Agua-suelo-clima

 

Capability of grass in recovery of a degraded area after coal mining

 

Capacidad de las gramíneas en la recuperación de una área degradada después de la minería del carbón

 

Lizete Stumpf^1* , Eloy A. Pauletto², Rafaela Costa de-Castro³, Luiz F. Spinelli-Pinto², Flavia Fontana-Fernandes², Tiago Stumpf da-Silva⁴, Jordano Vaz-Ambus⁴, Gabriel Furtado-Garcia⁴, Claudia L. Rodrigues de-Lima², Márcio R. Nunes⁵

 

¹ Post Graduate in Agronomy. Federal University of Pelotas. Street Campus Universitário Capão do Leão, s/n. Post office 354, CEP: 96900-010. Capão do Leão/RS. * Author for correspondence (zete.stumpf@gmail.com).

² Soil's Department. Federal University of Pelotas. Street Campus Universitário Capão do Leão, s/n. Post office 354, CEP: 96900-010. Capão do Leão/RS. (pauletto_sul@yahoo.com.br) (lfspin@uol.com.br) (f_flavia_fernandes@yahoo.com) (brclrlima@yahoo.com.br).

³ Federal University of Pelotas. Street Campus Universitário Capão do Leão, s/n. Post office 354, CEP: 96900-010. Capão do Leão/RS.(rcostadecastro@gmail.com).

⁴ Faculty of Agronomy. Federal University of Pelotas. Street Campus Universitário Capão do Leão, s/n. Post office 354, CEP: 96900-010. Capão do Leão/RS. (tiago.stumpf@hotmail.com) (jv.ambus@gmail.com) (gabrielgarciag2@hotmail.com).

⁵ Post Graduate in Management and conservation of water and soil. (marcio_r_nunes@yahoo.com.br). Federal University of Pelotas. Street Campus Universitário Capão do Leão, s/n. Post office 354, CEP: 96900-010. Capão do Leão/RS.

 

Received: November, 2013.
Approved: June, 2014.

 

Abstract

Surface coal mining causes an intense topographic and hydrologic change in the area with suppression of local vegetation, degrading the environment. One main problem presented by the rehabilitated soil refers to physical disruption of the soil, which delays the regrowth of plant species in the mining area. The objective of this study was to evaluate the influence of four grass species in the retrieval of the rehabilitated soil's physical properties at a recovered coal mine area in Candiota, Rio Grande do Sul State, southern Brazil. With the data an ANOVA was performed and treatments means were compared using Tukey test (p ≤0.05). The treatments were: T1-Hemarthria altissima; T2-Paspalum notatum; T3-Cynodon dactylon; T4-Urochloa brizantha; a rehabilitated soil without vegetation cover was used as control. Preserved soil samples after 78 months were collected in the 0.00-0.10 m and 0.10-0.20 m layers to determine bulk density, total porosity, macroporosity, microporosity and available water capacity. The process of rehabilitation of coal mined areas was slow due to the high degree of degradation that occurs during the extraction of coal and the topographic recovery of the area, adversely affecting the structural quality of the soils. The assessed grasses showed potential to reclaim these areas, since improvements were observed in the values of the soil attributes over time, especially in the 0.00-0.10 m layer. After 78 months of evaluation, U. brizantha showed the higher potential to recover the physical properties of the compacted rehabilitated mine soils.

Key words: Hemarthria altissima, Paspalum notatum, Cynodon dactylon, Urochloa brizantha, rehabilitated soils, physical properties.

 

Resumen

La minería del carbón en superficie provoca un intenso cambio topográfico e hidrológico en la zona por la supresión de la vegetación local, degradando el ambiente. Un problema principal presentado por el suelo rehabilitado es la perturbación física del suelo, lo que retrasa el rebrote de las especies vegetales en el área minera. El objetivo de este estudio fue evaluar la influencia de cuatro especies de gramíneas en la recuperación de las propiedades físicas del suelo en un área minera de carbón rehabilitada en Candiota, Rio Grande do Sul, sur de Brasil. Con los datos se realizó un ANDEVA y las medias de los tratamientos se compararon con la prueba de Tukey (p≤ 0.05). Los tratamientos fueron: T1-Hemarthria altissima; T2- Paspalum notatum; T3-Cynodon dactylon; T4-Urochloa brizantha; un suelo rehabilitado sin cubierta vegetal se usó como testigo. Muestras de suelo protegido se recolectaron después de 78 meses en capas de 0.00-0.10 m and 0.10-0.20 m para determinar densidad aparente, porosidad total, macroporosidad, microporosidad y capacidad de agua disponible. El proceso de rehabilitación de las zonas de minas de carbón fue lento debido al grado alto de degradación producido durante la extracción de carbón y la recuperación topográfica de la zona, lo que afecta negativamente la calidad estructural de los suelos. Las gramíneas evaluadas mostraron potencial para recuperar estas áreas, ya que se observaron mejoras en los valores de los atributos del suelo en el tiempo, especialmente en la capa de 0.00 a 0.10 m. Después de 78 meses de evaluación, U. brizantha mostró el potencial mayor para recuperar las propiedades físicas de los suelos rehabilitados de las minas.

