Midwest soil carbon monitoring requires deeper sampling, study finds

Soil organic carbon changes in long-term Midwest agricultural fields can differ sharply with sampling depth and carbon fraction, according to an analysis of archived soils from three field trials. In the study, permanganate oxidizable carbon, or POXC, consistently tracked gains and losses in total soil organic carbon more closely than particulate organic carbon, mineral-associated organic carbon or mineralizable carbon. Extending sampling below the surface also revealed larger changes than measurements limited to topsoil captured.

The researchers analyzed samples from two long-term Nebraska experiments that compared crop-residue removal with retention in continuous corn, lasting 10 and 19 years, respectively. A third experiment, conducted for nine years at the Great Lakes Bioenergy Research Center, compared continuous corn with perennial switchgrass. These agricultural trials provided soil samples spanning multiple depths, allowing the team to examine whether surface measurements alone captured the carbon trajectories observed across the soil profile.

At the 19-year Nebraska site, increasing plant inputs through residue retention enhanced total soil organic carbon, particularly in surface soils. The depth pattern also differed at the Great Lakes Bioenergy Research Center. Surface soil organic carbon declined under corn, while switchgrass exhibited losses through the whole soil profile. These contrasting observations demonstrate how a restricted sampling depth can change the apparent distribution of carbon beneath agricultural land. They do not, by themselves, establish an overall performance ranking for corn and switchgrass.

The carbon fractions showed distinct responses across sites. Particulate organic carbon varied, mineral-associated organic carbon changed minimally, and mineralizable carbon often declined. Total soil organic carbon did not move in parallel with every measured fraction, so increases or decreases in one component could not be used as a direct proxy for the entire carbon stock. At both Nebraska experiments, POXC more closely followed changes in total soil organic carbon than particulate organic carbon, mineral-associated organic carbon or mineralizable carbon did.

These results support a combined approach to carbon monitoring in the studied systems: sample across multiple depths, track total soil organic carbon and examine more than one fraction or soil-health metric. POXC emerged as the most reliable indicator of overall gains and losses in this dataset, but the evidence comes from three Midwest trials with different crops, durations and management conditions. The findings therefore identify POXC as a promising metric for carbon-focused monitoring while showing why deeper and fraction-specific measurements remain important for accurately describing soil-carbon trajectories.

Subject of Research: Soil organic carbon monitoring across depths and fractions in long-term Midwest agricultural trials

Article Title: Tracking soil organic carbon trajectories: depth and fractions matter for carbon monitoring in US midwestern long-term no-tilled systems

Article References: Naasko, K., Smychkovich, A., Mahmood, S., Lin, Y., Lavallee, J. M., Córdova, S. C., Schmer, M. R., & Sprunger, C. D. (2026). Tracking soil organic carbon trajectories: depth and fractions matter for carbon monitoring in US midwestern long-term no-tilled systems. Biogeochemistry. https://doi.org/10.1007/s10533-026-01382-4

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01382-4

Keywords: soil organic carbon, carbon monitoring, soil depth, POXC, particulate organic carbon, mineral-associated organic carbon, no-till agriculture, corn systems, switchgrass, Midwest agriculture, Tracking, soil