Abstract: Background and Objective: The use of cover crop residue for improving soil quality has been widely applied. Nevertheless, the effectiveness for improving ex-mining soil quality and crop performance at ex-mining soils is rarely documented. This study investigated the effect of cover crop residue on soil quality enhancement and corn production established in ex-nickel mining soils. Materials and Methods: An experiment comprising three treatment of cover crops residue, including Eleusine indica, Centrosema pubescens and Calopogonium mucunoides, arranged in a completely randomized design with three replications. The soil improvement process was evaluated by several parameters, such as soil acidity, soil organic carbon, total nitrogen, exchangeable potassium, exchangeable magnesium and heavy metals. On the other side, corn's growth performance was assessed using some attributes, i.e. height, diameter, total leaves, leaf area and biomass accumulation. Results: The results demonstrated that the cover crops residue had the potential to improve ex-nickel mining soil quality. The highest soil improvement was recorded in total nitrogen (700-800%). The treatments also showed a positive advantage to reduce heavy metals content, particularly for Fe, Mn and Zn by approximately 51.58-85.74%. No significant difference in corn growth performance was found in this study (p>0.05). However, the utilization of crop residue from Calopogonium mucunoides exhibited relatively higher total biomass than other treatments by around 3.08±1.99 g plant–1. Conclusion: Despite the treatments had no significant effect on corn performance. This study realized that cover crop residue could improve soil conditions for providing better environmental conditions for agriculture development.
INTRODUCTION
Integration of mining reclamation and food security currently become the most crucial issue in tropical land management1, including in Indonesia. In this context, the implementation of mining reclamation is expected to accelerate the land cover process by conducting revegetation and providing additional benefits for rural development and poverty alleviation2. This objective can be realized by converting the ex-mining area to agricultural land. However, the scenario is not easy to conduct since the soil characteristics at the ex-mining area have low fertility and high soil acidity due to contamination3-5. Moreover, the soil quality at the ex-mining area also has an excessive amount of heavy metals6-8. This condition is not suitable for supporting crop cultivation because many soil parameters become the limiting factors for plant growth and development. To anticipate the problem, soil amendment strategies can enhance soil quality at the ex-mining area, one of them is using cover crop residue9.
Several previous studies report that the use of cover crop residue for improving soil quality at the ex-mining site has been intensively conducted in many regions10-12, primarily at the location of ex-coal mining. The use of cover crop residue for mining reclamation provides benefits in improving soil structure, increasing soil organic matter, maintaining soil moisture and accelerating the activity of soil microorganisms10,13. In addition, the utilization of this treatment for phytoremediation in ex-mining soil highly increases the phosphorus availability by around 40-50%14. Those explanations confirm the potential use of cover crop residue as one of the methods for soil amendment at the ex-mining area. However, cover crop residue as a soil amendment treatment at the ex-mining area has to examine in other locations since every type of mining result in different problems of soil contamination. Furthermore, the occurrence of site interaction among soil, climate and topography may also affect the effectiveness of cover crop residue for improving soil quality at the ex-mining location15.
Moreover, the soil type in this area is predominantly ultramafic soil that naturally has low fertility due to high heavy metals content16. In this case, the application of cover crop residue is also directed to anticipate this challenge. It’s a hypothesis that covers crop residue provides a meaningful role in improving soil quality and the growth performance of corn at the ex-nickel mining soils.
This study aims to evaluate the effect of cover crop residue on soil improvement and corn performance at the ex-nickel mining soils located in Southeast Sulawesi. The research is important to implement because many ex-nickel mining lands still required reclamation activity in this location.
MATERIALS AND METHODS
Study area: This study was conducted at the laboratory level for facilitating the measurement process periodically. The study site was located at the field nursery managed by the Department of Soil Science, Faculty of Agriculture, Halu Oleo University from September, 2018-July, 2019. Topography was relatively flat with a slope level of 0-8%. Altitude reached 55 m above sea level, with a mean daily temperature was 27.68°C with a minimum of 23.87°C and a maximum of 32.05°C. Annual rainfall varied from 1,600-2,500 mm year–1 during the last five years from 2016-2020. The highest rainfall occurred in January. Dry periods were relatively more extended than five months from May-September. The average air humidity was 80.9%, with a minimum of 73% and a maximum of 86%.
Experimental design: An experiment comprising three treatments of cover crop residue was set up using a Completely Randomized Design (CRD) with three replications for every treatment. The source of cover crop residue was from three different species that commonly used in plantations, namely Eleusine indica, Centrosema pubescens and Calopogonium mucunoides. Two species were categorized as family Leguminosae (C. pubescens and C. mucunoides), while another species was classified into family Gramineae (E. indica).
