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Journal of Resources and Ecology >

2025 , Vol. 16 >Issue 1: 115 - 123

DOI: https://doi.org/10.5814/j.issn.1674-764x.2025.01.011

Plant Resources and Plant Ecology

The Role of Plant Diversity in the Soil Aggregate Stability of a Degraded Coal Mine

  • Rabia ZAFAR , 1, 2, *
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  • 1. School of Biological Sciences, The University of Sydney, Sydney 2006, Australia
  • 2. Environmental Science Department, Sardar Bahadur Khan Women’s University, Quetta 08763, Pakistan
*Rabia ZAFAR, E-mail:

Received date: 2024-02-19

  Accepted date: 2024-10-22

  Online published: 2025-01-21

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Abstract

The function of mine spoil following restoration depends on the development of soil aggregation. Two models may contribute to consideration of how spoil may be restored: the hierarchical model and the interdependence model. This study examined the relationship between plant diversity and water stable aggregate in mine spoil. Four plant species Dodonaea viscosa (Jacq), Pittosporum phillyraeoides DC, Hymenosporum flavum (Hook) F. Muell, and Pandorea pandorana (Andrew Steenis) inoculated with eight different Arbuscular mycorrhizal (AM) fungi were either grown as a single species or a mixture of all species in pots containing mine spoil amended with 18% compost. Soil aggregation was measured as mean weight diameter (MWD) and organic carbon was determined after 12 months. Aggregation of spoil from under the mixed plant treatment was in the middle of the range for single species, for overall MWD and weight of each of the water-stable aggregate fractions. MWD of soil under P. phillyraeoides was higher than for all other single plant species and plant mixture. Organic carbon in the water-stable micro-aggregate sized fraction was greater in all planted than the unplanted treatment. The mean soil carbon concentration of P. pandorana was higher than all other single plant treatments and plant mixture. The study suggests that the development of aggregation of mine spoil amended with compost is independent of plant diversity.

Key words： mine soil rehabilitation; plant-fungal interaction; soil structure; soil organic carbon; plant diversity

Cite this article

Rabia ZAFAR . The Role of Plant Diversity in the Soil Aggregate Stability of a Degraded Coal Mine[J]. Journal of Resources and Ecology, 2025 , 16(1) : 115 -123 . DOI: 10.5814/j.issn.1674-764x.2025.01.011

