By David D'Amore
Soils are complex assemblages of factors that play a role in their formation. Climate, biological organisms, geologic parent material, topography, and time are the soil ‘state factors’ that lead to the emergent property of a soil. This complexity results in properties that allow soils to provide important ecological functions that support life through their role in the growth of plants, habitat for animals and bacteria, and enhancing water suppliesthrough natural filtration. Soils also serve to store large amounts of carbon. The majority of terrestrial carbon is found in soils; estimates of global soil carbon stocks have a median across all studies of ~1500 Pg C to 1m depth (1Pg = 1015 g and up to 2300 Pg to 3m depth). A large proportion of soil carbon is estimated to be in forested soils, which account for approximately one-third the land area of the US but store 50 to 70 percent of soil carbon.
Soils accumulate these large amounts of reduced organic carbon through the residues of biological accumulation of carbon dioxide (CO2) from the atmosphere. Photosynthetic accumulation of carbon by plants and subsequent storage in biomass and soils in the terrestrial ecosystem provides a buffer against increasing atmospheric concentrations of carbon dioxide. The forest carbon sink is responsible for accumulating approximately 16 percent of annual carbon dioxide emissions in the United States. However, the rates, fate, and overall balance in the many diverse terrestrial ecosystems that occur across the landscape are large sources of uncertainty when quantifying the amount of C accumulated in soils and reconciling the role of passive carbon accretion in soils. Predicting the response of soil properties to increasing temperature and variability in precipitation is one of the greatest challenges facing soil carbon cycle scientists.
Organic matter, minerals, and carbon
Understanding soil carbon cycles begins by examining the organization within soil profiles and the modes of formation within soil types. Soils store carbon in two distinct vertical components: the organic horizon and the mineral horizon. Organic horizons consist of plant material in varying states of decomposition. This organic material can be deposited as leaf litter, woody debris, or soluble carbon consisting of organic acids leached from vegetation. The concentration of carbon in this organic material is very high, but bulk density is low due to the large pore spaces between the particles. Therefore, the depth of the organic horizon controls the overall carbon storage. In many situations, the organic horizon is not deep because the soil organic matter is mineralized by microbes that use the reduced organic carbon as a source of energy and release essential nutrients in the soil organic matter.
Soluble organic acids, which pass through the organic horizon and flow downward to the lower soil horizons, move carbon deeper into the soil profiles. Roots also deliver organic matter to the mineral horizon through exudates and tissue senescence, and root exudates are important in transferring carbon into the subsoil where different processes act on the organic matter.
Mineral horizons are formed from weathering rock or sediments that have been redistributed by gravity, water, or ice. Weathering of the primary minerals in these parent materi- als creates secondary minerals called clays, which have greater surface area and reactivity. Clay minerals provide substrate for carbon, organisms, and water in the soil matrix. As minerals weather and the soil progresses further in development, metals, such as iron (Fe), Aluminum (Al), and Silica (Si), are released and can combine with the soluble organic acids to create metal-organic aggregates through chemical complexation. Carbon can accumulate in this soil matrix as the primary minerals weather and the soil horizon deepens. However, there is also physical and chemical erosion of the soil and a tension between soil formation and carbon accumulation and loss.
The evolution of understanding soil carbon
Soil carbon was believed to be a stable carbon pool, but our understanding of soil carbon is undergoing a reexamination as conceptual models and measurement techniques change. Original conceptual ideas regarding soil carbon proposed that the decomposition process was governed by alteration of soil organic matter and the formation of humic substances. Measurements of soil organic matter used extracts that converted the organic matter to either soluble or insoluble forms, which were interpreted to act as stable or labile (unstable) pools. However, recent research has challenged the idea that soil organic matter molecular conversion actually occurs and that the assumption of carbon stability resulted from the byproducts of the extraction procedure.
A range of research has led to a new conceptual model that highlights the soil conditions, rather than the structure of the organic matter, as controlling soil organic matter turnover. Therefore, the susceptibility of the organic matter to accumulation or decomposition is related more to the complex emergent properties within the soil and is governed by the soil-forming factors. While the organic molecules are composed of reduced organic carbon and are susceptible to decomposition, the arrangement of the organic matter within the soil matrix and physical and chemical conditions influence the fate of the material.
