We sliced open radioactive particles from soil in South Australia and found they may be leaking plutonium

National Archives of Australia

Barbara Etschmann, Monash University; Joel Brugger, Monash University, and Vanessa Wong, Monash UniversityAlmost 60 years after British nuclear tests ended, radioactive particles containing plutonium and uranium still contaminate the landscape around Maralinga in outback South Australia.

These “hot particles” are not as stable as we once assumed. Our research shows they are likely releasing tiny chunks of plutonium and uranium which can be easily transported in dust and water, inhaled by humans and wildlife and taken up by plants.

A British nuclear playground

After the US atomic bombings of Hiroshima and Nagasaki in 1945, other nations raced to build their own nuclear weapons. Britain was looking for locations to conduct its tests. When it approached the Australian government in the early 1950s, Australia was only too eager to agree.

Between 1952 and 1963, Britain detonated 12 nuclear bombs in Australia. There were three in the Montebello Islands off Western Australia, but most were in outback South Australia: two at Emu Field and seven at Maralinga.

British nuclear tests left behind a radioactive legacy.
National Archives of Australia

Besides the full-scale nuclear detonations, there were hundreds of “subcritical” trials designed to test the performance and safety of nuclear weapons and their components. These trials usually involved blowing up nuclear devices with conventional explosives, or setting them on fire.

The subcritical tests released radioactive materials. The Vixen B trials alone (at the Taranaki test site at Maralinga) spread 22.2 kilograms of plutonium and more than 40 kilograms of uranium across the arid landscape. For comparison, the nuclear bomb dropped on Nagasaki contained 6.4 kilograms of plutonium, while the one dropped on Hiroshima held 64 kilograms of uranium.

These tests resulted in long-lasting radioactive contamination of the environment. The full extent of the contamination was only realised in 1984, before the land was returned to its traditional owners, the Maralinga Tjarutja people.

Hot potatoes

Despite numerous cleanup efforts, residual plutonium and uranium remains at Maralinga. Most is present in the form of “hot particles”. These are tiny radioactive grains (much smaller than a millimetre) dispersed in the soil.

Plutonium is a radioactive element mostly made by humans, and the weapons-grade plutonium used in the British nuclear tests has a half life of 24,100 years. This means even 24,100 years after the Vixen B trials that ended in 1963, there will still be almost two Nagasaki bombs worth of plutonium spread around the Taranaki test site.

Plutonium emits alpha radiation that can damage DNA if it enters a body through eating, drinking or breathing.

Read more:
Dig for secrets: the lesson of Maralinga’s Vixen B

In their original state, the plutonium and uranium particles are rather inactive. However, over time, when exposed to atmosphere, water, or microbes, they may weather and release plutonium and uranium in dust or rainstorms.

Until recently, we knew little about the internal makeup of these hot particles. This makes it very hard to accurately assess the environmental and health risks they pose.

Monash PhD student Megan Cook (the lead author on our new paper) took on this challenge. Her research aimed to identify how plutonium was deposited as it was carried by atmospheric currents following the nuclear tests (some of it travelled as far as Queensland!), the characteristics of the plutonium hot particles when they landed, and potential movement within the soil.

Nanotechnology to the rescue

Previous studies used the super intense X-rays generated by synchrotron light sources to map the distribution and oxidation state of plutonium inside the hot particles at the micrometre scale.

To get more detail, we used X-rays from the Diamond synchrotron near Oxford in the UK, a huge machine more than half a kilometre in circumference that produces light ten billion times brighter than the Sun in a particle accelerator.

Studying how the particles absorbed X-rays revealed they contained plutonium and uranium in several different states of oxidation – which affects how reactive and toxic they are. However, when we looked at the shadows the particles cast in X-ray light (or “X-ray diffraction”), we couldn’t interpret the results without knowing more about the different chemicals inside the particles.

To find out more, we used a machine at Monash University that can slice open tiny samples with a nanometre-wide beam of high-energy ions, then analyse the elements inside and make images of the interior. This is a bit like using a lightsaber to cut a rock, only at the tiniest of scales. This revealed in exquisite detail the complex array of materials and textures inside the particles.

