Positive priming effect explained by microbial nitrogen mining and stoichiometric decomposition at different stages

The priming effect is an essential mediator in the soil carbon (C) cycle. There is a growing concern about the priming effect induced by labile C input. However, the driving factors of the priming effect under agroecosystems with different historical tillage management remain unclear. By conducting a laboratory incubation experiment, the priming effect and the fate of 13C-labeled glucose (1.658 atom%) were quantified in two soils (Cambisols and Phaeozems) that underwent the 12-year or 13-year tillage managements: rotary tillage without straw retention (RTN), and no-tillage with straw retention (NTS), and a grassland (GRL). After the 31-day incubation, RTN that had nitrogen (N) limitation emitted 26.7% more total C and 35.0% glucose-derived C than NTS across the two soil types. In Cambisols, the RTN exhibited 21.2% and 47.5% higher priming effect than NTS and GRL, respectively; while in Phaeozems, the RTN had 29.5% and 34.8% lower priming effect than NTS and GRL. Moreover, RTN showed 81.8% lower and 26.7% higher microbial C use efficiency (CUE) than NTS in Cambisols and Phaeozems, respectively. The higher N availability, CUE, and labile C retention which consists of the sum of glucose-derived microbial biomass C, total dissolved organic C, and organo-mineral C were the major contrib-utors to the lower priming effect under long-term tillage management. A positive priming effect was observed across all treatments after glucose addition as a result of stimulating microbial activities, and then the increased microbial activities promoted co-metabolism. At the early stages, the priming effect was driven by microbial N -mining, and the driving force was dominated by microbial stoichiometric decomposition at later stages. These findings provide a more accurate understanding of soil organic C dynamics. The results can be used to predict the change of soil organic C under long-term tillage management, which are critical for sequestrating the elevated atmospheric C concentrations.

Automated summary

The priming effect is an important factor in the soil carbon cycle, but the driving factors in agroecosystems with different tillage management remain unclear. This study found that long-term tillage management with straw retention can reduce the priming effect by increasing nitrogen availability, microbial carbon use efficiency, and labile carbon retention.