Forms of nitrogen deposition shift soil microbial resource limitation and carbon use efficiency in temperate forest

Nitrogen (N) deposition in forests shifts the nutrient balance of entire ecosystems, promoting acidification, loss of soil organic carbon (SOC) and plant biodiversity, and altering microbial activity. Here, we studied the effects of 14 years of N deposition (10 g N m⁻² y⁻¹) in multiple forms on nutrient stoichiometry in soil and microbial biomass (MB) pools, as well as changes in enzyme activities, in a mixed temperate forest in northeastern China. The experiment followed a completely randomized design: control (water only), inorganic N (NH₄NO₃, IN), organic N (urea:glycine = 1:1, ON), and mixed N (IN:ON = 7:3, Mix-N). Under ON, SOC in litter and mineral soil horizons (0–10 and 10–20 cm) increased by 8–42 % compared to the control. In the topsoil, MBC:MBP increased by 66 %, and MBN:MBP increased by 58 % under ON, indicating shifts in microbial demand for C and P. Enzyme activities increased by 48 % for C and 57 % for P under ON in topsoil, since microorganisms responded to resource limitation. Organic N mitigated microbial C and P limitation by altering soil nutrient cycles, suppressing fungal abundance, and reducing the fungal-to-bacterial ratio. Enhanced microbial C use efficiency was positively associated with nutrient availability and contributed to SOC sequestration under ON. Thus, ON deposition has a crutial role in regulating microbial processes and nutrient cycling, thereby promoting soil carbon sequestration and ecosystem stability in temperate forests.