With many agricultural soils at risk of degradation, the world’s food supply might soon depend on the success of remediation science.
It can take hundreds of years for microbes and animals, such as worms, to break down organic matter and create just a few centimetres of fertile surface soil — also known as topsoil. But, according to the Food and Agriculture Organization of the United Nations, 90% of agricultural topsoil is at risk of degradation and erosion1 by 2050.
Deyi Hou at Tsinghua University is working on ways to remediate a growing number of brownfield sites so that they can be used as productive land.
Given that almost all of the world’s food is grown in soil,2 it’s alarming that the United Nations estimates that as much as 40% of the Earth’s land is degraded, says Deyi Hou, director of the division of soil and groundwater environment at the School of Environment at Tsinghua University in Beijing. This affects poses a threat to roughly half of the global GDP3, he points out.
China is a major agricultural producer, and Hou is working to protect its valuable agricultural soils using sustainable approaches. “Black soil is generally considered richer in nutrient content, and is suitable for most agricultural purposes,” explains Hou.
Northeast China is also one of the world’s largest areas of black soil. But between 1996 and 2019, an additional 116,000 square kilometres was planted with crops.4 Unsustainable agricultural practices can lead to pesticide and heavy metal pollution, which accelerates erosion, says Hou.
“A national soil survey in China in 2014 revealed that more than 16% of our soil is contaminated,” says Hou. “This led the government to establish legislation for soil remediation. In 2019, for example, China enacted a law that requires state agencies to supervise and control soil contamination.”5 An ongoing soil survey — planned for completion in 2025 — should provide insight on the impact of such laws.6
Deyi Hou (forth from left) and his team are working on solutions for remediating soil contaminated by multiple heavy-metal and organic pollutants.
Protecting microbes
“Soil degradation is the result of many factors, including soil pollution, increases in salt or acid concentrations, and mechanical water and wind erosion,” Hou explains. These all reduce the amount of the life in the soil, while water and wind erosion leach essential nutrients and damage the soil structure.
A huge problem is that many businesses — from petrol stations to factories — abandon polluted industrial sites rather than remediating them, which results in vast swaths of unhealthy soil, or ‘brownfields’. These can contain toxic heavy metals and volatile organic compounds released from waste, leaking pipelines, broken storage tanks and wastewater ponds. There are more than five million potentially contaminated brownfield sites globally, notes Hou.
His experience working with brownfields began early in his career. Before joining the faculty at Tsinghua University, Hou had been working as a consultant on soil remediation in the United States and the United Kingdom, including at locations that had formerly been rubbish tips.
Hou notes that “achieving long-term and sustainable remediation of contaminants at such sites is exceptionally challenging” due to difficult geophysical conditions and complex pollutants.
Today, Hou’s team is working on another type of brownfield that is contaminated by multiple heavy-metal pollutants — including arsenic, cadmium, and mercury. These can be caused by everything from coal-burning power plants and oil refineries, to chemical and pesticide-producing facilities.7
“Heavy metal pollution is widespread and they often evident on sites that have mixed and complicated contamination issues. These are challenging to remediate when compared to soils contaminated by single elements,” Hou says.
From the ashes
One solution Hou is working on involves adding biochar to soils. Biochar is a form of charcoal made by breaking down biomass at high temperatures (typically above 300 °C) in the absence of oxygen.
One of the things biochar does is increase the microbial content of soil, which helps break down organic contaminants in soil, Hou explains. Studies have shown that this is due to biochar’s high porosity, which provides a higher density of living space in the soil for bacteria and fungi.8
Biochar’s porosity can also draw in large amounts of organic and inorganic contaminants, says Hou.9 “Additionally, biochar can be engineered to possess certain surface properties, allowing it to be customized for specific types of contaminants, such as heavy metals or organic pollutants.”
Adding biochar can help improve soil nutrient content and the amount of water it can hold, protecting it from erosion. Hou’s team are leveraging engineered biochars and clay-based additions, such as layered double hydroxides, to remediate soils that contain multiple contaminants.
Biochar composites and modified layered double hydroxides have been shown to capture both heavy metals and organic contaminants, including herbicides, Hou explains.10,11 Field-scale trials of this type of remediation are underway in many regions from southwest to northeast China, says Hou. If successful, these amendments will be potentially developed for use by industry, he adds.
