There is no information on how organisms influence hydrostructural
properties of constructed Technosols and how such influence will be affected
by the parent-material composition factor. In a laboratory experiment,
parent materials, which were excavated deep horizons of soils and green
waste compost (GWC), were mixed at six levels of GWC (from 0 to 50 %).
Each mixture was set up in the presence/absence of plants and/or earthworms,
in a full factorial design (
Pedogenesis results from the dynamic interaction between climate, parent rock, and organisms. The most important factor(s) has been debated for a long time (Wilkinson et al., 2009) and studied independently (Jenny, 1941), but their interactions remain little understood (Paton, 1978; Amundson et al., 2007). Understanding of the influence of bioturbation (physical displacement by organisms) is not straightforward on soil formation (Amundson et al., 2007; Wilkinson et al., 2009). Some authors consider biotic mixing agents as a secondary cause of soil formation (Carson and Kirkby, 1972), while others argue that bioturbation plays a major role in forming soil (Paton 1978; Wilkinson and Humphreys, 2005).
Soils developed on non-traditional substrates and largely influenced by human activity are now referenced as Technosols in the World Reference Base for Soil Resources. When technogenic materials or artifacts are assembled deliberately to create soils, they are referred to as constructed Technosols (IUSS Working Group WRB, 2015). Many urban planners and green space enterprises are interested in constructed Technosols because these materials could be used as an alternative to topsoil material uptake from the countryside and the damage implied on the collecting site which need 10 000 years at least for reconstruction. Also, transportation costs and downsides could be avoided. Moreover, Technosols offer an opportunity to recycle urban waste, such as excavated deep horizons/backfills from enterprises of the building sector, sewage sludge from waste water plants, or green waste from greens pace enterprises or local authorities. In this regard, Technosols offer another life to these materials, which accumulation is urgent to cope with, due to health and environmental problems (Nemerow, 2009; Marshall and Farahbakhsh, 2013), while they could be used to improve urban ecosystem services (Morel et al., 2014) and form a closed loop that reduces the impact of cities on the environment. Constructed Technosols are different from other soils because they are designed assemblages of technogenic materials. Thus, the evolution of Technosols is different compared to the pedogenesis of natural soils (soils that generally show genetic relationships between the horizons they are composed of, and in which transitions among soils' types are visible. Humanity does not influence their formation process; Lehmann and Stahr, 2007). However, Technosols exhibit some formation processes similar to those observed in natural soil pedogenesis, such as decarbonization and aggregation (Séré et al., 2010; Jangorzo et al., 2014).
The pedogenesis of a constructed Technosol is particularly interesting. It begins with the mixing of parent materials in a proportion chosen by the experimenter, whereas the initial state of natural soils is never under the control of researchers.
Parent materials strongly influence the type of soil formed (Charman and Murphy, 2000). Organo-mineral composition of constructed Technosols determines several soil chemical and physical properties (pH, cationic exchange capacity, texture, etc.) and affects their quality (Molineux et al., 2009; Olszewski et al., 2010; Arocena et al., 2010; Rokia et al., 2014). The Influence of organic matter and texture on compactability of Technosols (Paradelo and Barral, 2013) and the formation of the organo-mineral complex in newly formed soil (Monserie et al., 2009) have also been documented. However, hydrostructural properties have not yet been investigated. This is of particular importance since constructed Technosols are often influenced by compaction (Jangorzo et al., 2013). Moreover, they are expected to provide water regulation services and to supply vegetation requirements. Therefore, we were interested in determining influences of different functional groups of organisms on soil hydrostructural properties. We focused on two kinds of organisms with different impacts on soil physical structure. Earthworms make an important contribution to soil function by influencing chemical, biological, and physical soil processes (Lavelle and Spain, 2001; Edwards, 2004), with consequences for ecosystem services (Blouin et al., 2013). Their major physical contributions are due to their high consumption rates and burrowing activity that affect soil structure, aggregation, and aeration (Blanchart et al., 1997), which influence the hydric properties of soil (Schrader and Zhang, 1997; Shipitalo and Butt, 1999). These modifications of hydrostructural properties by earthworms have tremendous consequences for plant growth (Scheu, 2003; Eisenhauer et al., 2007; Van Groenigen et al., 2014). Plant roots and rhizosphere inhabitants (microorganismes) also have a significant influence on aggregates and their stability (Jastrow et al., 1998; Rillig et al., 2002), sometimes more significant than that of earthworms (Blanchart et al., 2004). Roots penetrate the soil and create macropores which guarantee the exchange of gases in the vadose zone (Beven and Germann, 1982). Roots also create weak zones that fragment the soil and form aggregates, whose formation is strengthened by wetting–drying cycles due to water uptake by the plant (Angers and Caron, 1998). In addition, plant root residues provide a food source for microorganisms and fauna, which contribute to soil structure formation and stabilization (Innes et al., 2004). In return, microorganism-mediated changes in soil structure affect plant growth, mostly by modifying the root's physical environment (Dorioz et al., 1993).
