Grassland soils store carbon

Grasslands are more resilient to drought and fire than forests, making them more reliable for carbon storage in regions like California (Dass et al., 2018; Neary and Leonard, 2020). Grasslands store 24–80% of their biomass belowground (Byrne et al., 2021), and many species have fire adaptations, such as quickly resprouting or emerging from the seed bank post-burn (Dass et al., 2018). In some old-growth prairies, stands of native bunchgrasses like purple needle grass (Stipa pulchra) can live for more than 100 years contributing to a reliable carbon sink (Jackson et al., 2007). Fire is known to stimulate fine root production and root turnover, increasing soil carbon (Dixon et al., 2019; Neary and Leonard, 2020).

The legacy of agricultural land use and recovery of soil carbon along the central California coast

Most prairies along the Central Coast have a land use history that has left a legacy effect, such as previous histories of farming or grazing that have impacted the soil structure. Jackson et al. (2007) estimate that 450,000 ha of grazed land and 110,000 ha of irrigated cropland in Monterey County were previously coastal grasslands, which has resulted in decreased soil quality and a loss in soil carbon. Soil carbon is typically slow to recover. A couple of studies have shown no recovery in soil carbon 10 years after restoring perennial bunchgrasses (Jackson et al. 2007; Valadez, 2024). No study has yet documented how fire  affects soil carbon in the long-term within this region. To learn more about soils in the Coast Range Region, see UC Agriculture and Natural Resources – Ecology and Management of Rangeland Soils Series.

How does  grassland burning affect the carbon flux?

When grasslands burn, their above-ground biomass is converted into carbon dioxide (Dixon et al., 2019), but the above-ground biomass typically regrows quickly. Frequent fires can lead to a reduction in soil carbon content at 0–5 cm depth (Xu et al., 2022); however, lower severity and intensity burns can increase carbon through the incorporation of unburned or partially-burned fragments, as well as ash (Alcañiz et al., 2018). Ultimately, carbon efflux is a balance between soil carbon inputs and removal (Strong et al., 2017). In general, lower intensity burns with moderate fire frequencies have a net positive impact on soil carbon, whereas higher intensity and frequent burns can have a negative impact on soil carbon. No study has yet documented how fire affects soil carbon in the long-term within this region.

Fire impacts on other soil properties

Fire can impact soil health in several ways, described below. These effects are influenced by the timing and intensity of the burns.

• Soil Structure. Fire can affect the soil structure through the alteration of clay minerals and organic matter combustion (Neary et al., 1999). When fires burn hot enough, this can increase the coarseness of the soil which decreases the soil absorption capacity contributed by the clay fraction (Ngole-Jeme, 2019). Such conditions affect the cation exchange capacity (CEC), which is important for nutrient retention (Mukhopadhyay et al., 2019). Additionally, soil bulk density may increase with fire because of the release of soil aggregates following combustion of organic matter (Alcañiz et al., 2018). 
• Soil Nutrients. Fire typically increases the short-term availability of many soil nutrients due to the deposition of nutrient-rich ash, which contains macronutrients (e.g., calcium, magnesium, potassium, phosphorus), heavy metals, silica, and organic carbon (Alcañiz et al., 2018; Docherty et al., 2012). However, the effect of fire on soil nitrogen is more variable. This variability is partly due to nitrogen’s high volatility during combustion and also to the patchy spatial distribution of ash across the burn area, which creates uneven nutrient inputs (Dixon et al., 2019).
• Soil pH. Fire tends to increase soil pH (make it less acidic), particularly in the upper 20 cm of the soil (Xu et al., 2022). This is due to the release of cations, like calcium and magnesium, combustion of the organic soil horizon, and oxidation of organic matter; yet low intensity burns or single treatment burns might show little pH change (Alcañiz et al., 2018). 
• Soil Moisture and Temperature. Fire can increase soil moisture by reducing plant water uptake through the removal of plants, as well as accumulation of ash that leads to higher water absorption capacity (Xu et al., 2022). However, soil moisture can decrease when the ash is hydrophobic, as well as from degraded soil structure and elevated soil temperature (Xu et al., 2022). Less than 5% of the heat from a surface fire is transmitted in the soil (Neary and Leonard, 2020). In moist soils, heating duration and maximum soil temperature are generally lower than in drier soils (Badía et al., 2017), so the timing of burns relative to rainfall has a strong impact on soil temperature, nutrient cycling, and the soil seedbank.
• Microbes and Mesofauna. Microbes are one of the most important drivers of ecosystem recovery following fires (Yang et al., 2020). Whereas heating from burns can kill microbes, the release of previously unavailable nutrients can increase microbial activity (Dixon et al., 2019; Docherty et al. 2012). In general, fires decrease fungi while bacteria are more resistant (Docherty et al., 2012; Glassman et al., 2023; Pressler et al., 2019), likely because fungi are more directly tied to plant mortality (Glassman et al., 2023). The patchiness of burning is important for soil mesofauna (e.g., Collembola, Acari), as unburned refugia host more soil arthropods; otherwise burning kills mesofauna (Pressler et al., 2019). 
• Seed banks. Soil heating up to a certain temperature can also stimulate seeds that lie dormant for several years, breaking their thick seed coats (Neary and Leonard, 2020). However, if the temperature is too hot, this can kill seeds, particularly if the soil is moist. While this varies species to species, a review on grassland soils with fire found that lethal temperatures for seeds are approximately 70°C for wet soils and 90°C for dry soils (Neary and Leonard, 2020).