1 Introduction

Soil structure plays a crucial role in the functioning of terrestrial ecosystems, and its ability to support plant and animal life along with regulating the environmental quality (Lal et al., 2021; Francaviglia et al., 2023). In brief, soil structure defined as size, shape, arrangement, and continuity of solids/voids, plays a major role in the functions of soil, enabling root elongation, storage of water and carbon, and gas exchange with the atmosphere (Miller and Jastrow, 2000; Barrios, 2007; Gougoulias et al., 2014; Abdalla et al., 2023).
Since the industrial revolution, anthropogenic activities such as land use, soil/crop management and mining have significantly reduced overall soil health leading into soil degradation (Feng et al., 2019; Bandyopadhyay and Maiti, 2022; Huang et al., 2022). Surface mining, including open- cut coal mining casuses more environmental damage to soil structure due to the use of heavy mechanised mining operations and production of large waste (Feng et al., 2019). In open-cut coal mining, surface layers of soil are removed to access coal seams (Scott et al., 2010; Litvin et al., 2017). The exposed subsurface material or rearranged soil also termed as overburden or mine spoil or technosol is a degraded mine soil lacks organic carbon, microbiota, plant seeds and adequate minerals necessary for plant growth and development (Johnson, 2003; Rambabu et al., 2020). The restoration of the degraded coal mine spoil is a common theme of environmental mining reclamation (Aradottir and Hagen, 2013; Dong et al., 2019; Maus et al., 2020) due to the importance of mine spoil systems in regulating the plant, water and landscape subsystems of mining area (Adhikari and Hartemink, 2016; Almenar et al., 2021). Different restoration and rehabilitation techniques including the use of cover cropshave been used to restore degraded mine soil systems (Tautges et al., 2019; Bhattacharyya et al., 2022). However soil structure stability which is the capacity of soil to retain its arrangement of particles and pores against external pressure and stresses (Oades, 1993; Wiesmeier et al., 2019) and soil organic carbon reserves have taken centre stage in management practices targeting the restoration of degraded mining land (Turmel et al., 2015; Fatichi et al., 2020; Or et al., 2021).
Soil aggregation and organic carbon are two critical indicators of soil structure (Oades, 1984; Jastrow et al., 1998; Six et al., 1998; Franzluebbers et al., 2001; Wiesmeier et al., 2019; Hartmann and Six, 2023). Aggregation influences both the structural and functional properties of soil (Amézketa, 1999; Khan et al., 2007; Stătescu et al., 2013). Structurally, aggregation protects organic materials from decomposition and in return organic matter physically protects aggregates from degradation, leading to an accumulation of carbon, especially in the micro-aggregate fraction (Tisdall and Oades, 1982; Oades, 1984; Six et al., 2002; von Luetzow et al., 2008; Grosbellet et al., 2011; Rabot et al., 2018; Cotrufo and Lavallee, 2022; Hartmann and Six, 2023). In addition, aggregation influences soil structural properties such as density and pore size distribution which in return affect soil water movement, retention and gas exchange (Lipiec et al., 2007; Menon et al., 2020).
Hierarchical model (HM) provides a framework to associate various aggregate sizes with different binding agents such as AM fungi and fibrous plant roots (Tisdall and Oades, 1982; Oades, 1984; Rabot et al., 2018; Lavallee et al., 2020; Angst et al., 2021; Cotrufo and Lavallee, 2022). These binding agents are particularly associated with the stabilization of water-stable macro-aggregates (operationally defined as >250 µm). In addition, plant roots exudates (Watt et al., 1993; Angst et al., 2021), hyphae of AM fungi (Rillig et al., 2010; Morris et al., 2019; Wu et al., 2024) along with the bacteria (Cheshire and Hayes, 1990) and saprotrophic (Daynes et al., 2012; Erktan et al., 2020) are the key contributor of organic materials associated with the maintenance of water-stable micro-aggregates (Six et al., 2000; Totsche et al., 2018; Kleber et al., 2021). Besides contributing organic materials in micro-aggregates soil fraction, AM fungi also influence plant community composition (Klironomos et al., 2000; Yang et al., 2018). AM fungi regulation of plant community composition is based on the degree the various plant species depend on their mycorrhizal symbionts for growth and development in the specific environment (van der Heijden et al., 1998a; Asmelash et al., 2016). Similarly, the role of AM fungi diversity for the productivity of the plant community has also been reported (Klironomos et al., 2000; Yang et al., 2018; Soudzilovskaia et al., 2020). For instance, increase in the productivity of single plant species (Maherali and Klironomos, 2007; Wagg et al., 2019; Tedersoo et al., 2020) or a community of grassland plants (van der Heijden et al., 1998a; Bardgett and Van Der Putten, 2014) due to AM fungi diversity or either one to one relationship in which a plant is beneficial from one AM fungi taxon than the mixture of several AM fungi (van der Heijden et al., 1998a; van der Heijden et al., 1998b; van der Heijden et al., 2003; Vogelsang et al., 2006; Semchenko et al., 2022). Thus, the diverse communities of either plants or AM fungi functions via complementarity or selection effect depend on the factors that can change species ability to acquire resources (van der Putten et al., 2016; Marro et al., 2022; Semchenko et al., 2022). Resource acquisition ability of species is a critical factor on which biodiversity effect depends (Wagg et al., 2015; Barry et al., 2019).
Furthermore, the mutual interdependence model (Bever et al., 1997; Bever, 1999; Tedersoo et al., 2020) argues that plant-AM fungal interactions contribute to diversity within both the plant and fungal community. Implicit in the mutual interdependence model is the assumption that functions in addition to community composition will also be modified. These functions include biomass accumulation by fungi and plants, hyphal length and phosphorus capture (Bever et al., 1997; Bever, 1999; Crawford et al., 2019; Tedersoo et al., 2020; Fei et al., 2022).
Together, plants and AM fungi contribute to the development of water-stable aggregates and the content of organic materials in soil. Previously the combined role of diverse plant species, and diverse AM fungi contribution to soil structure in a degraded mine soil has not been discussed in greater detail. Thus, the primary objective of this study is to understand the effect of plant diversity on the restoration of mine soil amended with diverse AM fungi and compost.

