Increasing and deepening root inputs into soils has been proposed as a mechanism to increase soil carbon. It remains unclear to what extent and under which environmental conditions this will be an effective strategy. This makes it challenging to determine how viable roots are as a solution to increase soil carbon, which can have many positive benefits such as soil health and fertility.

How carbon gets into soil and whether it stays or moves out of the soil system is a complex process. We published a new study in Communications Earth and Environment that sought to test a longstanding assumption that the more carbon a plant takes up and transfers to its roots, the more carbon is ultimately stored underground in soils.

We focused specifically on fine root carbon (FRC), which comprises the carbon stored in the roots smaller than 2 mm in diameter. FRC can help store carbon belowground as soil organic carbon (SOC), but it can also facilitate the opposite.

When plant roots release organic compounds into the soil, this can promote SOC loss by stimulating microbial activity in response to these root exudates. This is a process known as priming.

These opposing dynamics formed the basis of our study, which examined the SOC–FRC relationship across the United States at a continental scale.

Soil information and other ecological data from the National Ecological Observatory Network (NEON) dataset were analyzed at 43 sites, primarily forests and grasslands, across the United States. These sites spanned environmental gradients and encompassed several ecosystem types, with each sample measuring up to 2 meters belowground. We expected a positive relationship between FRC and SOC across both ecosystems. However, the story ended up being much more nuanced.

Grasslands strongly drove the SOC and FRC relationship. In this ecosystem, for a 1 kg/m² increase in FRC, the SOC increased by 15-23 kg/m². This relationship was even more pronounced in deeper soils below 30 cm, where carbon is often more stable and less likely to be lost to the atmosphere.

In contrast, the relationship between FRC and SOC in forests was less consistent. SOC levels were highly variable, and no clear correlation with FRC was observed. This could have been a result of forests experiencing a much wider range of controls on SOC beyond FRC, such as higher aboveground litter inputs, variable turnover rates of roots, different mineralogy, and complex mycorrhizal associations.

These findings suggest that grassland may play an outsized role in promoting stable SOC, and this SOC is closely related to FRC, especially at depth.

Understanding how carbon cycles belowground across different landscapes is critical for well-informed strategies to improve soil health, including factors such as fertility and water holding capacity. Our unexpected findings underscore the importance of considering ecosystems differences in not only on how much carbon plants take in, but also on what happens once it’s beneath our feet and out of sight.