Ten Facts about Soil Biodiversity with Professor Richard Bardgett

1. Soil is the most speciose habitat on earth

A few attempts have been made to estimate the percentage of the Earth’s biodiversity which inhabits soil, and generally it has been difficult to enumerate because of the many groups of organisms about which we still have limited knowledge. A recent comprehensive study estimated that almost 60% of all species on Earth can be found within the soil, indicating that soil is the most biodiverse habitat on Earth.

Soils are highly complex in their biodiversity, and while we often talk about the more familiar microbes and larger animals, such as earthworms, there’s a wide range of different types of microbes and animals living within the soil. For example, in addition to being home to approximately 45 to 50% of all bacterial species on Earth, and 90% of fungi, soil is home to a vast diversity of viruses, protists, nematodes, microarthropods, and more.  These different types of organisms form a a complex food web, vast both in its abundance and diversity.

To put this vastness into perspective, a handful of soil from any field or a garden likely contains billions of bacterial cells and potentially tens of thousands of individual species of bacteria.

Soil is so species rich because it is so spatially and temporally diverse itself. If you walked across a field, the soils you’d find, and the way they are structured, will vary tremendously, giving organisms an unrivalled opportunity for specialising in and inhabiting different niches which happily coexist.

2. Enumerating soil health is complex

Calculating the biodiversity present in soils is challenging due to soils’ own complexity. For a start, it can be difficult to remove animals and microbes from the soil to count them – many are microscopic, and identifying them within physically heterogeneous soil is costly, time consuming and often requires input from specialists to handle the resulting data. Added to this, many microbes cannot be cultured in the laboratory, so we rely on the use of molecular tools to characterise the diversity of soil life.

Another equally important consideration is not only understanding what is in the soil, but also knowing what each individual organism is doing, how they interact with one another and with above-ground organisms.

Less conventional approaches to understanding soil composition can also provide valuable data. For example, the public were recently invited to perform a ‘worm dance’ to encourage worms to the soil’s surface, and then to count and report the number they saw. Earthworms, which are relatively easy to measure, can be a good proxy for other indicators of soil health, and such initiatives can generate a baseline dataset, drive public engagement and raise awareness around the critical importance of diversity in soil.

3. Soil biodiversity is likely in decline, but it is key to successful above ground biodiversity

Because soil biodiversity can be so difficult to measure, it is hard to obtain an accurate picture of the current levels of life within our soils. Currently there is no systematic measuring of soil biodiversity in the UK, and while there are some surveys that can give us an idea, we simply don’t know whether populations are improving or declining.

That said, there are other indicators that would suggest that soil biodiversity is in decline. For example, data on land use intensification and the conversion of natural or semi-natural habitats to agriculture would suggest a decline in the diversity of organisms. Figures in England and Wales suggest that four million hectares of soil are at risk of compaction and two million are at risk of erosion, and with degradation comes a decline in biodiversity.

Declines in soil biodiversity are likely linked to areas of intensive agriculture or urbanisation. However where we restore and convert ecosystems, there is the potential for high levels of soil biodiversity to be maintained or even increase. The UK, alongside many other nations, has committed to protecting and conserving 30% of land and sea for biodiversity by 2030 – known as 30×30. Though higher diversity above-ground doesn’t always mean higher diversity below-ground, in general the restoration of habitats will likely benefit the biodiversity within the soil. As such, it is important not to ignore soil biodiversity when setting biodiversity targets, as organisms in the soil play a vital role in the re-creation and restoration of above-ground habitats. This is especially relevant to new initiatives such as Biodiversity Net Gain, where the potential of soil biodiversity could be enormous.

4. Intensive agriculture, urbanisation and climate change pose threats to soil biodiversity

As mentioned, when land use changes for agriculture or urbanisation, soil tends to be degraded and therefore biodiversity is lost, and this conversion of natural habitats poses the biggest threat to soil biodiversity due to things like continuous cultivation, fertiliser use, and pollution. Heavy fertiliser use, for example, essentially bypasses the need for plants to invest in things like mycorrhizal fungi and is associated with less organic matter, which is a key resource for soil organisms. That said, it is important to note that not all groups of organisms are impacted in the same way – for example, bacterial diversity tends to be higher in more intensively managed systems.

Climate change and resulting climate extremes (floods, droughts, heatwaves) also impact the soil, and current research is looking at how these events affect the diversity and composition of underground communities and the functions they perform. Communities are very diverse, which means they are usually able to bounce back from extreme weather events. However, concern is growing as such events are becoming more frequent, and there may be legacy effects on their ability to recover. Ongoing research mimicking these weather patterns to assess their impact has shown that when soils are repeatedly hit with cycles of extreme wet and dry weather, their biodiversity and nutrient cycling abilities are impaired. In the future as these weather patterns become more intense, there could be similar impacts on the diversity of life in the soil.

5. Soil biodiversity is important for soil function and soil biodiversity loss and soil degradation are intricately linked

As with levels of biodiversity itself, the impact of changing biodiversity levels on soil function is hard to measure. When biodiversity levels change, the factors causing that change are also likely to have impacted the soil environment itself, and it is hard to disentangle these. If we consider soil degradation as a cause of reduced biodiversity, we must also consider how that degradation changes the physical and chemical properties of the soil.

Research has shown that over large areas, areas of high soil biological diversity tend to be associated with more efficient soil nutrient and carbon cycling, and conversely areas with lower diversity areas tend to be associated with lower function.