Plutonium and uranium show up as bright lumps embedded in darker iron-aluminium alloy in this electron microscope image.
Cook et al (2021), Scientific Reports, Author provided

Much of the plutonium and uranium is distributed in tiny particles sized between a few micrometres and a few nanometres, or dissolved in iron-aluminium alloys. We also discovered a plutonium-uranium-carbon compound that would be destroyed quickly in the presence of air, but which was held stable by the metallic alloy.

This complex physical and chemical structure of the particles suggests the particles formed by the cooling of droplets of molten metal from the explosion cloud.

In the end, it took a multidisciplinary team across three continents — including soil scientists, mineralogists, physicists, mineral engineers, synchrotron scientists, microscopists, and radiochemists — to reveal the nature of the Maralinga hot particles.

From fire to dust

Our results suggest natural chemical and physical processes in the outback environment may cause the slow release of plutonium from the hot particles over the long term. This release of plutonium is likely to be contributing to ongoing uptake of plutonium by wildlife at Maralinga.

Even under the semi-arid conditions of Maralinga, the hot particles slowly break down, liberating their deadly cargo. The lessons from the Maralinga particles are not limited to outback Australia. They are also useful in understanding particles generated from dirty bombs or released during subcritical nuclear incidents.

Read more:
Friday essay: the silence of Ediacara, the shadow of uranium

There have been a few documented instances of such incidents. These include the B-52 accidents that resulted in the conventional detonation of thermonuclear weapons near Palomares in Spain in 1966, and Thule in Greenland in 1968, and the explosion of an armed nuclear missile and subsequent fire at the McGuire Air Force Base in the USA in 1960.

Thousands of active nuclear weapons are still held by nations around the world today. The Maralinga legacy shows the world can ill afford incidents involving nuclear particles.The Conversation

Barbara Etschmann, Research officer, Monash University; Joel Brugger, Professor of Synchrotron Geosciences, Monash University, and Vanessa Wong, Associate Professor, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Dishing the dirt: Australia’s move to store carbon in soil is a problem for tackling climate change


Robert Edwin White, University of Melbourne and Brian Davidson, University of Melbourne

To slow climate change, humanity has two main options: reduce greenhouse gas emissions directly or find ways to remove them from the atmosphere. On the latter, storing carbon in soil – or carbon farming – is often touted as a promising way to offset emissions from other sources such as energy generation, industry and transport.

The Morrison government’s Technology Investment Roadmap, now open for public comment, identifies soil carbon as a potential way to reduce emissions from agriculture and to offset other emissions.

In particular, it points to so-called “biochar” – plant material transformed into carbon-rich charcoal then applied to soil.

But the government’s plan contains misconceptions about both biochar, and the general effectiveness of soil carbon as an emissions reduction strategy.

Emissions rising from a coal plant.
Soil carbon storage is touted as a way to offset emissions from industry and elsewhere.

What is biochar?

Through photosynthesis, plants turn carbon dioxide (CO₂) into organic material known as biomass. When that biomass decomposes in soil, CO₂ is produced and mostly ends up in the atmosphere.

This is a natural process. But if we can intervene by using technology to keep carbon in the soil rather than in the atmosphere, in theory that will help mitigate climate change. That’s where biochar comes in.

Making biochar involves heating waste organic materials in a reduced-oxygen environment to create a charcoal-like product – a process called “pyrolysis”. The carbon from the biomass is stored in the charcoal, which is very stable and does not decompose for decades.

Plant materials are the predominant material or “feedstock” used to make biochar, but livestock manure can also be used. The biochar is applied to the soil, purportedly to boost soil fertility and productivity. This has been tested on grassland, cropping soils and in vineyards.

A handful of biochar.
Biochar is produced by burning organic material in a low oxygen environment.

But there’s a catch

So far, so good. But there are a few downsides to consider.

First, the pyrolysis process produces combustible gases and uses energy – to the extent that when all energy inputs and outputs are considered in a life cycle analysis, the net energy balance can be negative. In other words, the process can create more greenhouse gas emissions than it saves. The balance depends on many factors including the type and condition of the feedstock and the rate and temperature of pyrolysis.

