Response of nitrogen transformation to glucose additions in soils at two subtropical forest types subjected to simulated nitrogen deposition

Abstract

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. 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, N0; low N (30 kg 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 °C, and a 60% water-holding capacity for 21 days. The results showed that, after 21 days 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 < 0.05) in soils from the EBF than from the CFF site and increased with N in soils subjected to simulated N deposition. However, the net N transformation rates also decreased with C addition and had the minimum value at G1000. 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.