Purpose Soil nitrogen (N) transformation is an important phenomenon in forest ecosystems and it is regulated by carbon (C) input. This study aimed to evaluate the impacts of labile C levels on soil N mineralization under different simulated N deposition rates.
Materials and methods Soils at 0-15-cm depth were collected from two contrasting subtropical forests, a coniferous fir forest (CFF) and an evergreen broadleaf forest (EBF); both soils had been subjected to 3 years of artificial NH4NO3 input or deposition (no-N control, N-0; low N (30kg N ha(-1) year(-1); N30), and high N (100 kg N ha(-1) year(-1)); N100). The impacts of external glucose-C (G) on N mineralization of these N-deposited soils were investigated by the addition of six C rates (mg C kg(-1) dry weight soil-0, G0; 100, G100; 300, G300; 1000, G1000; 2000, G2000; and 5000, G5000), at a temperature of 25 degrees C, and a 60% water-holding capacity for 21 days.
Results and discussion The results showed that, after 21days of incubation, concentrations of inorganic N (NH4+-N and NO3--N) decreased significantly (P <0.05) with increasing C rates and reached a minimum value when the added C rate was G1000. The lowest NH4+-N under G1000 was 9.2 mg kg(-1) in all of these three N-deposited soils at the CFF site while 11.6 mg kg(-1) in the N30 soil at the EBF site. The concentration of NO3- -N was decreased to 0 under G1000 and G2000 in the CFF and EBF soils, respectively. These results revealed that the higher the soil NO3--N concentration was, the greater the NO3--N reduced, with a maximum decrease of 80 mg NO3--N kg(-1) in the N100 soil from the EBF site. In addition, the soil mineralization and nitrification rates were significantly higher (P
Conclusions Our results suggested that there could be a critical C level at which N transformation being altered in certain soils, based on their N status, and that the impacts of C on soil N mineralization were independent of soil N availability.