C and net N mineralisation in a coniferous forest soil: the contribution of the temporal variability of microbial biomass C and N

The microbial biomass is a very important factor that influences C and N mineralisation in soils. In the present study, we investigated the C and net N mineralisation rates, fluxes of C and N through the microbial biomass and a possible connection of the two in the upper 20 cm of an acidic and nutrient poor coniferous forest soil. The experiment was a field study under partially controlled conditions. We used roofed microcosms containing undisturbed soil profiles, exposed to natural temperature conditions and watered by an artificial throughfall of constant quality and quantity. nic N concentration (NO3–N+NH4–N) and the ratio NO3–N to inorganic N in the leachates as well as respired CO2–C were correlated to temperature. We found a net N mineralisation rate of 50.5 kg ha−1 a−1 and a C mineralisation rate of 690 kg ha−1 a−1. Microbial biomass C and N and their ratio varied from 158 to 249, 18 to 51 and 5.0 to 8.1 kg ha−1, respectively, during the 16 months of the experiment. However, the microbial properties were not related to temperature or to C and net N mineralisation rates. The flux of C through the microbial biomass was calculated by two methods. The calculation according to Joergensen (1995) resulted in a C turnover rate of 2.7 a−1 and a C flux through microbial biomass of 537 kg ha−1 a−1. The calculation method of McGill et al. (1986) was shown to give unreliable results. We therefore warn to calculate turnover rates and fluxes through microbial biomass by summing the absolute changes in microbial biomass among sampling dates.