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Research Focuses on Soil Additive’s Seemingly Contradictory Benefits

There is a growing global movement among gardeners, farmers, and others to add ground charcoal, or biochar, to soil to both boost crop yields and counter global climate change. A new study released this week by researchers at Colorado College and Rice University could help settle the debate about one of biochar's biggest benefits - the seemingly contradictory ability to make clay soils drain faster and sandy soils drain slower.

"Understanding the controls on water movement through biochar-amended soils is critical to explaining other frequently reported benefits of biochar such as nutrient retention, carbon sequestration, and reduced greenhouse gas emissions," said Rebecca Barnes, assistant professor of environmental science at Colorado College and the lead author of the study. She began the research while serving as a postdoctoral research associate at Rice.

The study, available online this week in the journal PLOS ONE, offers the first detailed explanation for the hydrological mystery.

Biochar can be produced from waste wood, manure, or leaves, and its popularity among do-it-yourselfers and gardening buffs took off after archaeological studies found that biochar added to soils in the Amazon River Basin more than 1,000 years ago was still improving the water- and nutrient-holding abilities of those poor soils today.

Studies over the past decade have found that biochar soil amendments can either increase or decrease the amount of water that soil holds, but it has been tough for experts to explain why this occurs, due partly to conflicting results from many different field tests.

In the new study, biogeochemists conducted side-by-side tests of the water-holding ability of three soil types - sand, clay, and topsoil - both with and without added biochar. The biochar used in the experiments, which was derived from Texas mesquite wood, was prepared in the lab of Rice geochemist Caroline Masiello, a study co-author, to ensure comparable results across soil types.

Barnes said the team chose to make its comparison with simple, relatively homogenous soil materials in order to compare results to established hydrologic models that relate water flow to a soil's physical properties, such as bulk, density, and porosity.

"This is what helped us explain the seeming disconnect that people have noted when amending soils with biochar," Barnes said. "Biochar is light and highly porous. When biochar is added to clay, it makes the soil less dense and it increases hydraulic conductivity, which makes intuitive sense. Adding biochar to sand also makes it less dense, so one would expect that soil to drain more quickly as well; but in fact, researchers have found that biochar-amended sand holds water longer."

Barnes said the highly porous structure of biochar makes each of these pathways more torturous than the pathway that water would take without it. Moreover, the surface chemistry of biochar - both on external surfaces and inside pores - is likely to promote absorption and further slow the movement of water.

"By adding our results to the growing body of literature, we show that when biochar is added to sand or other coarse-grained soils there is a simultaneous decrease in bulk density and hydraulic conductivity, as opposed to the expected result of decreased bulk density correlated with increased hydraulic conductivity that has been observed for other soil types," Barnes said.

The study is the latest from Rice's interdisciplinary Biochar Research Group, which formed in the wake of Hurricane Ike in 2008 when the city of Houston called for ideas about how to get rid of an estimated 5.6 million cubic yards of fallen trees, broken branches, and dead greenery left behind by the storm.

Report an issue - Last updated: 12/16/2020