Second, while biochar may improve the soil carbon status at a new site, the sites from which the carbon residues are removed, such as farmers’ fields or harvested forests, will be depleted of soil carbon and associated nutrients. Hence there may be no overall gain in soil fertility.

Read more:
A pretty good start but room for improvement: 3 experts rate Australia’s emissions technology plan

Third, the government roadmap claims increasing soil carbon can reduce emissions from livestock farming while increasing productivity. Theoretically, increased soil carbon should lead to better pasture growth. But the most efficient way for farmers to take advantage of the growth, and increase productivity, is to keep more livestock per hectare.

Livestock such as cows and sheep produce methane – a much more potent greenhouse gas than carbon dioxide. Our analysis suggests the methane produced by the extra stock would exceed the offsetting effect of storing more soil carbon. This would lead to a net increase, not decrease, in greenhouse gas

Beef cattle grazing in a field
Farmers would have to increase stock numbers to benefit from pasture growth.
Dan Peled/AAP

A policy failure

The government plan refers to the potential to build on the success of the Emissions Reduction Fund. Among other measures, the fund pays landholders to increase the amount of carbon stored in soil through carbon credits issued through the Carbon Farming Initiative.

However since 2014, the Emissions Reduction Fund has not significantly reduced Australia’s greenhouse gas emissions – and agriculture’s contribution has been smaller still.

Read more:
Carbon dioxide levels over Australia rose even after COVID-19 forced global emissions down. Here’s why

So far, the agriculture sector has been contracted to provide about 9.5% of the overall abatement, or about 18.3 million tonnes. To date, it’s supplied only 1.54 million tonnes – 8.4% of the sector’s commitment.

The initiative has largely failed because several factors have made it uneconomic for farmers to take part. They include:

  • overly complex regulations
  • requirements for expensive soil sampling and analysis
  • the low value of carbon credits (averaging $12 per tonne of CO₂-equivalent since the scheme began).
A farmer inspecting crops.
For many farmers, taking part in the Emissions Reduction Fund is uneconomic.

A misguided strategy

We believe the government is misguided in considering soil carbon as an emissions reduction technology.

Certainly, increasing soil carbon at one location can boost soil fertility and potentially productivity, but these are largely private landholder benefits – paid for by taxpayers in the form of carbon credits.

Read more:
Climate explained: are we doomed if we don’t manage to curb emissions by 2030?

If emissions reduction is seen as a public benefit, then the payment to farmers becomes a subsidy. But it’s highly questionable whether the public benefit (in the form of reduced emissions) is worth the cost. The government has not yet done this analysis.

To be effective, future emissions technology in Australia should focus on improving energy efficiency in industry, the residential sector and transport, where big gains are to be made.The Conversation

Robert Edwin White, Professor Emeritus, University of Melbourne and Brian Davidson, Senior Lecturer, Department of Agriculture and Food Systems, University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

To predict droughts, don’t look at the skies. Look in the soil… from space

Siyuan Tian, Australian National University and Albert Van Dijk, Australian National University

Another summer, another drought. Sydney’s water storages are running on empty, and desalinisation plants are being dusted off. Elsewhere, shrunken rivers, lakes and dams are swollen with rotting fish. Governments, irrigators and environmentalists blame each other for the drought, or just blame it on nature.

To be sure, Australia is large enough to usually leave some part of our country waiting for rain. So what exactly is a drought, and how do we know when we are in it?

This question matters, because declaring drought has practical implications. For example, it may entitle those affected to government assistance or insurance pay-outs.

But it is also a surprisingly difficult question. Droughts are not like other natural hazards. They are not a single extreme weather event, but the persistent lack of a quite common event: rain. What’s more, it’s not the lack of rain per se that ultimately affects us. The desert is a dry place but it cannot always be called in drought.

Ultimately, what matters are the impacts of drought: the damage to crops, pastures and environment; the uncontrollable fires that can take hold in dried-up forests and grasslands; the lack of water in dams and rivers that stops them from functioning. Each of these impacts is affected by more than just the amount of rain over an arbitrary number of months, and that makes defining drought difficult.

