Seasonal changes in microbial nitrogen in an old broadleaf forest and in a neighbouring young plantation

Soil microorganisms are actively involved in many processes of the soil N cycle and are strong competitors with plants for soil N. Therefore, microbial dynamics are important factors in controlling forest productivity. Nevertheless, they are poorly studied especially in relation to forest age, which can produce strong effects on the microbial community by affecting the forest floor environment. In the present study, seasonal variations of soil microbial N (N_mic) were monitored in an old floodplain hardwood forest (270 years) and in a young hardwood plantation (19 years) in two soil horizons (0–15 and 15–30 cm). Although the differences according to time of sampling and soil horizon were statistically significant, N_mic was significantly higher in old than in young forest, especially for the deeper soil layer. However, the highest percentage of total N (N_tot) immobilised in microbial biomass was found in the surface soil layer of the young plantation. Soil organic C (C_org) explained 23% of the spatial–temporal variation of N_mic over all sampling periods in the old forest, whereas the linear combination of N_tot, total extractable soil N (N_totex) and the C/N ratio explained 59% of variation in N_mic when considering only the growing season. In contrast, C_org and N_totex explained 59% of variation in N_mic in the young stand when considering all sampling periods and 75% when the analysis was limited to growing season. Soil moisture did not show any significant correlation with N_mic in either site. The sensitivity of N_mic to variation in C_org and N_tot seems to be affected by forest age, being higher in young than in old forest. Finally our results indicate that during the growing season, when the N_totex availability is low, the dynamics of N_mic and N_totex are temporally interdependent, suggesting the existence of a reciprocal control whose mechanisms deserve to be elucidated.