Priming effect of stable C pool in soil and its temperature sensitivity

It is important to understand the effects of exogenous carbon (C) additions on the mineralization of native soil C in agricultural soils under changing global temperatures. The slowly mineralizable C pools represent the major component of soil carbon. Once the labile fraction of soil C is depleted through mineralization, additions of exogenous labile C may enhance decomposition of slowly mineralizable soil C through the so-called priming effect (PE). The effect of exogenous C on mineralization of slowly mineralizable C in soils that have undergone extended substrate depletion is not well understood. The aim of this study was to assess the effects of exogenous C addition on the PE and its temperature sensitivity (Q₁₀) in two substrate depleted soils. The Mollisols with two levels of C 6.8% and 2.9% were pre-incubated at 15 and 25 ℃ for 864 days, followed by 42-day incubations with and without ¹³C-enriched corn (Zea mays) leaf addition at 15 and 25 ℃. Soil organic carbon (SOC) decreased by 7%−9% in low and 1%−3% in high C soil after the long-term pre-incubation, respectively. In the 42-day incubation following the long-term pre-incubation, the incubation at the higher temperature (25 ℃) significantly increased cumulative CO₂ production compared to that at the lower temperature (15 °C) for both high and low C soils with or without exogenous C addition. Furthermore, corn leaves addition stimulated CO₂ production derived from resistant OC in both soils, resulting in a positive PE. Higher incubation temperature resulted in a greater PE in both soils independent of pre-incubation temperature. The temperature sensitivity (Q₁₀) of the PE in the low C soil pre-incubated at 25 °C was greater than that pre-incubated at 15 °C, but in the high C soil, the Q₁₀ of PE was lower after pre-incubation at 25 °C. The results confirmed that exogenous C addition and a higher temperature accelerate native SOC mineralization and suggested that the temperature sensitivity of the PE depended on the soil C content and the long-term temperature regime a soil is subjected to.