Cooling minerals pull CO2 into Mojave soils every night, Berkeley researchers find

A physical carbon flux is easiest to mistake for a biological one in a desert. Amundson said respiration tends to run slowly there because plants are sparser than in humid regions such as the tropics ^[19]. Scientists read soil-air exchange mainly as respiration: microbes feed on organic carbon, and the CO2 they make diffuses out of the ground ^[3]. "It's always been assumed that this is entirely driven by a biological process," Amundson said ^[4].

The study pairs two records gathered independently. The field record comes from soil sensors the team has run in the Mojave since 2017 ^[5]. Jennifer Mills, a co-author, found that at night the CO2 concentration 5 centimeters down fell below the concentration at the surface, so gas moved into the soil ^[8]. Respiration pushes CO2 upward, and a gradient pointing down at night has the opposite sign. "There's no biological explanation for this," Amundson said ^[7].

The lab record took three years. Anna Abramova, the lead author, took Mojave samples to Lawrence Berkeley National Laboratory and measured how much CO2 they adsorbed across a range of temperatures and CO2 concentrations ^[11]. The desert surface can reach 60 degrees Celsius by day and fall below 20 at night ^[9], a swing of more than 40 degrees ^[10]. Across temperatures spanning that night and afternoon, the soils took up more CO2 as they cooled and released it as they warmed ^[12]. "We designed these lab experiments to quantify adsorption rates and compare that to how it occurs in the Mojave," Abramova said ^[14]. Her data showed that adsorption could explain what the field sensors had recorded ^[23]. "It was exciting to see how two completely different, independently obtained data sets are very much aligned," she said ^[15].

The global figure needs its denominator checked. The authors estimate that 1 to 3 gigatons of carbon move into and out of desert soils each year, roughly a tenth to a third of annual human emissions ^[16]. Human emissions are a one-way addition to the air. The desert figure counts carbon that enters the soil at night and leaves it by day ^[2], so it measures turnover. The high end is three times the low end ^[22], and the estimate rests on Mojave soils alone; the authors say it could be higher for that reason ^[17]. The same uptake has been observed in cold deserts, including Antarctica ^[18].

Abramova said the cycle offers an alternative explanation for how soils exchange carbon with the air ^[21]. I think the concern for dryland carbon budgets holds in direction. Desorption tracks temperature ^[12], so a flux record that reads daytime desert efflux as respiration would credit microbes with CO2 that minerals took up the night before ^[2][3]. The thing this doesn't tell you is how a given site's afternoon efflux splits between the two. Phys.org's account of the study does not estimate that share for any desert.