Increasing nitrogen availability does not decrease the priming effect on soil organic matter under pulse glucose and single nitrogen addition in woodland topsoil

Nitrogen (N) availability has been suggested to affect the mineralization of soil organic carbon (C). However, it remains unclear how N supply affects the decomposition of native soil organic C induced by labile C input (i.e., priming effect). This study aimed at examining whether the magnitude and direction of the priming effect depended on the N supply rates and how this interacted with soil layers differing in soil organic C content. Nitrogen was supplied from 0 to 160 mg N kg−1 as Ca(NO3)2 prior to incubation, and 13C-glucose was supplied weekly to induce the priming effect. The priming effect increased with increasing N addition up to 20 mg kg−1, and then plateaued between 20 and 140 mg kg−1 of N addition. In the surface layer of topsoil (0–5 cm), the priming effect at 160 mg N kg−1 decreased by 16% from the peak but was still higher than that in the nil-N control. The effect of N addition on increasing the priming effect lasted for four weeks during which soil mineral N concentrations decreased and the microbial biomass C:N ratio maintained low. By Week 6, N addition stimulated the priming effect to the greatest extent. This N effect was greater in the surface than subsurface layer (5–10 cm) of the topsoil (53% vs. 25%) in the first four weeks but similar afterwards. Our results show that soil organic C in woodland topsoils is susceptible to loss via the N-enhanced priming across a range of N rates.