2 Methodology

2.1 Materials

Mine spoil (overburden) obtained from an open-cut coal mine (Latitude 33.52ºS, Longitude 151.12ºE), located at Mount Owen, in New South Wales (NSW), Australia, was sieved through a 2 mm grid to remove large particles (Daynes et al., 2013). The mine spoil and compost were sterilised to remove any microbes by autoclaving 3 times for 2 h over a 72 h period. The mine spoil was then mixed with compost (18% weight/weight (w/w): approx 5% carbon) derived from bulk household garbage via the Bedminster process (Daynes et al., 2013) at the SITA Environmental Solutions Advanced Resource Recovery Facility, Raymond Terrace, NSW, Australia. Table 1 shows the physiochemical properties of mine spoil and compost.
Table 1 Physical and chemical properties of spoil and compost materials <2000 µm diameter
Parameter Unit <2000 µm diameter Parameter <2000 µm diameter
Mine spoil Compost Mine spoil Compost
Physical properties Metal contents (mg kg‒1)
pH (1:5 H2O) 9.2 7.3 Trace metal
pH (1:5 0.01M CaCl2) 8.8 6.8 Total lead 235
Electrical conductivity µS cm‒1 105 3265 Total mercury 0.38
Total soluble salt mg kg‒1 347 10774 Cobalt 0.7 0.3
Base saturation % 100 91 Copper 2.7 38
Coarse sand % 48 Iron 691 1065
Fine sand % 31 Molybdenum 0.6 1.2
silt % 12 Zinc 2.9 275
clay % 9 Manganese 160 798
Chemical properties Metal & non metal
Total organic matter % 0.2 53 Calcium 1106 3752
Total organic carbon % 0.1 27 Sodium 106 4349
CEC cmol(+) kg-1 7.2 19 Magnesium 245 439
Adjusted.CEC cmol(+) kg-1 7.2 14 Potassium 113 2219
Exchangeable. Ca cmol(+) kg-1 4.8 5.8 Nitrogen 0.2 9
Exchangeable. H2 cmol(+) kg-1 <0.1 4.8 Phosphorus 2.3 134
exchangeable Mg cmol(+) kg-1 1.8 1.1 Boron 0.1 12
Exchangeable. K cmol(+) kg-1 0.3 1.8
Exchangeable. Na cmol(+) kg-1 0.4 5.8
exchangeable Na % 5.6 30
Calcium: Magnesium ratio 2.7 5.1

Note: This table was modified from Daynes et al. (2013).

2.2 Experiment

Approximately 8 kg mine spoil mix was placed in each pot. Single seedlings of four plant species Dodonaea viscosa (Jacq), Pittosporum phillyraeoides DC, Hymenosporum flavum (Hook) F. Muell, and Pandorea pandorana (Andrew Steenis) inoculated with eight different AM fungi (Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe Isolate NBR 4.1 and BUR11A, Glomus sp. Isolate PP1, Glomus sp. Isolate PM1.2, Rhizophagus irregualris. N.C. Schenck & G.S. Sm. and Acaulospora sp. Isolates CHL5, CHL11 and CHL13) were then transplanted to the amended mine spoil in pots, packed and watered. Eight isolates of AM fungi used in this experiment were isolated from highly disturbed sites in the Sydney basin and from agricultural soils in central NSW, Australia. Each planted pot had four plants, consisting of either one plant species or one each of all four plant species hereafter called plant mixture. The experimental design is comprised of five replicate treatments of each Pandorea pandorana, Hymenosporium flavum, Dodonea viscosa, Pittosporum phillyraeoides, and the plant mixture all inoculated with the mixture of 8 AM fungi. Each control pot lacked seedlings and AM fungi. Each pot was placed outside in a northerly aspect and was watered (reverse osmosis) at regular intervals for three months through a drip irrigation system, at the rate of 4 L hr-1. From three months until the plants were harvested, the pots were only watered after observing the first sign of wilting. Four replicates of P. phillyraeoides, the plant mixture and the control and five replicates of the other three single plant species survived to harvest.

