Contrasting priming effect intensities and drivers in single and repeated glucose additions to a forest soil receiving long-term N fertilization

Soil nitrogen (N) availability plays an important role in regulating priming effects (PE). In this study, we aimed to investigate the effects of long-term N fertilization on PE under single and more frequent additions of labile carbon (C) inputs (C-13-labeled glucose). The soils collected from a 6-year-long inorganic N fertilization experiment in a subtropical Phyllostachys edulis forest in China were incubated at a constant temperature of 25 & DEG;C for 90 days. Regardless of the mode of glucose addition, the cumulative primed C was lower in the previously fertilized soil and showed a positive relationship with soil pH and peroxidase activity. When comparing the amount of cumulative primed C and the remaining added glucose-C after 90 days of incubation, the fertilized soil had a higher net C increase than the control. These results indicate that atmospheric N deposition and N fertilization might reduce the PE caused by labile organic matter inputs, thereby increasing soil organic C sequestration. The effect of N fertilization on PE under a single glucose addition (reduced by 11%) was smaller than that of repeated additions (reduced by 24%). Previously fertilized soil with a single glucose addition exhibited higher microbial P deficiency, which had a negative impact on PE; in contrast, in soil with repeated glucose additions, higher abundance of Gram-positive bacteria contributed to the negative PE. As such, the frequency of glucose addition affects the intensity and regulation of soil PE. Regardless of the N fertilization treatment, soil with a single glucose addition exhibited stronger PE but lower soil net C increases than those with repeated additions, which might be related to higher microbial activities. Collectively, our findings provide insights regarding soil PE in subtropical forests under the combined effects of increasing N deposition and glucose addition.