Current research has focused on the potential for soil carbon compounds to degrade rapidly by microbial processing. Although all soil organic matter is subject to decomposition and has a strong connection to increased temperatures, it can be protected through either physical occlusion or chemical complexation with elements such as Fe and Al. Al and Fe activities can interact with soluble organic acids to form organo-mineral complexes that are chemically sequestered in aggregates within the soil matrix. This is the defining process in Spodosols, which are prominent in northern temperate forests. The reactive spodic horizons can extend deep into the soil profiles that promotes the complex mix of organic carbon associated with secondary clay minerals, along with what are known as ‘amorphous’ minerals where Al, Fe, Si and organic matter combine as highly weathered aggregated complexes.
Physical alterations can occur through weathering processes where crystalline rock minerals are transformed to secondary clays through dissolution and precipitation reactions. New surface areas are created that have a charge associated with them that can facilitate the attachment of organic compounds. Newly produced organic matter does not bind to mineral soil surfaces linearly, but rather is facilitated by surfaces preconditioned by physical alteration or where older organic matter exists. Biologically active microsites are favorable to the presence of active microbial communities that can quickly exploit the new organic material.
A soil carbon case study
One of the densest areas for soil carbon storage in the world is the Northeast Pacific coastal temperate rainforest that extends from northern California to southeast Alaska and includes coastal mountain ranges, large glacially carved valleys, and lowlands with extensive peatlands. The perhumid sub-region (pCTR) along coastal British Columbia and southeast Alaska provides a compelling case study for examining current and future soil carbon stocks. Soil carbon densities across the pCTR can be >200 Mg Ha-1. Total stock estimates for portions of the region range from 1.9 to 4.8 Pg C. The most recent assessment also provided a highly resolved spatial map of carbon across the pCTR and an overall stock estimate of 4.5 Pg. The majority of this stock is contained in the mineral soils along with dense pockets of deep organic soil peatlands. While the total stock is not as large as the vast boreal forests, the density within small areas has implications for the future trajectory of this landscape due to varying fates across landforms and soil types.
The soil types of the region are dominated by Histosols, which are almost completely composed of decayed plant material, and Spodosols, which are soils dominated by organic-metal complexes. The Histosols can have enormous amounts of carbon due to the great depth of accumulation of 4 m or more. While their extent is not as large as mineral soils, the lack of good estimates for depth in these areas leave this as one of the largest sources of uncertainty in regional carbon accounting. The organic soils actually have a period of moisture deficit, which causes the surface horizons to become aerobic and changes the chemical reactions that influence carbon decomposition. Decomposition increases under rising temperatures and may result in a potential loss of the total carbon stock in these Histosol soils. The Spodosols are extensive and can be quite deep and rich in organic carbon concentration. These soils also have a great deal of reactive components and the potential to store large amounts of carbon. Hydromorphic, or soil saturation, alters Fe from crystalline to amorphous forms with consequent impacts to the soil carbon environment.
While these soils can sequester carbon, they are also some of the most susceptible to carbon loss with increased temperatures. Current research is underway to examine the association of carbon with clay minerals to understand the patterns of organo-metallic complexes across different parent material. Another focal area is determining the influence of biological weathering on soil development and subsequent carbon stabilization through an approach using mesh bag mineral material buried in soils.
What is still to learn?
A key outstanding question of the forest soil carbon cycle is how climate warming will impact the emergent soil properties that govern the persistence of carbon in the terrestrial ecosystem. Research in the pCTR provides some insights for the future of soil carbon. The soil carbon storage, its potential change, and the proximity of soluble carbon export to the coastal ocean all make understanding the mechanisms of carbon cycling in pCTR soils important. There is evidence that carbon losses have a greater potential to outpace carbon gains in areas with large stocks. As a percentage of the entire stock, this might be equivalent to other regions, but in terms of regional soil carbon losses, it could be substantial. The new conceptual model holds that changes in soil conditions could lead directly to changes in the soil organic matter turnover. The activity of microbial communities will stimulate growth with not only mineralization of soil organic carbon, but also increasing competition for nitrogen in an already N-limited environment.
Young-growth forest stands rely on the standing stock of nutrients and water in the organic-rich forest floor to sustain vigorous growth rates. Alteration of the soil organic matter may influence the ability of young forests to maintain growth due to a reduction in vigor if nutrient supplies are impacted by loss of labile soil organic matter. This in turn will affect aboveground terrestrial carbon stocks. This relationship demonstrates the connection between carbon pools and that when discussing carbon, a holistic approach is needed.
David D’Amore is a research soil scientist with the USDA Forest Service Pacific Northwest Research Station. He can be reached at (907) 586-9755 or email@example.com.