2.3 Soil analysis

After 12 months, shoots were removed at ground level and dried at 100 ℃ overnight and then weighed. The pots were left to dry for a week. The soil was sampled using an open-ended stainless steel sampling tube (length 35 cm, outer diameter 10 cm). Soil samples were collected top down from the centre of the pot. The sharp cutting edge of the tube was forced down through the central section of the pot between plants giving a longitudinal core. Each sample was transferred into split PVC tubes designed to allow access to any portion of the core. Samples were immediately dried for 48 hrs at 80 ℃ and transferred to a 6 ℃ cold room for storage. The soil cores from each pot were cut into sub-sections. The top (0‒1.7 cm) and bottom sub-section (14.9-16 cm) were disturbed during transferring soil core into split PVC tubes and therefore were not used for measuring soil aggregation and soil organic carbon. From each soil core, the sub-section 3.7‒5.5 cm was used to determine soil water-stable aggregates by wet sieving and distribution of carbon.

2.4 Measurement of soil aggregate stability in water

The water-stability of soil structure was measured by wet sieving. The soil cross-section (3.7-5.5 cm) was gently passed through a 5 mm grid and then through a 1 mm grid. A bank of sieves 20 cm in diameter with mesh apparatus of 53, 250, 710 and 2000 µm (Essa, Perth, Australia) was used in wet sieving. The two soil aggregate fractions (>1 mm and <1 mm) were labelled and weighed. Soil aggregate samples greater than 1 mm were wet sieved through 4 sieves of 2000, 750, 250 and 53 μm grid opening while soil aggregate samples less than 1 mm were wet sieved through grid openings of 750, 250 and 53 μm (Yoder, 1936). Dry samples were placed on the top sieve and immersed in 15 L tap water. Soil samples were left to equilibrate for 5 min. Floating debris (inorganic impurities in the compost) was collected, oven dried and discarded with the dry weight subtracted from the starting weight of soil. Samples were lifted and lowered in water for 8 min. at 30 rpm with 2.5 cm vertical displacement. After 8 min, samples from each sieve were transferred to labelled plastic flasks, dried at 80 ℃, weighed and transferred into clean labelled plastic bags and returned to the cold room. The resultant fractions were used to calculate the mean weight diameter (MWD) and the proportion of water-stable aggregates (%WSA). MWD is commonly used index in soil sciences relating aggregate size to soil stability (Nimmo and Perkins, 2002). Higher MWD shows the greater soil aggregate stability due to the presence of less erodible, larger aggregate in the soil (Piccolo et al., 1997; Ferreira et al., 2019).
$MWD=\sum\limits_{i=1}^{n}{\overline{{{X}_{i}}}}{{W}_{i}}$
MWD is a mathematical technique used for expressing data on aggregation in the form of a single parameter (Eq. (1)). MWD is equal to the sum of products of the mean diameter; $\overline{{{X}_{i}}}$of each size fraction and the proportion of the total sample weight; Wi occurring in the corresponding size fraction, where the summation is carried out over all n size fractions, including the one that passes through the finest sieve (Van Bavel, 1949; Youker and Mcguinness, 1957).

2.5 Soil organic carbon analysis

Soil carbon was measured in the water-stable aggregate size fractions of 710-2000, 250-710, 53-250 µm generated from <1 mm soil samples by wet-sieving. 5 g of soil sample from each size fraction was hand ground to <53 µm using a mortar and pestle and then stored in clean labelled plastic bags. The carbon (%) in each soil fraction was then determined using a CNS analyzer (VarioMAX CNS analyzer, Elementar Analysensysteme GmbH, Hanau, Germany).

2.6 Statistical analysis

All statistical analyses were undertaken using SPSS v 17.0 (SPSS Inc., Chicago). Data were assessed for normality and found to be normally distributed. Homogeneity of variance was tested (Levene’s F test). Analysis of variance (ANOVA) was performed to test the effect of different treatments. Means were separated by using a Least Significant Difference (LSD) test at 5% confidence level. In case of applying one-way ANOVA, the plant species were taken as a fixed effect.

