Turnover of carbon and nitrogen in coniferous forest soils of different N-status and under different 15NH4---N application rate

The aim of the study was to compare the allocation of applied 15NH4---N into soil microbial biomass and soil organic matter (SOM) in mor humus samples from a N-poor and a N-rich plot in an experimental Scots pine (Pinus sylvestris L.) forest to clarify the N immobilisation mechanisms. Intact humus cores were incubated for 147 days in the laboratory after addition of 4, 20 and 100 μg N g−1 OM of 15N-labelled (NH4)2SO4. CO2---C evolution, extractable organic C, microbial C, labelled and unlabelled NH4---N, NO3---N, extractable organic N, microbial N, and N in soil organic matter (SOM) were determined. Initially, 15NH4---N was rapidly immobilised in all treatments. Microbial assimilation of 15N increased with increased 15N application rate in both the N-poor and N-rich substrate, indicating that NH4---N availability was limiting. No substrate effect was found. Recovery of 15N in microbial biomass ranged from 0.6 to 25% of added 15N and was characterised by an increase in 15N during the first 14 days and a gradual decrease thereafter for nearly all treatments. The recovery of 15N in microbial biomass and SOM showed reciprocal trends after day 14, suggesting a considerable exchange of N between these pools. At the end of the incubation, 36–74% of added 15N occurred in SOM, 2.4–25% was recovered as 15NH4---N, but almost no 15NO3---N was found, indicating no net nitrification. CO2---C evolution was almost constant during the incubation period and was not affected by substrate differences or 15N application rate. Microbial biomass C was highest in the N-poor substrate and was not affected by the 15N application rate. In conclusion, the study showed that lowering the C:N-ratios from 37 to 24 did not significantly alter the turnover of C and N. The retention of 15N in soil microbial biomass and SOM were similar in both substrates when an equal amount of N was added. Microbial N assimilation followed by incorporation of N in SOM were the main N immobilising mechanisms, both in the N-poor and N-rich humus layers.