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 (NH4)-N-15-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 mu g N g(-1) OM of N-15-labelled (NH4)(2)SO4. 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, (NH4)-N-15-N was rapidly immobilised in all treatments. Microbial assimilation of N-15 increased with increased N-15 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 N-15 in microbial biomass ranged from 0.6 to 25% of added N-15 and was characterised by an increase in N-15 during the first 14 days and a gradual decrease thereafter for nearly all treatments. The recovery of N-15 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 N-15 occurred in SOM, 2.4-25% was recovered as (NH4)-N-15-N, but almost no (NO3)-N-15-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 N-15 application rate. Microbial biomass C was highest in the N-poor substrate and was not affected by the N-15 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 N-15 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.