Nitrous oxide emissions and nitrate leaching in relation to microbial biomass dynamics in a beech forest soil

In a 140 year-old beech forest with undergrowth of Allium ursinum, field measurements of N2O and CO2 emissions in conjunction with measurements of microbial biomass-N, extractable mineral nitrogen and NO3−-leaching were made during four vegetation growth periods. We examined whether pulses of nitrate leaching could be accompanied by enhanced N2O emission rates. Highest N2O emission rates (>150 μg N m−2 h−1) were recorded during July, when substantial nitrate leaching also was evident from ion exchange resins and suction cup lysimeters. These nitrogen losses were preceded by mineralisation of decaying Allium leaves and microbial proliferation in June. In July, a significant decline of microbial biomass nitrogen occurred and up to 113 kg N ha−1 were released. Microbial biomass carbon, as determined from substrate induced respiration, also declined in July. With this method, we identified recurring cycles of microbial growth triggered by soil wetting. Soil microbial biomass carbon related inversely to concentrations of extractable sugar carbon substrates. Our study suggests that within nitrogen-enriched forests, nitrate leaching and N2O emissions may be linked during the plant growing season. Nitrogen losses appeared to be strongly affected by biomass turnover and microbial mineralisation.