Biosphere 2 in Arizona. Image credits: Wikimedia Commons.
During a controlled drought inside Arizona’s Biosphere 2, scientists found that this exchange could change direction as the soil dried. When soil moisture fell below about 19%, soil that had been acting as a net sink for several volatile organic compounds (VOCs) began releasing some of them into the air. The finding came from the Biosphere 2 Water, Atmosphere and Life Dynamics (B2-WALD) experiment, which examined the effects of prolonged drought and subsequent rewetting in the facility’s enclosed tropical rainforest in Arizona, according to research published in Nature Microbiology . Soil moisture fell from about 29% to 12.5%, allowing the team to track how the soil’s ability to absorb and release VOCs changed under increasing water stress. The controlled conditions of Biosphere 2 also allowed researchers to regulate drought duration and rainfall during the recovery phase, providing a way to isolate the effects of changing soil moisture. After 65 days without normal rainfall, researchers rewetted the rainforest.
A rainforest floor may look quiet, but beneath the vegetation, soil microbes are constantly exchanging gases with the atmosphere. Researchers manipulated the rainforest’s water supply and continuously monitored soil moisture and VOC exchange as the drought progressed. The experiment also examined what happened when water came back. The return of water produced a rapid burst of carbonyl emissions, followed by a more prolonged release of sulfur-containing compounds. The first response appeared to involve largely abiotic processes, while the later emissions were linked more strongly to biological activity. The response shows that drought does not simply push an ecosystem from one stable state into another. Drying and rewetting can produce different chemical reactions, with the timing and composition of emissions changing as the soil moves between wet and dry conditions.
Biosphere 2’s tropical rainforest is a large enclosed ecosystem in Oracle, Arizona, with roughly 1,940 square metres of rainforest vegetation rooted in soil up to several metres deep. During the experiment, soil moisture declined from about 29% before the drought to 12.5% at its driest point. Nature Microbiology also notes that researchers used carbon-13-labelled pyruvate to follow how soil microbes processed carbon under normal and drought conditions.
In other words, more of these compounds were being taken up by the soil than released from it. During drought, emissions of several volatile metabolites, including acetate, acetone and diacetyl, increased, while carbon dioxide released from microbial processing declined. Unlike a natural rainforest, where drought develops alongside changes in temperature, rainfall and atmospheric conditions, the enclosed system allowed researchers to isolate the effects of prolonged drying. Scientists continuously measured VOCs leaving or entering the soil using specialised chambers connected to a proton-transfer-reaction time-of-flight mass spectrometer. This allowed them to track changes in gas exchange as the drought progressed. Under wetter conditions, the soil acted as a net consumer of several VOCs, including isoprene, monoterpenes, carbonyl compounds and alcohols. The researchers then looked more closely at what was happening inside the soil. Their evidence pointed towards microorganisms as an important driver of the change. During wetter conditions, soil microbes consumed some of the VOCs present in the soil-atmosphere system. As drought intensified, microbial activity declined overall, but the organisms did not simply become inactive. Instead, their use of carbon changed. The researchers interpreted this as evidence that drought altered microbial carbon metabolism, leaving more carbon in volatile compounds that could escape into the atmosphere. This helps explain why the change in VOC behaviour was more complicated than simply saying that dry soil produces more gases. The drought affected both production and consumption. Some microbial pathways became less active, while other processes associated with stress and the accumulation of intermediate metabolites became more prominent.
The implications extend beyond the glass walls of Biosphere 2. Tropical forests are major contributors to the global pool of biogenic VOCs, and climate change is expected to increase the frequency or duration of drought in some regions. If prolonged drying reduces the ability of soils to consume atmospheric VOCs while increasing the release of certain compounds, the overall balance between the forest floor and atmosphere could change.