Soil carbon balance by priming differs with single versus repeated addition of glucose and soil fertility level

Labile carbon (C) inputs strongly alter soil organic matter (SOM) turnover by priming, thus affecting soil C dynamics and soil fertility in the long term. The frequency of labile C inputs is expected to affect the intensity of priming effect. We evaluated the effects of single versus repeated additions of 13C-labeled glucose (with added 13C corresponding to 2% of soil organic C content) to five soils with increasing fertility level on the intensity of priming effect and soil C balance. Repeated glucose addition induced 61–108% greater C mineralization due to positive priming than the single addition in all soils. The intensity of priming effect declined with soil fertility level and mineral nitrogen (N) content, but increased with higher activities of N-acquisition enzymes (N-acetyl-glucosaminidase and leucine amino peptidase). Consequently, the decrease in N availability strongly increased the intensity of priming effect presumably by enhancing microbial N mining from SOM. Considering the C balance between SOM losses due to priming and glucose-C retention, glucose addition was responsible for net C losses in the low and moderate fertility soils (−14.0 to −0.30 mg C g−1 SOC after 14 weeks) but led to C gains in the high fertility soils (+0.44 to +4.36 mg C g−1 SOC). Greater retention of glucose-C was attributed to higher N availability and more intensive microbial growth, thus supporting microbial necromass formation and soil C sequestration. Compared to single addition, repeated glucose addition caused larger net C losses or lower C gains due to stronger SOM priming. In conclusion, the frequency of substrate addition and soil fertility are important regulators of SOM priming and the C balance. This knowledge should be informative to interpret soil C dynamics in response to labile C inputs in agroecosystems.