An important thing to remember is that each organism has its own role within the soil, so losing one species will have a different impact on overall soil function than if you lost another. There is a lot of evidence showing that soil responses are quite specific to the species lost, and the soil conditions themselves. Simply put though, the simplification of the soil food web and the reduction of soil biodiversity is never going to be a good thing for the soil overall or the ecosystems it supports.

6. Soils must be considered holistically

Soil biodiversity is just one aspect of soil health, and it is essential to consider the biodiversity in soils and their various roles holistically. Physical and chemical environments matter as well, and it can be helpful to think of soil biodiversity as making up a complex food web of individuals whose roles and functions need to be considered collectively.

As discussed, soil health and soil biodiversity can serve as proxies for each other. However, when addressing the crisis in our soils, focusing solely on managing soil biodiversity will not be sufficient to resolve all issues. Efforts to improve soil biodiversity will not always be good for overall soil function, and vice versa. For example, bacterial diversity tends to be higher in more intensively managed systems, which would normally be considered as having negative consequences for overall soil health. Ultimately, we need to consider what is going to be best for each soil type, on each land use type, in each specific context.

To consider soils holistically, we also need an improved understanding of the entire soil profile, as most of what we know focuses on the topsoil only. The topsoil is likely to be where most organisms live as it’s where there is the most organic matter. However, understanding what’s happening deeper in the soil is an important and interesting area to explore and this will support what happens higher up.

7. Efforts to improve soil biodiversity must consider soil type and land use

Soil type is likely the most significant factor determining soil biodiversity, meaning it is crucial to understand soil types and tailor interventions accordingly. The soil is the physical structure, or the ‘house’ in which the organisms live, playing a vital part in shaping the communities of organisms which can be found in it.

Land use type also has a significant impact on levels of soil biodiversity and can dictate the overall impact of improvement efforts. When determining where to target efforts in terms of land use, we should consider the co-benefits that an improvement to soil biodiversity will provide. For example, by transitioning from an intensively managed agricultural system to more sustainable methods, not only will you benefit the soil biodiversity, but also the crops themselves and wider ecosystem services.

Targeting efforts must also take into account the historical management of the land, as this will influence the outcomes when land management practices are altered. A recent study found that across grasslands that had been grazed by sheep at low stocking rates for centuries, soil biodiversity declined when the grazing ceased, serving as a reminder that it doesn’t necessarily follow that interventions that might be considered good for above-ground diversity will have similar effects below-ground.

8. There are important links between soil biodiversity and soil carbon increases

Soil organisms play a crucial role in carbon cycling, with different groups of organisms performing a distinct functions – some animals break dead plant litter down, making it more available for microbial attack, some stabilise that carbon, and others release the carbon back to the atmosphere. These organisms depend on organic matter to survive by fuelling them with the resources they need to survive.

The microbial contribution to carbon cycling is an area of rapidly advancing research. Recent studies have demonstrated that the relationship between soil biodiversity and soil carbon levels is more complex than simply serving as proxies for each other. One such area of study is on necromass (dead microbial matter), where it has been shown that it can constitute almost half of the total organic carbon pool. Literature emerging over the past few years points to the importance of microbial turnover in particular as a driver of soil carbon sequestration, and also the stabilisation of carbon in the soil. The more we understand this process, the more we may be able to manage them in a way which will encourage the processes of necromass formation, and hence the stabilisation of carbon within the soil.

Again, different organisms perform different functions, and it is hard to link biodiversity in general to any of soils’ processes because so many different organisms are doing so many different things. In many cases, there are key groups of organisms that are driving particular functions, and understanding these organisms is key to understanding how they contribute to the sequestration of carbon in soils.

9. Regenerative agriculture is a good thing for soil biodiversity

On the whole, the regenerative agriculture movement is a good thing for soil biodiversity, not least because, as a concept, it gets people thinking about their soil, what lives in it, and how best to manage it. Reduced tillage is a big win for soil biodiversity, but it really gets exciting when we start integrating biodiversity into a farm-scale system. In a regenerative system, you are not farming solely for healthy soil, but also for the co-benefits brought by healthy soils, such as farm resilience, disease suppression and so on.

Regenerative interventions can mean reduced yields in the short term, and work that has looked at restoring species diversity in grasslands has shown that interventions that are best for biodiversity tend to be those which are associated with declines in hay yield. However, those yields can become more stable over time and resilient to climate shocks, so the longer-term benefits are what can really have an impact.

We need the scientific knowledge to really pinpoint and evidence the impact of these different measures, and policy should be there to guide that evidence to say definitively what works for biodiversity across different landscapes – again going back to the need to tailor soil management for different situations.

10. We know a lot about soil, but technology can help us learn more

The technology that helps us to understand soil is getting better and more efficient all the time, allowing a greater understanding of the composition of soil communities and what they do. Although they remain expensive, techniques have developed enormously, particularly molecular tools, which allow us to interrogate biodiversity and get a much fuller picture of what lives below-ground. Meta-genomics is an example of a technology that allows us to link changes in microbial communities to actual functions, and things like their ability to withstand drought.

Another area of particular interest is the study of the plant root microbiome and the growing understanding of root characteristics, how they impact different organisms, and how that has implications for things like carbon storage and nitrogen cycling. This bridges plant ecology and soil ecology and highlights again why above- and below-ground biodiversity need to be considered together.

Developing tools such as machine learning and AI can be hugely advantageous in this field too, because the data tends to be vast and complex, and these technologies allow us to analyse and interrogate data to understand these complex communities and how they are changing in space and time.

Although there’s still much to discover, we do already have a significant understanding of soils, as we continue to build on this it’s exciting to anticipate the knowledge that’s yet to come!