Temperature sensitivity (Q) of stable, primed and easily available organic matter pools during decomposition in paddy soil

The response of stable and labile C pools to global warming is uncertain, especially in paddy soils with very low oxygen availability and the dominance of electron acceptors with low efficiency. To clarify the response of organic matter decomposition to warming, flooded paddy soil was incubated at four temperatures (5, 15, 25, and 35 °C) for 75 days. The 13C-labelled Na-acetate was used as an analogue for root exudates and as a methane (CH4) source. Soil with acetate had higher C availability to microorganisms leading to 2–2.7 times and 2–153 times higher emission of carbon dioxide (CO2) and CH4 on day 75 than from soil without acetate, respectively. Incubation temperature explained >40% of the variance of CO2 and CH4 effluxes. Acetate stimulated microbial activities and turnover and so, increased soil organic matter (SOM) mineralisation in the first week, especially at low temperatures (<15 °C) with slow acetate consumption and longer oxygen (O2) availability. The priming effects measured as CH4 emissions were especially sensitive to temperatures from 5 to 15 °C. The high Q 10 value of primed CH4 (Q 10  > 10) at low temperature indicates that flooded paddy fields will contribute greatly to the greenhouse effect in warm winters, which have become common from 1970s. Caution is necessary for interpretations of previous estimates of the temperature sensitivity of SOM decomposition because the priming effect was ignored, especially that of CH4 under the condition of limited O2 availability in paddy and other wetland soils.