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When thinking about ways to mitigate climate change through carbon storage, trees are often one of the first methods that come to mind. However, forest soil beneath the ground is just as important. For instance, in temperate forests, like those found in some parts of the Northwest Climate Hub region, about half of the total carbon is stored in the soil. Despite the importance of forest soil carbon, there is still a lot of uncertainty about how it will respond to climate change.

What is forest soil carbon?

Plants sequester carbon from the atmosphere to create carbon-rich leaves, stems, and roots. As these plant parts decay, soil organisms feed on the dead material, locking carbon underground. Some CO2 is also released back into the air as part of this process. Credit: Climate Central.

Soil formation is influenced by five key factors: parent material, climate, topography, organisms, and time. Soil forms continuously, but slowly, from the gradual breakdown of rocks through weathering. Over time, primary minerals, which come from the weathering of parent material, break down to form secondary minerals, like clay. Clay is an important component in forest soils because clay minerals attract nutrients like carbon and nitrogen. When these nutrients interact with mineral surfaces, they become chemically bound. Northwest forests such as those found in the Willamette Valley in Oregon have clay-rich soils.

In forest ecosystems, soil organic matter—the primary form of soil carbon—forms through the decomposition of plant material such as leaves, wood, and roots. This process begins when invertebrates like earthworms break large pieces of plant material into smaller ones. Then, fungi and microbes (microscopic organisms like bacteria) break them down further. Plant organic matter is approximately 50% carbon, and a portion of this organic carbon is transferred to the soil. The remaining portion returns to the atmosphere as carbon dioxide.

Root exudates, carbon-rich substances released by plant roots, influence how carbon is stored in soil. Root exudates feed microbes, influencing decomposition. When microbes break down organic matter near reactive minerals, carbon can chemically bind to mineral surfaces. When soil carbon is chemically bound to mineral surfaces, it is less available for microbes to break down. In contrast, when carbon is physically protected within soil aggregates, it remains more accessible to microbes, which can lead to carbon being returned to the atmosphere as carbon dioxide through microbial decomposition.

Another important pathway involves the transfer of carbon to mycorrhizal fungi. Mycorrhizal fungi exchange nutrients with plants. Carbon transferred to mycorrhizal fungi is either released into the soil by living fungi or incorporated into the soil as dead biomass.

What factors influence forest soil carbon stocks?

Different forest ecosystems store and cycle carbon in unique ways. The amount of carbon stored in the soil depends on the balance between carbon gained from plant growth and carbon lost through decomposition. Climate is a key factor in how much carbon forest soils can hold. Temperature and moisture levels in different regions affect both plant growth and the speed of organic matter breakdown. Local factors, including soil moisture, temperature, elevation, and slope, also influence decomposition and carbon storage. Additionally, the soil parent material plays a role in determining its ability to store carbon over time.

Different forest types store varying amounts of carbon above and below ground. In the Northwest Climate Hub region, coastal forests in western Oregon, Washington, and Alaska align with the 'Cool temperate moist' category. Interior mixed-conifer forests east of the Cascades fit the 'Warm temperate dry' category, while boreal forests in southeastern Alaska correspond to 'Boreal moist' and forests North of the Brooks Range in Alaska to 'Boreal dry.' Figure by: Janowiak et al., 2017.

• Forest soils in the boreal forests of Alaska are characterized by the presence of permafrost, which can remain frozen year-round. Because of extremely cold temperatures and short growing seasons, tree and plant growth are limited. As a result, most of the carbon in northern Alaska is stored in the soil. 
• In contrast, temperate forests in coastal areas of Oregon, Washington, and southcentral and southeast Alaska have a mild, wet climate that promotes steady plant growth for much of the year. This provides consistent carbon inputs into the soil, allowing for a lot of above and belowground carbon. 
• The dry interior mixed-conifer forests found east of the Cascade Range are in a more arid climate. These dry conditions cause faster decomposition and less plant growth, which limits how much carbon can be stored in the soil. As a result, this region stores less soil carbon compared to coastal forests.
• Globally, wet tropical forests (though not found in the Northwest) provide another example of how climate can affect forest soil carbon storage. In these ecosystems, warm and humid conditions support rapid plant growth but also speed up decomposition. As a result, most of the carbon is stored aboveground in the dense forest canopy, with relatively little carbon stored in the soil compared to the vegetation above.

How can Northwest forest soils mitigate climate change?

The soils in the forests of the Northwest Climate Hub region help mitigate climate change by storing a large amount of carbon. However, climate change is making it more difficult for these soils to store carbon. Longer, hotter summers increase the temperature of the forest floor which accelerates the breakdown of organic matter in the soil. Warmer conditions can also dry soil, increasing the risk of wildfires, which can burn organic matter and return carbon to the atmosphere.

Longer growing seasons associated with climate change may increase productivity in some forests. If productivity increases, the amount of organic matter in the soil increases, simultaneously increasing the amount of carbon in the soil. However, as temperatures rise and disturbances including drought and wildfire become more common, the carbon gained through increased productivity could be offset.

As fire regimes shift, the characteristics of soils could also change. Clay-rich soils, like those found in Oregon's Willamette Valley, have a high capacity for long-term carbon storage due to reactive mineral surfaces that bind and protect organic matter. However, as fires occur more frequently, soils lose moisture, weakening the bonds between organic matter and minerals. Fires can degrade or destroy the clay mineral surfaces to which carbon can bind. After severe wildfires, the risk of soil erosion also increases because vegetation that stabilizes the soil is burned. Without plant roots to anchor the soil or a forest canopy to shield soil from rainfall, the exposed soil becomes susceptible to erosion. The loss of thin, nutrient-rich topsoil reduces soil carbon.

A map of the contiguous U.S. showing soil organic carbon stocks, estimated from Forest Service Forest Inventory and Analysis plots. Idaho, Oregon, and Washington have some of the highest soil organic carbon stocks. Image from: USDA Forest Service Northern Research Station.

Rising temperatures, drought, and insect outbreaks may change the species of trees that are able to grow in parts of the Northwest. For example, longer hotter summers in the coastal forests of Oregon and Washington could lead to a shift in dominant forest types. This shift in tree species is important because different tree species produce organic matter with distinct chemical compositions, which strongly influence the decomposition process. In addition, drought conditions increase evaporative demand and draw more moisture from both plants and soils. This is important in dry forests such as juniper woodlands, where the combination of heat stress and moisture diminishes tree survival and recruitment. 

Though strategies to increase carbon storage in trees (e.g., reforestation, afforestation) are well established , improving carbon storage in the soil is less certain. This is because soil carbon stocks vary, even within the same forest. Differences in soil makeup, root growth, moisture, and micro-climatic conditions all affect how much carbon is stored in the soil. Sampling methods, such as the depth of soil sampling or tool choice, can also affect results. Thus, it is difficult to measure and understand how much carbon is stored in forest soils.

Despite these uncertainties, there are some practices known to promote forest soil carbon storage. Planting trees on land where trees have been logged or burned can help restore soil carbon. Avoiding physical disturbance, such as soil compaction, can also promote soil carbon, especially in soils with high clay content. More comprehensive data from future research will improve carbon models and strengthen soil carbon management strategies.