Read more:
Is Australia’s current drought caused by climate change? It’s complicated

Scientists and governments alike have been looking for ways to measure drought in a way that relates more closely to its impacts. Any farmer or gardener can tell you that you don’t need much rain, but you do need it at the right time. This is where the soil becomes really important, because it is where plants get their water.

Too much rain at once, and most of it is lost to runoff or disappears deep into the soil. That does not mean it is lost. Runoff helps fill our rivers and waterways. Water sinking deep into the soil can still be available to some plants. While our lawn withers, trees carry on as if there is nothing wrong. That’s because their roots dig further, reaching soil moisture that is buried deep.

A good start in defining and measuring drought would be to know how much soil moisture the vegetation can still get out of the soil. That is a very hard thing to do, because each crop, grass and tree has a different root system and grows in a different soil type, and the distribution of moisture below the surface is not easy to predict. Many dryland and irrigation farmers use soil sensors to measure how well their crops are doing, but this does not tell us much about the rest of the landscape, about the flammability of forests, or the condition of pastures.

Not knowing how drought conditions will develop, graziers face a difficult choice: sell their livestock or buy in feed?

Soils and satellites

As it turns out, you need to move further away to get closer to this problem – into space, to be precise. In our new research, published in Nature Communications, we show just how much satellite instruments can tell us about drought.

The satellite instruments have prosaic names such as SMOS and GRACE, but the way they measure water is mind-boggling. For example, the SMOS satellite unfurled a huge radio antenna in space to measure very specific radio waves emitted by the ground, and from it scientists can determine how much moisture is available in the topsoil.

Even more amazingly, GRACE (now replaced by GRACE Follow-On) was a pair of laser-guided satellites in a continuous high-speed chase around the Earth. By measuring the distance between each other with barely imaginable accuracy, they could measure miniscule changes in the Earth’s gravitational field caused by local increases or decreases in the amount of water below the surface.

By combining these data with a computer model that simulates the water cycle and plant growth, we created a detailed picture of the distribution of water below the surface.

It is a great example showing that space science is not just about galaxies and astronauts, but offers real insights and solutions by looking down at Earth. It also shows why having a strong Australian Space Agency is so important.

Read more:
The lessons we need to learn to deal with the ‘creeping disaster’ of drought

Taking it a step further, we discovered that the satellite measurements even allowed us to predict how much longer the vegetation in a given region could continue growing before the soils run dry. In this way, we can predict drought impacts before they happen, sometimes more than four months in advance.

Map showing how many months ahead, on average, drought impacts can be predicted with good accuracy.
author provided

This offers us a new way to look at drought prediction. Traditionally, we have looked up at the sky to predict droughts, but the weather has a short memory. Thanks to the influence of ocean currents, the Bureau of Meteorology can sometimes give us better-than-evens odds for the months ahead (for example, the next three months are not looking promising), but these predictions are often very uncertain.

Our results show there is at least as much value in knowing how much water is left for plants to use as there is in guessing how much rain is on the way. By combining the two information sources we should be able to improve our predictions still further.

Many practical decisions hinge on an accurate assessment of drought risk. How many firefighters should be on call? Should I sow a crop in this paddock? Should we prepare for water restrictions? Should we budget for drought assistance? In future years, satellites keeping an eye on Earth will help us make these decisions with much more confidence.The Conversation

Siyuan Tian, Postdoctoral fellow, Australian National University and Albert Van Dijk, Professor, Water and Landscape Dynamics, Fenner School of Environment & Society, Australian National University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Forest soil needs decades or centuries to recover from fires and logging

File 20190121 100288 15v1q9i.jpg?ixlib=rb 1.1

David Blair, Author provided

Elle Bowd, Australian National University and David Lindenmayer, Australian National University

The 2009 Black Saturday fires burned 437,000 hectares of Victoria, including tens of thousands of hectares of Mountain Ash forest.

As we approach the tenth anniversary of these fires, we are reminded of their legacy by the thousands of tall Mountain ash “skeletons” still standing across the landscape. Most of them are scattered amid a mosaic of regenerating forest, including areas regrowing after logging.

