Electron transfer capacity of soil dissolved organic matter and its potential impact on soil respiration

Soil dissolved organic matter (DOM) as the labile fraction of soil organic carbon (SOC) is able to facilitate biogeochemical redox reactions effecting soil respiration and carbon sequestration. In this study, we took soil samples from 20 sites differing in land use (forest and agriculture) to investigate the electron transfer capacity of soil DOM and its potential relationship with soil respiration. DOM was extracted from 20 soil samples representing different land uses: forest (nos. 1-12) and agriculture (nos. 13-20) in Guangdong Province, China. Chronoamperometry was employed to quantify the electron transfer capacity (ETC) of the DOM, including electron acceptor capacity (EAC) and electron donor capacity (EDC), by applying fixed positive or negative potentials to a working electrode in a conventional three-electrode cell. The reversibility of electron accepting from or donating to DOM was measured by applying switchable potentials to the working electrode in the electrochemical system with the multiple-step potential technique. Carbon dioxide produced by soil respiration was measured with a gas chromatograph. Forest soil DOM samples showed higher ETC and electron reversible rate (ERR) than agricultural soil DOM samples, which may be indicative of higher humification rate and microbial activity in forest soils. The average soil respiration of forest soil (nos. 1-12) and agricultural soil (nos. 13-10) was 26.34 and 18.58 mg C g(-1) SOC, respectively. Both EDC and EAC of soil DOM had close relationship with soil respiration (p < 0.01). The results implied that soil respiration might be accelerated by the electroactive moieties contained in soil DOM, which serve as electron shuttles and facilitate electron transfer reactions in soil respiration and SOC mineralization. DOM of forest soils showed higher ETC and ERR than DOM of agricultural soils. As soil represents one of the largest reservoirs of organic carbon, soil respiration affects C cycle and subsequently CO2 concentration in the atmosphere. As one of the important characteristics of soil DOM related to soil respiration, ETC has a significant impact on greenhouse gas emission and soil carbon sequestration but has not been paid attention to.