Priming effect of labile carbon on the decomposition of recalcitrant nitrogen via heterotrophic nitrification in a subtropical acidic forest soil

Recalcitrant organic nitrogen (N) decomposition is crucial for soil fertility and ecosystem function. Although it is well-established that plant root exudates, containing various labile carbon (C) sources, can stimulate recalcitrant N decomposition, the specific contribution of different labile C in driving this process through heterotrophic nitrification have yet to be fully elucidated. This study investigated the effects of simple C treatments—i.e., citric acid (CA), catechol (CT), and glucose (GLU); as well as combined carbon and nitrogen (C + N) treatments—i.e., glucose + ammonium sulphate (GSA), glucose + glycine (GA), glucosamine (ASS), and glycine (GLY)—on recalcitrant organic N heterotrophic nitrification (ONrec) and mineralization (MNrec) in a subtropical acidic forest soil. Inorganic nitrogen (as ammonium sulfate, SA) was included as a reference treatment for glycine (N treatment). The Ntrace model was employed to estimate the gross rate of ONrec and MNrec. Results unveiled that the highest ONrec developed in response to the C + N treatments (ranging from 0.143 to 1.953 mg N kg−1 d−1), followed by the N treatments (0.043 mg N kg−1 d−1), C treatments (ranging from 0.003 to 0.009 mg N kg−1 d−1), and the control (CK) (0.003 mg N kg−1 d−1). The positive impact of C + N treatments on ONrec was primarily driven by an increase in soil dissolved organic carbon (DOC), likely due to enhanced relative activity of C-:N-hydrolyzing extracellular enzymes (e.g. (CBH + BG):LAP)). Furthermore, the increased abundance of fungi and potential heterotrophic nitrifiers (e.g., Penicillium, Trichoderma, and Mortierella) in the C + N treatments also contributed to their higher ONrec rates. Conversely, the highest MNrec was observed in the N treatments (53.664 mg N kg−1 d−1), followed by the C + N treatments (ranging from 25.438 to 59.088 mg N kg−1 d−1), simple C treatments (ranging from 0.194 to 0.690 mg N kg−1 d−1), and CK (0.587 mg N kg−1 d−1). Unlike ONrec, MNrec was primarily driven by an increase in total nitrogen (TN), which fulfilled the N demand for most soil microorganisms. These findings redefined our understanding of recalcitrant N decomposition through the pathway of ONrec and MNrec, especially in plant-soil ecosystems.