Soil organic matter oxidation by hydroxyl radical: an overlooked route of greenhouse gas production in drylands

The rewetting of dry soils by rainfall pulses boosts the release of greenhouse gases over a short time period and is the primary pathway for greenhouse gas emissions in dryland ecosystems. However, the intrinsic mechanisms underlying such emission pulses of greenhouse gases are not clear, especially in areas covered by biological soil crusts with strong microbial activity. Here, we simulated rain events in bareland, cyanobacteria/lichen, and moss crusted soil using triple isotope labeling (¹³C, ¹⁵N, and ¹⁸O) to explore the sources of the hydroxyl radical (·OH) and their effects on greenhouse gas production. We found that the ·OH was produced after a rainfall event via rapid activation of microorganisms in the soils. The carbon dioxide (CO₂) and nitrous oxide (N₂O) production significantly decreased after ·OH removal, whereas the methane (CH₄) production was not affected. We revealed that ·OH synergy with enzymatic reactions of microorganisms increased CO₂ production from the soil by 25%. The ·OH also stimulated the conversion of NH₄⁺ to NO₃⁻ and dominated the N₂O production (80%). Our results confirm the pivotal role of ·OH in the production of greenhouse gases and indicate that microbially mediated ·OH oxidation mechanisms are an overlooked dominated pathway for the emission of greenhouse gases in dryland ecosystems.