A view from above : the global soil community

The Earth’s soils hold the largest amount of organic matter in the world, storing as much carbon as above-ground vegetation and the atmosphere combined. While plants perform a vital role in fixing this carbon from the atmosphere, it is the combination of different organisms in the soil which are so important for influencing the climate. They play a central role in influencing the fertility of soils, plant growth, and the exchange of carbon (and other gases) with the atmosphere. Our recent research on the global distribution of one soil organism, nematodes, shows that these animals are the most abundant on the planet – four out of every five of all animals on Earth is a nematode worm – which goes to show how significant this below ground world is!

Due to the hyper-diverse, highly complex and unique nature of individual soil communities in different regions around the world, soil science research has historically had to focus on local scale studies. In predicting the variation in how different soils function around the world, scientists and practitioners have often used environmental factors as a proxy for how they might behave in different climates. However, our review of over 300 published research papers shows that it is the organisms – not just the environment – that can influence how effectively soils function (e.g. in cycling carbon).

Soil communities around the world are so complex that we don’t necessarily expect to see any trends in how they behave at a global scale. However, when we take a birds-eye view of the world, we start to see clear patterns. The accumulation of carbon on land is negatively correlated with the temperature of that ecosystem!

Our review shows that the highest metabolic activity of soil communities is found in warm, moist tropical regions. Whilst this leads to higher carbon and nitrogen fixation rates, it also shows that warm regions emit soil carbon into the atmosphere at a faster rate.

In contrast, colder ecosystems slow down the rate of decomposition significantly, which has led to a huge build-up of undecomposed organic material over time. This represents a huge store of the world’s carbon.

This graph shows how the storage of carbon is spread across the world, both above-ground (in green) and below-ground (in brown), at different latitudes - from the Amazon Jungle up to the northern latitudes of Siberia.  Whilst the greatest diversity of soil microbes is in the tropical regions, this graph shows us that deep soils in cold northern regions hold much more microbial biomass, and therefore carbon. But this represents a significant risk – a ticking time bomb. As the climate warms these soil organisms will start to become more active, breaking down the dead organic material around them and respiring carbon into the atmosphere. By speeding up the rate of carbon emissions from the soil, this will accelerate further warming of the climate which, in turn, will drive additional loses of carbon from the soil in the atmosphere. This is known as a ‘positive feedback’, which is predicted to accelerate the rate of climate change by up to 17%.

The composition of soil communities plays a role in determining the functional potential. Whether it’s fungi, bacteria, archaea, protists or animals, the mix of organisms in a community will drive the carbon and nutrient cycling which are so critical for regulating soil fertility, atmospheric composition and the climate.  

Ecologists have started to identify patterns of global soil communities and have built four research areas to describe this: biomass and abundance, functional group composition, taxonomic diversity and composition, and functional trait expression. These research areas will help us to understand global soil communities and in turn, support healthy communities that maximize ecosystem health and carbon storage.

From our review of previously published studies, we show that there are clear, consistent patterns in soil biodiversity and soil carbon storage at a global scale. In identifying these patterns, we show that changes in the soil community strongly correlate with changes to the distribution of carbon on Earth. With this knowledge we can begin to see that there is great potential to manage soil communities to maximize ecosystem health.

By looking at these global patterns, we can also see for the first time just how important the different types of soil communities are for carbon storage. For instance, soils that are dominated by bacteria have fast carbon cycling and release a lot of carbon into the atmosphere. In contrast, those dominated by fungi generally trap more carbon in the soil for long periods of time. As such, if we can manage soils to promote the dominance of long-lived fungi (e.g. using soil inoculants or no-tillage practices), we can promote the long-term storage of carbon in the soil.

From a scientific perspective, the findings of our review show the large potential to further improve current models in the field of soil ecology today. In addition, we continue to discover critical mechanisms governing the turnover of organic matter at local scales. In order to place these mechanisms into context, it is important we continue to expand our understanding of the global biogeography of soil organisms. We need to step back from local, taxonomic-based and context-dependent research on soil communities to, instead, look at global patterns of groups of soil organisms. Expanding our perspective of global soil communities will enable us to better identify consistent patterns in soil biodiversity and soil carbon cycling, which will be needed to effectively manage soils in a changing world.

From a practical perspective, the research already shows that soils are the largest terrestrial repository of both biodiversity and carbon, so effective management of soil is among our most powerful weapons in the fight against biodiversity loss and climate change. Furthermore, the reviewed studies consistently show that the management of the soil community is possible, and when selected correctly, can promote healthy vegetation in both agricultural and natural settings. By illustrating what types of soil communities and ecosystems are best suited to any given location on Earth, our review---in tandem with work by hundreds of other soil scientists---can help inform local management goals and ensure that soils are managed appropriately. For example, we can now use soil inoculants to selectively assist the development of healthy soil communities into natural or agricultural settings. Similarly, by selecting the right types of trees, plants, and grasses, and by adopting responsible agricultural practices, land managers can shape the types of organisms that inhabit the soil, which not only promotes healthy, native ecosystems, but also maximizes carbon storage in soils.