Palabras clave: Hemarthria altissima, Paspalum notatum, Cynodon dactylon, Urochloa brizantha, suelos rehabilitados, propiedades físicas.

 

INTRODUCTION

Altered ecosystems in the case of surface mining are related to the removal of the original vegetation and layers of soil and rocks, resulting in topography changes and the hydrological regime of the area (Rodrigues et al., 2007). The degraded ecosystem may not return to its previous state or its return may be slow (Alves and Souza, 2011).

Coal mining in Candiota, Brasil, is the surface mining type with the removal of topsoil (soil horizons A, and sometimes B) and then overburden (geological strata-sandstones, shales, siltstones, and claystones). For the topographic rehabilitation of the area, overburden is placed back into the previous open pit and leveled. The landscape rehabilitation is completed by replacing the topsoil removed before excavation.

During the soil rehabilitation process there is an intense traffic of heavy machinery over the area compacting the layer of soil replaced on the overburden. According to Horn et al. (1995) repeated wheeling induces a denser rearrangement of soil aggregates and the formation of a platy structure.

The rehabilitation of degraded areas from mining requires the use of specific techniques, according to the region and type of ore exploited, and there is no definitive model for the recovery of these areas (Silva et al., 2006). The growth of a vegetative cover on mined surfaces is the most common measure of recovery (Silva and Corrêa, 2010), but it is a slow and difficult process, requiring the choice of plants that are good for growth and development in these degraded environments (Alves et al., 2007).

Grass coverage is of great interest in the recovery process because it plays an important role in rebuilding the physical and chemical characteristics of the substrate (Amaral et al., 2012). Grasses, due to their capacity of soil protection against the impact of rain water, wind and their role in promoting soil aggregation through the supply of organic matter (Avaretto et al., 2000), as well as through their abundant and extensive root system in constant renewal in the soil (Campos et al., 1999), are used in regeneration programs of degraded areas. According to Angers and Caron (1998), one of the most significant plant-induced changes in soil structural form is the formation of continuous macropores by penetrating roots in the soil (Bronick and Lal, 2005). In this context, the aim of this study was to analyze the influence of four grass species in recovering the density, porosity and water holding capacity of a rehabilitated soil over time in the area of a coal mining in Candiota/RS.

 

MATERIAL AND METHODS

This study was conducted on a rehabilitated coal mine area belonging to the Companhia Riograndense de Mineração (CRM), in Candiota, Rio Grande do Sul State. The layer of replaced soil in the experimental area had been taken from a B horizon of the soil of the pre-mined area, a Rhodic Lixisol as indicated by the reddish color (2.5 YR 3.5/6), with a low organic matter content and clay textural class in the 0.00-0.20 m layer (Table 1).

The soil of the experimental area was scarified with a bulldozer to a depth of 0.15 m due to the highly compacted condition, derived from uncontrolled topsoil deposition and leveling operations made by 20 t trucks and a 38 t bulldozer (3.6 m² contact area). The soil was then prepared with a disk harrow, limed with 10.4 t ha^-1 of limestone (100 % ECCE) and fertilized with 900 kg ha^-1 5-20-20, according to the results obtained by the soil analysis.

The soil was rehabilitated in early 2003 and the experiment started in November/December 2003. For the experiment 20 m² plots (5 m x 4 m) were used, with four repetitions per treatment in a randomized block design, and treatments (T) were: T1) Hemarthria altissima (Poir.) Stapf & C.E. Hubb.; T2) Paspalum notatum Alain ex Flüggé cv. Pensacola; T3) Cynodon dactylon (L.) Pers. cv. Tifton 85; T4) Urochloa brizantha (Hochst. ex A. Rich.) R.D. Webster. Seedlings were used for T1 and T3 treatments, spaced between 0.10-0.15 m in the row and 0.40 m between rows. For T2 and T4, 200 g and 140 g of seeds were used per plot, corresponding to 50 and 35 kg ha^-1, respectively. The results from each grass treatment were compared to results from a rehabilitated bare control soil located next to the experimental area.