Read more:
Comic explainer: forest giants house thousands of animals (so why do we keep cutting them down?)

But while we can track the obvious visible destruction of fire and logging, we know very little about what’s happening beneath the ground.

In a new study published in Nature Geoscience, we investigated how forest soils were impacted by fire and logging. To our surprise, we found it can take up to 80 years for soils to recover.

Logging among the charred remains of Mountain ash after the 2009 fires.
David Blair, Author provided

Decades of damage

Soils have crucial roles in forests. They are the basis for almost all terrestrial life and influence plant growth and survival, communities of beneficial fungi and bacteria, and cycles of key nutrients (including storing massive amounts of carbon).

To test the influence of severe and intensive disturbances like fire and logging, we compared key soil measures (such as the nutrients that plants need for growth) in forests with different histories. This included old forests that have been undisturbed since the 1850s, forests burned by major fires in 1939, 1983 and 2009, forests that were clearfell-logged in the 1980s or 2009-10, or salvage-logged in 2009-10 after being burned in the Black Saturday fires.

We found major impacts on forest soils, with pronounced reductions of key soil nutrients like available phosphorus and nitrate.

A shock finding was how long these impacts lasted: at least 80 years after fire, and at least 30 years after clearfell logging (which removes all vegetation in an area using heavy machinery).

However, the effects of disturbance on soils may persist for much longer than 80 years. During a fire, soil temperatures can exceed 500℃, which can result in soil nutrient loss and long-lasting structural changes to the soil.

We found the frequency of fires was also a key factor. For instance, forests that have burned twice since 1850 had significantly lower measures of organic carbon, available phosphorus, sulfur and nitrate, relative to forests that had been burned once.

Sites subject to clearfell logging also had significantly lower levels of organic carbon, nitrate and available phosphorus, relative to unlogged areas. Clearfell logging involves removing all commercially valuable trees from a site – most of which are used to make paper. The debris remaining after logging (tree heads, lateral branches, understorey trees) is then burned and the cut site is aerially sewn with Mountain Ash seed to start the process of regeneration.

Fire is important to natural growth cycles in our forests, but it changes the soil composition.
David Lindenmayer, Author provided

Logging compounds the damage

The impacts of logging on forest soils differs from that of fire because of the high-intensity combination of clearing the forest with machinery and post-logging “slash” burning of debris left on the ground. This can expose the forest floor, compact the soil, deplete soil nutrients, and release large amounts of carbon dioxide into the atmosphere.

Predicted future increases in the number, frequency, intensity and extent of fires in Mountain Ash forests, coupled with ongoing logging, will likely result in further declines in soil nutrients in the long term. These kinds of effects on soils matter in Mountain Ash forests because 98.8% of the forest have already been burned or logged and are 80 years old or younger.

To maintain the vital roles that soils play in ecosystems, such as carbon storage and supporting plant growth, land managers must consider the repercussions of current and future disturbances on forest soils when planning how to use or protect land. Indeed, a critical part of long-term sustainable forest management must be to create more undisturbed areas, to conserve soil conditions.

Read more:
New modelling on bushfires shows how they really burn through an area

Specifically, clearfell logging should be limited wherever possible, especially in areas that have been subject to previous fire and logging.

Ecologically vital, large old trees in Mountain Ash forests may take over a century to recover from fire or logging. Our new findings indicate that forest soils may take a similar amount of time to recover.The Conversation

Elle Bowd, PhD scholar, Australian National University and David Lindenmayer, Professor, The Fenner School of Environment and Society, Australian National University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

We need more carbon in our soil to help Australian farmers through the drought

File 20181011 72127 z4wp9y.jpg?ixlib=rb 1.1
Healthy soils can hold water even during droughts.
Evie Shaffer/Unsplash

Nanthi Bolan, University of Newcastle

Australia has never been a stranger to droughts, but climate change is now super-charging them.

Besides taking a toll on human health, droughts also bake the earth. This means the ground holds less water, creating a vicious cycle of dryness.

Our research has investigated ways to improve the health and structure of soil so it can hold more water, even during droughts. It’s vital to help farmers safeguard their soil as we adapt to an increasingly drought-prone climate.

