Climate and Management Effects on Soil Organic Carbon in Temperate Managed Ecosystems

Protecting soil carbon is crucial for effective carbon management, as reversing losses is slow and difficult. This resource considers various effects on soil carbon in temperate managed ecosystems and can help land managers maintain and enhance carbon storage, supporting ecosystem resilience.

What is Soil Carbon? Why is it Important? 

Soils store more carbon than the atmosphere and vegetation combined, an impressive fact that underscores the importance of considering soils when managing for ecosystem carbon. Not only do soils store vast amounts of carbon, but soil carbon also persists in the ecosystem for much longer than other pools of carbon. The mean residence time of soil carbon is on the order of decades to millennia, compared to vegetation carbon which cycles back to the atmosphere over timescales of years to centuries. The slow cycling of soil carbon also means accrual rates of new soil carbon are slow (Schlesinger 1990), while disturbance (e.g., land use change, erosion following biomass removal) can cause large and rapid site-level soil carbon losses (Guo and Gifford 2002, Berhe et al. 2018). Therefore, protecting existing soil carbon stores is foundational to managing for carbon because reversing soil carbon losses through management takes decades at a minimum, and is sometimes not even possible. 

Soil organic carbon is primarily derived from plants, which convert atmospheric carbon dioxide (CO2 ) into organic matter via photosynthesis. Carbon initially fixed by plants and then consumed by other organisms such as animals and fungi also contributes to soil carbon. In non-arid systems, most soil carbon is organic carbon (hereafter, “soil carbon” refers to soil organic carbon) and is the focus of this primer. In contrast, arid systems have significant amounts of inorganic carbon in the form of calcium carbonate deposits, commonly referred to as “caliche”. 

Plant organic matter is approximately 50% carbon, and a portion of this plant carbon is transferred to the soil. The remaining portion returns to the atmosphere as CO2 during plant respiration and microbial decomposition. There are three main pathways through which carbon moves from plants to soils: 1) exudation of carbon-rich molecules such as simple sugars from living plant roots into the soil, 2) decomposition of above- and below-ground dead plant tissue, and 3) transfer of carbon to mycorrhizal fungi (in exchange for resources such as nitrogen and phosphorus) which in turn is either released into soil by living fungi or incorporated into the soil as dead microbial biomass.

In general, carbon entering the soil from belowground roots or fungi contributes disproportionately to long-term soil carbon storage (Rasse et al. 2005, Jackson et al. 2017, Sokol and Bradford 2019). This is due in part to the proximity of belowground carbon inputs to soil minerals. Additionally, roots and fungi play large roles in promoting soil aggregation. Soil aggregation enhances soil carbon persistence because carbon occluded within aggregates is physically protected from microbial decomposition. Soil aggregation is hugely important to soil carbon storage and also contributes to soil water holding capacity and soil structure. Meanwhile, carbon from roots and fungi can also feed the microbial community, thereby accelerating microbial decomposition and loss of existing soil carbon. While this illustrates the complexity of soil carbon, overall, belowground inputs generally promote soil carbon storage (Verbruggen et al. 2013).

Soil Carbon Fractions 

Carbon is stored in many different forms in the soil, affecting soil carbon sequestration and storage. To aid in characterizing and managing soil carbon, scientists commonly differentiate two broad fractions of soil carbon based on their distinct physical properties: particulate organic matter (POM), typically plantlike materials in various stages of decomposition ranging from recognizable plant debris to highly decomposed muck-like materials, and mineral associated organic matter (MAOM), which is mostly microbially-sourced organics that are preserved by bonding with mineral surfaces. These two procedurally defined soil fractions also vary in their process of formation, mean residence time, and response to climate change and land management (Cotrufo and Lavallee 2022), making the distinction between carbon in POM versus MAOM fractions useful for linking ecological processes with carbon management outcomes (Fig. 1)(Lavallee et al. 2020).

POM is formed by the physical transfer of plant, animal, and microbial products to the soil, while MAOM is formed from dissolved organic carbon (DOC) or microbial products adsorbing to soil mineral surfaces. Therefore, much of the carbon that initially enters the soil is in the form of POM, with some DOC leached or exuded from plant tissues directly entering the MAOM carbon pool. Over time, as POM is decomposed, a fraction of POM carbon is transferred to the MAOM pool while the remaining carbon is released into the atmosphere as CO2 . To illustrate, consider a deciduous leaf that falls to the ground in autumn. As the leaf slowly decomposes over weeks and months, carbon-rich leaf fragments will enter the POM pool. A proportion of carbon will quickly leach from the leaf and become DOC. Some DOC will move into the mineral soil and associate with soil minerals to become MAOM carbon, while the remaining DOC will leach out of the soils and drain into the watershed. Over time, the leaf fragments in the POM pool will be further decomposed by microbes, and this microbially-processed carbon will become part of the MAOM pool.

Figure 1- Illustration of soil carbon pools and fluxes. Three soil carbon pools are shown, including dissolved organic matter (DOM), particulate organic matter (POM), and mineral associated organic matter (MAOM), with relative turnover represented by differences in the size of the clock icon. Soil carbon fluxes are depicted by yellow arrows, including plant inputs to soil carbon (both aboveground and belowground plant tissues), internal cycling of carbon between soil carbon pools, and outputs of carbon to the atmosphere and through leaching. Note that MAOM turnover time varies widely, with some MAOM turning over quickly. Also note that the relative importance of different carbon fluxes vary between ecosystems.

Historically, our understanding was that more complex carbon compounds (such as ‘tougher’ lignin-rich plant tissues) persist in the soil for longer. Now it is understood that carbon residence time is more influenced by protection from microbial processing (i.e., physically inaccessible to microbes or stuck strongly to soil mineral surfaces) than the chemical makeup of the organic matter. Inputs that decompose more quickly (e.g., fine roots) contribute more to MAOM carbon and long-term soil carbon storage compared to slower-decomposing carbon tissues such as downed woody debris (Cotrufo et al. 2013, 2015). Therefore, these relatively simple and fast-decomposing tissues still have a long residence time.

MAOM carbon is more resistant to carbon losses compared to POM due to binding with soil minerals, although POM contained in soil aggregates can also persist if soil structure is maintained. With increasing soil depth, relatively more carbon is held in MAOM and less in POM. As deeper soils are less prone to disturbance and decomposition, this pattern also lends to MAOM persistence. In general, MAOM has a longer mean residence time and therefore contributes more to long-term soil carbon storage compared to POM. Note that this distinction in residence time between MAOM and POM broadly holds true, but a significant portion of MAOM can turn over quickly.

Various Effects on Soil Carbon