Cosmos Q&A: Threat to our soils

The Australian Academy of Science this week called for greater attention to be paid to the significant damage bushfires cause to the soil, releasing an Expert Brief outlining the threats to agricultural productivity and the risk that some native vegetation may not recover.

Cosmos spoke to the authors, Professor Alexander McBratney, from the University of Sydney, and Professor Rob Fitzpatrick, from the University of Adelaide.

Bushfires have always been a part of life in Australia. Is there a new sense of urgency?

Australian soils have been impacted by fire for tens of thousands of years. Interestingly, Indigenous peoples have used fire as a beneficial practice to successfully manage Australian landscapes for millennia and to prevent “mega fires” like what was observed in 2019-20. Unfortunately, this practice was interrupted due to the arrival of European settlers in the late eighteenth century.  Bushfires can be beneficial to soil because the combustion of leaf litter and other organic matter can increase the availability of some nutrients, but there are many downsides.

However, there is an increasing sense of concern because the current extreme heat waves, drought conditions and high accumulative fuel loads at the soil surface have resulted in fires burning at higher than usual temperatures during the Australian 2019/20 bushfires. These bushfires severely damaged millions of hectares of land across Australia.

Sufficiently intense bushfires can cause long-term and sometimes even irreversible transformations to soil condition and severely damage local ecosystems. Significantly, these consequences have implications on how we can manage our fragile soil resources.

The changes in soil condition following a bushfire event have clear implications for soil fertility and, therefore, Australia’s agricultural productivity and the recovery of native vegetation and ecosystems more generally.

It is difficult to develop a management plan for soil condition post-fire without first implementing plans to accurately monitor ecosystems before fires, and documenting fire intensity and severity. Accordingly, we believe there is now an urgency to commence “a nationally consistent framework for monitoring the effects of bushfire on soil condition” – with a specific focus on an Australia-wide campaign for soil data observation, storage and accessibility. More work is required to advance and hasten soil recovery from bushfires.

Can soil fully recover or is some bushfire damage irreparable?

Fire is a natural ecological process and many ecosystems are adapted to it or are even dependent on fire disturbance.  In such circumstances, post-fire, there is a logical plant succession characterised by the simultaneous growth of herbaceous species, shrubs and trees. As they grow, one or more species will become dominant due to exposure to the sun or competition for water and soil nutrients.  

In such ecosystems, soil can fully recover and it has also been shown that post-fire management is not always necessary if the aim is to recover the same vegetation type, and that the forest was prepared for this natural disturbance.

Decomposition of soil organic matter, a result of bushfire, begins at 200-250 degrees Celsius and complete degradation occurs around 460 degrees. Some soil nutrients are more recalcitrant to heat degradation and are not as readily affected as others. While we can lose soil organic matter in one fire episode it will probably take decades to regenerate.

Under low oxygen conditions below the soil surface plant roots etc. can undergo thermal decomposition in a limited supply of air (oxygen) to for a kind of charcoal called biochar, which is relatively stable to further degradation.

Fire can  cause irreversible mineral alterations to soil, which includes the permanent conversion of some minerals into new minerals under a range of temperature conditions.

For example, following severe bushfire, coarse soil fractions comprising hard, ceramic-like fragments are formed within clayey soils with high organic matter (e.g. burn peaty clays). These soil types have been identified in Australian soils and has led to the introduction of “burnt soil materials” in the Australia Soil Classification, now classified as “fusic material”. Extremely high temperature fires (>800 degrees for more than one hour or 600 degrees for 80 hours) have also been shown to melt other salt-rich soil types (e.g. saline acid sulfate soils) to form masses of glass-like groundmass. 

These solid masses reduce the chemical, physical, and consequently biological characteristics of soil condition.  Similarly, hyroxylapatite (a potential source of phosphorus, which is an essential molecule for plants and microbes that live in the soil), in bone from humans, livestock and wild animals during intense bushfire, is permanently transformed into a crystalline structure and becomes inaccessible to plant uptake.

Fire in areas of vegetation leads to permanent soil loss by soil erosion. It is the root system of the above ground vegetation that aids to hold soil in place. The replacement of perennial trees and shrubs lost during a bushfire could take 95-100 years to recover. Although not irreversible, these are extremely long timescales.

Additional impacts include the loss of harvestable fruit and smoke taint (particularly grapes for wine).

Last summer’s fires were particularly intense. Does that necessarily mean a greater impact on soils?

The short answer is: yes. The primary factors affecting soil condition during a bushfire are peak fire intensity and fire duration, which were very intense during last summer’s fires.  These variables are strongly controlled by season, soil type, plant species, moisture of fuel, fuel load, air temperature and humidity, wind speed and topography of the site.

During a bushfire, soil condition is first impacted by the loss of vegetation and surface organic matter. As surface temperatures increase and exceed 95 degrees, soil moisture and soluble nutrients held in soil water are lost through vaporisation. Soil surface temperatures can rise to 200-300 degrees, sometimes 500-700 degrees, and up to 850-1000 degrees during intense bushfires. 

