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Nematodes uncovered: soil organisms and carbon cycling

Logo https://crowtherlab.pageflow.io/nematodes-uncovered-soil-organisms-and-carbon-cycling-adef354c-2c44-4b90-84ba-e44e22b5322f

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Soil organisms are crucial in the terrestrial biosphere. They play a central role in every aspect of global biogeochemistry, influencing the fertility of soils and the exchange of CO2 and other gasses within the atmosphere. As the primary players within the soil food web, they are excellent indicators of soil health and functioning.  

Of these soil animals, nematodes are by far the most abundant. In fact, nematodes account for nearly 4/5 of all animals on Earth! Just one handful of soil can contain 50,000 or more of these tiny worms.

Given their importance in soils, understanding global distribution patterns of nematodes is crucial for climate modelling and, ultimately, environmental decision making. However, to date, we have had no quantitative information of the active belowground community on a large scale. To address this, we created the first global map of soil nematode distribution patterns. This map adds a crucial layer of information to the holistic view of ecosystem functioning and its response to climate change – knowledge that is essential for environmental decision making and effective restoration projects
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At local scales, the composition of soil communities can be extremely variable, ranging from just a few nematodes per gram of soil to several hundreds. Historically, this high heterogeneity has prevented soil ecologists from exploring global patterns. In the current era of big data and artificial intelligence, we can go past these limitations. To do so, we set up a global network of 70 soil ecologists, each providing their local data on soil nematode abundance.

This collaboration resulted in a comprehensive dataset of 6,759 samples, taken across the world. Next, we paired this dataset with global information on soil, vegetation and climate characteristics. We then used sophisticated geospatial modelling approaches, including machine learning algorithms, to generate the first global map soil nematode abundance. This map not only shows spatial distribution patterns of soil nematodes, it also provides valuable insights into the driving forces of specific environmental variables, such as soil physiochemical and climatic conditions. As these variables can shift due to a changing climate, our map highlights the regions that are most vulnerable to climate change.
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Our research shows that nematodes are abundant in soils across the globe, in even higher numbers than previously anticipated. We calculate that there are 4.4 x 10^20 nematodes present in the Earth’s topsoil, or in other words: for every human on the planet, there are 60 billion nematodes. Their sheer population size also means that nematodes process much more organic carbon than previously thought. We estimate that in the peak growing season, the time during which plants are most active, nematodes are responsible for a monthly turnover of 0.14 Gigatons carbon, of which 0.11 Gigatons are respired into the atmosphere. To put this into context, this is equal to approximately 10% of human monthly carbon emissions.

Another surprising result of our research was the geospatial distribution pattern of soil nematodes. Where aboveground organisms are typically most abundant in warm, tropical areas, our maps reveal that soil nematodes are present in highest numbers in cold regions. These include the tundra and boreal forests in Alaska, Siberia and Scandinavia. In contrast, their numbers are much lower in tropical regions.
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In total, our models include 73 layers of environmental information, on climate, soil physiochemical, vegetative, topographic and anthropogenic variables. By exploring the predicted nematode abundances across regions, we can now start to investigate the relative importance of environmental variables in driving soil community composition.

In contrast to aboveground animals, where climatic conditions are strong drivers for their distribution, soil characteristics are key in regulating soil nematode abundance. By far the most important driving variable of soil nematode abundance is soil carbon. The high soil organic carbon stocks found in tundra and boreal forest soils are the prime reason for the high nematode abundances in these regions. However, nematodes are abundant in warm regions too, particularly in tropical peatlands, for example in the Peruvian Amazon. Just as sub-arctic regions, these soils are characterized by high carbon stocks.
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This strong control of soil characteristics on soil nematode abundance suggests that the effect of climate is indirect. In sub-arctic regions, low temperatures reduce the decomposition of soil organic material, resulting in increased soil carbon and nutrient levels. In tropical peatlands, high amounts of precipitation lead to saturation of soil with water, also reducing decomposition rates. Ultimately, it is the high carbon and nutrient concentrations in these soils that result in high nematode abundances. 

To date, the low temperatures in sub-arctic regions has restricted activity of soil organisms, including nematodes. With climate models unanimously predicting elevated temperatures in these regions, this might change. Higher temperatures can lead to increased decomposition rates, due to in raised activity and respiration rates of soil organisms. As a result, soils might lose carbon even faster than previously anticipated.
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