Differences in Soluble Organic Carbon Chemistry in Pore Waters Sampled from Different Pore Size Domains
Protecting soils to mitigate climate change.
Soil has networks of pores and channels that weave through it like interconnected straws. These networks are formed underground by the different minerals that compose soil and as a result of movements or growth by roots, insects, and other living organisms. Soil pores house gases and liquids such as soil organic carbon (SOC) and water. SOC plays a vital role in the carbon cycle. A recent study found that carbon complexity differs with the size of the pore that contains it, yet its decomposability is driven by its proximity to microorganisms, not its chemistry.
These findings could provide a powerful framework for building a new generation of models simulating SOC dynamics and composition. The findings also provide insights for using natural processes to protect SOC so that it remains or decomposes in the soil rather than returning to the atmosphere.
In the natural water cycle, the hydrologic connectivity of soil pores surges as soil water content increases, and when pore channels fill with water, SOC and other nutrients can mix and redistribute. Furthermore, when the soil is saturated, soil pores become increasingly connected (making them straw-like) by water, allowing movement of dissolved SOC between pores. This movement increases the likelihood that stored carbon will be transported to microbial-rich locations more favorable to decomposition. This diverse distribution of microbial decomposers throughout soil indicates that metabolism or persistence of SOC compounds is highly dependent upon short distances— think “sprints”—of transport between pores, via water, within the soil.
To demonstrate this process, researchers at Pacific Northwest National Laboratory saturated intact soil cores and extracted pore waters with increasing suction pressures to sequentially sample them from increasingly fine pore domains. The soil solutions were held behind coarse and fine pore “throats,” and revealed more complex soluble carbon in finer pores than in coarser ones. Analysis of the same samples—incubated with fungi Cellvibrio japonicus, Streptomyces cellulosae, and Trichoderma reseei—showed that the more complex carbon in fine pores is not more stable; rather, it is at least as easily decomposed as the simpler forms of carbon found in coarse pores. In fact, the decomposition of complex carbon led to greater losses of it through respiration than the simpler carbon found in coarse pore waters. This finding suggests that repeated cycles of drying and wetting in soils may be accompanied by repeated cycles of increased carbon dioxide emissions. All this raises a question: Is SOC persistence primarily a function of its isolation in different-sized pores?
All the study’s incubated samples demonstrated that the fungi could decompose the SOC in pore waters within the first 48 hours of colocation, meaning that the proximity of microbes with the substrate is the controlling factor in protecting carbon within the soil. The challenge is to use this information to improve predictions of carbon persistence in soils and perhaps determine if and how these natural processes within the soil could be exploited on a much bigger scale so that carbon releases to the atmosphere are reduced.
Pacific Northwest National Laboratory
This work was supported by the Terrestrial Ecosystem Science program of the Office of Biological and Environmental Research within the U.S. Department of Energy (DOE) Office of Science. A portion of this research was performed using the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility located at Pacific Northwest National Laboratory.
Bailey, V., et al. “Differences in soluble organic carbon chemistry in pore waters sampled from different pore size domains.” Soil Biology and Biogeochemistry 107, 133–43 (2017). [DOI:10.1016/j.soilbio.2016.11.025].