How much oxygen is needed for acetylene to be consumed in soil?

Abstract

Acetylene (C2H2) is employed for the quantification of important biological processes such as nitrogen fixation, nitrous oxide reduction, ammonium and methane oxidation, and methanogenesis. Although acetylene is not a natural product, the ability of bacteria to grow on C2H2 is a phenomenon common to soils and sediments. Our experiment was designed to study the modification of CO2 production, O2 uptake and microbial biomass (Cmic) in soil in response to the consumption of added acetylene. Two soils (peat-muck and Eutric Cambisol) were incubated under well aerated (60% water holding capacity [WHC]) or flooded conditions, and enriched with C2H2 in the range 0.1–10 kPa (initially, 0.434–63.4 mmol C2H2 kg−1) at a constant temperature of 20°C. Gases were measured chromatographically, while Cmic was measured by the physiological SIR (substrate-induced respiration) method based on the initial response of microorganisms to glucose amendment. We used a simple calculation of net CO2, net O2 and net Cmic (as differences between amended and not amended soils) to estimate the contribution of C2H2 to the total respiration and microbial growth during the incubation. Peat-muck soil consumed more C2H2 than Cambisol (maximum 54.03 vs. 19.25 mmol kg−1, p < 0.001). Acetylene utilization was faster and larger in flooded than in wet soils (16.2 and 7.81 mmol kg−1, respectively, p < 0.05), and followed the tendency observed for Cmic. Depending on C2H2 enrichment, air–water conditions, and soil tested, both reduction and stimulation of measured activities were observed in response to acetylene consumption. Low C2H2 uptake, especially in Cambisol incubated at 60% WHC, resulted in the reduction of soil respiration and biomass (by 1–29%). Large C2H2 consumption in flooded soils stimulated CO2 production, O2 uptake and Cmic, even by 78%, 72% and 43%, respectively. Net CO2, net O2, and net Cmic were linearly positively related to the quantity of consumed C2H2 (p < 0.001). Acetylene utilization was a combined effect of initial C2H2, soil properties, and air–water status. The amount of consumed C2H2 was the highest in flooded peat-muck soil enriched with 10 kPa, and lowest in Cambisol at 60% WHC with 0.1 kPa C2H2. Consumption of 1 mol acetylene induced production of 0.5 mol CO2, and uptake of 0.4 mol O2. Low acetylene consumption (<6 mmol kg−1) resulted in reduction of both soil respiration and microbial biomass.