In order to analyze the effect of liming and fertilization after 78 months of experiment, disturbed samples were collected in the 0:00 to 0:10m and 0.10-0.20 m layers to determine: water pH, organic carbon, calcium (Ca), magnesium (Mg), aluminum (Al), potential acidity (H+Al), potassium (K), sodium (Na), and phosphorous (P), according to Tedesco et al. (1995). To determine of bulk density (Blake and Hartge, 1986), total porosity, macroporosity (Embrapa, 2011), and the available water capacity, 160 undisturbed samples were collected in the 0.00-0.10 m and 0.10-0.20 m layers (4 blocksX5 treatmentsX2 repetitions per plotX 2 soil layers) with steel cylinders (0.030 m high and 0.0485 m diameter).

The undisturbed samples were saturated with water by capillarity for 24 h, and then placed in a tension table, where they were equilibrated at a pressure of 6 kPa. Afterwards, the samples were equilibrated at pressures of 10, 33, 102 and 1530 kPa in Richards pressure chamber (Klute, 1986), and then dried at 105 °C to constant weight. With the data, the values of macroporosity (tension of 6 kPa), microporosity, bulk density and estimated available water capacity were calculated by considering the water content at tensions of 10 kPa and 1530 kPa, respectively, representing field capacity and permanent wilting point.

To calculate the estimated available water capacity, the water retention curves were fitted to the model of van Genutchen (1980) by the program SWRC (Soil Water Retention Curve) (Dourado-Neto et al., 2001).

With the data an ANOVA was performed and treatments means were compared using Tukey test (p≤ 0.05). To assess the influence of the cover plants in the recovery of soil physical attributes over time, the results obtained at 5, 41 and 78 months after starting the experiment were compared using MIXED. All statistical analysis was performed using SAS (1985).

 

RESULTS AND DISCUSSION

The values of soil chemical variables indicate that the liming and fertilization were more effective in the 0:00 to 0:10 m layer, with little effect in the 0.100.20 m layer (Table 2). This occurred due to highly compacted condition of the soil, which limited the incorporation of limestone and fertilizer.

According to CQFS (2004), the data indicate that the pH in the 0:00 to 0:10 m layer is above the reference range for the development of perennial grasses (pH>5.5), as well as the values of Ca and Mg (>4.0 and >1.0 cmol_c kg^-1, respectively). The CEC is average (5.1 to 15 cmol_c kg^-1), whereas K and P values are high (from 61-120 mg kg^-1, and from 9.0-18 mg kg^-1 for clay soils, respectively) for the treatments, except P in T4.

Regarding the layer below 0.10 m, pH values are below 5.0 for most of the treatments; nevertheless, the amounts of Ca and Mg are high and those of Al range from low to medium (CQFS, 2004). The CEC is average, the values of K ranging from medium to high, and P values are low (Table 2).

The organic carbon for both layers is low, which is explained by the degradation promoted during the topographic recovery of the area, since carbon losses can be attributed to mechanical mixing of the B with the A horizon. The amounts of carbon (Table 2), nevertheless, improved in relation to the original condition (Table 1).

After 78 months, treatments except T2 (P. notatum) showed lower soil density (BD), and higher values of total porosity (TP) and macroporosity (MA) than the control in the recovered topsoil (0.000.10 m) (Table 3).

It is common to relate root growth in compacted soil with its density, which is dependent on its textural class (Reinert et al., 2008). For clayey soils the critical limit of BD is 1.30-1.40 Mg m^-3 (Reichert et al, 2007). In the present study, all treatments had values above the critical threshold in both rehabilitated soil layers except U. brizantha (T4), which showed BD of 1.37 Mg m^-3 in the 0.00-0.10 m layer (Table 3). This result may indicate a better adaptation of T4 to the post-mining environment and its efficiency in improving the constructed soil structure, probably because of its aggressive root system, rapid establishment and tolerance to drought periods (Fontanelli et al., 2012).

The high degree of compaction in the 0.10-0.20 m layer is indicated by BD values much higher than 1.40 Mg m^-3, TP values below 50 %, and MA values lower than 10 % (Table 3), considered critical to plants growth (Reichert et al., 2007; Tormena et al., 1998; Girardelo et al., 2011).