3 Result and discussion

Changes in species diversity in one guild of organisms can have multiple effects on organisms in different guilds (Soliveres et al., 2016). On the same note, greater species diversity of either plant or fungal community might result into selection or complementarity effect among each other species (Smith and Read, 2010; Eisenhauer, 2012; Wagg et al., 2015). The necessity to explore the relationship is not only to understand the role of biodiversity in ecosystem functioning (Hooper et al., 2012; Wagg et al., 2014) but also to find its significance in the restoration of degraded mined land (Bandyopadhyay and Maiti, 2022).
The interdependence model (Bever et al., 1997; Bever, 1999) allowed us to predict that increasing plant diversity would increase aggregation of compost-amended mine spoil and deposition of carbon in the presence of a complex community of AM fungi. The present study did not support the interdependence model prediction (Tables 2, 3, 4) for the expected increase in mine spoil aggregation and soil carbon. Minor responses in aggregation and soil carbon were observed in mine spoil amended with 18% compost after 12 months growth of well-watered seedlings inoculated with eight AM fungi (Tables 3, 4).
Table 2 Mean (± standard error) above ground biomass (g) of 4 different plant species and plant mixture, inoculated with 8AM fungi after one year in mine spoil amended with compost (18% w/w)
Treatment n Mean biomass species grown alone (g) Mean biomass species grown in a mixture (g)
Pandorea pandorana 5 65±2.2 d 19±6 bc
Hymenosporum flavum 5 99±6.8 c 27±10 b
Dodonaea viscosa 5 170±1.8 a 65±13 a
Pittosporum phillyraeoides 4 128±15 b 12±6.8 cd
Plant mixture 4 123±7.4 b

Note: Data are means±standard error of n replicates. Different letters indicate significant differences between means according to least significant difference (LSD) multiple comparisons post-hoc test following ANOVA (P<0.05).

Table 3 Mean Weight Diameter (MWD) and Water-stable aggregate size fractions (± standard error, g) of amended mine spoil from under one of four plant species and plant mixture inoculated with 8 AM fungi after one year in mine spoil amended with compost (18% w/w)
Treatment MWD (g) Mean aggregate size fraction > 2000 µm Mean aggregate size fraction 710-2000 µm Mean aggregate size fraction 250-710 μm Mean aggregate size fraction 53-250 μm
Pandorea-pandorana 863.4±31.2 bc 23.8±4.6 a 23.4±2.1 b 23.1±2.6 a 23.2±2.7 ab
Hymenosporum-flavum 939.8±32.0 ab 26.1±3.3 a 34.7±3.7 a 24.8±1.7 a 24.1±1.8 ab
Dodonaea-viscosa 824.6±27.9 c 20.8±2.1 a 25.8±1.5 ab 25.7±2.1 a 28.1±2.4 a
Pittosporum-phillyraeoides 998.5±53.1 a 18.9±4.3 a 25.4±6.2 ab 13.1±3.3 b 13.0±3.1 c
All four species 950.3±24.0 ab 22.0±1.9 a 33.9±1.1 a 20.2±1.0 a 18.7±0.4 bc
Unplanted control 925.5±15.6 abc 27.6±1.3 a 26.8±2.3 ab 22.3±0.5 a 20.2±0.4 b

Note: Different letters indicate significant differences between means in each column according to least significant difference (LSD) multiple comparisons post-hoc test following ANOVA (P<0.05).

Table 4 Mean carbon concentration (±standard error) of 710-2000 µm, 250-710 μm, 53-250 μm fraction of amended mine spoil from under one of four plant species and plant mixture inoculated with 8 AM fungi
Treatment Carbon content in the 710-2000 µm fraction (%) Carbon content in the 250-710 µm fraction (%) Carbon content in the 53-250 µm fraction (%) Total mean carbon concentration (%)
Pandorea pandorana 7.3±0.2 a 5.3±0.1 a 4.0±0.1 a 16.7±0.4 b
Hymenosporum flavum 5.9±0.1 d 4.3±0.2 b 3.5±0.1 b 13.7±0.4 a
Dodonaea viscosa 6.5±0.2 bc 4.5±0.2 b 3.2±0.2 d 14.2±0.4 a
Pittosporum phillyraeoides 6.4±0.2 bc 4.6±0.1 b 3.3±0.2 c 14.4±0.4 a
All four species 6.9±0.1 ab 4.6±0.1 b 3.4±0.1 b 14.9±0.4 ab
Unplanted control 6.4±0.2 bc 4.2±0.1 b 2.7±0.2 e 13.3±0.4 a

Note: Different letters in each column indicate significant differences between means according to least significant difference (LSD) multiple comparisons post-hoc test following ANOVA (P<0.05).