Read more:
Australia moves to El Niño alert and the drought is likely to continue

Soil moisture is key

The immediate effect of drought is complete loss of soil water. Low moisture reduces soil health and productivity, and increases the loss of fertile top soil through wind and water erosion.

To describe how we can improve soil health, we first need to explain some technical aspects of soil moisture.

Soil with good structure tends to hold moisture, protecting soil health and agricultural productivity.
Author provided

Soil moisture is dictated by three factors: the ability of the soil to absorb water; its capacity to store that water; and the speed at which the water is lost through evaporation and runoff, or used by growing plants.

These three factors are primarily determined by the proportions of sand, silt and clay; together these create the “soil structure”. The right mixture means there are plenty of “pores” – small open spaces in the soil.

Read more:
How to fight desertification and drought at home and away

Soils dominated by very small “micropores” (30-75 micrometres), such as clay soil, tend to store more water than those dominated by macropores (more than 75 micrometers), such as sandy soil.

If the balance is skewed, soil can actually repel water, increasing runoff. This is a major concern in Australia, especially in some areas of Western Australia and South Australia.

Improving soil structure

Good soil structure essentially means it can hold more water for longer (other factors include compaction and surface crust).

Farmers can improve soil structure by using minimum tillage, crop rotation and return of crop residues after harvest.

Another important part of the puzzle is the amount of organic matter in the soil –it breaks down into carbon and nutrients, which is essential for absorbing and storing water.

There are three basic ways to increase the amount of organic matter a given area:

  • grow more plants in that spot, and leave the crop and root residue after harvest

  • slow down decomposition by tilling less and generally not disturbing the soil more than absolutely necessary

  • apply external organic matter through compost, mulch, biochar and biosolids (treated sewage sludge).

Typically, biosolids are used to give nutrients to the soil, but we researched its impact on carbon storage as well. When we visited a young farmer in Orange, NSW, he showed us two sites: one with biosolids, and one without. The site with biosolids grew a bumper crop of maize the farmer could use as fodder for his cattle; the field without it was stunted.

The farmer told us the extra carbon had captured more moisture, which meant strong seedling growth and a useful crop.

Read more:
On dangerous ground: land degradation is turning soils into deserts

This illustrates the value of biowastes including compost, manure, crop residues and biosolids in capturing and retaining moisture for crop growth, reducing the impact of drought on soil health and productivity.

Improving soil health cannot happen overnight, and it’s difficult to achieve while in midst of a drought. But how farmers manage their soil in the good times can help prepare them for managing the impacts of the next drought when it invariably comes.

The author would like to thank Dr Michael Crawford, CEO of Soil CRC, for his substantial contribution to this article.The Conversation

Nanthi Bolan, Professor of Enviornmental Science, University of Newcastle

This article is republished from The Conversation under a Creative Commons license. Read the original article.

We have no idea how much microplastic is in Australia’s soil (but it could be a lot)

File 20180521 42200 6pmsdu.jpg?ixlib=rb 1.1
Microplastic in the soil is extremely difficult to track (or remove).
Florida Sea Grant, CC BY-NC-SA

Alisa Bryce, University of Sydney; Alex McBratney, University of Sydney; Budiman Minasny, University of Sydney; Damien Field, and Stephen Cattle, University of Sydney

Microplastics in the ocean, pieces of plastic less than 5mm in size, have shot to infamy in the last few years. Governments and businesses targeted microbeads in cosmetics, some were banned, and the world felt a little better.

Dealing with microbeads in cosmetics is a positive first step, but the reality is that they are just a drop in the ocean (less than a billionth of the world’s ocean).

Read more:
Film review: A Plastic Ocean shows us a world awash with rubbish

Microplastics in soil may be a far greater problem. Norwegian research estimates that in Europe and North America, between 110,000 and 730,000 tonnes of microplastic are transferred to agricultural soils each year.

Here lies the issue: we know almost nothing about microplastics in global soils, and even less in Australian soils. In this article we take a look at what we do know, and some questions we need to answer.

