)</td><td></td><td>53.7 ± 17.1 (f)</td><td>35 ± 0.7 (fg)</td><td></td></tr><tr><td>2018</td><td>118.8 ± 41.3 (a)</td><td>34.3 ± 6.1 (cdef)</td><td>68 ± 46.5 (bc)</td><td>30.2 ± 1.8 (cdef)</td><td>53.7 ± 5 (f)</td><td>29.8 ± 1.4 (fg)</td><td></td><td></td><td></td><td></td></tr></tbody></table>
the one hand and the presence of coarse particles on the other hand likely contributed to an overall volume loss, while preserving porosity.

### 4.3. Leaching / particle transfer

The overall SOC increase in mineral lower layers and that of N and P stock in all lower layers implies the transfer of organic particles and solutes down the Technosol profiles. The intense irrigation may have promoted this transfer and accelerated changes of initial layers observed resulting horizons. Bouzouidja et al. (2016) highlighted a similar process in a four-year study of an extensive experimental green roof substrate ranging from 7.5 to 9.5 cm in thickness. Burrowing activities by earth-worms may also have contributed to such transfers but were not monitored in the present study.

Adopting a common garden practice also suggested for Technosols, our experiment utilized so-called lasagna beds features (Lanza, 1999) designed to mimic natural soils with contrasting horizons. Over time, we observed the formation of a new intermediate horizon (Fig. 5), likely resulting from the downward transfer of particles and solutes from the upper layer. This rapid pedogenesis in young Technosols, also observed by Bouzouidja et al. (2018, 2016) and Séré et al. (2008), demonstrates the influence of time and parent material composition on the vertical and structural organization of these constructed Technosols.

### 4.4. Lixiviation

Over the three-year monitoring period, all Technosols acted as sources of nitrogen and dissolved organic carbon, which were released in the drainage waters. This efflux likely resulted from the biodegradation and mineralization of the parent materials, exacerbated by the high level of irrigation. The rooftop farm studied by Harada et al. (2018) also was a source of nitrogen in drainage water with an annual drainage output of 104 kg N.ha⁻¹.y⁻¹, which they largely explained by the mineral fertilizer inputs. In contrast, our study, which involved no fertilizer inputs, exhibited substantially higher nitrogen losses, i.e. from 416 and 711 kg N.ha⁻¹.y⁻¹ for OP and MP respectively and from 1058 to 1823 kg N.ha⁻¹.y⁻¹ for O and M treatments in average over the three years. These large losses might be explained by the larger nutrient content of our parent materials, particularly in the case of the biowaste compost. Regarding early pedogenesis, one consequence of the lixiviation of organic compounds was the increase over time in the SOC content of lower horizons (Table 1) and the creation of the intermediate horizon (Fig. 5).

While the temporal patterns of N losses did not exhibit any trend (S4-Fig. 3) the loss of dissolved organic carbon exhibited a clearer temporal pattern with an important loss during the first year and slower decrease during the second and third growing season. This resulted in a consequent mass loss of 5.6–9.4 tC.ha⁻¹ the first year and of 1 to 1.2 tC.ha⁻¹ in average during the second and third year, which contributed on to the overall loss of OC of Technosols over time. Other studies on green roofs reported variable but lower losses of dissolved organic carbon (in concentration and mass). As an illustration, studying a semi-intensive green roof, Zhang et al. (2015) reported concentrations of 60.3 ± 50.2 mg C. l⁻¹ of DOC whereas on extensive and intensive green roof, Berndtsson et al. (2009) reported 2 to more than 40 mg C.l⁻¹ of DOC. In our study, DOC varied for MP from 1367 mg C.l⁻¹ the first year to 324 and 111 mg C.l⁻¹ the second and third year respectively, with similar trends for other treatment. These observations highlight the necessity for further research and development aimed at optimizing technosol composition and conceiving appropriate technical installations on rooftops to minimize the degradation of drainage water quality.

### 4.5. Biology driven pedogenesis processes

We hypothesized that plant presence would alter particle arrangement and reduce leaching via water uptake. While we found no

significant effect of plants on bulk density (Table 1) and, consequently, porosity (S2-Table1), planted Technosols exhibited reduced leaching (Table 4), likely due to enhanced water and nutrient uptake.

We did not study the full spectrum of potential early pedogenesis processes. Soil macro- and mesofauna are known drivers of pedogenesis through the promotion of biodegradation, aggregation (Watteau et al., 2010) and bioturbation (Jangorzo et al., 2014). We did not study these factors despite the evidence that Technosols host a diversity of soil organisms (Deeb et al., 2016; Joimel et al., 2018) and while we observed earthworms and milepedes, acari and collembola in all treatments. We posit that these organisms contributed in particular to the biodegradation of the parent materials. The influence of soil fauna deserves further research to understand how soil organisms affect Technosols development and how fauna colonization could foster specific pedogenesis processes.

