Report on the Impact of Kiwifruit Branch Compost on Rare Earth Elements in Agricultural Soils with Emphasis on Sustainable Development Goals (SDGs)

Abstract

Rare earth elements (REEs) such as lanthanum (La) and cerium (Ce) are extensively used in various industries due to their unique magnetic, optical, and electrical properties. However, their accumulation in the environment poses potential risks to human health and ecosystems. This study investigates the effect of compost made from kiwifruit branches and sheep manure on the bioavailability and chemical forms of La and Ce in agricultural soils. Using Tessier’s sequential extraction method, the study found that compost addition significantly reduced the proportions of La and Ce in their effective states (exchangeable, bound to carbonates, and bound to iron and manganese oxides), accelerating their conversion to more stable forms. The transformation was most pronounced between the 14th and 21st day after compost application. The effect was proportional to the compost quantity, with higher compost ratios yielding greater stabilization. For environmental remediation, higher compost ratios are recommended, while a 10% compost ratio balances agricultural productivity and cost-efficiency.

Introduction

Rare earth elements (REEs) are critical to modern technology and agriculture but pose environmental and health risks due to their persistence and bioaccumulation. The Sustainable Development Goals (SDGs), particularly SDG 3 (Good Health and Well-being), SDG 6 (Clean Water and Sanitation), SDG 12 (Responsible Consumption and Production), and SDG 15 (Life on Land), underscore the importance of managing such contaminants sustainably.

This study addresses the environmental challenge of REE contamination by exploring sustainable soil remediation methods using organic compost derived from agricultural waste, aligning with SDG 12 and SDG 15.

Materials and Methods

Materials

• Soil samples were collected from farmland near rare earth tailings in Southwest China.
• Compost was prepared from kiwifruit branches and sheep manure in a 1:1 ratio.
• REE concentrations and forms were analyzed using Tessier’s sequential extraction method.
• Analytical instruments included ICP-MS, centrifuge, oven, and shaker.

Experimental Design

• Soil and compost were mixed at ratios of 0%, 10%, 20%, 30%, 40%, and 50% compost by weight.
• Samples were incubated outdoors for 28 days with daily watering.
• Sequential extraction was performed to quantify La and Ce in five chemical forms: exchangeable (EC), bound to carbonates (BC), bound to iron and manganese oxides (BIM), bound to organic matter (BO), and residual state (RS).
• Sampling occurred at multiple time points to assess temporal changes.

Results

Effect of Compost on Soil pH

The compost exhibited alkaline properties (pH 8.42), increasing the soil pH from an initial 5.04. Soil pH rose proportionally with compost ratio and over time, influencing REE speciation and bioavailability.

Impact on Lanthanum (La) Forms

1. Exchangeable (EC) La: Increased with compost up to 30% then decreased, peaking at 0.021 mg/kg (0.107%).
2. Bound to Carbonates (BC) La: Decreased consistently with compost increase, lowest at 50% compost.
3. Bound to Iron and Manganese oxides (BIM) La: Declined with higher compost ratios.
4. Bound to Organic Matter (BO) La: Showed a complex trend with initial decrease then increase, influenced by compost ratio.
5. Residual State (RS) La: Increased with compost, indicating stabilization of La in soil.

Impact on Cerium (Ce) Forms

1. Exchangeable (EC) Ce: Similar biphasic trend as La, peaking at 30% compost.
2. Bound to Carbonates (BC) Ce: Decreased with compost increase.
3. Bound to Iron and Manganese oxides (BIM) Ce: Declined with compost addition.
4. Bound to Organic Matter (BO) Ce: Varied with compost ratio, lowest at 10% compost.
5. Residual State (RS) Ce: Increased at 10% compost, indicating enhanced stabilization.

Transformation of Effective States

The effective states (EC, BC, BIM) of La and Ce decreased significantly with increasing compost ratios, demonstrating a shift towards more stable, less bioavailable forms. This transformation was most rapid between days 14 and 21 post-compost addition.

Temporal Dynamics

• Exchangeable forms of La and Ce showed a biphasic response with a minimum at day 14.
• Effective states peaked at day 14 then declined sharply by day 21, stabilizing thereafter.
• Recommendations include planting crops after 21 days to minimize REE bioavailability or initiating phytoremediation within 14 days to maximize uptake.

Discussion

This study highlights the role of organic compost in modulating the bioavailability of REEs in agricultural soils, contributing to SDG 15 by promoting sustainable land management and SDG 12 through responsible waste utilization. The compost-induced increase in soil pH and organic matter content influences REE speciation, reducing their mobility and uptake by plants, thereby mitigating potential health risks (SDG 3).