How microplastics get into agricultural soil

Sewage sludge and plastic mulch are the two biggest known contributors of microplastics to agricultural soil. Australia produces about 320,000 dry tonnes of biosolids each year, 55% of which is applied to agricultural land. Biosolids, while controversial, are an excellent source of nutrients for farmland. Of the essential plant nutrients, we can only manufacture nitrogen. The rest we must either mine or recycle.

Sewage treatment plants receive water from households, industry, and stormwater, each adding to the load of plastics. Technical clothing such as sportswear and quick-dry fabrics often contain polyesters and polyamides that break off when clothes are washed. Tyre debris and plastic films wash in with the stormwater. Treatment plants filter microplastics out of the water, retaining them in the sludge that is then trucked away and spread over agricultural land.

Read more:
Sustainable shopping: how to stop your bathers flooding the oceans with plastic

In agriculture, plastic mulch suppresses weeds, keeps the soil warm and damp to assist germination, and improves yield. Over time, these mulches break down, and some fragment into smaller pieces.

Biodegradable bioplastic mulches are designed to break down into carbon dioxide, water, and various “natural substances”. Environmentally friendly plastics are often more expensive, raising the question of whether businesses will be able to afford them.

Other potential sources of plastics in agricultural soil include polymer sealants on fertilisers and pesticides, and industrial compost. Unsold food is often sent to the composting facility still in plastic packaging, and with plastic stickers on every apple and kiwi fruit.

The Australian Standard for composts tacitly recognises that microplastics are likely to be present in these products by having acceptable levels of “visible contamination”. Anyone who has bought compost or garden loam from a landscaping supplier may have noticed pieces of plastic in the mix.

In horticulture, particularly as green walls and green roofs grace more buildings, polystyrenes are used deliberately to make lightweight ‘soil’.

There might be other pathways we don’t know about yet.

What happens once microplastics are in the soil?

Here we stand at the edge of the cavernous knowledge gap, because we don’t know the effect of microplastics in our soil. The overarching question, physically and biologically, is where do microplastics go?

How plastics fragment and degrade in the soil depends on the type of plastic and soil conditions. Compostable, PET, and various degradable plastics will behave differently, having different effects on soil physics and biology.

Fragments could move through soil cracks and pores. Larger soil fauna might disperse fragments vertically and laterally, while agricultural practices such as tillage could push plastics deeper into the soil. Some fragmented plastics can absorb agrochemicals.

Soil microbes can break down some plastics, but what are the byproducts and what are their effects? Newer, biodegradable bioplastics theoretically have limited impact as they break down into inert substances. But how long do they take to break down in different soil and climatic conditions, and what proportion in the soil are non-degradable PET plastics?

Both the main form of carbon in soil and polythene (the most common type of plastic) are carbon-based polymers. Could the two integrate? If they did, would this prevent plastics from moving deeper into the soil, but would it also stop them breaking down?

Could plastics be a hidden source of soil carbon storage?


Bioaccumulation is when something builds up in a food chain.

Research into microplastic accumulation on land is sparse at best. A 2017 study in Mexico found microplastics in chicken gizzards. In the study area, waste management is poor and most plastics were ingested directly from the soil surface as opposed to having bioaccumulated.

Nematodes can take up polystyrene beads suggesting some potential for bioaccumulation, however earthworms have reduced growth rate and increased mortality when they ingest microbeads.

Larger microplastics are unlikely to cross plant cell membranes, but it’s possible that plants can absorb the chemicals formed when plastic degrades. Plants have natural mechanisms to keep contaminants out of their fruiting bodies – pieces of plastic in apples or berries is highly unlikely – but root vegetables and leafy greens are a different story.

Metals can accumulate in leafy greens and the skin of root vegetables – could plastics or their byproducts do the same?

This is before we even get to nanoplastics, which are 1-100 nanometres wide. Can plant roots can absorb nanoplastics, and can they pass through an animal’s gut membrane?

Where to now?

The first step is to quantify how much plastic is currently in the soil, where it is, and how much more to expect. This is more difficult in land than water, as it’s easier to filter plastics out the ocean than to separate them from soil samples. The smaller the plastics are, the harder they’ll be to track and identify – which is why research must start now.

