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 (O_Nrec) and mineralization (M_Nrec) 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 O_Nrec and M_Nrec. Results unveiled that the highest O_Nrec 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 O_Nrec 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 O_Nrec rates. Conversely, the highest M_Nrec 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 O_Nrec, M_Nrec 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 O_Nrec and M_Nrec, especially in plant-soil ecosystems.