A 3‐year continuous record of nitrogen trace gas fluxes from untreated and limed soil of a N‐saturated spruce and beech forest ecosystem in Germany: 1. N₂O emissions

Abstract

For 3 years we followed the complete annual cycles of N 2 O emission rates with 2‐hour resolution in spruce and beech plantations of the Höglwald Forest, Bavaria, Germany, in order to gain detailed information about seasonal and interannual variations of N 2 O emissions. In addition, microbiological process studies were performed for identification of differences in N turnover rates in the soil of a spruce and a beech site and for estimation of the contribution of nitrification and denitrification to the actual N 2 O emission. Both pronounced seasonal and extreme interannual variations of N 2 O emissions were identified. During long‐term frost periods, while the soil was frozen, and during soil thawing, extremely high N 2 O emissions occurred, contributing up to 73% to the total annual N 2 O loss. The enormous N 2 O releases during the long‐term frost period were due to high microbial N turnover rates (tight coupling of ammonification, nitrification, denitrification) in small unfrozen water films of the frozen soil at high concentrations of easily degradable substrates derived from the enormous pool of dead microbial biomass produced during the long‐term frost period. Liming of a spruce site resulted in a significant increase in ammonification, nitrification, and N 2 O emissions as compared with an untreated spruce control site. The beech control site exhibited 4–5 times higher N 2 O emissions than the spruce control site, indicating that forest type itself is an important modulator of N 2 O release from soil. At all sites, nitrification contributed ∼70% to the N 2 O flux, whereas denitrification contributed markedly less (∼30%). There was a significant positive correlation between amount of in situ N input by wet deposition and magnitude of in situ N 2 O emissions. At the beech site, 10% of the actual N input was released from the soil in form of N 2 O, whereas at the spruce site the fraction was 0.5%. N 2 O emission rates were positively correlated with net nitrification rates. The results demonstrate the need for long‐term measurements over several years for more precise estimates of annual N 2 O losses from forest ecosystems. On the basis of our results we conclude that the importance of temperate and boreal forests for the global N 2 O source strength may have been significantly underestimated in the past and that these forests contribute most likely ≫1.0 Tg N 2 O N.