Research needs to address the different types of plastics, including beads and other synthetic fibres. Each is likely to act differently in the soil and terrestrial ecosystems.

Understanding how these plastics react will inform the next obvious questions: at what quantity do they become hazardous to soil, plant and animal life, and how can we mitigate this impact?

The ConversationPlastics in soil represent artefacts of human civilisation. Soils are full of human artefacts; if this was not the case then there would be no field archaeology. However, the effects of microplastic may persist far longer than our own civilisation. We must fill in our knowledge gaps swiftly.

Alisa Bryce, Research Affiliate, University of Sydney; Alex McBratney, Professor of Digital Agriculture & Soil Science; Director, Sydney Institute of Agriculture, University of Sydney; Budiman Minasny, Professor in Soil-Landscape Modelling, University of Sydney; Damien Field, Associate professor, and Stephen Cattle, Associate professor, University of Sydney

This article was originally published on The Conversation. Read the original article.

Eyes down: how setting our sights on soil could help save the climate

Budiman Minasny, University of Sydney; Alex McBratney, University of Sydney; Brendan Malone, University of Sydney, and Uta Stockmann, University of Sydney

The world’s soils could be a key ally in the fight to limit global warming to 2℃, thanks to their ability to store carbon and keep greenhouse gases out of the atmosphere.

France’s agriculture minister Stéphane Le Foll has founded an ambitious international research program, called “4 pour mille” (“4 per 1000”), which aims to boost the amount of carbon-containing organic matter in the world’s soils by 0.4% each year.

The program was launched officially today at the United Nations climate summit in Paris, with the hope to sign up as many nations as possible.

How much carbon do soils store? A lot. At about 2.4 trillion tonnes of carbon, soil is the largest terrestrial carbon pool, and the top 2 metres of the planet’s soils hold four times as much carbon as all the world’s plants. Carbon stored in soil can also stay there for a very long time relative to carbon in plants.

Thanks to recently published maps of global soil carbon stocks, we can work out how much extra carbon needs to be stored in soils (and where) in order to meet the target.

The size of the task

There are roughly 149 million square kilometres of land in the world, so if all the world’s soil carbon were dispersed evenly there would be 161 tonnes per hectare. Hitting the 0.4% target would mean increasing soil carbon stocks by 0.6 tonnes (600 kg) of carbon per hectare per year, on average.

But of course, soils around the world vary widely in carbon storage – tropical peat soils, for example, hold about 4,000 tonnes of carbon per hectare, whereas sandy soils in arid regions may only hold 80 tonnes per hectare. The type of above-ground vegetation and how quickly the soil microbes use the carbon can also affect the amount of storage. Generally speaking, only a quarter of organic matter added to soil ends up being stored as carbon in the long term.

Farmers and other landowners would need detailed information about what exactly they will need to do to their own soils to boost their stored carbon by the required amount.

Is the target achievable?

Studies around the world suggest that soil carbon can potentially be stored at a rate of 500 kg of carbon per hectare per year – slightly below the average target – by reducing tillage and planting legume cover crops.

These estimates change with soil type and climatic regions. Our research suggests that some cropland areas of the world have the potential to hit the 0.4% target, locally at least, through more modest overall increases in carbon storage. Restoring the soil’s carbon content in these areas is a win-win situation, as it will offset greenhouse gas emissions and boost soil quality at the same time.

One such place is Australia, where current soil carbon estimates suggest that the 0.4% target could be met by boosting soil carbon by just 220 kg per hectare – something that could easily be delivered in places that are not suffering drought.

The “4 per 1000” aspiration is an ambitious one, but perhaps even more important is the effect this initiative will have on promoting good soil management, which in turn can help to mitigate climate change. By encouraging farming practices that store more carbon, we can also help farmers improve the quality of their soils and boost food security at the same time.

The Conversation

Budiman Minasny, Associate Professor in Soil Modelling, University of Sydney; Alex McBratney, Professor of Soil Science, University of Sydney; Brendan Malone, Research fellow, University of Sydney, and Uta Stockmann, , University of Sydney

This article was originally published on The Conversation. Read the original article.