The soil materials used in this research were taken from the ex-nickel mining areas located in Konawe District. Soil properties were quantified first to evaluate its characteristics before starting the application of treatments (Table 1). This step was importantly conducted to obtain the preliminary information about soil characteristics as basic data to assess the effectiveness of treatment application for soil improvement.
| Table 1: Characteristics of soil materials from ex-nickel mining area used in this study | |||
| Parameters | Symbol |
Unit |
Value |
| Actual soil acidity | pH |
- |
5.64 |
| Soil organic carbon | SOC |
g kg–1 |
0.07 |
| Total nitrogen | TN |
g kg–1 |
0.03 |
| Exchangeable potassium | Exc-K |
mg kg–1 |
0.22 |
| Exchangeable magnesium | Exc-Mg |
mg kg–1 |
0.04 |
| Iron | Fe |
mg kg–1 |
7.44 |
| Manganese | Mn |
mg kg–1 |
7.44 |
| Zinc | Zn |
mg kg–1 |
2.68 |
| Data were presented in pH: Actual soil acidity, SOC: Soil organic carbon, TN: Total nitrogen, Exc-K: Exchangeable potassium, Exc-Mg: Exchangeable magnesium, Fe: Iron, Mn: Manganese and Zn: Zinc | |||
After obtaining the initial soil data, site preparation was implemented to create a homogeneous condition in the experiment. This effort was exceptionally required to minimize the bias observations due to the influence of environmental conditions outside the treatment17.
Afterwards, the soil material was placed in the bucket and provided a name tag to guarantee the specific code for every treatment. Then, the cover crop residue was mixed into the bucket with a dose of 200 g for each treatment. A month later, the corn seed was planted in the bucket. Corn was selected as an alternative crop species since this plant was highly sensitive to soil fertility. In this study, the number of plants in every plot was only one to derive more accurate observations at the individual plant level. The additional fertilizer was not given to observe the natural influence of cover crop residue on soil improvement and the growth performance of corn. However, the maintenance activities such as weed control and the watering process were still conducted to support the early growth of corn.
Data collection: Data were collected gradually in a chronological manner. Soil characteristics were measured two times, i.e. before treatment application and after corn harvesting. Several parameters were selected to quantify the soil properties, including soil acidity, soil organic carbon, total nitrogen, exchangeable potassium, exchangeable magnesium and heavy metals content (iron, manganese and zinc). Soil acidity was determined by a digital pH meter while soil organic carbon was quantified using Walkley and Black method18. Total nitrogen was calculated using the Kjeldahl method19 while exchangeable potassium was estimated using a spectrophotometer (Hitachi Double Beam U-2900, Kendari, SE Sulawesi, Indonesia). The exchangeable magnesium and heavy metals were quantified using atomic absorption spectrophotometry (Hitachi Z-2000 Tandem Type Atomic Spectrophotometry, Kendari, SE Sulawesi, Indonesia). The protocol of soil analysis was processed following the guidance of soil and water analysis20.
The growth performance of corn was measured periodically every two weeks. The harvesting period of corn was conducted in the sixth week. Our study focused on the growth performance of corn at the final observations because it represented the adaptability of corn toward the soil improvement process due to the effect of cover crop residue. Some parameters were measured in this period, such as height, diameter, total leaves, leaf area and biomass distribution. The measurement of plant biomass was conducted in every corn component, namely root, shoot and flower.
Data analysis: A descriptive test was applied to identify the range of data distribution21. The normality of data was examined using the Shapiro-Wilk test22. The homogeneity of variance among treatments was evaluated by the Fligner-Killeen test23. The comparison of soil improvement among treatments was presented descriptively by the actual value and percentage unit of improvement in every parameter. Meanwhile, the growth performance of corn among treatments was analyzed by the Kruskal-Wallis test and followed by the Kruskal-Nemenyi test24. There was a relationship between soil improvement and the growth performance of corn, the correlation analysis was applied using the Pearson method with a pallet matrix25. The process of data analysis was conducted using R software version 4.0.2. with a significant level of 5%.
RESULTS AND DISCUSSION
Soil improvement: Summarized results of the observation demonstrated that the application of cover crop residue showed a positive role in improving soil quality from the ex-nickel mining soils (Table 2). The use of cover crop residue from all treatments had a potential contribution to reducing soil acidity and increasing soil organic carbon, total nitrogen, exchangeable potassium as well as exchangeable potassium. Moreover, this study observed that the utilization of cover crop residue as organic matters substantially declined the heavy metal content in the ex-nickel mining soils by approximately 50-80%, particularly for iron, manganese and zinc. Interestingly, our study recorded the highest soil quality improvement among eight parameters found in total nitrogen with a range of 700-800% (Table 2). On another side, the lowest enhancement of soil quality was discovered in soil acidity. According to the results, it was indicated that cover crop residue from family Leguminosae (C. pubescens and C. mucunoides) improved total nitrogen better than family Gramineae (E. indica). In contrast, the use of cover crop residue from family Gramineae resulted in a more significant decline of heavy metal content than Leguminosae, primarily related to iron and zinc. However, every treatment demonstrated a future result to facilitate the soil improvement process at the ex-nickel mining soils.
