Soil gross nitrogen transformations along a secondary succession transect in the north subtropical forest ecosystem of southwest China

• A 15N tracing study was conducted in situ to investigate gross N transformation rates along a secondary succession transect.
• Soil N availability and N retention capacity in the late successional stages become greater.
• Soil–plant interactions affect internal N cycles and enhance soil N supply and N retention.

Soil nitrogen (N) transformations in forest soils play a vital role in plant N availability. However, the effect on soil N dynamics of forest succession remains largely unknown. A 15N tracing study was conducted in situ to investigate gross N transformation rates along a secondary succession transect (grassland GL, coniferous forest CF, coniferous-broad-leaved mixed forest CBF, broad-leaved forest BF, natural broad-leaved forest NF) under a north subtropical monsoon climate. Gross N mineralization and NH4+ immobilization rates were significantly higher in the BF soil than in the NF, CBF, GL and CF soils, although no significant differences were found among the latter four soils. Gross NO3− immobilization rates in the GL and CF soils were negligible, and were significantly lower than those in the three late succession stage soils. In all soils, NO3− was produced almost exclusively by heterotrophic nitrification. The ratio of the gross NO3− immobilization rate to the gross nitrification rate was significantly lower in the GL and CF soils than in the other three soils, indicating a higher NO3− retention capacity in the three late successional stage soils. The combination of higher gross N mineralization, NH4+ and NO3− immobilization turnover and higher NO3− retention promotes a faster N turnover, resulting in greater N availability and N retention capacity in the late successional stages. Such results also suggested that during succession, internal N cycles adapt and develop mechanisms (i.e. increasing soil organic C and N concentration) in response to soil–plant interactions that enhance the soil N supply and N retention.