Soil compaction occurs when an applied soil stress exceeds the strength of the soil, with the increasing vehicular pressures exerting their compactive effect more and more into the subsoil. This subsoil compaction proves to be very persistent (Lipiec et al., 2003). The compaction causes a structural change in the soil, increasing its density and reducing the total porosity and macroporosity (Stone et al., 2002). According to Neto et al. (2008), soils with low values of macropores present serious constraints to plant growth, gas exchange and water infiltration, increasing runoff and, consequently, the risk of hydric erosion.

When compared with the control, treatments T4, T3 and T1 presented lower values of BD and higher TP and MA in the 0.00-0.10 m layer (Table 3). This can be explained by the soil coverage effect, preventing or reducing the direct action of raindrops impact, as well as by the added biomass from the roots and the aerial part of the plants, helping to create an environment more favorable to aggregation (Campos et al., 1999).

The occurrence of improvements in the rehabilitated soil structure due to the presence of cover plants, can also be noted when evaluating the effect of grasses over time (05, 41 and 78 months), with most of the treatments showing positively changes in BD, TP and MA (Table 4).

The lower values of BD and the higher values of MA observed in the 0.00-0.10 m layer after 5 months compared with 41 months (Table 4), are probably the result of the conventional tillage operations performed for starting the experiment. Similar effect, but less pronounced, was observed in the 0.10-0.20 m layer.

The effect of scarification is considered temporary, the length of which being variable according to the soil type (Reichert et al., 2007). Initially, both operations (scarification and tillage) promote soil cultivation with the reduction of BD and increasing of MA; however, natural soil densification occurs with time in the absence of sequential tillage (Vieira and Klein, 2007). The slow establishment of cover plants in the first two years due to successive drought periods could also influenced this result.

From 41 to 78 months BD decreased and MA increased in the 0.00-0.10 m layer of the constructed soil (Table 4). Possibly, after 41 months more active root systems of the plant species studied in the surface layer improved the structural quality of the rehabilitated soil. In agricultural soils under no tillage, BD decreases with the time of adoption of the system (Vieira and Klein, 2007), which is explained by the development of galleries in the soil from the death of the root systems of previous crops (Hickmann et al., 2012). This contributes to the increase of water infiltration and diffusion of gases, improving the physical quality of the soil for subsequent crops (Foloni et al., 2006).

In the 0.10-0.20 m layer a slight decrease of the BD and a practically no change of the MA, from 41 to 78 months of the experiment (Table 4), indicates an unfavorable environment for root growth in this layer. The T4 was the only treatment that showed some improvement in MA in the layer for that period, indicating a higher potential of the U. brizantha for root growth in compacted layers. Plants modify soil structure mainly when their roots grow into the dense layers, then later die and decay leaving behind pores or channels which are called biopores (Yunusa and Newton, 2003).

Alternating shrinking and swelling and wetting and drying cycles, common to the clay soils, probably played an important role in the process of soil structure recovery in the present study. In agricultural soils, the formation of soil aggregates is dependent upon both abiotic and biotic factors; the former is mainly related to soil clay content and the capacity for natural structure forming processes (Topp et al., 1997). The first aggregation process is always governed by soil shrinkage and results in vertical cracks defining a prismatic structure; repeated wetting-drying cycles would further, by shear strength, induce the formation of blocky and subangular-blocky structure (Seguel and Horn, 2006).

Root penetration is often associated with soil fragmentation as it creates zones of failure. Wetting and drying cycles also influence the extent of soil fragmentation and aggregate formation. Drying produces cracks and induces fracture of aggregates. The drying of soil by the roots may also act synergistically with the aggregate binding material produced in the rhizosphere and increase soil structural stability (Angers and Caron, 1998). According to Bronick and Lal (2005), roots and hyphae enmesh and release organic compounds that act as glue to hold particles together. Particles can be rearranged during enmeshment, whereas wet-dry cycles help to stabilize the aggregates.

In fact, little is known about why some cover and agronomic crops are more effective in structural development than others (Bronick and Lal, 2005). Therefore, research should focus on a better understanding the interaction between rehabilitated mine soils and plant root systems in order to improve the performance of plants with the potential to recover these degraded areas.

 

CONCLUSION

The limitations for plant growth are due more to physical than to chemical conditions of the rehabilitated soil, especially in the 0.10-0.20 layer.

The process of rehabilitation of coal mine areas is slow due to the high degree of degradation that occurred during the extraction of coal and the topographic recovery of the area, adversely affecting the structural quality of the soils. However, the assessed grasses showed potential to reclaim these areas, since they improved in the values of the soil attributes over time, mainly in the 0.00-0.10 m layer.

After 78 months of evaluation, Urochloa brizantha showed the higher potential to recover the physical properties of the rehabilitated mine soils.

 

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