Findings suggest that compost amendments can be optimized to balance environmental remediation and agricultural productivity, supporting SDG 2 (Zero Hunger) by ensuring safe and sustainable food production.

Conclusion

• Compost from kiwifruit branches and sheep manure effectively reduces the bioavailability of La and Ce by transforming them from effective to stable states in soil.
• The effect is proportional to compost quantity, with rapid transformation occurring between days 14 and 21.
• For environmental remediation, higher compost ratios are recommended; for agricultural use and cost control, a 10% compost ratio with planting after 21 days is optimal.
• This approach aligns with multiple SDGs by promoting sustainable agriculture, environmental health, and responsible waste management.
• Further research on plant growth and soil microbial diversity is recommended to fully assess biological impacts.

Implications for Sustainable Development Goals (SDGs)

• SDG 2 (Zero Hunger): Enhances soil quality and safe crop production by reducing REE bioavailability.
• SDG 3 (Good Health and Well-being): Mitigates human health risks associated with REE contamination.
• SDG 6 (Clean Water and Sanitation): Limits REE leaching into groundwater, protecting water quality.
• SDG 12 (Responsible Consumption and Production): Promotes recycling of agricultural waste into compost for soil remediation.
• SDG 15 (Life on Land): Supports sustainable land use and biodiversity by reducing soil contamination.

1. Sustainable Development Goals (SDGs) Addressed or Connected

1. SDG 2: Zero Hunger
   • The article discusses agricultural soils and the impact of compost on soil quality, which relates to sustainable agriculture and food security.
2. SDG 3: Good Health and Well-being
   • The article highlights potential threats to human health from rare earth elements (REEs) entering the food chain and causing various health disorders.
3. SDG 6: Clean Water and Sanitation
   • REEs contamination in soil and groundwater is discussed, impacting water quality.
4. SDG 12: Responsible Consumption and Production
   • The study investigates the use of compost made from agricultural waste (kiwifruit branches and sheep manure) for soil remediation, promoting waste valorization and sustainable production.
5. SDG 15: Life on Land
   • The article addresses soil contamination by heavy metals (REEs), their bioavailability, and remediation strategies to protect terrestrial ecosystems.

2. Specific Targets Under the Identified SDGs

1. SDG 2: Zero Hunger
   • Target 2.4: By 2030, ensure sustainable food production systems and implement resilient agricultural practices that increase productivity and production, help maintain ecosystems, and strengthen capacity for adaptation to climate change.
2. SDG 3: Good Health and Well-being
   • Target 3.9: By 2030, substantially reduce the number of deaths and illnesses from hazardous chemicals and air, water and soil pollution and contamination.
3. SDG 6: Clean Water and Sanitation
   • Target 6.3: By 2030, improve water quality by reducing pollution, minimizing release of hazardous chemicals and materials, halving the proportion of untreated wastewater, and substantially increasing recycling and safe reuse globally.
4. SDG 12: Responsible Consumption and Production
   • Target 12.5: By 2030, substantially reduce waste generation through prevention, reduction, recycling and reuse.
   • Target 12.4: By 2020, achieve the environmentally sound management of chemicals and all wastes throughout their life cycle.
5. SDG 15: Life on Land
   • Target 15.1: By 2020, ensure the conservation, restoration and sustainable use of terrestrial and inland freshwater ecosystems and their services.
   • Target 15.3: By 2030, combat desertification, restore degraded land and soil, including land affected by desertification, drought and floods.

3. Indicators Mentioned or Implied to Measure Progress

1. Concentration and Proportion of Rare Earth Elements (La and Ce) in Soil
   • Measured in different chemical states: Exchangeable state (EC), Bound to Carbonates (BC), Bound to Iron and Manganese oxides (BIM), Bound to Organic Matter (BO), and Residual state (RS).
   • Used to assess bioavailability and mobility of REEs in soils.
2. Soil pH Levels
   • Changes in soil pH with compost addition are measured, influencing REEs bioavailability and migration.
3. Effectiveness of Compost Ratios
   • Different compost ratios (0%, 10%, 20%, 30%, 40%, 50%) and their impact on REEs forms and soil pH are indicators of remediation efficiency and agricultural suitability.
4. Time-based Changes in REEs Forms
   • Monitoring changes in REEs forms over time (0, 7, 14, 21, 28 days) to evaluate the dynamics of bioavailability and stabilization.
5. Bioavailability of REEs
   • Implied through the proportion of effective states (EC, BC, BIM) versus stable states (BO, RS) in soil, indicating potential uptake by plants and entry into the food chain.

4. Table of SDGs, Targets, and Indicators

Source: nature.com