3.1 Above ground plant dry biomass

After one year mean shoot dry weight (g) of D. viscosa was significantly heavier than all five planted treatments i.e. P. pandorana, H. flavum, P. phillyraeoides, and the plant mixture (P<0.01: Table 2). The shoot dry weight of P. phillyraeoides was similar to the shoot dry weight of the plant mixture (Table 2). Mean shoot dry weight for each plant in all four plant mixtures was also determined. Mean shoot dry weight (g) of all four plants in plant mixture differed significantly (P=0.007, Table 2). In all plant mixture treatments, D. viscosa was significantly heavier (P<0.05) and P.phillyraeoides was significantly lighter (P<0.05) than all other plant species (Table 2). Based on the assumption of the mutual interdependence model we predicted increase in the productivity of plant communities as plant diversity increases, due to the contribution from (different) AM fungi to each plant species. Again, no significant increases in shoot biomass were observed. The shoot dry weight of the mixed plant treatment is between the heaviest and lightest of the plant species grown as a monoculture (Table 2). In this study, shoot biomass results show both complementarity and selection effect among diverse plant species (Table 2). For instance, the heavier biomass of D. viscosa within plant mixture treatment indicates the selection effect. However, in the same plant mixture treatment, all plant species survived after 12 months and none of them disappeared completely which indicates the complementarity among plant species.

3.2 Mean weight diameter of soil aggregates

Mean weight diameter (MWD) of soil under all planted treatments differed significantly (P=0.01, Table 3) among planted treatments. MWD of soil under P. phillyraeoidesis significantly higher than for D. viscosa and P. pandorana (P=0.01, Table 3).
MWD for plant mixture was significantly higher (P<0.05) than for D. viscosa. MWD of the unplanted treatment was similar to all planted treatments (Table 3). For the water-stable macro-aggregate size fractions >2000 µm (P=0.47, Table 3) and 710-2000 µm (P=0.07, Table 3) no significant differences were observed across treatments. However, within the 710-2000 µm fraction, the mean water-stable aggregate size fraction for soil under the plant mixture and H. flavum were larger than P. pandorana. Statistically significant differences (P=0.006, Table 3) were found among treatments for the 250-710 μm water-stable micro-aggregate size fraction. The difference among all treatment was due to the lighter fraction in soil from under P. phillyraeoides. Treatments differed significantly (P=0.001, Table 3) for water-stable micro-aggregate size fractions (53-250 µm). The water-stable micro-aggregate size fraction for D. viscosa was significantly larger (P<0.05) than for P. phillyraeoides, plant mixture, and the unplanted treatments. The water-stable micro-aggregate size fractions for P. pandorana and H. flavum were similar (P<0.05). For P. phillyraeoides the water-stable micro-aggregate size fraction was significantly less (P<0.05) than all other planted treatments (Table 3). Based on (Bever et al., 1997, Bever, 1999) interdependence models two specific responses were predicted: increased water-stable aggregation, particularly water-stable macro-aggregation, and increased carbon. MWD of soil under the plant mixture was higher than only one of the treatments with single plant species (Table 3). In the water-stable macro-aggregate fractions 710-2000 µm and 250-710 µm, the weight of soil under the plant mixture was larger than only one treatment planted with a single species in each fraction: P. pandorana in 710-2000 µm and P. phillyraeoides in 250-710 µm (Table 3). Furthermore, if it is assumed that the contribution to aggregation of the community of AM fungi is similar across planted treatments, then it would appear that the contribution of each plant species to aggregation differs. The differences between plant species are evident as MWD and weight of the water-stable microaggregate fraction (Table 3).