The cover crop residue was classified as an organic matter that contained an amount of nutrients26. The accumulation of nutrients in the organic matter would be released into the soil when the decomposition process occurred27. It confirmed why the utilization of cover crop residue had a potential contribution to reducing soil acidity and improving macro nutrients in the ex-nickel mining soils.
| Table 2: Details soil improvement for every parameter in three different treatments of cover crop residue | ||||||||
| E. indica | C. pubescens | C. mucunoides | ||||||
| Parameter | Symbol |
Unit |
Actual value |
Percentage improve (%) |
Actual value |
Percentage improve (%) |
Actual value |
Percentage improve (%) |
| Actual soil acidity | pH |
- |
6.82 |
20.92 |
6.97 |
23.58 |
6.68 |
18.43 |
| Soil organic carbon | SOC |
g kg–1 |
0.36 |
414.28 |
0.36 |
414.28 |
0.4 |
471.42 |
| Total nitrogen | TN |
g kg–1 |
0.24 |
700 |
0.27 |
800 |
0.27 |
800 |
| Exchangeable potassium | Exc-K |
mg kg–1 |
0.86 |
290.9 |
0.74 |
236.36 |
0.6 |
172.72 |
| Exchangeable magnesium | Exc-Mg |
mg kg–1 |
0.14 |
250 |
0.16 |
300 |
0.14 |
250 |
| Iron | Fe |
mg kg–1 |
3.07 |
58.71 |
3.6 |
51.58 |
3.41 |
54.13 |
| Manganese | Mn |
mg kg–1 |
1.26 |
83.05 |
1.12 |
84.93 |
1.06 |
85.74 |
| Zinc | Zn |
mg kg–1 |
0.9 |
66.35 |
1.05 |
60.74 |
1 |
62.61 |
| Data were presented in the actual value and percentage unit | ||||||||
| Table 3: Growth performance of corn at the treatment of cover crop residue | |||||
| Treatment | |||||
| Parameter | Unit |
E. indica |
C. pubescens |
C. mucunoides |
p-value |
| Height | cm |
74.30±3.75a |
78.26±2.54a |
78.20±21.06a |
0.429 |
| Diameter | cm |
2.33±0.25a |
2.37±0.23a |
2.53±0.06a |
0.47 |
| Total leaves | - |
6.00±0.00a |
6.00±0.58a |
6.00±0.58a |
0.564 |
| Leaf area | cm2 |
699.07±84.55a |
675.49±82.29a |
583.97±46.74a |
0.193 |
| Root biomass | g |
0.80±0.04a |
0.74±0.21a |
0.65±0.30a |
0.874 |
| Shoot biomass | g |
1.89±0.27a |
2.07±1.26a |
2.31±1.81a |
0.956 |
| Flower biomass | g |
0.11±0.03a |
0.14±0.03a |
0.11±0.02a |
0.415 |
| Total biomass | g |
2.81±0.34a |
2.95±1.40a |
3.08±1.99a |
0.956 |
| Data were demonstrated in mean ± standard deviation. The similar letter in the row indicated there was not a significant difference based on the Kruskal-nemenyi test | |||||
| Table 4: Comparison of the growth performance of corn between the crop residue from family Gramineae and Leguminosae | |||
| Parameter | Unit |
Gramineae |
Leguminosae |
| Height | cm |
74.3 |
78.23 |
| Diameter | cm |
2.33 |
2.45 |
| Total leaves | - |
6 |
6.33 |
| Leaf area | cm2 |
699.07 |
629.74 |
| Root biomass | g |
0.8 |
0.7 |
| Shoot biomass | g |
1.9 |
2.19 |
| Flower biomass | g |
0.11 |
0.13 |
| Total biomass | g |
2.81 |
3.02 |
Cover crop residue could also decrease the heavy metals content because it formed complex compounds and made the heavy metals immobile28. In the case of mining reclamation, particularly for ex-nickel mining soils, the decreasing soil acidity and heavy metals content became the primary challenge in the process of revegetation. These parameters were the limiting factors that influenced plant survival29. Thus, the effort of site preparation was exceptionally required to accelerate the soil improvement process before planting activities. Referring to the results, the application of cover crop residue as the organic matter could become one of the alternative strategies for site preparation in the activity of mining reclamations.
