Soil's memory: particle size, not fertilizer type, dictates carbon chemistry after 32 years

A three-decade study in southern China reveals that the physical arrangement of soil particles has a greater influence on the biochemical nature of stored carbon than the type of fertilizer applied.

A long-term agricultural experiment has revealed that the physical structure of soil is more important than the type of fertilizer used in determining the chemical makeup of stored organic carbon. After 32 years of consistent fertilization treatments, researchers from Tianjin Normal University and partner institutions discovered that the size of soil particles where organic matter is stored has the primary influence on its biochemical properties, a finding with significant implications for soil management and carbon sequestration strategies.

The 32-Year Experiment

The study, published in the journal Carbon Research, was conducted at the Jinxian experimental station in southern China on a type of red soil known as Ferralic Cambisol. Beginning in 1986, scientists led by Yidong Wang maintained four distinct fertilization treatments on plots of land used for a maize-maize double-cropping system. The treatments included an unfertilized control, nitrogen-only fertilizer, a balanced mix of nitrogen, phosphorus, and potassium NPK, and an NPK mix supplemented with pig manure. This extended timeframe allowed the team to observe the cumulative effects on soil organic matter.

Carbon Buildup with Room for More

Over the three decades, all treatments resulted in an increase in soil organic carbon compared to the initial 1986 levels. The plots receiving both mineral fertilizer and manure saw the largest gain, with a 52% increase in soil organic matter. The study also determined that this subtropical red soil has not yet reached its carbon storage capacity. The efficiency of converting carbon from plant residue and manure into stable soil organic carbon was a modest 6.8%, suggesting that these soils still have substantial potential to store more carbon.

It's All About the Size

The most significant finding emerged when scientists analyzed the soil's composition. They separated soil organic matter into three physical fractions based on particle size: coarse particulate organic matter cPOM, fine particulate organic matter fPOM, and the smallest fraction, mineral-associated organic matter MAOM. The analysis showed that the biochemical composition and oxidation state of the organic matter were mainly governed by which physical fraction it belonged to, not by the specific fertilizer treatment it had received for 32 years.

A Tale of Two Compounds

The chemical differences between the fractions were distinct. The larger particulate fractions, which are less protected and turn over more quickly, were enriched with tough, plant-derived compounds like aromatics and lignin. In contrast, the smallest MAOM fraction, where organic matter binds to mineral surfaces for long-term stability, accumulated nitrogen-containing compounds. These nitrogen-rich compounds are largely byproducts of microbial activity, indicating that microbes are key players in creating the most stable forms of soil carbon.

The Dominance of Microbial Processing

Researchers also investigated the origin of the soil organic matter. Across all treatments and particle sizes, the vast majority 60 to 90 percent of organic matter was found to be derived from microbes rather than directly from the original plant or manure inputs. This shows that soil microorganisms extensively rework and transform organic materials, ultimately leaving their own chemical signature on the carbon that is stored long-term in the soil.

Implications for Soil Health and Climate

These findings suggest that for improving long-term soil carbon storage, focusing on the quality of organic inputs alone may not be enough. Agricultural practices that promote the formation of stable soil structures, particularly the association of organic matter with fine mineral particles, could be more effective. The work by Yang Chen, Yidong Wang, and their colleagues provides a deeper understanding of the mechanisms that control carbon persistence in agricultural soils, offering valuable information for developing better soil management practices to enhance fertility and mitigate climate change.

Corresponding Author:

Yidong Wang

Original Source:

https://doi.org/10.1007/s44246-022-00034-0

Contributions:

YC: Formal analysis; Investigation; Literature collection and analysis; Writing- original draft. KLL: Investigation; Funding acquisition; Resources. NH: Writing- reviewing and editing. YLL: Conceptualization; Funding acquisition. FW: Writing- reviewing and editing. YDW: Conceptualization; Funding acquisition; Project administration; Supervision; Writing- reviewing and editing. All authors read and approved the final manuscript.