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.




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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?
Shutterstock

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.




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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.




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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:
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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


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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.




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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.




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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.




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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)



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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).




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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.




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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

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.