Rapid formation of abiotic CO2 after adding phenolic gallic acid, to agricultural soils

Abstract

Abiotic efflux of CO₂ from soil is typically attributed to weathering of carbonates but also arises from concurrent oxidation of organic matter and reduction of metal oxides. Little is known, however, about the magnitude of the latter reaction in soil environments. We observed rapid formation of CO₂ from soils treated with a simple phenolic acid (gallic acid, [GA]), consistent with redox reactions catalyzed by Mn or Fe oxide. We measured CO₂ formed during 4-h incubations of soil from different management systems (n = 5), archived benchmark soils (n = 18), and samples of reagent-grade metal oxides (n = 4). Treatments included water, pH 4 phthalate buffer, glucose (0.029 M), or GA (0.025 M). Little CO₂ was formed when samples were treated with water or glucose, but CO₂ quickly evolved with GA. Adding buffer elicited CO₂ in some samples. Soil from a 5-year rotation produced less net CO₂ (p ≤ 0.05) than other crop rotations or pasture. Net responses from benchmark samples ranged broadly. The CO₂ from some soils was attributable to an acid-carbonate reaction, while for other soils CO₂ was inferred to derive from oxidation of GA by metal oxides. Unlike other tested oxides, Mn(IV) oxide produced a CO₂ response similar to that seen in soil. Redox reactions producing CO₂ can occur in a variety of soils after inputs of GA, a simple phenolic constituent of root exudates, and be influenced by management. Such processes, catalyzed by Mn(IV) oxide, might be significant abiotic sources of CO₂ from agricultural land.