Gross nitrogen transformations in the organic layer of acid forest ecosystems subjected to increased atmospheric nitrogen input

Gross nitrogen transformation rates were simultaneously determined in acid forest litter with the use of 15N pool dilution. The litter was sampled in two coniferous and a deciduous forest ecosystem in The Netherlands at four different times of the year. In addition, in a second experiment the vertical distribution of gross N transformations and microbial respiration in the F + H layer of a Douglas fir forest soil were studied. The gross nitrogen transformation rates indicated clear differences between the forest litter types. Highest gross N mineralization and immobilization rates were measured in the deciduous oak-beech forest litter. Within the coniferous litter, the mixed Scots pine-Douglas fir forest litter revealed the lowest N mineralization and immobilization rates compared to the Douglas fir forest litter. Although N transformation rates fluctuated between the sampling dates, the differences between the sites remained evident throughout the year. The observed differences in N turnover between the forests coincided with differences in substrate quality as indicated by the lignin content, the lignin:nitrogen ratio and the pH of the litter. All N transformation rates were found to be higher in the F1 than in the F2 sub-layer of the organic layer of the Douglas fir site. Differences in nitrification rate between litter from the different forests could not be explained by the occurrence of differences in competition for ammonium. The simultaneous measurement of gross N transformation and (carbon) respiration rates allowed the estimation of model parameters linking N and C dynamics. In the F1 and F2 sub-layers of the Douglas fir site, microbial (carbon) efficiency during this short term incubation experiment was estimated to be 0.54 and 0.59, respectively. The C:N ratio in microbial biomass was estimated to be 11.7 and 12.7 for the F1 and F2 sub-layers, respectively.