Similarly, we did not investigate other pedogenesis processes which had been shown to occur rapidly in newly constructed Technosols, such as weathering (Séré et al., 2010) because our parent materials were predominantly organic.

### 4.6. Consequences on ecosystem services provided by the Technosols

Many ecosystem services are nowadays expected from the installation of green roofs within cities. The Technosols studied here have demonstrated their capacity to deliver several ecosystem services such as food production, regulation of water runoff quantity, recycling of organic wastes (Grard et al., 2018) and biodiversity provision (Joimel et al., 2018). The sustained high yields observed in this experimental setup (results not shown) and in comparable layered Technosols (Grard et al., 2020, 2018, 2015) show that the characteristic of the parent materials and their subsequent first steps of pedogenesis ensure a favorable physical environment for roots development. Specifically, low bulk density, high porosity and substantial water retention capacities were maintained over three years despite the observed shrinkage and mass loss of the Technosol. The ongoing biodegradation and mineralization of the mostly organic parent materials of these Technosols continuously provided nutrients to the plants. Regarding water runoff mitigation, While the current experimental set-up, involving over-irrigation, was not well adapted to assess water runoff mitigation, measurements after 3 years showed that 46 (M) to 54 (% (O) of the incipient water was retained by the Technosols in the absence of plants and 79 % (MP) to 86 % (OP) in the presence of plants. It showed a good maintenance of this service, enhanced by plant presence.

However, the current study revealed significant trade-offs regarding drainage / runoff water quality. The biodegradation and mineralization of the organic parent materials, in both upper and lower layers resulted in important losses of dissolved organic C and of mineral nitrogen, primarily as nitrates. While Harada et al. (2018) estimated that the nitrogen loads originating from even largely implemented rooftop farms would represent only a small part of the citywide nitrogen loss, it is a priority to address the identified tradeoffs. Adressing the tradeoffs inherent to the pedogenesis of these Technosols: minimizing biodegradation mineralization of organic materials to protect drainage water quality, while optimizing these processes to ensure an adequate mineral nutrition of plants is a key challenge for future research and innovation. This observation highlight the need for further investigation addressing this trade-off by adjusting Technosols composition (e.g., proportion and nature of organic wastes) and layout, as well as developping technical solutions such as the recycling of percolating waters for irrigation.

## 5. Conclusion

Technosols made of urban waste materials demonstrate a significant potential for delivering multiple ecosystem services, including the production of substantial edible biomass. Considering the origin of the currently used material, particularly their carbon footprint, re-using
urban waste as parent material for constructed soil in cities is a neces-
sity. The present study demonstrated the rapid changes of Technosols
made of organic and mineral urban wastes, evident even within the first
year. This early steps of pedogenesis comprised (i) biodegradation and
mineralization of organic materials, (ii) rearrangments and shrinkage
driven by biodegradation and water flow, from rain and irrigation, and
(iii) vertical transport of organic compounds and mineral nitrogen via
particle transfers and lixiviation. Irrigation, because not optimally
managed may have accelerated these processes. These early pedogenic
processes were similar to that of natural soils. Consequently, the Tech-
nosol properties after three years were different from initial ones, while
still being favourable to plant growth and allowing the delivery of
multiple ecosystem services. Indeed, the evolved materials remained
porous and biodegradable ensuring adequate nutrient provision for
edible plants. Our results, however, emphasize the delicate balance to be
managed between biodegradation of organic matter and its stabiliza-
tion. The significant losses of dissolved carbon and nitrogen observed
the necessity to further investigate strategies to minimize these losses
while maintaining the crucial equilibrium between organic matter
biodegradation and stabilization, showing the need for pedological en-
gineering of Technosols.

The relatively short duration of the study does not allow to conclude
on the long-term development of the Technosol profiles and related
important ecosystem services such as carbon storage, water retention
and clean water provision. Increasing our knowledge about the pluri-
annual changes of Technosol made from urban wastes is crucial for
designing sustainable and functional green infrastructure in cities.

Funding sources

This work was supported by AgroParisTech, the Chaire Eco-
conception and the DIM ASTREA (funding scheme) from the Ile de
France Region.

CRediT authorship contribution statement

**B.J.P. Grard:** Writing – original draft, Visualization, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **A. Barrier-Guillot:** Investigation, Formal analysis. **C. Chenu:** Writing – review & editing, Validation, Supervision, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Acknowledgements

The authors would like to acknowledge the contribution of Eugène
Pipraud, Anaïs Lostier, Antoine Juvin, Christine Aubry, and Charlène
Zahir in setting up and monitoring the rooftop AgroParisTech Urban
microfarm. The authors also would like to thank BioYvelinesServices
and La Boite à Champignon for providing freely their growing substrate.

**Appendix A. Supplementary data**

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.geoderma.2025.117343.

Data availability

Data will be made available on request.

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