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Soil protection in solar photovoltaic farms by revegetation with mycorrhizal native species.

Introduction

Installation of a solar photovoltaic plant represents a change in land use. Installation involves significant earthworks, with consequent modification of the original soil properties, acceleration of erosion and a decrease in infiltration capacity (Albaladejo et al. 1998), removal of natural vegetation and, consequently, the disappearance of natural populations of mycorrhizal fungi (Pera and Parlade 2005).

In addition to the change in land use, the presence of solar panels has a clear detrimental effect on the nature of the landscape because of the removal of vegetation cover. It is known that the removal of natural vegetation in Mediterranean areas leads to a decrease in the organic matter content in the soil and in the stability of the aggregates, as well as an increase in the bulk density of the soil (Albaladejo et al. 1998). Therefore, installation of a photovoltaic plant has a significant negative impact on vegetation and soil, and these effects can be reduced by revegetation of the affected area.

With the initial aim of protecting the environment, reducing air pollution and contributing to sustainable development, the installation of solar photovoltaic farms must fulfil a set of legislative rules for the implementation and execution of solar installations within a given territory. For example, under Spanish legislation, regeneration of the original vegetation is recommended or required. This task is achieved by revegetation of a percentage of the land occupied by the photovoltaic farm in order to minimise the impact of the farm on the soil. This revegetation process involves the planting of native species with a high survival percentage because the plants must survive the first summer after planting in order to ensure adequate cover, although this process is also affected by climate and soil factors (Perez-de-los-Reyes et al. 2013).

It is universally accepted that mycorrhizal symbiosis is essential for good nutrition of plants and soil quality (Smith and Read 2008; Azcon-Aguilar et al. 2009). In addition, mycorrhizal symbiosis improves the health of plants by increasing protection against environmental stress, either biotic (e.g. attack by pathogens) or abiotic (e.g. drought, salt, heavy metals, organic pollutants), and by developing soil structure through the formation of aggregates (Alguacil et al. 2004; Barea et al. 2005; Caravaca et al. 2005a). The results of several studies have shown that the diversity of mycorrhizal fungi in the soil can improve the diversity and productivity of plants, which, in turn, supports the stability and sustainability of the ecosystem (Vogelsang et al. 2006).

The removal of vegetation cover because of natural effects or human activities leads to the disappearance of natural populations of mycorrhiza (Barea et al. 2011). Reforestation and revegetation processes should be considered through the reintroduction or reinstatement of these fungi (Caravaca et al. 2002, 2005a; Alguacil et al. 2004; Pera and Parlade 2005). Mycorrhizal plants can be used to improve the survival percentages of certain species because, in addition to improving the survival and growth of the species, they increase the structural stability of degraded soils. There is extensive scientific literature on mycorrhizal species (Barea et al. 2011), although most of the experiments have been conducted under controlled nursery conditions in order to improve the quality of the plant (Barea et al. 2005; Pera and Parlade 2005). For this reason, the aim of the present study was to analyse the effects of mycorrhizal colonisation of native species used in the revegetation of solar photovoltaic farms in the field. The effect of this natural relationship between native plants and mycorrhizal fungi on both survival percentage and the apical growth of several species was evaluated.

Materials and methods

Location, climate and soil characteristics

The work was performed at the facilities of the Institute of Concentration Photovoltaic Systems (ISFOC) in the village of El Villar in Puertollano (Ciudad Real, Castilla-La Mancha, Spain).

The study area has annual average temperatures between 14 and 15[degrees]C and mean annual rainfall in the range 400-450 mm. According to the Papadakis classification, the climate is 'temperate Mediterranean' (MMA 2004).

