Scientists Urge New Model for Soil Carbon Adapted from article

Author: Adapted from an article by Blaine Friedlander in the Cornell Chronicle, Nov. 2015 by Lauren Gwin, Small Farms Program, Oregon State University

Publish Date: Spring 2016

A growing number of “carbon ranchers” and “carbon farmers” across the U.S. and globally are implementing land management practices specifically to sequester more carbon in the soil. By “locking up” carbon in long-term storage in the soil, these innovators aim to slow or even reverse climate change. Such practices include cover cropping, reducing tillage, and managed grazing systems.

The agroecological value of these practices, including their connection with soil carbon management, is not in dispute. Neither is the importance of the relationship between soil organic matter and carbon.

However, the idea that carbon can be locked up in long-term storage in the soil is no longer valid, according to new soil science research.

In a recent article in the journal Nature, soil scientists from Oregon State University and Cornell University argue convincingly that the “humic” model of soil carbon is not accurate. Johannes Lehmann (Cornell) and Markus Kleber (OSU) have proposed a new soil carbon model supported by laboratory analysis techniques that weren’t available when the humic model was developed.

The “humic model” is based on the idea that a stable substance called “humus,” once it is formed from decaying leaves, grass, and plant matter, can sequester large, complex molecules of carbon for hundreds or even thousands of years.

But as co-author Johannes Lehmann explained, “This understanding could not be confirmed by modern analytical tools. In the last 10 years, soil scientists have clearly shown that humic substances and large complex molecules are not formed in soil.”

The emerging understanding of soil organic matter accounts for underlying microbial processes. These new concepts – such as the “soil continuum model” – could assist today’s scientists by accurately accounting for soil carbon, thus helping to forecast climate change and warming temperatures.

Nutrient, energy, and carbon exchanges between soil organic matter, the soil environment, aquatic systems, and the atmosphere are an engine that drives agricultural productivity, water quality, and climate.

“Soil organic matter makes up and absorbs more carbon than the world’s vegetation and the atmosphere combined,” Lehmann said. “So small changes in the soil carbon content have huge impacts on the climate.”

Accurate predictions of soil carbon’s behavior are therefore essential. “That’s only possible,” Lehmann said, “if we have the right kind of model and we can mathematically predict what could happen in 50 or 100 years from now.”

For a copy of the paper, contact Lauren Gwin, OSU Center for Small Farms & Community Food Systems,

Abstract of the paper, “The contentious nature of soil organic matter,” by J. Lehmann and M. Kleber, published in Nature, 12/3/2015, v. 528.

The exchange of nutrients, energy, and carbon between soil organic matter, the soil environment, aquatic systems, and the atmosphere is important for agricultural productivity, water quality, and climate. Long-standing theory suggests that soil organic matter is composed of inherently stable and chemically unique compounds. Here we argue that the available evidence does not support the formation of large molecular-size and persistent “humic substances” in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds. We discuss the implications of this view of the nature of soil organic matter for aquatic health, soil carbon-climate interactions, and land management.

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