Consistent effects of inorganic and organic N addition on microbial necromass carbon in subtropical forest soils: A mesocosm study

Atmospheric nitrogen (N) deposition can substantially influence the accumulation of microbial residues in forest soils. However, the effects of deposited N forms on soil microbial necromass carbon (MNC) and the underlying microbial mechanisms remain unclear. In this study, an N addition experiment was conducted in a subtropical plantation to investigate the effects of inorganic N (NH4Cl) and organic N (urea and glycine) on soil microbial biomass, hydrolytic enzyme activities, and MNC at 0–10 cm and 10–20 cm depths. The results showed that, despite unchanged soil organic carbon (SOC) content and mineralization, additions of NH4Cl, urea, and glycine reduced soil MNC by 20.7%, 17.4%, and 20.1%, respectively. This decline in soil MNC was mainly attributable to a reduction in fungal necromass carbon, whereas bacterial necromass carbon remained unchanged following N addition. Regardless of N form, N addition increased microbial biomass, intensified microbial carbon and phosphorus limitations, and shifted microbial communities from r- to K-strategists. Furthermore, both inorganic and organic N addition decreased the MNC:SOC ratio, indicating a reduced contribution of microbial residues to the SOC pool. Shifts in microbial community composition toward K-strategists, together with aggravated phosphorus limitation, accounted for the decline in soil MNC under N addition. Overall, these findings suggest that, irrespective of N form, atmospheric N deposition weakens the contribution of MNC to SOC by regulating microbial life-history strategies and nutrient limitations in subtropical plantations.