Having characterised the soil morphology, horizons of the different samples were collected, air dried and sieved (2 mm) before analysis (texture, pH and organic matter) using the methods described by Porta et al. (1986) using the hydrometer method, a pH meter in a 1 : 2.5 soil: water solution and by potassium dichromatc oxidation and titration of remaining dichromatc with ammonium ferrous sulfate, respectively. The soil had a profile with four horizons: Ap (0-16cm), Btgl (16-41 cm), Btg2 (41-82 cm) and C (>82cm). The soil had a sandy clay loam texture in the Ap horizon (20.1% clay; 25.7% silt; 54.2% sand) passing through a clay loam in Btgl (35.3% clay; 21.2% silt; 43.5% sand) to clay in the Btg2 horizon (51.9% clay; 17.7% silt; 30.4% sand). The soil had a slightly basic pH in water (7.5 in Ap, 7.9 in Btgl and 7.2 in Btg2) and a low organic matter content in the Ap horizon (1.1%). The soil was classified as Typic Rhodoxeralf (Soil Survey Staff 2006) or Haplic Luvisol (FAO 2006).

Experimental design

Planting was carried out in April 2012 with 1-year-old plants from a registered nursery. The species used were Cistus monspeliensis L., Lavandula stoechas L., Thymus vulgaris L. and Rosmarinus officinalis L. One month before planting, half the plants were inoculated with 1 [cm.sup.3] of an ecocompatible mycorrhizal suspension of Rhizophagus irregularis DAOM24403 (known as Glomus intraradices) cultured in vitro (marketed under the name Glomygel, Mycovitro S.L., Spain) with 2000 propagules [cm.sup.-3] spores, hyphae and mycorrhizal root fragments. This suspension was a semisolid gel, applied by injection into the soil around each quick-pot plant.

The experiment was a randomised block design with two factors (inoculation or non-inoculation, and species) and two replication blocks. Each block consisted of 12 plants of each species (48 plants per block). The plants were planted with 0.5 m between plants and 1 m between lines. The survival percentage and apical growth were studied annually for each species. Apical growth values were calculated as the difference between the final and initial lengths of the longest shoot and considering only positive height changes, as suggested by Oliveira et al. (2011).

Following the methodology suggested by a specialist laboratory (Mycovitro), in April 2013 root samples were taken from five randomised plants of each species and inoculation treatment, mixed and sent to the laboratory, where a blind assessment of the mycorrhizal colonisation of each species and inoculation treatment was undertaken by the gridline intersect method after staining with Trypan blue.

The plantation was managed on a commercial basis. Support irrigation was performed after planting, and watering was performed in June. Manual weeding was performed in the spring of the following year (2013) in order to control weed growth.

Statistical analysis

Statistical analyses were performed using STATGRAPHICS Plus 5.1 (Statistical Graphics Corporation, Princeton, NJ, USA) and EXCEL (Microsoft, Redmond, WA, USA). Survival data for the different species were tested using Chi-squared analysis for the hypothesis of independence (P < 0.01). Apical growth data were analysed by two-way analysis of variance (ANOVA) using the multiple range test (Fisher's least significant difference) with significance set to P<0.05.

Results

Mycorrhiza percentage

Inoculated plants of the species C. monspeliensis L., L. stoechas L., T. vulgaris L. and R. officinalis L. had higher percentages of mycorrhiza in their roots than non-inoculated plants (80%, 30%, 50% and 90%, respectively, for inoculated plants; 5%, 5%, 3% and 50%, respectively, for non-inoculated plants; Fig. 1).

The results obtained by the specialist laboratory in the blind evaluation of mycorrhizal colonisation of the various species showed that some non-inoculated plants had mycorrhizae. This could be explained by natural colonisation, a common phenomenon in nature (Alguacil et al. 2004; Caravaca et al. 2005a; Pera and Parlade 2005; Barea et al. 2011).

Survival percentage

The survival percentages of the species used in the test are shown in Fig. 2. The species with the highest survival at the end of the study period (2014) was L. stoechas L. (95.8% for mycorrhizal plants and 100% for plants without mycorrhizae). The lowest survival was recorded for C. monspeliensis L. (12.5% in mycorrhizal plants and 8.3% in non-mycorrhizal plants).

The Chi-squared test for the hypothesis of independence showed that the inoculation factor did not affect the survival of plants in either 2013 or 2014. However, the species factor affected the survival of plants with a 99% confidence level in each year of the study under the test conditions ([chi square] = 83.38, d.f. = 5, P=0.000 in 2013; [chi square] = 75.97, d.f. = 5, P=0.000 in 2014).

