Document Type : Research Article
Authors
1
Assistant Professor, Department of Desert and Arid lands Management, Faculty of Natural Resources and Environment, Ferdowsi University of Mashhad, Mashhad, Iran
2
Associate Professor, Department of Desert and Arid lands Management, Faculty of Natural Resources and Environment, Ferdowsi University of Mashhad, Mashhad, Iran
3
Professors of Department of Soil Science, College of Agriculture & Natural Resources, University of Tehran, Karaj, Iran
Abstract
These days, pollution of soils by heavy metals has become an important environmental issue. These pollutants can cause the gradual elimination of beneficial organisms from soils, which may lead to a decrease in biodiversity—a defining characteristic of the dynamics, self-regulation, balance, and stability of soil ecosystems. The total metal concentrations in the environment do not necessarily reflect environmental conditions such as bioavailability and toxicity, as these depend on soil properties. Understanding the effective processes governing the bioavailability of metals can help in modeling and assessing the environmental effects of these elements in the soil.
To investigate the influence of land use type on the bioavailability of Cu, Mn, Fe, and Zn, 116 surface soil samples were collected from Chitgar Forest Park across three land uses: coniferous forest, broadleaved forest, and rangeland. The DTPA- and nitric acid-extractable forms of Cu, Mn, Fe, and Zn, as well as soil properties such as calcium carbonate content, pH, and organic carbon, were measured.
The results of the ANOVA analysis revealed that land use type significantly impacted the bioavailable forms of Cu, Mn, and Zn, whereas the available form of Fe did not differ significantly between the land uses. Additionally, land use type affected soil properties, especially soil organic carbon. Furthermore, the results of the Pearson correlation analysis showed a significant positive correlation between soil organic carbon and some heavy metals, indicating that plantation type and land use influence the availability of heavy metals in these soils.
Extended Abstract
Introduction
Pollution of soils by heavy metals has become an important environmental issue in recent years. These pollutants can cause the gradual elimination of beneficial organisms from soils, leading to decreased biodiversity—a fundamental factor in maintaining the dynamics, self-regulation, balance, and stability of soil ecosystems. Total metal concentrations in the environment do not necessarily reflect environmental conditions such as bioavailability and toxicity, as these depend on soil properties. Understanding effective processes affecting the bioavailability of metals can help in modeling and understanding the environmental effects of these elements in the soil. Studies have shown varying effects of different land uses on heavy metal concentrations, but research primarily focuses on farming, industry, or urban development. The effects of changing land use on the bioavailable forms of heavy metals remain unclear. This study aims to explore the impact of converting rangeland into broadleaf and needleleaf forests on the concentrations of bioavailable metal forms in soils over 45 years.
Material and Methods
The study area, covering 665 ha in Chitgar Forest Park, western Tehran, includes needleleaf forest (Pinus eldarica), broad-leaved forest (Robinia pseudoacacia, Fraxinus rotundifolia), and rangeland (Alhagi maurorum, Hordeum murinum, and other grasses). Stratified grid sampling identified 116 sites (250 × 250 m grid, 0–20 cm depth). Available forms of Zn, Cu, Mn, and Fe were determined using atomic absorption spectroscopy. Soil particle size was measured by the hydrometer method. Organic carbon, nitrogen, and total neutralizing value (TNV) were determined by wet oxidation, Kjeldahl, and volumetric methods, respectively. Electrical conductivity was measured in saturation extracts using an EC meter. The effects of different land-use types on the bioavailability of heavy metals were analyzed using one-way ANOVA.
Results and Discussion
The ANOVA analysis indicated that land-use type significantly influenced the bioavailability of Cu, Mn, and Zn, while Fe availability remained unchanged across different land uses. Soil characteristics, particularly soil organic carbon, were affected by land use, with a substantial positive correlation between soil organic carbon and certain heavy metals identified through Pearson correlation analysis. This suggests that land use and planting practices impact heavy metal concentrations in soils by affecting soil properties. Zn was more prevalent in broadleaf and needleleaf forests, whereas Mn and Cu were highest in rangelands, with rangelands having significantly more available Mn compared to needleleaf forests. Mn availability was influenced by total concentration, clay content, and organic carbon, though CaCO₃ reduced the impact of organic matter on Mn availability. Cu levels were highest in rangelands, with availability controlled by total Cu concentration, clay, and TNV. Zn concentrations were highest in broadleaf forests, strongly correlating with organic carbon. Land-use types influenced Zn concentrations through their effect on organic matter. Fe showed significant correlations with soil organic matter but did not differ significantly between land uses.
Conclusion
After forty-five years of converting rangeland into needleleaf and broadleaf forests, this study explored the availability of four heavy metals under three land uses. The results revealed significant differences in the available forms of Zn, Cu, and Mn due to the distinct effects of land-use types on soil properties. Land-use type notably influenced soil characteristics, particularly soil organic carbon. Additionally, Pearson correlation analysis identified a substantial positive relationship between soil organic carbon and heavy metals. These findings underscore the critical role of land use and planting practices in determining heavy metal availability in soils, with important implications for environmental management and agricultural practices.
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