Temporal variability in soil net nitrogen mineralization among forest regeneration patterns in eastern Tibetan Plateau

The dynamic effects of forest regeneration pattern after clear-cutting primary forest on soil N mineralization (Nmin) have not been well investigated and whether the effects vary with season is unclear in eastern Tibetan Plateau. In this study, we compared soil net Nmin, i.e. net ammonification rate (net Ra) and net nitrification rate (net Rn) among Picea asperata-broadleaf mixed forest (natural regeneration after planting P. asperata, MF), natural secondary forest (natural without assisted regeneration, NF) and P. asperata plantation (reforestation after clear-cutting primary forest, PF) from June to November, and measured soil microbial community and enzyme activities. Forest regeneration pattern significantly altered net Ra as well as net Nmin in June and November. Net Ra of MF, NF and PF in June was -0.29, -0.52 and 0.04 mg kg- 1 d-1, respectively. NF was likely to have higher net NH4+ consumption in June presumably by the greater microbial utilization (biomass and enzymes) directly drove by the larger initial NH4+ concentration. NF had higher net NH4+ production (0.30 mg kg- 1 d-1) in November than MF (0.15 mg kg- 1 d-1) and PF (0.03 mg kg- 1 d-1). Net Rn was insignificantly different among the regenerated forests and ranged between -0.13 and 0.39 mg kg-1 d-1 with a unimodal seasonal pattern. Nitrate dominated the inorganic N pool in PF while it was similar to ammonium in MF and NF in the middle of growing season, possibly suggesting an alteration of N preferences among the regenerated forests. Independent edaphic factors (including soil organic matter and available N) had greater explanations of net Ra variability (19.0%), while net Rn variability was more determined by enzyme activities (21.8%) and microbial community (13.1%). Overall, our results highlighted that the variations in soil net Nmin among three regenerated forests were dependent on season. Ammonium concentration before the start of growing season might play an important role in soil N dynamics. NF exhibited a larger N demand with regard to net Ra, microbial induces and N-acquisition enzymes. These provided essential information for understanding soil N dynamics and for developing management practices of subalpine forests in eastern Tibetan Plateau.