Healthy soil is rich in organic matter, but scientists have yet to fully understand exactly how that organic matter is formed. Now a team of University of New Hampshire scientists have uncovered evidence that microbial pathways – not plants – are the chief originator of the organic matter found in stable soil carbon pools.
The new insight provides promise for designing agricultural systems that promote microbial communities to optimize soil organic matter formation.
The research was conducted by Cynthia Kallenbach, former UNH graduate student now at Colorado State University, her advisor, Stuart Grandy, associate professor of natural resources at UNH, and Serita Frey, professor of natural resources at UNH. Their results were published in the paper “Direct evidence for microbial-derived soil organic matter formation and its ecophysiological controls” in the journal Nature Communications, and comes from work supported by the NH Agricultural Experiment Station.
In the paper, UNH scientists suggest that soil organic matter accumulates from inputs of dead microbial cells and microbial byproducts formed when microbes eat plant roots and residues, rather than from plants themselves, as previously thought.
In the past, scientists thought the best way to build soil organic matter was to slow down or inhibit decomposition using plants that soil microbes find difficult to decompose. The idea was that the undecomposed plant parts would gradually become soil organic matter, especially if the soil microbial community was inactive.
However, this pool of soil organic matter, decomposing plant parts, doesn’t last long and can quickly disappear as carbon dioxide, sometimes within a year. This left the question: how do we form pools of soil organic matter that persist for decades, and that are so critical for sustaining healthy, productive soils?
Soil organic matter is a massive reservoir of carbon – containing more than twice as much carbon as there is atmospheric carbon dioxide. Thus, even relatively small changes in soil organic matter can have a large impact on atmospheric carbon dioxide, a critical greenhouse gas. Soil organic matter is also fundamental for plant growth and healthy agricultural systems.
"Soil carbon, in my mind, is the fulcrum around which all aspects of soil revolve," Kallenbach said.
The UNH research paints a completely different pathway to building soil organic matter. This has been challenging for scientists to demonstrate, because once soil organic matter is formed, identifying whether it was most recently a plant or microbial cell is impossible.
In the lab, the scientists demonstrated the accumulation of significant amounts of chemically complex, persistent soil organic matter from microbial materials, in the absence of any plant inputs. Microbes unexpectedly produced soil organic matter that was almost identical to natural, field-derived soil organic matter when fed nothing but table sugar. Further, soil organic matter accumulation is greatest when more–not less–active microbial biomass is formed. This is especially true when that biomass is produced more efficiently, meaning more of the substrate is converted to biomass rather than carbon dioxide.
“Cynthia showed that the amount of organic matter formed depends heavily on the characteristics and physiology of the microbial community,” Grandy said. “Challenging another long-held view, Cynthia also found that the characteristics of the microbial community are even more important for soil organic matter formation than soil type.”
This material is based upon work supported by the National Science Foundation Doctoral Dissertation Improvement Grant (DEB-1311501) and the NH Agricultural Experiment Station through joint funding of the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award numbers 1003421 and 1007001, and the state of New Hampshire.