Apical growth

Two-way ANOVA shows that inoculation factor and the interaction between inoculation and species did not affect the apical growth of plants.

The apical growth of plants of different species is represented in a box plot in Fig. 3, where it can be seen that the species with the highest apical growth in the period 2012-14 was R. officinalis L. (19 cm in mycorrhizal plants and 15.2 cm in plants without mycorrhizae). The species with the lowest growth was T. vulgaris L. (5.83 cm in mycorrhizal plants and 2.8 cm in plants without mycorrhizae).

Conversely, the results of the ANOVA indicated that there was a significant difference among apical growth at a 95% confidence level. However, the results of a multiple range test indicated just two homogeneous groups, with C. monspeliensis L., L. stoechas L. and R. officinalis L. in one group and T. vulgaris L. in the other.

Discussion

As far as the inoculation of plants and subsequent mycorrhisation are concerned, it has been shown in several studies that inoculation with mycorrhizal fungi does not necessarily lead to significant differences between inoculated and non-inoculated plant because 90% of cultivated plants establish this relationship naturally (Pera and Parlade 2005; Barea et al. 2011). In this sense, when field experiments with mycorrhizal inoculated and non-inoculated plants have been performed, a natural colonisation of a percentage of non-inoculated seedlings has been observed. This colonisation depends on both the time after planting and the ability of the shrubs to enrich the soil with mycorrhizal propagules (Caravaca et al. 2002, 2005a; Alguacil et al. 2004). These facts are consistent with those obtained in the present study, where some plants that were not initially inoculated became colonised in this way. All the species studied are mycotrophic and, during the year in which they were established in the field, the non-inoculated plants also showed symbiosis. However, it is worth highlighting that, as expected, in the present study the inoculated plant species had a higher percentage of mycorrhizal colonisation than the plants that were not inoculated.

The next parameter studied was survival percentage. A high survival percentage will depend on several factors, such as the shock suffered by the seedlings on being transferred from the nursery to field conditions. In Mediterranean areas, this shock is related to a shortage of available water (Castro et al. 2002; Vilagrosa et al. 2003) because the stress of transplantation is essentially a water stress (Burdett 1990). Not surprisingly, the newly planted specimens are susceptible to water stress. The restored contact between root and soil, and the initiation of water absorption are critical factors that determine short-term survival and, for this to occur, root growth has to reset (Burdett 1990). The presence of weeds also increases competition for water (Rey Benayas et al. 2005). As a result, in many cases the success of revegetation and reforestation processes is limited by a low survival percentage (Badano et al. 2009). This parameter is important in the case of cover crops that will not receive additional care and will be planted on bare soil without natural vegetation (i.e. under hostile environmental conditions; Maestre et al. 2001; Castro et al. 2002; Perez-delos-Reyes et al. 2013). In contrast, a high survival percentage ensures the fulfilment of legal requirements and reduces the cost of planting (Rey Benayas et al. 2005).

Pera and Parlade (2005) indicated that the use of inoculated plants improved the survival percentage after seedlings were transferred under field conditions. In contrast, Caravaca et al. (2005a) showed that 2 years after planting, plant survival was approximately 90% for all treatment (non-inoculated, inoculated with a mixture of fungi and inoculated with Glomus claroideum) and plant species (Olea europaea L., Pistacia lentiscus L., Retama sphaerocarpa L. and Rhamunus lycioides L.). In another experiment, Caravaca et al. (2005A) reported that there were no significant differences in plant survival between treatments (inoculated vs non-inoculated Cistus albidus L. and Quercus coccifera L. plants).

In the present study, C. monspeliensis L. had the lowest survival percentage, although in previous studies this species showed a high survival percentage (Perez-de-los-Reyes et al. 2013); in this case, the results in the present study can be explained by the poor condition of the plants from the nursery. Plant quality is one of the factors that determines the appropriate development of revegetation (South 2000). Duryea (1985) defined a quality plant as one that is able to achieve optimal development (growth and survival) in a particular environment and therefore meet targets in a revegetation plan. This outcome did not occur with the other species that reached the plantation with suitable plant quality. Lavandula stoechas L. had a high survival percentage and, in fact, had a higher survival rate than it normally shows (~30% according Perezde-los-Reyes et al. 2013). The ecological diversity of the Mediterranean area may explain the differences in the behaviour of the different species with regard to survival percentage.

