Native soil labile organic matter influences soil priming effects

The input of exogenous labile carbon into the soil is an important factor affecting soil organic carbon dynamics by accelerating or inhibiting the decomposition of native soil organic matter, known as the priming effect. Many factors affect the priming effect, including biological factors, such as soil microorganisms, plants, and soil fauna, as well as, abiotic factors, such as soil structure, nutrients, water, and temperature. Although the mechanisms responsible for the occurrence and maintenance of priming effects are well-understood, most previous studies only considered the impact of input of exogenous available organic carbon on priming effects. How the native soil labile organic matter (LOM) affects the direction and magnitude of the priming effect remains unclear. In this study, mineral layer soil was collected from a Chinese fir plantation in Qianyanzhou, Jiangxi. Native LOM concentrations in the soil were adjusted using vacuum soil solution extraction. The priming effect was quantified by adding ¹³C-labeled glucose at two different concentrations. Changes in the concentration of native LOM affected the native microbial biomass in the soil, in turn altering the response of soil microorganisms to exogenous available organic carbon. In the absence of glucose, soil with a high concentration of native LOM contained high levels of soil microbial biomass carbon. After adding glucose, soil microbial biomass increased, which promoted the decomposition of soil organic carbon and produced a positive priming effect. At two glucose addition levels, soil with the native LOM concentration of 50 % showed a significantly lower priming effect with values of 1.00 ± 3.10 and 0.69 ± 2.99 mg C g⁻¹ SOC at 5 % and 20 % MBC, respectively. The priming effects did not significantly differ between the other native available organic matter concentration treatments. Thus, the influence of the native LOM concentration on the soil priming effect may be much more complex than previously considered.