Nitrogen input in different chemical forms and levels stimulates soil organic carbon decomposition in a coastal wetland

•Six-year N input significantly stimulates plant biomass, height and density.•Stimulation of plant growth increases soil carbon storage under 6-year N input.•N-input enhances soil carbon storage and accelerates SOC decomposition.•Stimulation of SOC decomposition is N form dependent.•Microbial biomass carbon is positively related to SOC decomposition. Nitrogen (N) input significantly regulates soil organic carbon (SOC) storage in N-limited ecosystems. However, the regulatory direction, magnitude, and mechanisms of SOC decomposition under continuous N input in different chemical forms and levels in coastal wetlands are poorly understood. We investigated the impact of 6-year N input in a coastal wetland of the Yellow River Delta, examining the effects on plant growth and soil properties for different chemical forms (NH: ammonium, NO: nitrate, and NN: ammonium-nitrate) and levels (5 g N m−2 yr−1, 10 g N m−2 yr−1, and 20 g N m−2 yr−1) at two depths (0–10 cm, 10–20 cm). We also set up an incubation experiment with N-treated soil to investigate SOC decomposition under different treatments. Six-year N input led to overall increases in the soil nutrients (i.e., total N, ammonium-N (NH4+-N), and nitrite-N (NO3—N)), stimulation of plant growth (i.e., plant biomass, height, and density), enhancement of soil C storage (i.e., total carbon, SOC, dissolved organic carbon, and microbial biomass carbon), and decreases in the electrical conductivity (EC). The incubation experiment revealed that N input-induced SOC decomposition stimulation is N-form dependent; NO input was the strongest stimulator of decomposition, when compared to NH and NN input. The stimulation differences among the dominant-form N inputs increased with increasing N supply levels. Structural equation modeling (SEM) analysis indicated that the stimulation of SOC decomposition by N input was associated with N-related changes in the soil nutrients, vegetation, soil C storage, and soil environment. Together, these results aid the evaluation of soil C cycling under future N deposition scenarios in coastal wetlands.