In terms of apical growth, the results of a study by Pcra and Parlade (2005) showed that inoculated and non-inoculated mycorrhizal plants were not significantly different. Furthermore, significant differences were not observed in the inoculation factor for each of the species. However, other studies reported significant differences in the apical growth between inoculated and non-inoculated plants (Alguacil et al. 2004; Caravaca et al. 2005a, 2005A). This fact depended on mycorrhizal fungi and species plants.

In the present study, R. officinalis L. had the highest apical growth at the end of the study period. This can be explained by the fact that R. officinalis L. is one of the most widely used species for the revegetation of semi-arid environments because it is a good protector of the soil (Bochet et al. 1998; Casermeiro et al. 2004) and it is a species with high growth, as reported by Perez-de-los Reyes et al. (2013). In contrast, T. vulgaris L. had the lowest apical growth, a fact that is easily explained by the morphology of this plant. It is known that apical growth should not be a decisive factor in the choice of species for soil revegetation because the morphology of the plant, rather than its apical growth, is an essential attribute that influences soil protection (Bochet et al. 1998; Casermeiro et al. 2004). In all revegetation processes it is recommended that a multilayered cover with different plant species is implemented in order to ensure effective protection of the soil (Andreu et al. 1998).

As a final point, we have to remember that the findings reported in the present study relate to a Mediterranean area. The ecological diversity of this region implies that situations or issues that occur in a given area arc not necessarily applicable to other areas, although in some cases they may seem similar. This is why this type of research contributes to the study of the sustainability of Mediterranean ecosystems that have been degraded by the installation of photovoltaic solar farms. Knowing how to manage this native vegetation in different environmental conditions could prove useful in the future to provide guidance to the solar photovoltaic industry towards a more sustainable activity.

Conclusions

Under the conditions of the present study, mycorrhisation of the inoculated plants was satisfactory, but there was a natural colonisation in some plants that had not been inoculated initially. This finding is verified by the results of a statistical analysis of the data, which did not show significant differences between mycorrhizal and non-mycorrhizal plants in either survival rate or apical growth.

The species that were best adapted to the test conditions were L. stoechas L., which had the highest survival percentage, and R. officinalis L., which had the highest apical growth at the end of the study period. The species factor had a significant effect on survival rate and apical growth of the species studied.

The choice of species is a key factor for the success of the revegetation process, but it can be concluded that this success depends mainly on climatic, soil and agronomic factors, all of which can change the outcome of this process.

http://dx.doi.org/10.1071/SR15026

Acknowledgements

The authors thank Oscar de la Rubia and Angel Hipolito from the Institute of Concentration Photovoltaic Systems for their contributions to this work. The authors also extend their appreciation to Dr Neil Thompson for assistance with language editing. The study was supported by the research project 'SIGMAPLANTAS: innovation in plants and models of concentrated photovoltaic systems in Spain', Ministry of Science and Innovation of Spain (IPT-2011-1468-92000).

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Monica Sanchez Ormeho (A), Sara Hervas (A), Jose Angel Amoros (A), Francisco Jesus Garcia Navarro (A), Juan Campos Gallego (A), andCaridad Perez-de-los-Reyes (A,B)

(A) Escuela de Ingenieros Agronomos, Universidad de Castilla-La Mancha, Ronda de Calatrava, 7, 13071 Ciudad Real, Spain.

(B) Corresponding author. Email: Caridad.Perez@uclm.es

Received 28 January 2015, accepted 20 July 2015, published online 4 March 2016
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Author:Ormeno, Monica Sanchez; Hervas, Sara; Amoros, Jose Angel; Navarro, Francisco Jesus Garcia; Gallego,
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Date:Mar 1, 2016
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