In this study, we were interested in the effect of two soil-forming factors,
i.e., parent materials and organisms, on hydrostructural parameters via
measurements of soil shrinkage curves (SSCs) which represents the concomitant
decrease in soil volume and water mass during drying (Haines,
1923). The influence of parent-material properties (especially clay content
and type) (Boivin et al., 2004), organic matter
(Boivin et al., 2009), and organisms
(Kohler-Milleret et al., 2013; Milleret et al., 2009) on shrinkage properties has already been
studied in natural soils. This study addresses the question of
material–organism interaction on the hydrostructural properties of a
constructed Technosols in a 5-month microcosm experiment with four
“organism” treatments (control, plants, earthworms, plants
The mineral material excavated from deep horizons of soil (EDH) used in this
study was provided by the ECT Company (Villeneuve sous Dammartin, France).
This material is typically what is found when foundations are dug in the
Île-de-France. It is mainly the result of the weathering of carbonated rock
fragments of the Parisian Basin (France) from the Eocene. For our study, we
collected 500 kg of EDH at eight locations from the base of ECT's landfill
site, in order to have a composite sample representative of what may be used
to construct Technosols around Paris. EDH is classified as carbonated sandy
soil (Nachtergaele, 2001). Our material was composed of 880 g kg
Mean
EDH and GWC were mixed using a concrete mixer to prepare six different
mixtures with specific volumetric percentages of GWC at 0, 10,
20, 30, 40, and 50 %. One liter of each mixture was placed in a
microcosm of 13
Plants were sown 24 h after watering the pots; and earthworms were
introduced 24 h after sowing. Each percentage of GWC was combined with
four treatments: a control without organisms (C), a treatment with two
individuals (0.5
Microcosms were kept 21 weeks in a climate chamber (S10H, Conviron, Canada)
under the following conditions: photoperiod of 12 h, luminosity of
500
Technosol samples were collected from the surface of each microcosm at the
end of the experiment using a 5 cm high, 5 cm diameter cylinder and were
placed on a wet porous plate for saturation with deionized water according
to the manual instructions of Eijkelkamp (referee) for 7 days by applying
a water potential of 0 kPa at the base of the sample. The shrinkage curve
was continuously measured according to Braudeau et al. (1999) by using the
RETRACTOMETER© apparatus. Water-saturated
Technosol samples were placed in an oven at a constant temperature
(30
Configurations of water partitioning in macropores and micropores
related to the shrinkage phases of a standard shrinkage curve (water content
At the end of the measurement, samples were dried in an oven at
105
The three transition points separating the four pseudo linear shrinkage
phases (Fig. 1) are points L, M, and N, which are at the intersection of the
tangent straight lines of the linear phases. According to this model of SSC
(Braudeau et al., 1999, 2004), the value of the
water content at each point is equal to the value of max (
All hydrostructural parameters were transformed with Eqs. (4) and (5) and
thus became the moisture ratio at macropore saturation (
Considering these hydrostructural parameters (Braudeau et
al., 2004), the ratio of the maximum available water for plants from
macropores (
Plants were cut at the soil surface 21 weeks after sowing. Fresh leaves were
weighed, dried in an oven at 50
Dry root biomass distribution among diameter classes was determined
according to the method of Blouin et al. (2007). It is based on the granulometry method used to assess soil
texture: roots are dried, cut transversely with a mixer, and placed on a
column of sieves with decreasing mesh size. During the shaking of the sieve
column, root fragments with a section diameter smaller than the mesh size
pass through this mesh and stop on the first sieve with a mesh size below
that of the root section diameter. Biomass distribution is assessed by
weighing the biomass recovered in each sieve. Five diameter classes were
chosen according to sieve mesh size: 0–100, 100–200, 200–400, 400–800, and
Two-way ANOVA showing the effects of the presence/absence of
earthworms (E) and the proportion of green waste compost (GWC) in the
mixtures on plant dry biomasses, shoot : root ratio, and root system structure
(thick root
The number in the table are the
We calculated means and standard errors of hydrostructural parameters for
all treatments by fitting the curves with the hydrostructural model (Table S2).
