Purpose: This study investigated the differences between net and gross rates of N transformation in subtropical forest soils under aerobic and anaerobic conditions. We aim to explore the factors responsible for the differences observed and to evaluate the ability of net and gross N transformation measurements to reflect the occurrence and stimulation of N transformation in subtropical forest soils.

Materials and methods: Subtropical soil samples from adjacent native forest (NF) and two plantation forest sites (Pinus caribaea var. hondurensis (PCH) and Hoop pine (HP), Araucaria cunninghammii), with different initial pool sizes of NH4 + and NO3 −, were collected from South East Queensland, Australia. Soil samples were incubated at 25 °C to determine net and gross N transformation rates (15N pool dilution method) under aerobic and anaerobic conditions.

Results and discussion: Under aerobic and anaerobic conditions, gross N transformation rates differed significantly from net N transformation rates. NH4 + immobilization occurred under aerobic and anaerobic conditions for all soils. NO3 − immobilization occurred under aerobic conditions in the NF soils but was negligible in both the PCH and HP soils. Gross ammonification rates were significantly correlated with the C/N ratios (R 2 = 0.714, p < 0.001). Gross nitrification rates were significantly correlated with soil carbon composition (13C, R 2 = 0.569, p < 0.01). The ratios of the gross rates of NH4 + immobilization to the gross ammonification rates (R_i a/g a) were significantly correlated with δ13C (R 2 = 0.829, p < 0.001). The results indicated that in the subtropical forest soils, both the variability of gross N production and immobilization mainly influenced the net N transformation rates. Forest type affected the gross production or immobilization of NH4 + and NO3 − via C turnover, which was indicated by the relationship between the N transformation rates and soil C/N ratio or soil 13C.

Conclusions: Although subtropical forest soils are generally an N-unlimited system, the immobilization of NH4 + and NO3 − under aerobic or anaerobic conditions played an important role in the N transformations. Net rates of NH4 +/NO3 − accumulation are not suitable to describe the occurrence and stimulation of N transformations in subtropical soils when immobilization (or other consumption processes) of NH4 + and NO3 − is occurring, especially under anaerobic conditions.