Growth performance of corn: The use of cover crop residue did not provide a significant influence on the growth performance of corn for all observation parameters (p>0.05) (Table 3). Nevertheless, this study noted that the utilization of cover crop residue from family Leguminosae demonstrated a higher average corn performance than cover crop residue from the family Gramineae, except in leaf area and root biomass (Table 4). The data of Table 4 show that treatment using cover crop residue from family Leguminosae, the plant height reached 78.23 cm, stem diameter reached 2.45 cm, total leaves reached 6.33 pieces, shoot biomass reached 2.19 g, flower biomass reached 0.13 g and total biomass reached 3.02 g. While the treatment using cover crop residue the family Gramineae, the plant height only 74.30 cm, stem diameter only 2.33 cm, total leaves only 6.00 pieces, shoot biomass only 1.90 g, flower biomass only 0.11 g and total biomass only 2.81 g. Among the treatment applications, the highest average biomass of corn was recorded in the treatment of cover crop residue from C. mucunoides (3.08±1.99 g) and followed by C. pubescens (2.95±1.40 g) and E. indica (2.81±0.34 g) (Table 3). It usually occurred because the accumulation of soil organic carbon and total nitrogen at the treatment of C. mucunoides was relatively higher than other treatments (Table 2).
| Fig. 1(a-b): | (a) Relative contribution to the total biomass and (b) Correlation analysis among soil characteristics and the growth performance of corn H: Plant height, D: Plant diameter, L: Leaf number, LA: Leaf area, RB: Root biomass, SB: Shoot biomass, FB: Flower biomass, TB: Total biomass, pH: Actual soil acidity, CO: Soil organic carbon, NT: Total nitrogen, Mg: Exchangeable magnesium, K: Exchangeable potassium, Fe: Iron, Mn: Manganese, Zn: Zinc |
This finding was also confirmed by the results of correlation analysis wherein there was a strong correlation between soil organic carbon and total nitrogen with a total biomass of corn Fig. 1(a-b). The previous studies also supported it wherein the higher availability of soil organic carbon and total nitrogen significantly increased the biomass production of plant30-32.
The data of Fig. 1(a-b) show that the relative contribution of root biomass to the total biomass at the treatment of E. indica was relatively greater than other treatments. This trend could occur since the improvement of exchangeable potassium in this treatment was substantially higher than in other treatments. As one of the macronutrients, potassium availability played an essential role in plant physiology33, mainly related to the translocation process between source and sink34. In the marginal soils, the higher potassium availability could stimulate the more effective root development process35.A study reported that in facing stress conditions due to the impact of environments, mainly from heavy metals, potassium availability could support plant adaptability36.
The activity of soil management became the principal challenge in the context of mining reclamation, particularly at the ex-nickel mining soils. Besides having a serious problem related to soil contamination, mining reclamation had to face low soil fertility due to the high soil acidity and deficient nutrients availability. Consequently, the process of revegetation in the ex-mining area required expensive cost and long-time consuming. Therefore, most mining companies seek the most efficient method to accelerate the revegetation in the ex-mining area.
This study has evidenced the potential use of cover crop residue to accelerate the soil improvement at the ex-nickel mining soils. Besides improving nutrient availability, this method could also reduce soil acidity and heavy metals content. Most importantly, the use of cover crop residue had a more affordable cost than other soil amendment strategies. The results of the soil improvement process could be observed at a relatively short period, around 2-3 months. This method could become an alternative strategy for supporting the mining reclamation, primarily at the ex-nickel mining areas in Southeast Sulawesi.
CONCLUSION
This study concluded that the use of cover crop residue had a high potential contribution to improving the soil quality at ex-nickel mining soils in Southeast Sulawesi. The application of treatments provided a substantial role in recovering soil acidity, soil organic carbon, total nitrogen and exchangeable base as well as reducing heavy metals content. Thus, the treatments also gave good advantages for stabilizing better environmental conditions in the context of agriculture development, mainly related to crop cultivation. However, our study did not find a significant effect of cover crop residue on the growth performance of corn. Nevertheless, the application of cover crop residue from Calopogonium mucunoides exhibited relatively higher corn biomass than other treatments.
SIGNIFICANCE STATEMENT
This study discovers the use of land cover crop residues that have a high potential to restore ex-mining soil conditions to provide better environmental conditions for agricultural development. This study will help the researcher uncover the critical areas of rehabilitation of post-mining land using land cover residue for agricultural uses, which many researchers could not explore. Our finding revealed that the land cover crop residue has potential for land restoration, especially in post-mining land rehabilitation through adaptive cover crops. Finding adaptive cover crop types is an essential step in the ex-mining land rehabilitation strategy.