The hydrostructural parameter representing the slope of the interpedal
Belowground biomass ranged from 1.7 to 3.6 g and aboveground biomass from
2.9 to 4.4 g, which amounted to a total biomass of 4.6 to 8.1 g (Fig. 2).
Two-way ANOVA showed that both GWC percentage and the presence of
earthworms had a positive effect on dry belowground, aboveground, and total
biomasses (Table 2). GWC percentage had almost no influence from 0 to
30 % on total biomass but increases plant production at 40 and 50 %
(Fig. 2a–c). Earthworm presence had a positive effect on belowground
biomass only at 50 % GWC, whereas aboveground biomass was affected only in
the 0–30 % GWC range. As a result total biomass was always significantly
higher in the presence of earthworms, except at 40 % GWC. On average,
earthworms increased total plant biomass of 21 % (Fig. 2c). The best
treatment for plant growth was clearly the mixture of 50 % GWC with
earthworms, with a total dried plant biomass of 8.1 g, which was
significantly higher than all other mixtures, except for 40 % GWC with
earthworms. There was no interaction between the effects of GWC percentage
and earthworms on plant biomasses, which means that these two effects are
additive. All parameters describing biomass allocation inside the plant,
such as the root : shoot ratio and the thick (
All our Technosols exhibited the classical sigmoid shape of the shrinkage curve reported for most natural soils (Laurizen, 1948; Braudeau et al., 1999; Peng and Horn, 2005) (Figs. 3 and 4); thus, shrinkage phases (residual, basic, structural, and the saturating shrinkage phase) were easy to recognize. All the parameters deduced from SSC are given in Table S2.
High GWC percentage caused moisture ratio
Averaged shrinkage curves (
Averaged shrinkage curves (
RDA performed on eight hydrostructural parameters of the Table S2 showed
that the factors “GWC percentage” and “organisms” had an influence on
hydrostructural parameters. The total percentage of variance explained by
these factors was high: 72 % (
The LDA explained 76 % of hydrostructural properties' observed variance
(
Additional PCAs were performed to characterize the effect of organisms on
hydrostructural properties for each GWC percentage. The effect of plants
was not significant at 0, 10, and 20 % GWC (
Linear discriminant analysis of the influence of control,
earthworm, plant, and both earthworm and plant on hydrostructural
parameters. The first and the second axes explained 42 and 26 % of the
variance, respectively.
The complete ANOVA model with GWC percentage, earthworms, and plants had a
significant effect (
Moisture ratio at
The presence of earthworms influenced the effect of GWC percentage on
moisture ratio and total volumetric available water contents at macropore
and micropore. For example, in the absence of earthworms, GWC percentage had
a positive influence on moisture ratio at macropore for 0–40 % GWC, while
in the presence of earthworms, moisture ratio at macropore decreased at
percentages of 30–50 %. The presence of plants modified the influence of
GWC percentage on moisture ratios at micropore and macropore, and total
volumetric available water at macropore and micropore. For example, in the
absence of plants, the influence of GWC percentage on moisture ratio at
macropore was positive at percentages of 0–40 % and became negative at
50 %, whereas in the presence of plants, the influence of GWC was positive
regardless of its percentage (Fig. 4a). A similar influence was observed for
the interaction between plants and GWC percentage on macropore volumetric
available water (Fig. 6d). The interaction between earthworms and plants had
a significant effect only for moisture ratios in micropore and macropore but
not for total moisture ratio, suggesting an opposite effect on micropores and
macropores (Table 3). Indeed,
Three-way ANOVA testing the effect of GWC,
earthworms (E), and plants (P) on the maximum moisture ratio from macropores
(
The number in the table are the
Linear regressions between total plant biomass (g) and available volumetric
water content (cm
Shrinkage analysis was initially developed to describe hydrostructural properties of natural soils (Haines, 1923; Milleret et al., 2009), and it was used by Kohler-Milleret et al. (2013) and Milleret et al. (2009) to evaluate the influence of organisms in natural soils. However, the effect of organisms on hydrostructural properties of constructed Technosols has never been studied before. Our study shows that shrinkage curve analysis was relevant for describing Technosol structure and water-holding capacities. In our case, parent materials exhibited highly divergent behaviors: EDH showed a SSC with the typical sigmoid shape that reveals two levels of organization (presence of both micropores and macropores). However, the green waste compost shrinkage curve had a hyperbola shape (Deeb et al., 2016). Thus, the behavior of the mixtures was difficult to predict. Here, we showed two embedded levels of organization in the mixtures, with a sigmoid shape even at the highest GWC percentage (50 %, V/V). Because this organization is often, but not always, observed in natural soils, we conclude that after 5 months mixtures of mineral and organic materials behave as many natural soils from a hydrostructural viewpoint.
