Deirdre Griffin LaHue | Assistant Professor of Soil Health, Washington State University
Soil carbon sequestration is increasingly a focus in agricultural and environmental research and policy as a strategy for mitigating climate change. Indeed, soils are the largest terrestrial C sink, storing approximately 2500 gigatons (Gt, or billion metric tonnes) of carbon, 4-5 times more than the vegetative pool (~500 Gt) and ~3 times more than the atmospheric pool (~800 Gt; Trivedi and Singh, 2018). Carbon is stored in soils primarily as soil organic matter, which is 50-58% carbon. Globally, soil organic matter stocks have declined in some places as a result of intensive agricultural use, soil moving, and development/urbanization. Thus, there is an intense focus on building back soil organic matter, both for the purposes of soil carbon sequestration and for the many ecosystem and agronomic benefits that soil organic matter provides.
So how do we build soil organic matter? Many soil management practices have potential to build soil organic matter by adding above and belowground plant material (e.g., cover cropping), adding amendments of organic material (e.g., biosolids, compost, manure), and reducing soil disturbance, which can expose previously protected organic matter to decomposition by soil organisms. However, the potential for soil organic matter storage is strongly dictated by a soil’s texture (i.e., clay content) and the climate of the area.
The question for carbon sequestration then becomes, how do we build soil organic matter that is stable, or will persist in the soil? Clay content actually plays a much larger role in stabilizing organic matter than soil scientists previously thought. The old paradigm was that “stable” soil organic matter was created from addition of chemically complex material, like wood chips that are high in lignin and would therefore be difficult for soil microbes to break down. More recently, soil scientists realized that organic matter is actually stabilized through attachment to clay particles, and that it’s the smaller, simpler compounds (like sugars and amino acids) that become “mineral-associated”. These small compounds are usually produced by microbes, flipping on its head the old paradigm that microbial resistance was the way to store soil carbon (Figure 1). This mineral-associated organic matter can stick around in the soil for decades to centuries while the remaining particulate organic matter goes away more quickly, after a few years or decades (Lavallee et al., 2020).
Figure 1. Soil microbes act as a filter for carbon inputs to build stable soil organic matter. (Figure adapted from SoilLife.org)
In our research at the long-term biosolids trial in central Washington, biosolids have been applied every 4 years since 1994, and we are now quantifying how much soil carbon has been stored in the soil profile to date. In addition to carbon inputs from the biosolids, plant growth has typically been higher in the plots with biosolids, contributing increased plant residue inputs. Given the newer understanding of soil carbon storage, we are not only quantifying total organic matter pools (down to a depth of 2 ft), we will also be measuring how much organic matter is in the particulate and mineral-associated pools. This requires separating the fractions by size and/or by density (Figure 2). This work, though laborious, will be important for evaluating the carbon sequestration potential of biosolids in semi-arid dryland systems. We are still in the process of analyzing the 300 soil samples we collected in 2022, so stay tuned for more results on this project!
Figure 2. Conceptual diagram of major soil organic matter (SOM) pools fractionated based on density and/or size. POM: particulate organic matter; MAOM: mineral-associated organic matter; LMWC: low molecular weight compounds; DOM: dissolved organic matter. (Figure from Lavallee et al., 2020)
Works cited:
Trivedi, P., Singh, B.P., 2018. Chapter 1 – Soil Carbon: Introduction, Importance, Status, Threat, and Mitigation, in Soil Carbon Storage. Elsevier Inc. https://doi.org/10.1016/B978-0-12-812766-7.00001-9
Lavallee, J.M., Soong, J.L., Cotrufo, M.F., 2020. Conceptualizing soil organic matter into particulate and mineral-associated forms to address global change in the 21st century. Glob. Chang. Biol. 26, 261–273. https://doi.org/10.1111/gcb.14859