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

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), 10gNm -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 Nform 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.