Shrinkage curve analysis indicated a positive correlation between the amount of GWC percentage and the quantity of macropores and micropores. This is likely due to organic matter present in the GWC: an increase in total void ratio was also observed in natural soil amended with organic matter (McCoy, 1998; Marinari et al., 2000; Tejada and Gonzalez, 2003) and recently in Technosols (Paradelo and Barral, 2013). The addition of GWC to EDH seems a promising strategy to obtain useful hydric properties that match plant needs for water and are similar to those observed in natural organic soils.
Linear regression between total dry plant biomass and available
water (cm
Earthworms were responsible for a significant increase in total moisture
ratio (Fig. 5c). This was the result of an increase in moisture ratio at
saturated micropore, not macropore (Fig. 5). Through this mechanism,
earthworms are likely to have a positive impact in climates with occasional
droughts. Earthworms might thus help plants to face a water deficit in
drying Technosols and effectively contribute to water regulation. This
result was surprising: earthworms are generally known to affect
macroporosity through their galleries. Our results differed from those
obtained with
The general influence of roots on soil structure was observed by
Monroe and Kladivko (1987), Angers and Caron (1998), and Kautz et al. (2013). This positive effect is
mainly due to plants' abilities to create macro-aggregates and macropores.
Similar results have been reported in other studies
(Reid and Goss, 1982; Caron et al., 1996).
Moreover, the positive influence of plants on moisture ratio at macropore
increased with the presence of earthworms. It was not due to the direct
influence of earthworms, which improved moisture ratio at saturated
micropore (
We also showed how plants and earthworms can help confront one of the main problems encountered by Technosols: compaction. Technosols often tend to compact with time (Jangorzo et al., 2013). Organisms such as plants or earthworms are responsible for maintaining a high volume of voids and moisture per solid-volume unit (void and moisture ratios, respectively). By introducing these organisms at the very beginning of Technosol creation, i.e., before compaction, managers could initiate a virtuous cycle in which organisms maintain loose soil structure. This favors the establishment of other organisms that maintain their own habitats, which in turn could benefit from plants and earthworms by preventing later compaction.
Because the influence of plants on hydrostructural properties was significant at 30–50 % GWC, one had to consider the initial composition of mixtures of materials to benefit from this organismal positive feedback.
This study allows comparing the influence of the proportion of parent
materials (0–50 % GWC) and the presence of organisms (presence/absence of
plants and earthworms) on pedogenesis. These situations are far from
covering all kinds of parent materials and organisms but are a first
attempt to compare the relative importance of soil-forming factors under
experimental conditions based on parent materials that never experienced the
biological activity of macro-organisms such as plants and earthworms. We
found that variations in Technosol hydrostructural properties were poorly
explained by parent materials alone (14 % of explained variance) and by
organisms alone (19 % of variance), whereas material–organism
interaction explained more than the sum of their individual influences
(39 %
In a nutshell, we found that compost and plants play a positive role in macroporosity and microporosity in Technosols, while earthworms affect only microporosity. GWC percentage positively affected macroporosity up to a percentage of 30 %, and plants were responsible for extending this positive influence at 40 and 50 % GWC. The simultaneous presence of earthworms and plants was responsible for a synergistic, positive influence on macroporosity. These observations highlighted the need to consider plants not only as an output indicating the level of fertility, but also as an actor in Technosol construction, like earthworms. Organisms that physically modify their environment by creating, destroying, or maintaining ecological niches have been called “ecosystem engineers” (Jones et al., 1994). These ecosystem engineers can help restore ecosystems (Byers et al., 2006) and create new ecosystems such as constructed Technosols by assisting managers, who could “subcontract” one aspect of management. Therefore, instead of increasing the amount of compost, which is usually expensive, managers could avoid the difficult-to-explain negative influence of high percentages of compost by favoring conservation, recolonization, or inoculation of ecosystem engineers such as plants and earthworms, especially in combination (Blouin et al., 2013).
This study was conducted in collaboration with the Departmental Council of the Seine-Saint-Denis department, France, and the company Enviro Conseil et Travaux. The authors wish to thank the University of Damas, Syria, for financial support via a PhD scholarship. We also thank Thierry Desjardins, Gaghik Hovhannissian, and Pascal Podwojewski for their scientific advice and Florence Dubs for her help with statistical analyses. Michael Corson was responsible for post-editing the English. Edited by: A. Don