The extent of change of soil condition due to bushfire is highly dependent on the characteristics of the fire and the soil. During a bushfire, the surface organic matter and upper layers of the soil profile, which are often the most nutrient abundant, are generally most affected. Bushfires affect the physical, chemical and biological properties of soil condition. This has implications for soil fertility and therefore Australia’s agriculture productivity and the recovery of native vegetation. 

Do we know exactly how soils are affected and how they recover?

There is extensive literature on the effect bushfires can have on the  physical, chemical, and biological properties of soil. In addition to these soil changes, obvious impacts to above ground vegetation is also left in after the passage of the flames.

In general, the following examples provide a brief overview of our understanding on how soils are affected by bushfires and factors that contribute to their recovery. A soil with less vegetation or one stripped of all plant cover before a fire  is more protected from the immediate impact of fire and subsequent erosion from rainfall, wind and runoff events.  In contrast, a soil with high vegetation (tree) cover will be more affected by severe fires with increased fire duration and the development of non-wettable layers.  Non-wettable layers result in an increase in overland flow following rainfall runoff events. This results in greater soil erosion.

In addition, the eroded soil matter is transported elsewhere and can pollute streams and rivers. Furthermore, most soil dwelling organisms, including microorganisms, fungi and soil dwelling invertebrates reside within the top layers of soil. An immediate consequence of bushfire to these organisms is a reduction in their biomass and potentially species loss. Severe bushfire events with soaring temperatures can result in a complete sterilisation of the upper soil layer. In such cases, loss of the soil seed bank may also occur.

What can’t we do or don’t we know?

Not all soils respond in the same way to fire and some are especially vulnerable to degradation following post-fire management practices. Australia is a large continent with diverse soil types throughout. A future more in depth study of the physical, mineralogical, chemical and biological status of soil is essential to determine the degree of damage before carrying out post-fire treatments. It is also essential to study the seed bank, which is stored in the soil, and the capacity of the soil and the vegetation to recover.

The following are just three examples of what we can’t do or don’t know:

1. Adequately measure in situ fire intensity and severity, which will enable researchers and land managers to accurately predict ecosystem responses such as, soil erosion, water runoff, stream water quality and revegetation.

2 Adequately determine ecosystem recovery responses, which are correlated with fire severity.

3. Adequately determine responses to address soil condition, which may include prevention measures against soil erosion, stream/river pollution, faunal recolonisation, revegetation, or vegetation regeneration (through natural or artificial reintroduction).

Has technology made a difference?

Several advanced laboratory, digital soil mapping,  field spectrometry and remote sensing techniques have been developed to better characterise and map the effects of fire on environmental processes, as well as human activities, on soils. Some of these procedures and techniques allow us to better assess the effects of fire on soils. Satellite imagery has been useful in determining the extent to which areas of forest and underlying soils have been burned.

New technologies are being developed and are expected to enable better assessment of post-fire surface covers and the effects on vegetation and soil properties at regional and global scales (e.g. Multispectral or hyperspectral data). Some workers have drawn attention to new approaches that combine different data sources. For example, data can be combined to assess forest regrowth trajectory in the short- and long-term post-fire forest regeneration (e.g. LiDAR data combined with Landsat data).

Soil scientists are now also using advanced automated techniques (e.g. Australian Synchrotron powder diffraction beamline) to generate superior resolution of soil characteristics to quantify the poorly crystalline minerals (e.g. cristobalite and mullite) formed during bushfires.

Soil is part of a broader environment and ecosystem. Can we deal with it in isolation?

Bushfires are complex and dangerous natural phenomenon, involving intimate interactions of chemistry, physics and biology. In many Australian landscapes, bushfires are also a critical process for long-term ecological sustainability.  In fact, many species have evolved to cope with bushfires and some even require bushfires to propagate.  For these reasons, the impact of bushfire on soil should not be dealt with in isolation even when conditions are conducive.

Better surveying and monitoring of soil condition, generally called soil monitoring, will also benefit its related interlinked systems such as biodiversity and conservation programs, agriculture and horticultural industries and broader ecosystem services. This will better enable continual management and post-fire assessment and recovery of the whole ecosystem.  We need to set up long-term soil monitoring networks for this purpose.

A nationally consistent framework for soil data collection, storage and accessibility is important in underpinning the scientific evidence required for fire policy development. The successful recovery of soil condition extends further than the immediate soil ecosystem.

Climate change is expected to bring more and bigger fires. Will it also make soils more susceptible to damage?

The impact of climate change and its direct impact on soils is complicated and will need proper study and analyses. As such, it is not yet possible to give a simple yes or no answer.

However, we can make a first assessment based on the general understanding that the human impacts of global climate change are not only about rising sea levels, storms, droughts, and floods. Heatwaves and floods, which used to be once-in-a-century events are becoming more regular occurrences and it is now widely accepted that climate change is affecting agriculture.

But on the question of whether we should expect more fires in Australia and if this will make soils more susceptible to damage as the world continues to heat up, the answer is clear: yes. However, the magnitude of the impact will depend on how we adapt to the accelerated impacts of human-driven climate change (e.g. improved agricultural diversity will likely increase soil resilience under climate change scenarios).  As such, we have the knowledge to progressively start adapting to better protect our fragile Australian soils to future climate change scenarios.  We may need to design new surface covers for some of our more vulnerable soils.