UC Berkeley study finds Mojave soils take up carbon dioxide at night and release it by day

UC Berkeley researchers reported on September 23 that Mojave Desert soils in California take up carbon dioxide as they cool at night and release it as they warm during the day. Their study, published in Science Advances, links the pattern to gas temporarily attaching to soil particles. The finding matters because a nighttime carbon dioxide measurement in dry soil may reflect a physical exchange as well as biological activity.

The team combined years of measurements in the Mojave with laboratory tests on soil collected there. The field observations showed carbon dioxide moving into soil at night, while the laboratory samples held more of the gas at lower temperatures. The researchers say those results provide an explanation for a pattern that conventional accounts of soil respiration alone could not explain.

How cooling Mojave soil holds carbon dioxide

UC Berkeley says the team has monitored the Mojave with soil sensors since 2017. Earlier work by study co-author Jennifer Mills found that, at night, carbon dioxide concentration five centimeters below the surface fell below the concentration at the surface. That difference indicated gas moving inward, rather than simply escaping from the soil.

Lead author Anna Abramova took Mojave samples to Lawrence Berkeley National Laboratory and measured how much carbon dioxide they held at different temperatures and gas concentrations. The process, called adsorption, occurs when molecules attach to a surface. According to Berkeley, the laboratory results aligned with the field observations: cooling increased the soil's capacity to hold carbon dioxide, and warming released it.

What the result means for soil-respiration measurements

Soil respiration describes carbon dioxide released through processes involving roots and microorganisms. A sensor recording gas exchange at the soil surface captures the combined result of processes operating at that time. If minerals take up gas while the soil cools, a nighttime inward reading cannot be assigned to biology alone. Likewise, daytime release from those minerals may add to the outward flow measured as the soil warms.

The study reports that accounting for temperature-driven physical exchange reverses the apparent temperature hysteresis in its soil-respiration analysis. In practical terms, the authors say the measured relationship between temperature and carbon dioxide flow looks different once temporary adsorption and release are included. That result concerns the interpretation of their measurements; it does not establish the size of the effect in every desert soil.

Why the global carbon estimate remains uncertain

The study estimates annual adsorption of 16.7 to 19.3 grams of carbon, measured as carbon dioxide carbon, per square meter in the thermally active soil layer. Its estimate of up to 2.2 to 2.5 billion tonnes of carbon exchanged globally is an extrapolation from the studied setting, rather than a worldwide field measurement. Berkeley gives a broader estimate of roughly one to three billion tonnes moving into and out of desert soils each year and says that estimate rests on Mojave soils.

Those figures describe movement in both directions. Because the gas held overnight is released again as the soil warms, the observed daily cycle does not by itself show permanent carbon removal. The amount that other deserts might exchange depends on how closely their soils and conditions resemble the Mojave samples. The share of nighttime uptake attributable to adsorption across different deserts remains unmeasured in this study.

Earlier studies found other nonbiological desert-soil exchanges

The new adsorption finding sits alongside earlier evidence that desert carbon dioxide flows are not always explained by respiration. A 2017 Mojave study used carbon isotopes to find a stronger nonrespiratory source of carbon dioxide as dry soil heated, while wet soil showed stronger signs of biological respiration. That work associated daytime emissions with carbonate processes; it did not test or confirm the new study's mineral-surface adsorption mechanism.

A separate 2013 study of saline, alkaline soil found inward carbon dioxide flow at night and outward flow by day. Its authors attributed the pattern to temperature-driven dissolution into soil solution and subsequent release. In that study, the measured inorganic flow balanced over 24 hours. The authors identified soil pH and moisture as important controls, underscoring why a result from one kind of soil cannot automatically be extended to all deserts.

Berkeley says Abramova is now examining whether adsorption changes in soils with different amounts of organic carbon or minerals with greater capacity to hold carbon dioxide. Measurements across a wider range of soils would help establish how much of the nighttime signal comes from adsorption, how it interacts with other nonbiological processes, and how broadly the Mojave findings apply.