When roundworms lose, carbon emissions rise

Soil food webs play a key role in supporting grassland ecosystems, which cover about one-quarter of the land on Earth. Climate change poses a threat to these environments, partly because of the uncertainty of extremes in rainfall, which is projected to increase.

To learn more about the effects of these extreme events, a team of soil and plant ecologists, supported by funding from the National Science Foundation, studied nematodes, which play a key role in carbon and nutrient cycling and decomposition in soil.

Principal Investigator Osvaldo Sala is founding director of the Global Drylands Center at Arizona State University. We asked him about the study, out June 10, 2019, in Proceedings of the National Academy of Sciences.

What did you learn in this new study?

We expected that under drought conditions plants would allocate more resources to roots, which capture water, and less to leaves, which lose water. We found that this was true only in arid sites such as New Mexico but not in mesic regions such as Kansas. In this study, we learned more about why this happens.

We know that roots are eaten by herbivorous nematodes, which in turn are eaten by carnivorous roundworms. We found that the predatory roundworms were more sensitive to drought than their root-eating prey. When the roundworms declined because of drought, they freed herbivores from predation, allowing them to devour roots.

Predatory roundworms are generally less abundant in arid sites, so the effect of drought on these predators is less pronounced in drylands. As a result, roots are less damaged by drought in drylands than in mesic regions.

What can you use this information for?

When more carbon is stored belowground, less goes into the atmosphere to accelerate climate change. When there is drought in arid areas, we find more roots relative to leaves and twigs, which translates to more carbon stored underground.

We can use our findings to better predict the effects of climate-change-driven droughts on the distribution of roots and leaves and consequently the amount of carbon stored belowground.

The study’s lead author, Andre Franco of Colorado State University, is an affiliate of the Global Drylands Center.

What happens to the grasslands when the predator-prey balance is tipped in favor of the prey?

When the root-eating nematodes thrive, they overgraze roots, which makes plants more sensitive to drought. Not only do plants die off, but more carbon is released into the atmosphere through microbial respiration.

Why are healthy grasslands important to the health of the planet?

Drylands account for 40% of the terrestrial surface and are home for 30% of human population and 50% of global livestock. Drylands account for most of year-to-year variability in the global carbon cycle, emitting more carbon into the atmosphere during dry years and sequestering carbon during wet years.

Why are healthy grasslands important to human well-being?

Drylands sequester carbon, which maintains our atmosphere and climate in a range compatible with human life. They support a rich biodiversity and produce food to support thriving cultures. It is impossible to achieve global sustainability without achieving sustainability of drylands.

How might humans have to adapt if grasslands are damaged or destroyed?

In many regions, people migrate when grasslands are degraded or during severe droughts. People in Central America migrate to the US when drought ruins their harvest. A similar pattern occurs in Northern Africa and the migration to Europe. This is a difficult choice for the migrants with very challenging implications for the countries that take them in, and one of the very real reasons that we can’t achieve global sustainability without achieving sustainability of grasslands.