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Enhancement of Soil Carbon Sequestration

Author

James Amonette,
Pacific Northwest National Research Lab

Executive Summary 
Research at PNNL has focused on understanding the fundamental process by which humus is created and identifying practical management options to enhance the rate of humification in agricultural soils.



In a new approach to slowing global warming and restoring nutrient-depleted farmland, chemists have found they can promote soil's natural ability to sequester carbon. CASMGS helped fund research at the Pacific Northwest National Laboratory focused on understanding the fundamental process by which humus is created and identifying practical management options to enhance the rate of humification in agricultural soils. Recent experiments show that maintaining a proper alkalinity plus frequent wetting and drying cycles can coax soil to retain more carbon.  

     
The humification rate depends on many factors: enzyme stability, moisture, alkalinity, oxygen availability, microbial population and the physical properties of different soils. The apparent rate-limiting step in the process is the oxidation of polyphenols to quinones. This step is catalyzed by polyphenol oxidase and laccase enzymes produced by fungi. Minerals such as iron and manganese oxides also promote the oxidation step. The quinones then react with peptides and amino acids released by soil microbes to form complex, durable molecules called humic polymers which are not easily degraded by microbes and consequently are stable in the soil for decades.  

     
Experiments were designed to promote the activity of tyrosinase, a common polyphenol-oxidase enzyme. A near-neutral pH was found to be optimal for tyrosinase activity. More alkaline pHs, however, yielded even faster humification by promoting a nonenzymatic oxidation step and the quinone-amino acid polymerization step, in addition to the tyrosinase-mediated reaction. Adjusting the pH of acidic soils with an alkaline, porous material called “fly ash” to slightly alkaline pHs yielded the best results by promoting the oxidation and polymerization reactions and by providing small pores to protect humic polymers that were produced. Frequent cycles of wetting and drying also were important for fostering a rich microbial community that supplies many of the humic precursors and for aiding the formation of soil aggregates.  

     
The implications of these findings regarding reaction mechanisms and management strategies for enhancing soil-C sequestration were presented recently by the researchers in a paper at the American Chemical Society meeting. Next, the researchers will test their findings in the field in Charleston, SC by planting 72 pots containing various controlled mixtures of soil and catalysts.  

     

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