Soil Organic Carbon Sequestration Rates Under Question As New Methods Give Different Estimates

By Shweta Iyer on April 19, 2014 4:05 PM EDT

soil
Studies measuring the amount of carbon captured organically by soil, known as soil sequestration may have been misleading. Scientists have now developed a more accurate definition for the process. (Photo: RLHyde, CC BY-SA 2.0)

Changes in farming practices have been known to effect the process by which atmospheric carbon dioxide (CO2) is captured and stored, known as soil organic carbon sequestration rates. And for the past two decades, several studies have been trusted with estimating the amount sequestered from farming techniques like no-till systems. But recent research on no-till systems involving corn and soybeans grown without cover crops, small grains, or forages, may not be increasing at the published rates.  

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The recent study, led by Ken Olson, a soil scientist from the University of Illinois, looked at hundreds of articles published on soil science and tillage. The researchers, who came from multiple universities, selected 120 papers emphasizing soil organic carbon sequestration, storage, retention, and loss, for analysis. According to Olson, some studies showed that moldboard and no-till systems were both being depleted of organic carbon in soil over time.

Previous long-term studies only accounted for the difference in net soil organic carbon storage between no-till and moldboard plots between treatments, and the given estimates of sequestration may have been misleading. It is believed that no-till techniques retain more soil organic carbon because the soil is not disturbed from constant tilling and erosion.

While this may be true on sloping and eroding plots, which retain zero to 15 centimeters more soil organic carbon on the surface, Olson says that no-till subsurface layers often lose more soil organic carbon stock "over time than is gained in the surface layer," and that it is imperative to test and sample the subsurface layer to the depth of rooting at about one to two meters.

While researching the material, the team found that the discrepancies in previous studies were due to a variety of reasons, starting from the basic definition of soil organic carbon sequestration used by different researchers.

So, the team came up with a new definition for soil sequestration: The process of transferring CO2 from the atmosphere into the soil of a land unit through unit plants, plant residues, and other organic solids, which are stored or retained in the unit as part of the soil organic matter (humus). To claim soil organic carbon sequestration, management practices must lead to an increase in the net soil organic carbon from a previous pre-treatment baseline measurement and result in a net reduction in the CO2 levels in the atmosphere. Carbon not directly originated from the atmosphere (from outside the land unit) cannot be counted as sequestered soil organic carbon. These external inputs may include organic fertilizers, manure, plant residues, topsoil, or natural input processes such as erosion of a sloping soil and sediment-rich carbon deposition on a soil located on a lower landscape position or in a waterway. The land unit could be a plot, plot area, parcel, tract, field, farm, landscape position, landscape, wetland, forest, or prairie with defined and identified boundaries.

The other estimates that could lead to errors in calculating soil organic carbon sequestration rates included: inappropriate experimental methods; lack of sample testing from deeper layers of soil; lack of soil bulk density measurements; not measuring the carbon that is offloaded from external sources into the plot; varying laboratory methods for measuring soil organic carbon over the long-term study; effects of soil erosion, transport, and deposition on the experimental tillage plots; lack of sloping and eroding sites included in summary studies; natural variability that was not captured by the sampling scheme; only sampling the plot areas once when trying to determine rate of change; insufficient frequency of sampling; and finally, making estimates on the assumption that after 100 years of cultivation and before the tillage treatment was applied that the soil organic carbon had dropped 20 to 50 percent but was now at a steady state.

Olson said that aeration, drainage, tillage, disturbance, more intensive crop rotations, use of synthetic fertilizers, erosion, and lack of cover crops could all result in reduced soil organic carbon stocks. The accuracy of determining soil organic carbon sequestration depends on the method used, he said. The team's new definition of soil sequestration and some new recommendations for future studies may help improve accuracy.

"Because these long-term studies are used for crop-yield determinations they need to be re-started without interruption, and soil sampling can be done during the non-growing season," Olson said. "Then the long-term experiments can be used to measure soil organic carbon sequestration rates."

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