Decades of Organic Fertilizer Boost Soil Carbon — Study Reveals Where It’s Stored

Long-term organic amendments increase soil carbon and reveal its storage mechanisms

Sustainable farming is back in the spotlight after researchers at Kansas State University found that soil managed without tillage and fertilized only with manure or compost for 22 years stores more carbon than soil treated with chemical fertilizers or left unfertilized. The findings, reported in the Soil Science Society of America Journal, highlight both soil-health and climate benefits of long-term organic amendments.

The study went beyond measuring total carbon: using ultrabright synchrotron light sources, the team visualized where and how carbon is retained in the soil. Measurements were carried out at the Canadian Light Source (University of Saskatchewan) and the Advanced Light Source in Berkeley, California, allowing researchers to observe carbon preserved inside soil pores and bound to mineral surfaces.

Key findings

• Plots receiving only manure or compost for 22 years held more soil carbon than plots with chemical fertilizer or no fertilizer.
• Much of the extra carbon was physically protected inside soil pores, and a portion became chemically associated with soil minerals — both mechanisms that slow carbon loss.
• Advanced imaging with synchrotron beams enabled the team to see how carbon is spatially distributed and chemically connected to soil particles.

The researchers sampled soil from a Kansas cornfield managed under a no-till system and analyzed fine-scale carbon associations using spectroscopic imaging. Their work supports earlier studies linking organic amendments and regenerative practices to improved carbon sequestration, while adding important mechanistic detail about where that carbon resides.

"Collectively, studies like this are going to help us to move forward to more sustainable, more regenerative agriculture practices that will protect our soils and environment as well as help feed growing populations," said Dr. Ganga Hettiarachchi, KSU professor of soil and environmental chemistry, as quoted by Rowan Hollinger of the Canadian Light Source.

"Understanding the role of different minerals, chemicals, and microbes will help improve models for predicting how farming practices affect soil carbon storage," Hettiarachchi added.

While this study strengthens the case for incorporating organic amendments into farming systems, the authors note that soil carbon dynamics are complex: outcomes depend on climate, soil type, crop system, and management practices. Still, the combination of long-term field trials and advanced imaging tools provides actionable evidence that regenerative practices can both improve soil health and contribute to climate mitigation.