The Length
Arbuscular mycorrhizal fungi shuttle water, phosphorous and nitrogen to 70 percent of land plant species. In exchange, these fungi receive carbon from plants, drawing down the equivalent of roughly 4 billion tons of carbon dioxide, or 11 percent of the CO2 emissions from fossil fuels. Through this relationship, mycorrhizal fungi helped shape life on land for 450 million years. So how big must this global underground circulatory system be?
Bigger than you think—perhaps by a lot.
Each fungal network is made up of strands of tubular cells, called hyphae, which are 10 to 50 times thinner than a human hair. Nutrients and carbon travel through these hyphae. They may increase the foraging area of plant roots 100-fold.
And if every arbuscular mycorrhizal fungal hypha in the world from the top 15 centimeters of soil were laid end-to-end? The new work estimates they would stretch from Earth to the sun—about a billion times. That’s roughly 110 quadrillion kilometers of hyphae.
The mass
Biomass is a common way to measure the size of living things. For fungi that means estimating the mass of all hyphae in a section of soil, but using a method that can be extrapolated to reflect the variation in fungal species across the Earth. That requires finding an average hyphal density and radius for soils.
Density is the length of fungal hyphae in a teaspoon of soil. To determine it, researchers compiled data from 16,000 soil samples gathered by hundreds of scientists from more than 4,000 locations across many ecosystem types - forest, tundra, desert, farmland and more. To find a useful radius, an imaging robot developed at AMOLF, the Dutch biophysics institute, captured more than 300,000 measurements of hyphae from five strains of three common fungi. Scientists then combined that information with soil chemistry, climate and vegetation data and used machine learning models to make predictions.
The total biomass of all that underground infrastructure: about 300 million tons. That’s roughly 5 times the weight of every human alive.
The map
But where are the densest networks of arbuscular mycorrhizal fungi? The extraordinary extent of underground fungi was then mapped across the Earth, predicting likely hyphal densities for every square kilometer of land on all vegetated continents. That includes regions scientists have not yet even sampled.
The under-sampled areas have higher levels of uncertainty associated with the predictions (see uncertainty analyses in the Science publication). But there is no mistaking the patterns that emerge.
For instance, moist, tropical broadleaf forests are among the world’s most productive aboveground systems. Yet patterns are different below ground: the highest density of arbuscular mycorrhizal fungi, on average, were found in wild grasslands, more than a third denser than tropical broadleaf forests. In fact, fully 40 percent of arbuscular mycorrhizal fungi are found in grasslands such as those on the Tibetan Plateau, in Kansas’ Flint Hills Prairie, Florida’s Everglades or South Sudan’s Sudd Wetlands.
There is also evidence that we are changing this underground landscape: the data suggest that large-scale agricultural crop lands are predicted to be associated with about 50 percent lower network densities. The research doesn’t link particular farming practices to mycorrhizal health, but researchers worry lower fungal densities reduce soil resilience and lower its ability to cycle nutrients and store carbon.
The next step is getting these data into the hands of decision-makers to help move conservation and climate agendas toward including fungal networks.
Ahora podemos predecir los patrones de biodiversidad micorrízica con una resolución de 1 km² en todo el planeta. Estos análisis son el primer paso para poder utilizar los sistemas micorrízicos para guiar las estrategias de restauración, diseñar nuevos modelos de conservación y desarrollar sistemas agrícolas más sostenibles. Sin embargo, se necesitan medidas urgentes: hemos descubierto que más del 90 % de los ecosistemas de hongos micorrízicos subterráneos más diversos de la Tierra se encuentran desprotegidos. Esto es un problema: la disrupción de estas redes acelera el cambio climático y la pérdida de biodiversidad.