3.3 Mean soil carbon concentration

Mean soil carbon (710-2000 µm) for P. pandorana was significantly higher (P<0.05) than the mean soil carbon for the unplanted treatment, H. flavum, D. viscosa and P. Phillyraeoides (P<0.01, Table 4).
The mean soil carbon concentration in (710‒2000 µm) for D. viscosa and P. phillyraeoides were similar (P<0.05) to each other and the unplanted treatment (Table 4). The mean soil carbon concentration in the (250-710 μm) aggregate size fraction for P. pandorana was significantly higher (P=0.001, Table 4) than all other planted and the unplanted treatments (Table 4). The mean soil carbon concentration of the size fraction (53-250 μm) differed significantly (P= 0.001, Table 4). The mean soil carbon concentration of P. pandorana was significantly higher (P<0.05) than all other treatments. Mean soil carbon concentration for P. phillyraeoides and the plant mixture were similar to each other, while mean soil carbon concentration for unplanted treatment was significantly less (P<0.05) than all other treatments (Table 4). The total mean soil carbon for P. pandorana was significantly higher (P<0.05) than the mean soil carbon concentration for the unplanted treatment, H. flavum, D. viscosa and P. Phillyraeoides (P=0.0001, Table 4). The total mean soil carbon of P. pandorana was similar (P<0.05) to the total mean soil carbon concentration of plant mixture. Further, based on the mutual interdependence model hypothesis feedback between plant and AM fungus. The quantification of plant responses is relatively easy where biomass is a standard measure. In the present study the contribution by the plant to the AM fungi is quantified by the weight of water-stable aggregate fractions which indicates the contribution of hyphae of AM fungi to the enmeshment of water-stable macroaggregates. It is assumed that the increased weight of water-stable macro-aggregates would indicate increased enmeshment. The mine spoil was amended with 18% compost or approximately 5% added carbon and it has already been shown that the organic matter contributes to the formation of water-stable macroaggregates (Daynes et al., 2013). The added carbon appears to decline slowly probably because much of it is coarse: after 12 months, some 6.4% carbon remained in the coarse fraction and 4.2% in the other water-stable macroaggregate fraction (Table 4) of the unplanted treatment. As the unplanted treatment did not gain carbon from plants or plant-associated fungi, the experiment might be too short at 12 months for finer detail to become evident. However, significantly less carbon in the water-stable microaggregate fraction of the unplanted control indicates compost is degrading under the experimental conditions. This suggests that the mutual interdependence model may rely on limited carbon in the experimental soil for interdependence to become evident. Alternatively, plant community diversity may be influenced by fungal diversity without the functions of AM fungi being influenced by plant diversity. This synthesis is parallel to the work of (Wagg et al., 2015) who suggested that the greater plant diversity has no impact on the abundance of AM fungi in plant roots while the greater AM fungal diversity can enhance plant community performance but does not necessarily improve plant species complementarity. On the other hand, soil under P. pandorana had consistently more proportional carbon in fractions and in total but low MWD.
The sources of carbon in the experimental soils at harvest, apart from the compost, are unclear. If the contribution by plants is related to the shoot mass, then it can be predicted that soil under D. viscosa would contain more carbon because of the contribution from the root system. However, the relatively lightweight P. pandorana appears to be growing in soil which after 12 months has more carbon. Thus it can be hypothesised that the more carbon in the soil of P. pandoranais might be due to greater root turnover.
Finally, our understanding of carbon deposition in soil is largely based on the direct plant contribution. AM fungi also contribute carbon of plant origin to the soil. If the biomass of AM fungi turns over at a much faster rate than plant carbon, as would be expected of any microbe, then the contribution of fungi to standing carbon will remain extremely difficult to quantify without improved technologies. Plant species differently contribute to aggregation of soil. However, their contribution appears to be unrelated to the contribution of their AM fungal associations. Indeed, at least in soil aggregation and carbon deposition, plants and AM fungi do not appear to have an interdependent relationship.

4 Conclusions

The diversity in plants and AM fungi does not significantly increase aggregation or soil carbon in the compost amended mine spoil after 12 months. Instead of diverse plant species, the effect of individual plant species in the presence of diverse AM fungi have been dominant for MWD, water-stable microaggregate size fraction, and soil carbon. Thus, the current study does not support the mutual interdependence model; however, it does report both the selection and complementarity effect among diverse plant species in the presence of diverse AM fungi. Conclusively, the study emphasises the importance of individual plant species in soil structural development of degraded mine spoil than the diversity in the plant species.

Acknowledgments

This project could not have proceeded without the active support of my supervisor Professor Peter Allan McGee. The author is grateful to Dr Cathal N Daynes for his assistance during the experimental work. I am enormously grateful to the AusAID (Australia Pakistan Scholarships Program, APSP) (AusAID ID: ISL000348) for funding my Master stdies and the School of Biological Sciences, University of Sydney, Australia for providing the great facilities and outstanding research atmosphere.

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