The benefits of black soil
In the end, we must not forget that black soil is also a sustainable resource, says Hou. It retains moisture well, which helps the uptake of water by plants and reduces wind erosion. It is also full of nutrients, reducing need for chemical fertilizers. And black also holds its shape, providing a stable and fertile base for crops, contributing to long-term crop production, “which is crucial for global food security”, says Hou.
Beyond agricultural benefits, black soil can store large amounts of organic carbon, making it a significant global carbon sink. “Protecting this soil type is critical for carbon sequestration and mitigating climate change,” Hou says. “When black soil erodes or degrades, the stored carbon is released into the atmosphere, contributing to greenhouse-gas emissions.”
Environmentally sound approaches to remediating brownfields can reduce greenhouse-gas emissions by as much as 80%.12 These can range from 'sustainable immobilization’ — which involves adding low-impact binders to the soil (like biochar) — to bioremediation, which is when plants, bacteria, fungi or animals are used to extract, stabilize, degrade or move contaminants in the soil or groundwater.
But these remediation systems are also challenged by climate change, says Hou. Inundation at contaminated sites related to flooding or sea-level rise can cause the spread of pollutants, for example. We must pick remediation strategies that are resilient to these changes, says Hou.
He also points out that UN estimates that the speed of global land take for settlement will double by 2050. So, ideally brownfields are able to be reused and revitalized. This raises ethical and equality considerations, says Hou. For instance, low-income communities should not be forced to live on or near brownfields or poorly remediated brownfields, he says.
And to really address soil issues, nations must also work together, he says. “It’s essential to adopt a holistic perspective that transcends the site boundaries and considers a broader remit and time horizon.”
References
1. United Nations. FAO warns 90 per cent of Earth’s topsoil at risk by 2050 (2022) https://news.un.org/en/story/2022/07/1123462
2. Food and Agriculture Organization of the United Nations. Global symposium on soil erosion. (2019). https://www.fao.org/about/meetings/soil-erosion-symposium/key-messages/en/
3. United Nations Convention to Combat Desertification. Chronic land degradation: UN offers stark warnings and practical remedies in Global Land Outlook 2. (2022) https://www.unccd.int/news-stories/press-releases/chronic-land-degradation-un-offers-stark-warnings-and-practical
4. Hou, D. China: protect black soil for biodiversity Nature 604, 40 (2022) doi: 10.1038/d41586-022-00942-6
5. Law of the People’s Republic of China on Prevention and Control of Soil Contamination. (2019). https://english.mee.gov.cn/Resources/laws/environmental_laws/202011/t20201113_807786.shtml
6. State Council of the People’s Republic of China. Circular of the State Council on conducting the third national soil census State Council Gazette 7, 1762 (2022)
7. UN Environment Programme. Heavy metals. https://www.unep.org/cep/heavy-metals
8. Bolan, S., Hou, D., Wang, L., Hale, L., Egamberdieva, D. et al. The potential of biochar as a microbial carrier for agricultural and environmental applications Science of The Total Environment 886, 163968 (2023) doi: 10.1016/j.scitotenv.2023.163968
9. Guo, M., Song, W. & Tian, J. Biochar-facilitated soil remediation: mechanisms and efficacy variations. Frontiers in Environmental Science 8 (2020) doi: 10.3389/fenvs.2020.521512
10. Roy, A.S., Pillai, S.K. & Ray, S.S. Layered double hydroxides for sustainable agriculture and environment: an overview ACS Omega 7(24), 20428–20440 (2022) doi: 10.1021/acsomega.2c01405
11. Wang, L., Ok, Y. S., Tsang, D. C. W., Alessi, D. S., Rinklebe, J. et al. Biochar composites: Emerging trends, field successes and sustainability implications Soil Use and Management 38(1), 14-38 (2022) doi: 10.1111/sum.12731
12. Hou, D., Al-Tabbaa, A., O’Connor, D. et al. Sustainable remediation and redevelopment of brownfield sites Nature Reviews Earth & Environment 4, 271–286 (2023) doi: 10.1038/s43017-023-00404-1
Editor: Guo Lili
